1-1

CIBSE Guide B1
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heating
CIBSE Guide B1: 2016
The Chartered Institution of Building Services Engineers
222 Balham High Road, London, SW12 9BS
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heating
The rights of publication or translation are reserved.
© May 2016 The Chartered Institution of Building Services Engineers London
PDF amended November 2016 to incorporate corrections to pages 1-27, 1-30, 1-71,
1-76, 1-89, 1-90, 1-99, 1-100, 1-106, 1-119, 1-129
Registered charity number 278104
ISBN 978-1-906846-73-2 (Print)
ISBN 978-1-906846-74-9 (Online)
This document is based on the best knowledge available at the time of publication.
However no responsibility of any kind for any injury, death, loss, damage or delay
however caused resulting from the use of these recommendations can be accepted
by the Chartered Institution of Building Services Engineers, the authors or others
involved in its publication. In adopting these recommendations for use each
adopter by doing so agrees to accept full responsibility for any personal injury,
death, loss, damage or delay arising out of or in connection with their use by or on
behalf of such adopter irrespective of the cause or reason therefore and agrees to
defend, indemnify and hold harmless the Chartered Institution of Building Services
Engineers, the authors and others involved in their publication from any and all
liability arising out of or in connection with such use as aforesaid and irrespective
of any negligence on the part of those indemnified.
Design, layout and typesetting by CIBSE Publications
Printed in Great Britain by Page Bros. (Norwich) Ltd., Norwich, Norfolk NR6 6SA
Note from the publisher
This publication is primarily intended to provide guidance to those responsible
for the design, installation, commissioning, operation and maintenance of
building services. It is not intended to be exhaustive or definitive and it will be
necessary for users of the guidance given to exercise their own professional
judgement when deciding whether to abide by or depart from it.
Any commercial products depicted or describer within this publication are
included for the purposes of illustration only and their inclusion does not
constitute endorsement or recommendation by the Institution.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
No part of this publication may be reproduced, stored in a retrieval system or
transmitted in any form or by any means without the prior permission of the
Institution.
Foreword
Since the last edition of Guide B in 2005, the European Energy Performance of Buildings Directive
has been introduced. This requires national building energy regulations to be based on calculations
that integrate the impact of the building envelope and the building services systems, formalising
what was already recognised as good design practice. In addition, the use of voluntary energy
efficiency and sustainability indicators has increased.
These changes have influenced the content of Guide B, but the emphasis remains on system design.
The guidance in Guide B is not in itself sufficient to cover every aspect of the effective design of
HVAC systems. Energy (and carbon emission) calculations will also be needed, and a range of other
environmental criteria may specified by the client. These may, for example, include whole-life
costing or assessments of embodied energy or carbon. The balance between building fabric measures
and the energy efficiency of HVAC systems is important, as is the balance between energy use for
lighting and for heating, ventilation and cooling. More detailed information on energy efficiency
and sustainability can be found in Guides F and L respectively. The Guide does not attempt to
provide step by step design procedures: these can be found in appropriate textbooks.
Structure of the Guide
Guide B deals with systems to provide heating, ventilation and air conditioning services, and is
divided into several chapters which are published separately. It will usually be necessary to refer to
several – perhaps all – chapters since decisions based on one service will commonly affect the
provision of others.
——
Chapter B0: Applications and activities focuses on how different types of building and different
activities within buildings influence the choice of system. This chapter is not available in
printed form, but can be downloaded from the CIBSE website. For many activities and
types of building, more detailed design information is available in specialist guidance.
Chapters B1 to B4 address issues relating to specific services. There are usually several possible
design solutions to any situation, and the Guide does not attempt to be prescriptive but rather to
highlight the strengths and weaknesses of different options.
——
B1: Heating, including hot water systems and an appendix on hydronic systems, which is
also applicable to chilled water systems
——
B2: Ventilation and ductwork
——
B3: Air conditioning and refrigeration
——
B4: Noise and vibration control for building services systems (applicable to all systems)
When all chapters have been published, an index to the complete Guide B will be made available.
The focus is on application in the UK: though many aspects of the guidance apply more generally,
this should not be taken for granted. The level of detail provided varies: where detailed guidance
from CIBSE or other sources is readily available, Guide B is relatively brief and refers to these
sources. Examples of this are the treatment in the Guide of low carbon systems such as heat pumps,
solar thermal water heating and combined heat and power. On-site energy generation such as wind
power and photovoltaics are not covered.
Regulatory requirements are not described in detail in the Guide – the information varies between
jurisdictions and is liable to change more rapidly than the Guide can be updated. Instead, the
existence of regulations is sign-posted and their general scope explained. Sometime example tables
are shown, but readers should note that these are simply examples of the type of requirement that is
imposed and may not be current.
While there is some discussion of relative costs, no attempt is made to provide detailed cost figures
as these are too project-specific and variable with time and location.
Roger Hitchin
Chair, CIBSE Guide B Steering Committee
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Guide B provides guidance on the practical design of heating, ventilation and air conditioning
systems. It represents a consensus on what constitutes relevant good practice guidance. This has
developed over more than 70 years, with the Steering Groups for each edition of the Guide expanding
and pruning the content to reflect the evolution of technology and priorities.
Guide B1 Steering Committee
Trimble MEP Division
Paul Barnard
Evinox Energy
Robin Curtis
GeoScience Ltd
Richard Davies
Hoare Lea
Tony Day
International Energy Research Centre
David Hughes
MTT Consultants
Simon MitchellBeond
David Palmer
Campbell Palmer Partnership
Chris Parsloe
Parsloe Consulting Ltd
Martin Ratcliffe
Brunel University London
Martin Wilkinson
Spirotech
Paul WoodsEngie
Acknowledgements
The Committee acknowledges the particular contributions of Martin Ratcliffe as
technical author for this chapter of Guide B and Chris Parsloe as lead author of
Appendix 1.A1.
Permission to reproduce extracts from British Standards is granted by BSI Standards
Ltd. British Standards can be obtained in pdf or hard copy formats from BSI online
shop: www.bsigroup.com/Shop or by contacting BSI Customer Services for hardcopies
only: tel: +44 (0)20 8996 9001, e-mail: cservices@bsigroup.com.
Public information is reproduced under Open Government Licence v2.0.
Referees
Richard Brailsford
Explicatio Consulting
Peter ClackettSkanska
Will Pitt
NG Bailey
Editor
Ed Palmer
CIBSE Editorial Manager
Ken Butcher
CIBSE Technical Director
Hywel Davies
CIBSE Head of Knowledge
Nicholas Peake
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Mike Campbell (Chair)
Contents
1.1
1.3
1.4
1.5
1-1
1.1.1
General
1-1
1.1.2
Other CIBSE publications
1-1
Strategic design decisions
1-1
1.2.1
General
1-1
1.2.2
Purposes of heating systems
1-3
1.2.3
External design conditions
1-6
1.2.4
Site-related issues
1-6
1.2.5
Interaction with building design, fabric, services and facilities
1-7
1.2.6
Occupancy
1-7
1.2.7
UK regulatory requirements
1-7
1.2.8
Energy performance of building regulations outside the UK
1-8
1.2.9
Environmental performance targets
1-9
1.2.10
Economic considerations
1-9
1.2.11
Future requirements
1-10
Design criteria
1-10
1.3.1
General
1-10
1.3.2
Internal design conditions
1-10
1.3.3
Design criteria for human comfort and well being
1-11
1.3.4
Design criteria for other than human comfort
1-13
1.3.5
Environmental performance targets
1-15
1.3.6
Part L Building Regulations 2013 (England)
1-15
1.3.7
Energy and CO2 emissions benchmarks for existing buildings
1-19
1.3.8
NOx, SOx, particulates and greenhouse gases other than CO2
1-19
1.3.9
Environmental assessment schemes
1-19
Choice of system
1-20
1.4.1
General
1-20
1.4.2
System classification
1-21
1.4.3
Choice of centralised or decentralised systems
1-21
1.4.4
Particular applications
1-21
1.4.5
Choice of fuel or energy source
1-23
1.4.6
Choice of heat generator
1-26
1.4.7
Choice of heat emitters
1-28
1.4.8
Choice of distribution medium
1-31
1.4.9
Choice of domestic hot water system
1-31
Heating load calculations and sizing methodology
1-32
1.5.1
General
1-32
1.5.2
Calculation principles
1-32
1.5.3
Room design heating load
1-32
1.5.4
Mechanical ventilation heat loss
1-37
1.5.5
Domestic hot water
1-37
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1.2
Introduction
1.7
1.8
1.9
Distribution losses
1-37
1.5.7
Heat generator peak heating load
1-38
1.5.8
Design margins
1-38
1.5.9
Choice of number and duties of heat generators
1-39
Energy sources
1-39
1.6.1
General
1-39
1.6.2
Factors affecting choice of energy source
1-40
1.6.3
Gaseous fuels
1-41
1.6.4
Liquid fuels
1-42
1.6.5
Solid fuels
1-43
1.6.6
Electricity
1-43
1.6.7
Solar source
1-45
1.6.8
Handling and storage of fuels and regulations
1-45
Heat generators
1-59
1.7.1
Choice of heat source
1-59
1.7.2
Boilers
1-60
1.7.3
Gas-fired boilers
1-62
1.7.4
Oil fired boilers
1-63
1.7.5
Solid fuel boilers (general)
1-64
1.7.6
Solid fuel boilers (biomass)
1-64
1.7.7
Steam boilers
1-65
1.7.8
Combustion of fuels
1-67
1.7.9
Heat pumps
1-69
1.7.10
Combined heat and power
1-74
1.7.11
Solar water heating collectors
1-78
1.7.12
Chimneys and flues
1-78
1.7.13
Corrosion in boilers, flues and chimneys
1-84
Hydronic systems
1-85
1.8.1
General
1-85
1.8.2
Choice of flow and return water temperatures
1-86
1.8.3
General arrangement of LTHW systems
1-87
1.8.4
Secondary circuit(s)
1-88
1.8.5
Primary circuit
1-89
1.8.6
Interface between primary and secondary circuits
1-93
1.8.7
General arrangement of MTHW and HTHW systems
1-95
1.8.8
Integration of renewable/low carbon heat generators
1-96
1.8.9
Heat output rate of heat emitters
1-101
Steam systems
1-105
1.9.1
General
1-105
1.9.2
System design
1-106
1.9.3
Distribution
1-106
1.9.4
Condensate
1-110
1.9.5
Guidance and standards
1-112
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1.6
1.5.6
1.10
1.12
1.13
1.14
1-112
1.10.1
General
1-112
1.10.2
Heat sources
1-113
1.10.3
Distribution
1-114
1.10.4
Heating combined with air conditioning
1-114
1.10.5
Controls
1-116
1.10.6
Other standards and guidance
1-116
Unitary systems
1-116
1.11.1
General
1-116
1.11.2
Indirect gas- and oil-fired heaters
1-116
1.11.3
Direct electric heaters
1-117
1.11.4
Electric underfloor heating
1-118
1.11.5
Stand-alone heat pumps
1-118
1.11.6
Radiant systems characteristics
1-118
1.11.7
Convective heating characteristics
1-119
1.11.8
Controls
1-119
Domestic hot water systems
1-120
1.12.1
General
1-120
1.12.2
Classification of DHW systems
1-120
1.12.3
Regulations relevant to DHW systems
1-122
1.12.4
Generic DHW systems
1-123
1.12.5
Choice of DHW system
1-125
1.12.6
DHW demand and energy consumption
1-127
1.12.7
Solar hot water heating
1-128
1.12.8
Sizing of DHW systems
1-129
Connecting to heat networks
1-131
1.13.1
Introduction
1-131
1.13.2
Existing UK heat network performance
1-131
1.13.3
Key design points for heat networks
1-131
1.13.4
Network-consumer interface
1-132
1.13.5
Implications for design of building heating system
1-132
Operation, maintenance and energy management
1-133
1.14.1
General
1-133
1.14.2
‘Commissionability’ and ‘maintainability’
1-133
1.14.3
Life cycle issues
1-133
1.14.4
Construction (Design and Management) Regulations (UK)
1-133
1.14.5
Operation and maintenance manuals
1-133
1.14.6
Log books
1-134
1.14.7
Energy management, monitoring and targeting
1-134
Appendix 1.A1 Hydronic system design
1-143
Index
1-165
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1.11
Air systems
Introduction
1-1
Heating
1.1
Introduction
1.1.1
General
This chapter of the Guide deals with the selection, design,
commissioning, operation and management of most types of
heating systems in buildings. It deals specifically with nondomestic buildings though much of the contents will apply
to domestic communal heating. Such systems provide space
(including ventilation) heating and/or hot water services and
installations such as swimming pools. Virtually every
building (outside the tropics), contains a heating system. In
most cases its primary purpose is to produce acceptable levels
of thermal comfort – paramount for the health and wellbeing of building occupants and provide domestic hot water
– or to protect the building fabric or its contents.
Heating systems may contribute up to 40% of the capital
costs of the mechanical building services (see Figure 1.1),
and require regular inspections and maintenance.
In the UK, heating systems in non-domestic buildings are
responsible for about 5% (DECC, 2010) of the total
anthropogenic carbon dioxide emissions, together with
other pollutants such as oxides of nitrogen. Data for CO2
emissions for Europe (ECDGET, 2010) suggest that a
similar situation exists in other European states. CIBSE
Guide F contains energy consumption data for a range of
existing buildings in the UK: in existing naturally
ventilated offices, space heating and hot water energy
consumption is around 60% of total energy consumption
and a similar ratio is found for business hotels (without air
conditioning or pools).
Many countries in the world have or are implementing
building energy performance regulations. The Europeanwide Energy Performance of Buildings Directive (EU, 2010)
requires that member states set increasingly stringent
The proper selection, design, commissioning, operation,
maintenance and management of a building’s heating
system is therefore crucial for social, economic, regulatory
and environmental reasons. This Guide starts by
considering the strategic choices facing the heating system
designer, including the requirements imposed by the
intended use of the building, energy and environmental
targets, legal requirements and possible interaction with
other building services. The succeeding sections follow the
various stages involved in the design, installation and
operation of a building heating system. The content of each
section is summarised in Table 1.1 below.
1.1.2
Other CIBSE publications
There are a number of other CIBSE publications dealing
with heating systems in buildings that will be of interest to
the reader. These are set out in Table 1.2.
1.2
Strategic design
decisions
1.2.1
General
40
When using this Guide, the designer should firstly fully
map the design process that is being undertaken. The
process for each application will be unique, but will follow
the general format:
35
——
problem definition
——
choice of appropriate design criteria
20
——
ideas generation
15
——
analysis
——
selection of the final solution
——
detailed design.
45
30
25
10
Ho
te
ls
l
ai
Re
t
or
ts
Ai
rp
s
ie
sit
Un
iv
er
ho
Sc
O
ffi
0
ol
s
5
ce
s
Percentage of mechanical
services capital costs
national regulations governing the energy efficiency and
CO2 emissions associated with heating systems in both newbuild and refurbished buildings. Most buildings already
require an Energy Performance Certificate (EPC) on
construction, sale or rent and public buildings must display
in a prominent location a Display Energy Certificate (DEC).
It is proposed that by 2020, all new buildings be ‘nearly zero
energy’. In addition, Europe sets minimum energy efficiency
standards for energy using equipment including space and
domestic hot water heat generators (EU, 2009a).
Figure 1.1 Typical range of capital costs of heating systems as percentage
of mechanical services
This procedure is illustrated in Figure 1.2 in the form of an
outline flowchart. An online tool (CIBSE Design Compass,
www.cibsedesigncompass.org.uk) is available that includes
design, tendering, installation and commissioning stages.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1 1-2
Heating
Problem
definition
Select appropriate design criteria:
External design conditions (eg winter design temperatures for heating and frost
protection, humidity, wind speed)
Internal design conditions (eg operative temperature, air velocity, noise)
Maximum surface temperatures of heat emitters
Building pre-heat times
Environmental targets (eg CO2 and other emissions, BREEAM)
Maximum capital and/or life cycle costs
Consider improvements to building thermal performance:
Facade, insulation, air tightness, thermal mass
Ideas
Consider choice of system:
Room heat emitter types
Distribution medium (eg water, air)
Heat generation (eg boilers, heat pumps, CHP)
Fuel or energy (including renewable energy)
Method of domestic hot water generation
Assess potential solution in terms of meeting:
Regulations and planning policies
Indoor design criteria
Any special requirements
Spatial and site constraints
Economics
Environmental targets
Analysis
Determine heat loads:
Hot water demand
Room design heating load including any pre-heat
Ventilation design heat loss
Emitter duties
Distribution systems heat losses
Select final solution
System
selection
and
detailed
design
Carry out detailed design:
Plant/equipment number and sizing
Determine standby capacity
Produce control strategies and energy metering strategy
Assess commissionability and maintenance
Check environmental performance
Figure 1.2 Flow chart of design process
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Identify:
Purpose of the heating system
Areas to be heated
Building thermal performance
Fuels/energy sources available
Spatial limitations
Applicable Regulations (eg Building Regulations, Health and safety) planning policies
Economic constraints
Any special requirements
Strategic design decisions
1-3
Table 1.1 Summary of Guide B, chapter 1: Heating
Title
Comments
1.2
Strategic design decisions
This section is relatively broad ranging and discursive and is intended to be read from time to time as a
reminder of the key decisions to be taken at the start of the design process.
1.3
Design criteria
Sections 1.3 to 1.5 deal with design matters appropriate to all forms of heating. Section 1.3 sets out the
various design criteria for a heating system in terms of performance and compliance with regulations.
1.4
Choice of system
Section 1.4 discusses how a heating system is selected in terms of heat emitters, fuel, heat generator and
distribution medium.
1.5
Heating load calculations and
sizing methodology
Section 1.5 sets out the calculations required to size the components of a heating system with reference
to CIBSE Guide A.
1.6
Energy sources
1.7
Heat generators
Sections 1.6 and 1.7 deal with the various fuels available and heat generation plant including details of
boilers and flues, heat pumps, solar hot water, chp and district heating.
1.8
Hydronic systems
1.9
Steam systems
1.10
Air systems
1.11
Unitary systems
1.12
Domestic hot water systems
Sections 1.8 through 1.11 deal with the generic heating systems – hydronic, steam, air and unitary. Note
that the design of pipework for hydronic systems is dealt with in the Hydronic system design appendix of
this Chapter which also deals with cooling systems.
A separate section is provided for domestic hot water systems and includes advice on sizing specific to
dhw.
1.13
Connecting to heat networks
Briefly discusses the implications for the design of a building’s heating system if supplied from a
community or district heating network.
1.14
Operation, maintenance and
energy management
Relates to post-design issues and will be useful not only to facilities management staff but also to
designers to assist in the creation of a successful heating system.
1.A1
Hydronic system design
An overview of the main issues that need to be considered during the design of hydronic systems.
A successful heating system design will result in a system
that can be installed, commissioned, operated and
maintained at reasonable cost to deliver the indoor desired
design conditions whilst meeting environmental targets
and all applicable regulations and planning policies. When
in operation, it should operate safely and reliably and
should sustain its performance over its planned life without
excessive maintenance.
The following sections discuss the key questions that need
to be answered in ‘defining the problem’ as a first step in the
process to achieving a successful heating system design.
These are:
——
processes going on within the building including
swimming and spa facilities
——
the protection of the building’s contents
——
the prevention of condensation
——
frost protection of the building and the building
services systems
——
the production of domestic hot water.
1.2.2.1
Thermal comfort of human occupants
Thermal comfort for people requires that the thermal
environment of the building allows occupants to carry out
their activities whilst maintaining acceptable body and
skin temperature and rates of water loss through respiration
and perspiration. The quality of the thermal environment
is characterised by:
——
its purpose
——
site related issues and constraints
——
interaction with building and other services
——
occupancy
——
local air temperature and speed
——
legal and regulatory requirements
——
humidity
——
environmental performance targets
——
room radiant temperatures and distribution.
——
economic constraints
——
future requirements.
1.2.2
Purposes of heating systems
Heating systems in buildings may be required to provide
indoor conditions conducive to some or all of the following:
——
the well-being of occupants (humans, animals or
plants)
There are a number of methods of quantifying indoor
thermal environment which take some or all of these into
account and produce a single-valued index. CIBSE
recommends the use of operative temperature. This is
discussed further in section 1.3.3 of this Guide and in detail
in CIBSE Guide A, chapter 1. As the operative temperature
required for thermal comfort depends upon occupant
activity and clothing, recommended values are given for a
range of building types. The choice of design indoor
temperatures will have a significant impact on energy
consumption by the heating system.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Section
1-4
Heating
Table 1.2 CIBSE publications relevant to heating systems in buildings
Title
Scope and outline content
Guides
A: Environmental design
Thermal comfort criteria, external design conditions, thermal performance of buildings and
infiltration, heat loss calculations.
C: Reference data
Heat emissions from pipes, fuel characteristics.
F: Energy efficiency
Guidance on reducing energy consumption at design stage, heating energy consumption
benchmarks, energy management.
G: Public health and plumbing
Hot water demand calculations, safety issues and regulations. steam systems.
H: Building control systems
Heating system controls.
L: Sustainability
Guidance on assessing and reducing environmental impact of building services. (At time of
publication Guide L is withdrawn and under revision).
M: Maintenance engineering and
management
Maintenance of building services, monitoring of energy consumption, commissioning,
production of O&M manuals, owing and operating costs, condition surveys.
AM12: Small-scale combined
heat and power for buildings
Deals with the selection and design of chp systems in buildings of up to about 1 MWe.
Concentrates on spark ignition engine systems.
AM14: Non-domestic hot water
heating systems
A detailed guide to the design, installation and commissioning of lthw heating systems for
buildings. Deals with the particular requirements of both new-build and refurbishment projects.
AM15: Biomass heating
Detailed design guidance on biomass boiler systems and hydronic arrangements, primarily
covering boilers in the range of 50 kW to 5 MW burning woodchips or wood pellets.
TM13: Minimising the risks of
Legionnaires’ disease
Deals with the design and management of hot and cold water services in buildings and cooling
tower systems to minimise risk from Legionella bacteria.
TM29: hvac strategies for wellinsulated and air tight buildings
Examines the options for heating and cooling systems for buildings with very high performance
envelopes.
TM36: Climate change and
internal environment
Presents the results of computer simulations of hvac energy demand in the future. Gives
estimates of reductions in heating degree days.
TM38: Renewable energy
sources for buildings
Guidance given on the integration of solar thermal, photovoltaics, district heating, chp, ground
source heat pumps and biofuels in buildings. Based on the use of the CIBSE RESET software.
TM39: Building energy metering
Promotes good practice in energy metering in non-domestic buildings including UK Building
Regulation requirements. Intended for use by designers and facility managers.
TM41: Degree days
Provides data and guidance on the use of heating degree days for heating energy consumption
prediction and energy management.
TM48: The use of climate change
scenarios for building simulation:
the CIBSE future weather years
Provides details of how the try and dsy for future years were created. Applies to 14 UK sites up
to 2080. See also Probabilistic Climate Profiles - The effective use of climate projections in building
design (ProCliPs) (CIBSE, 2014).
TM51: Ground source heat
pumps
Offers an understanding of the technology used, comparisons of different heat pump systems
and clarity on Building Regulations surrounding the use of pumps.
TM58: Design and operation of
modern steam systems
Basic guidelines on how a modern steam system should be designed and commissioned,
including energy efficiency, training, maintenance.
KS4: Understanding controls
Provides an introduction to control systems for building services.
KS8: How to design a heating
system
An introduction to heating systems in buildings and choice of system
KS14: Energy efficient heating
systems
An introduction to the design of buildings and building heating systems to minimise energy
consumption and CO2 emissions.
KS15: Capturing solar energy
An introduction to the types of solar thermal and electric systems available, their applications
and their design, installation and management.
A: Air systems
Setting to work and commissioning of mechanical ventilation systems.
B: Boilers
Setting to work and commissioning of boilers for lthw and mthw systems.
W: Water distribution
Setting to work and commissioning of hydronic pipework systems.
CIBSE/BSRIA
Guides
Illustrated Guide to Mechanical
Building Services, BG31/2012
An introduction to a wide range of mechanical services including heating.
Software tools
CIBSE Design Compass
Deals with the design through to commissioning of a range of building services including
heating. Available as an online tool at www.cibsedesigncompass.org
Applications
Manuals
Technical
Memoranda
Knowledge
Series
Commissioning
Codes
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Series
Strategic design decisions
1-5
Table 1.3 Regulations relating to heating installations in England
Scope
Statutory Instruments 1974
No. 2160 Fuel and Electricity
(heating) (control) (HMSO,
1980)
This sets a maximum temperature of 19 °C to which a non-domestic building can be heated, except where a higher
temperature is required (e.g. health).
England and Wales:
Part L Building Regulations
Applicable to most building types. Sets minimum standards for energy efficiency of building fabric, plant and
systems and maximum emissions of CO2. Also sets minimum standards for system controls and commissioning.
Applies to new-build and refurbishment and the replacement of heating equipment. Energy Performance
Certificates are required on construction, sale or rent and public buildings in addition require Display Energy
Certificates. Part L is currently applicable to both England and Wales, though it should be noted that the Welsh
Government has issued amendments to the 2010 Building Regulations.
Scotland:
Scottish Building Standards
Northern Ireland:
Building Regulations
(Northern Ireland)
See section 1.2.7.1 for a discussion of this statute.
Scotland and Ireland produce their own regulations but are similar in substance to Part L.
For further details see section 1.3.6
England and Wales:
Part F Building Regulations
Sets minimum standards of ventilation in most new-build buildings. Part F is currently applicable to both England
and Wales, though it should be noted that the Welsh Government has issued amendments to the 2010 Building
Regulations. Scotland and Ireland produce their own regulations that are similar to Part F.
England and Wales:
Part J Building Regulations
Concerned with the safe installation of heat generators and flues. Part J is currently applicable to both England and
Wales, though it should be noted that the Welsh Government has issued amendments to the 2010 Building
Regulations. Scotland and Ireland produce their own regulations that are similar to Part J.
This is discussed further in section 1.7.
The Clean Air Act (1993)
Gives local authorities the right to set ‘smoke-free zones’ and so limit emissions of gaseous and particulate pollutants
from flues.
England, Scotland and Wales:
Fluorinated Greenhouse
Gases (FGG) Regulations
2009
Intended to minimise the emissions of a number of powerful greenhouse gases. These include fluorinated
hydrocarbons currently used as refrigerants in heat pumps.
Northern Ireland:
Fluorinated Greenhouse
Gases Northern Ireland)
Regulations 2012
Health and Safety
at Work Act
Workplace (health, safety and welfare) Regulations set minimum indoor temperatures that must be maintained
within the workplace for the health and well-being of building occupants during occupancy times. Building Bulletin
BB87 (DfE 2003) and BB101 (DfE, 2006) sets minimum standards of temperature and ventilation in schools
Control of Substances
Hazardous to Health
Guidance, in the form of an Approved Code of Practice (HSE, 2000) and explanatory booklets, on measures that
should be taken to reduce the risk of infection from bacterial growth in building services. CIBSE TM13: 2013 sets
out these requirements.
Animal Welfare Act
Sets minimum standards for the indoor environmental quality in buildings for the well-being of animals.
Welfare of Farmed Animals
Regulations
Set minimum standards for the indoor environmental quality in buildings for the well-being of animals.
Construction (Design and
Management) Regulations
Relate to the occupational health, safety and welfare in construction. They place duties on clients, designers and
contractors in relation to construction projects.
Construction (Design and
Management) Regulations
(Northern Ireland)
Regional and local planning
policies
May set minimum standards for energy efficiency or CO2 emissions which can have an impact on the selection and
design of heating systems. Some planning authorities have, or are intending to, implement policies that place greater
emphasis on the use of decentralised energy (particularly chp) and community or district heating networks and/or
the integration of renewable energy. Local planning policies are available from Local Authority websites.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Regulation/Policy
1-6
1.2.2.2
Heating
Thermal comfort of animals
1.2.2.3
Designing for building fabric,
contents or processes
In buildings (or parts of buildings) that are not normally
occupied by people, heating may not be required to
maintain comfort. However, it may be necessary to control
temperature or humidity in order to protect the fabric of
the building, its contents, processes going on in the building
or the heating system itself, e.g. from frost or condensation.
Where protection from frost or condensation is provided,
the operation of the heating system is likely to be highly
intermittent so that a dynamic thermal analysis should be
carried out. A methodology of assessing risk from surface
and interstitial condensation is given in CIBSE Guide A.
Swimming pools and spa facilities require heating systems
both for the heating of the water and the room in which
they are housed both for the comfort of users and order to
protect the building fabric from condensation. Temperature
requirements can vary considerably depending upon use.
Guidance on water temperatures are given in CIBSE Guide
G: Public health engineering. Guide G also gives
recommendations for the air temperature and humidity in
the pool hall.
Where the primary purpose of the heating system is to
create a suitable indoor environment for an industrial
process, specific requirements should be obtained from the
client. This might include limitations on the rate of change
of temperature with time and humidity.
1.2.2.4
Domestic hot water
This is the provision of hot water to serve taps, showers and
other sanitary fittings. Buildings such as hotels and leisure
centres tend to have very high demands for domestic hot
water, often exceeding space heat requirements. Storage
systems should be considered as a method of reducing peak
demand by extending generation times. Such hot water
demands will be year round rather than restricted to the
heating season. As such, consideration should be given to
separate systems for the generation of domestic hot water.
The design and operation of domestic hot water systems is
governed by Health and Safety regulations to reduce the
growth of bacteria. The Health and Safety Executive's
HSG274 (HSE, 2014) and CIBSE TM13 both deal with the
design, operation and maintenance of hot water systems to
minimise the growth of Legionella pneumophilia and other
bacteria. The temperature of the hot water at the point of
delivery in some buildings is governed by regulations
designed to prevent scalding.
Both centralised and local systems of hot water generation
are common. Centralised systems offer advantages in terms
of maintenance but require extensive distribution pipework
that can result in large heat losses and increased risk of
bacterial growth. Where hot water demand is relatively
small, as in offices, local generation of is common.
The selection, sizing, design, operation and management of
domestic hot water systems is discussed in section 1.12.
Further details are provided in CIBSE Guide G: Public
health engineering and in BS 8558 (BSI, 2011).
1.2.3
External design conditions
It is normal practice to select an external design temperature
that is higher than the minimum recorded. This introduces
a degree of risk related to the frequency with which outdoor
temperatures can be expected to fall below the design value
selected.
For space heating, a risk level of 1% is typical as the thermal
inertia of the building will mitigate the impact of outdoor
temperature falling below design. Section 1.3.3.4 gives
recommended outdoor design temperatures based on
location and the thermal inertia of the building. Heating
coils in ventilation plant may be susceptible to freezing and
so a lower outdoor design temperature than selected for
space heating is normal practice, often an average of the
extreme temperatures of the last 10 years is used.
Selecting for a higher design outdoor temperature will
reduce peak heating loads, physical size of plant, space
requirements and capital costs. The heating system is also
likely to operate with a higher seasonal efficiency. However
the increased risks and implications should be discussed
with the client.
Weather data is based on historical records. Consideration
should be given to making allowances for anticipated
climate change over the life of the building or heating
system, see section 1.2.10.
Wind speed affects the surface resistance on the outside
surfaces of buildings, with the resistance decreasing rapidly
with increasing wind speed. Coastal and hilly sites will
generally lead to higher wind speeds. Wind speed will also
increase with building height. For relatively poorly
insulated constructions, such as glazing, this can result in
significantly higher U-values and therefore increased heat
loss. CIBSE Guide A, chapter 3, gives corrections for
different exposures.
Wind speed will also strongly affect infiltration rates.
Infiltration is often the cause of much of the heat loss from
buildings, in some cases exceeding the fabric heat loss
Details are given in CIBSE Guide A, chapter 4.
1.2.4
Site-related issues
In addition to exposure, the number of buildings, their size
and height and the plant space available must be considered.
Site access for plant installation will influence type and size
of heat generating plant and associated equipment (e.g.
storage vessels). Consideration must also be given to
maintenance and replacement of plant.
Sites with multiple buildings might be better served by a
single, site-wide system with centralised plant rooms giving
greater scope in choice of fuel (e.g. chp or biomass with
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Animal houses for research or farming have specific
requirements and are often covered by regulations. These
are discussed in section 1.3.4 together with recommended
values for a range of applications.
As domestic hot water uses water continuously from the
water mains, issues of scaling in hard water areas must be
addressed.
Strategic design decisions
Large buildings will have higher heat loads and the need
for more extensive distribution systems that will influence
type of heating system. Tall buildings may require hydronic
systems to be hydraulically split to limit static pressures.
The availability of mains gas or heat supplies, such as
district heating or waste heat from processes, is a key factor
affecting the choice of fuel as are site access and space for
delivery and storage of fuel (e.g. liquid fuels and biomass).
1.2.5
Interaction with building
design, fabric, services and
facilities
The earlier the heating system designer can be involved in
the overall design process, the greater the scope for
optimisation. The layout of the building, the size and
orientation of windows, the extent and location of thermal
mass within the building, the levels of insulation of the
building fabric and the air tightness can all have a significant
effect on demand for heat. The airtightness of the building
shell and the way in which the building is ventilated are
particularly important, especially where the building is
well insulated as the fraction of the heat load imposed by
infiltration and/or ventilation increases. The designer
should therefore give consideration to improving the air
tightness and thermal performance of the building beyond
the minimum standards required by Building Regulations.
Buildings that are very well insulated and airtight may have
little or no net heating demand when occupied as heat gains
can offset heat losses. In such buildings, space heating
systems should be designed principally for pre-heating
prior to occupancy or for the provision of hot water and
mechanical ventilation. Conventional heating systems
might not be appropriate and separate hot water generation
plant may be best.
However, the designer is often faced with a situation in
which there is little or no opportunity to influence
important characteristics of the building that have a strong
bearing on the heating system, particularly in the
replacement of an existing heating system. For example,
there may be constraints on the area and location of plant
rooms, the space for and the routing of distribution
networks. There may also be a requirement to interface
with parts of an existing system, either for heating or
ventilation.
1.2.6
Occupancy
Where the building is to be occupied intermittently, as is
the case for most buildings, the annual heating energy
requirements can be reduced by operating the heating
system intermittently. The extent of such savings will
depend in part upon the thermal inertia of the building
(and individual rooms) with thermally lightweight
constructions resulting in the greatest savings in energy
and fuel costs. The peak output of intermittently operated
heating systems will however need to be increased beyond
the design steady-state heat loss to allow for acceptable preheat time, so increasing capital costs. At the same time,
shorter pre-heat periods will reduce energy consumption
prior to occupancy. There is therefore an optimum preheat
time that will result in the lowest life cycle cost. This is
discussed more fully in section 1.5.
Not all zones of the building may experience the same
occupancy patterns or level of thermal comfort. Centralised
heating systems may not be most appropriate or if used
should be able to provide independent (temperature and
time) control in different zones. More than one heating
system may be appropriate. Where domestic hot water is
required, a decision is required on whether it should be
heated by the same heat generator as the space heating, by a
separate centralised system or heated at the point of use.
It is common practice, particularly for intermittently
occupied buildings, to ignore useful heat gains from
processes, equipment and lighting in determining peak
heating demand. However, internal heat gains (together
with solar gains), will influence instantaneous heating
demand. The ability of the control system and speed of
response of the heat emitters, distribution system and heat
generation plant should be assessed.
1.2.7
UK regulatory requirements
Various strands of legislation affect the design,
commissioning, operation and maintenance of heating
systems. As an example, those applicable to the UK are
summarised in Table 1.3 above. In some instances,
regulations in England, Scotland, Northern Ireland and
Wales differ.
1.2.7.1
Statutory Instruments – fuel and
electricity
For applications in the UK it should be noted that the Fuel
and Electricity (Heating) (Control) (Amendment) Order (1980)
(HMSO, 1980) prohibit the use of fuels or electricity to heat
premises above 19 °C. This does not mean that the
temperature in buildings must be kept below 19 °C but only
that fuel or electricity must not be used to raise the
temperature above this level. For some applications, the
recommended winter design temperatures exceed 19 °C. In
these cases, it is assumed that these temperatures can be
maintained by contributions from heat sources other than
the heating system. These may include solar radiation, heat
gains from lighting, equipment and machinery and heat
gains from the occupants themselves. This does require
that adequate zone temperature controls are installed.
The regulations do not apply to dwellings and some nondomestic buildings are exempted where:
(1)
there exists any other regulation that specifies a
higher temperature must be provided
(2)
a higher temperature is required for the health or
well being of any person who is ill, disabled, infirm,
pregnant, under 5 years of age or over 60 years of
age
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
thermal storage) and plantroom location (with heavy and
potentially noisy plant sited further away from sensitive
areas) , and a reduction in total plant area. The requirement
for hydraulic separation of buildings should be assessed.
Alternatively, individual and independent heating systems
might better suit sites where there is wide variation in the
operating times of each building.
1-7
1-8
Heating
a higher temperature is required for the health of
any animal
states, e.g. California and Florida, have developed their
own codes that are considered more stringent.
(4)
industrial, agricultural or research processes or
food preparation requires a higher temperature
(5)
a higher temperature is required to prevent damage
to food, materials, crops or equipment.
The codes apply to new-build and refurbishment and cover
the external envelope (U-values, air tightness and avoidance
of condensation), and the efficiency of hvac, lighting and
dhw installations. Compliance is shown by means of
accredited software.
1.2.8
Energy performance of
building regulations outside
the UK
The following is not meant to be an exhaustive list of
countries that have implemented building energy efficiency
regulations but rather as an indicator of the extent of such
regulations worldwide and the different approaches
adopted. The International Building Energy Codes Portal
provides up to date information relating to around 20
countries worldwide (GBPN, 2016).
1.2.8.1
Member States of the European
Union
The European Union has set down a number of Directives
that have a bearing on the design of buildings and building
services. Directives set out objectives within a framework
and timescale that all member states must comply with
through the enactment of national regulations.
The Energy Performance of Buildings Directive (EU, 2010)
sets out requirements for reducing CO2 emissions in most
types of building by means of improved thermal insulation,
energy efficiency and the use of renewable energy and sets a
2020 deadline for new ‘nearly zero energy’ buildings. It also
sets the requirements for EPCs and DECs and how energy
performance is assessed.
The Eco-Design of Energy Related Products Directive (EU,
2009a) sets minimum energy efficiency standards for a wide
range of products including most building services
equipment (boilers, heat pumps, fans, pumps etc.), together
with means of testing and labelling. This includes a
program of gradual improvements in minimum efficiency.
These two frameworks form the basis for the UK Building
Regulations and those developed by other member states.
The European Renewable Energy Directive (EU, 2009b)
required each member state to produce an action plan for
achieving a 2020 target figure for the uptake of renewable
energy technology. In the UK this has been partially
responsible for the Feed-In Tariff and Renewable Heat
Incentive schemes (see section 1.2.10).
1.2.8.2
United States of America
The US Department of Energy produces a set of Building
Energy Codes (US DoE, 2013) for residential and nonresidential (‘commercial’) buildings. These set minimum
standards for the building and the building services and are
updated every three years. The codes are based on ASHRAE
90.1 (ASHRAE, 2013), extended to included low-rise
residential. Individual states are required to adopt and
adapt these codes, taking into account the wide-ranging
climates of the USA, or develop equivalent codes. Some
1.2.8.3
Canada
The National Energy Codes of Canada for Buildings
(NRCC, 2015) set out minimum energy efficiency
requirements for the external envelope, hvac, lighting,
dhw, electrical systems and motors. Compliance is shown
by means of accredited software. The current codes are
designed to give a 25% reduction in energy compared to
1997.
1.2.8.4
Australia
The Building Energy Code of Australia (ABCB, 2010)
applies to residential and non-residential new-build and
refurbishment. It sets out minimum standards for the
external envelope, hvac, lighting, dhw, swimming pools
and spas and covers access for maintenance and the
monitoring of energy use. Compliance is demonstrated by
meeting these minimum standards or by demonstrating a
reduction in energy consumption using modelling software.
Individual states are required to adopt and adapt the code
to suit their particular climate or produce their own
equivalent code.
1.2.8.5
China
The Ministry of Housing and Urban-Rural Development
has responsibility for the National Building Energy Codes.
These are developed and disseminated by the China
Academy of Building Research.
The Design Standards for Energy Efficiency of Public
Buildings (MHURD, 2005) applies to all non-domestic
buildings and concentrates on the building envelope and
hvac installations. The Design Standards are modified to
take into account the five climatic zones of China that vary
from severe cold to warm winters and hot summers.
China has plans for numerous ‘zero-carbon cities’.
1.2.8.6
Japan
The Rationalisation of Energy Use in Buildings (ANRE,
2006) sets two key targets that must be met:
(1)
Perimeter annual load: the annual energy used in
MJ/m2 per year.
(2)
Coefficient of energy consumption: ratio of energy
consumption to load.
Different targets are set for hvac, lighting, dhw and
vertical transportation depending upon the type of building
(e.g. hotel, office) and whether it is passively of mechanically
cooled.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
(3)
Strategic design decisions
1.2.8.7
India
The code applies to commercial buildings with a connected
load greater than 100 kW. Individual states may adapt the
code to suit their climatic zone.
1.2.9
Environmental performance
targets
Clearly, the performance of the heating system has a major
influence on energy consumption and carbon dioxide
emissions, particularly in an existing building with
relatively poor insulation. The designer has the opportunity
to influence it through adopting an appropriate design
strategy and choice of fuel, by specifying components with
good energy performance, and by devising a control system
that can accurately match output with occupant needs.
Particular aspects of energy efficiency are dealt with in
other sections of this Guide as they arise. The energy
efficiency of heating and hot water systems is dealt with in
detail in CIBSE Guide F: Energy efficiency in buildings.
Beyond strictly legal requirements, the client may wish to
meet energy and environmental targets, which can depend
strongly on heating system performance. Consideration
should be given to:
——
Achieving higher levels of energy efficiency and/or
lower CO2 emissions than required under current
building energy codes/regulations.
——
Applying
broader
ranging
environmental
assessments such as the UK Building Research
Establishment Environmental Assessment Methodology
(BREEAM) and the North American Leadership in
Energy and Environmental Design (LEED). These set
a series of best practice criteria against which
aspects of the environmental impactof a building
can be assessed including construction and use. A
high rating depends strongly on the performances
of the building fabric and heating system. Further
details of BREEAM and LEED are provided in
section 1.3.9.
——
——
Utilising low carbon energy sources. Carbon
dioxide emissions are proportional to the carbon
intensity factor (kgCO2/kW·h) of the fuel used. The
carbon intensity factor for most forms of fossil and
non-fossil fuels are prescribed in the Standard
Assessment Procedure (SAP) documentation
(BRE, 2012). Note that whilst some fuels may have
relatively low CO2 emissions, there may be other
environmental impacts, particular in relation to the
production of some biofuels.
Utilising fuels and heat generators with lower SOx,
NOx and particulate emissions than required by any
national or local regulations.
1.2.10
Economic considerations
Regulations will specify minimum permissible standards of
energy efficiency including the thermal insulation and air
tightness of the building fabric, the efficiency of heating
plant and equipment and provision of heating system
controls. However, higher standards may give economic
advantages. Economic appraisal of such higher standards
should be undertaken to show optimum levels of investment
according to the client’s own criteria, which may be based
on a simple pay­back period or a specified discount rate over
a given lifetime. This should also include options for
heating system type and fuels. Public sector procurement
policies may specifically require life cycle costing.
1.2.10.1
Renewable Heat Incentive
The Renewable Heat Incentive (RHI) is a UK Government
scheme that commenced in December 2011 and will be
reviewed every four years. It applies to England, Scotland
and Wales: Northern Ireland is currently reviewing such a
scheme. Certain renewable heat technologies qualify for
quarterly payments based on various tariffs and the amount
of renewable heat produced. The payments are intended as
incentives to increase the uptake of renewable heating and
complies with the European Renewable Energy Directive.
At present the following rules apply (Ofgem, 2014):
——
payments are for non-domestic systems or
community systems serving more than one dwelling
(single domestic schemes will be provided for under
a parallel scheme)
——
payments are only available provided no public
funds have been received towards the capital or
installation costs
——
the technology was installed and commissioned on
or after 15 July 2009.
——
the technology must be one of the following heating
technologies:
——
solid biomass;
——
ground or water source heat pump;
——
geothermal
——
solar thermal (<200 kW)
——
biogas combustion (<200 kW)
——
any installation below 45 kW must be meet the
requirements of the Micro Generation Certification
Scheme
——
the heat generated must be used in a building for
either space heating, water heating or process
heating.
Payments will be made quarterly for a period of 20 years
and based on metered readings of renewable heating
produced and a set of published tariffs (p/kW·h) that will
depend on size of installation and the type of renewable
technology installed.
The method of metering is set out in the RHI regulations
and will depend upon the type of renewable heating, the
installed capacity and the arrangement of heat distribution.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
India introduced its energy code for commercial buildings
in 2007 (GBPN, 2013). This is a voluntary code that can be
adopted by individual states in India. The code is produced
by the Bureau of Energy Efficiency. As of 2014 approximately
one-third of the states had made the code mandatory.
1-9
1-10
Heating
1.2.10.2
Feed-in tariffs
——
photovoltaics
——
wind turbines
——
hydro-electric
——
anaerobic digestion (producing biogas for use in
electrical generation)
——
micro-chp (<2 kW).
Tariffs paid vary with each type of technology and size of
installation. The anaerobic digestion could include
combined heat and power.
1.2.10.3
Enhanced Capital Allowance
A UK business can normally offset capital expenditure on
plant and equipment against tax liabilities at 20% of the
total cost per year. The Enhanced Capital Allowance scheme
allows the full cost to be claimed in the first year. To qualify,
the plant/equipment must meet minimum standards of
energy efficiency and be included on the Energy Technology
List maintained on behalf of the Department for Energy
and Climate Change by the Carbon Trust. The list includes:
——
boilers
——
combined heat and power
——
heat pumps
——
solar thermal systems
——
heat recovery
——
energy monitoring.
To assist in the production of specifications, minimum
standards of each are set out in detail.
1.2.10.4
Climate Change Levy
The Climate Change Levy is a tax applied to UK nondomestic users of electricity, natural gas and lpg (with some
exceptions for energy intensive industries and transport).
The levy is increased periodically in line with inflation.
Energy from renewable sources and electricity from
qualifying chp are exempt.
In the past, historical weather data has be used as the basis
for selecting appropriate outdoor design temperatures and
predicting energy consumption of heating systems. To
assist the designer in assessing the likely effects of climate
change, CIBSE has developed a set of future weather files.
These future hourly weather files, based on the existing
Design Summer Years (DSYs) and Test Reference Years
(TRYs) which incorporate the UKCIP09 climate change
scenarios, are available for 14 sites, for three time periods –
2020s (2011-2040), 2050s (2041-2070) and 2080s (2071–
2100). 10th, 50th, 90th percentiles are provided for the
following emissions scenarios:
——
2020s: High
——
2050s: Medium
——
2050s: High
——
2080s: Low
——
2080s: Medium
——
2080s: High
Supporting guidance on use of this data includes Probabilistic
Climate Profiles - The effective use of climate projections in
building design (ProCliPs) (CIBSE, 2014) and TM48: The
Use of Climate Change Scenarios for Building Simulation: the
CIBSE Future Weather Years (CIBSE, 2009).
The use of the building may also change within the life
span of the heating system, including the ability by the
client to reconfigure zones. The client should be consulted
to ensure that s/he can carry out such changes with minimal
work required on the heating system.
1.3
Design criteria
1.3.1
General
Having reviewed the principal strategic decisions, it is
necessary to establish the design criteria for the system .
These include:
——
internal design conditions
——
external design conditions
——
environmental targets.
For large users of energy in some sectors it is possible to
sign up to a Climate Change Agreement whereby the levy is
substantially reduced provided that the user meets agreed
energy saving targets.
These have been briefly introduced in section 1.2 and are
now discussed in greater detail below.
1.2.11
As discussed in the previous section, the choice of
appropriate internal design conditions will depend foremost
on the purpose of the heating system identified during the
problem definition stage. These include the creation of an
internal environment that provides for the:
Future requirements
UK Building Regulations are to be updated regularly and
with a staged program for reducing greenhouse gas
emissions from non-domestic buildings that will result in
1.3.2
Internal design conditions
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The UK Government has introduced a feed-in tariff
scheme, starting in April 2010, whereby the owners of
renewable electricity generation plant less than 5 MW
capacity are eligible for payments based on the total amount
of electricity generated plus an additional payment for any
electricity exported. Currently applies to England, Wales
and Scotland with Northern Ireland reviewing a scheme.
The technology is limited to:
‘nearly zero energy’ by or before 2020 in keeping with the
EPBD 2010. In addition, there are likely to be further
tightening of the minimum standards of building thermal
performance. Other European countries are likely to do the
same. Outside Europe, other countries have similar plans
in an attempt to meet their commitments to reducing CO2
emissions under the Kyoto agreement.
Design criteria
1-11
——
well being of animals
——
growing conditions for horticulture
——
protection of building fabric or contents from
condensation or frost
——
special requirements of sports facilities, clean
rooms, laboratories and processes.
These are discussed in turn including the basis of the choice
of internal design conditions and CIBSE recommendations.
Statutory regulations may also apply. These are listed in
section 1.2.
1.3.3
Design criteria for human
comfort and well being
Heating systems in most buildings are principally required
to maintain comfortable conditions for the people using the
building. This requires a balance between their metabolic
heat rate and rate of heat loss to their surroundings. The
human body exchanges heat with its surroundings by
convection, radiation and evaporation (respiration and
perspiration). Conduction is usually negligible. Thermal
comfort therefore depends on:
——
local air temperature
——
local air velocity
——
local radiant temperature
——
room humidity
——
occupant’s clothing
——
occupant’s metabolic rate (a function of activity, e.g.
office work, heavy physical work).
1.3.3.1
PMV index and PPD
A method of assessing human thermal comfort is the
predicted mean vote (pmv) index, as set out in the European
Standard BS EN ISO 7730 (ISO, 2005a) and described in
detail in CIBSE Guide A, chapter 1. An overview only is
given here. pmv incorporates all of the factors listed above.
The calculation produces a number (the pmv) which can be
positive or negative. A zero value indicates that the
perceived room thermal environment is neither too warm
nor too cold. Positive values indicate too warm and negative
too cool as shown in Table 1.4.
An algorithm, derived from empirical data, is used to
convert the value of pmv into the predicted percentage of
occupants likely to be dissatisfied (ppd) with the level of
thermal comfort provided. This is shown in Figure 1.3. For
example, a pmv of +0.5 will give a ppd of about 10% (too
warm). Note that even at a pmv of zero, 5% of occupants are
still predicted to be dissatisfied.
1.3.3.2
Operative temperature index
pmv is an excellent method of determining the suitability
of an existing thermal environment for a particular human
activity in a building. However, the building services
engineer is required to specify the thermal environment to
be achieved and in a way which is simple, unambiguous
and easily measured to prove compliance. For this, CIBSE
and ASHRAE have adopted the operative temperature
index (θc). This combines air dry bulb temperature, air
velocity and room mean radiant temperature to produce a
single value of room temperature. Room humidity is not
included.
Mathematically, operative temperature is defined as:
θc = {θai√ (10 υ)+ θr}/{1+√ (10 υ)}
(1.1)
where θc is the operative temperature (°C), θai is the room
dry bulb air temperature (°C), θr is the mean radiant
temperature (°C) and υ is the mean air speed in the occupied
zone (m·s–1).
Indoor air speeds in the occupied zone are typically about
0.1 m·s–1, when equation 1.1 simplifies to:
θc = (0.5 θai + 0.5 θr)
(1.2)
Table 1.5 in CIBSE Guide A gives recommended winter
operative temperatures for a range of building types and
activities. These assume activity and clothing levels typical
for those building types. Clients should be consulted to
establish whether there are any special requirements, such
as non-typical levels of activity or clothing. CIBSE Guide
A, chapter 1, includes methods for adjusting the operative
temperature to take account of such requirements. The
operative temperatures in Table 1.5 of Guide A are intended
to achieve a pmv of less than ±0.25, equivalent to about 5%
ppd. Where a higher ppd is acceptable, a lower minimum
design operative temperature may be used. This will result
in reduced energy consumption.
Table 1.4 Predicted mean vote scale
pmv
Thermal sensation
+3
Hot
+2
Warm
+1
Slightly warm
0
Neutral
-1
Slightly cool
-2
Cool
-3
Cold
80
60
Predicted percentage
dissatisfied (PPD) / %
thermal comfort and well being of human occupants
40
30
20
10
8
6
5
4
–2·0
–1·5
–1·0 –0·5
0
0·5
1·0
Predicted mean vote (PMV)
1·5
2·0
Figure 1.3 Predicted percentage dissatisfied as a function of predicted
mean vote (extracted from BS EN ISO 7730 (2005) with kind permission
of the British Standards Institution)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
1-12
1.3.3.3
Heating
Limitations to use of operative
temperature
——
room relative humidity lies between about 40 and
70%
——
local air speed is less than about 0.15 m/s
——
temperature difference between floor and head
height (temperature gradient) should ideally not
exceed 3 K
——
room air dry bulb temperature should not exceed
radiant temperature by more than about 3 K
——
the radiant temperature does not vary greatly with
direction (radiation asymmetry).
Room relative humidity has little impact on thermal
comfort provided it remains between about 40% and 70%.
During the heating season, outdoor air absolute moisture
content is low so that buildings with high infiltration or
ventilation rates may suffer from low relative humidity.
Below about 40% relative humidity there may be problems
with static electricity and, below about 30%, occupants may
complain of stuffiness, dust and other airborne irritants,
and dry eyes/nose/throat.
Local air movement can have a significant effect on the
perception of comfort with air speeds above 0.15 m/s giving
sensations of draught. Heating system design should take
account of the location of ventilation supply outlets and the
air movements they produce. Naturally ventilated buildings
need careful design to avoid draughts from natural
ventilation openings.
Temperature differences within the heated space may also
affect the perception of thermal comfort. Vertical
temperature differences are likely to arise from the
buoyancy of warm air generated by convective heating. In
general it is recommended that the vertical temperature
difference should be no more than 3 K between head and
feet (BS EN ISO 7730, 2005). If air velocities are higher at
floor level than across the upper part of the body, the
gradient should be no more than 2 K·m–1. Warm and cold
floors may also cause discomfort to the feet. In general it is
recommended that floor temperatures are maintained
between 19 and 26 °C, but that may be increased to 29 °C for
underfloor heating systems in circulation spaces.
For buildings with moderate to good levels of insulation,
which includes those constructed since insulation
requirements were raised in the 1980s, the difference
between air and mean radiant temperature is often small
enough to be insignificant for the building as a whole.
Nevertheless, it is important to identify situations where
these temperatures differ appreciably. As a general rule, this
difference is likely to be significant when spaces contain
large proportions of glazing or are heated non-uniformly or
intermittently. Air temperatures more than about 3 K
greater than radiant temperatures can give rise to sensations
of stuffiness.
—
proximity to cold surfaces, such as windows
——
proximity to hot surfaces, such as heat emitters,
light sources and overhead radiant heaters.
CIBSE Guide A recommends that radiant temperature
asymmetry should result in no more than 5% dissatisfaction,
which corresponds approximately to vertical radiant
asymmetry (for a warm ceiling) of less than 5 K and
horizontal asymmetry (for a cool wall) of less than 10 K.
The value for a cool ceiling is 14 K and for a warm wall is
23 K. It also gives recommended minimum comfortable
distances from the centre of single glazed windows of
different sizes.
On start-up of a heating system, the air temperature within
the room may rise very quickly but it may be several hours
before surface temperatures in the building increase
sufficiently to achieve the design operative temperature and
thermal comfort. This is referred to as pre-heat time. It is a
function, amongst other things, of building thermal inertia.
Pre-heat time is dealt with in section 1.5. Where the heat
emitters are controlled by sensors that detect only or
predominantly room air temperature, heat output from the
heat emitters may be reduced before the design operative
temperature is achieved. Surface temperatures will also be
strongly affected by choice of heat emitter. A heat emitter
with a high radiant to convective heat output will achieve
higher surface temperatures. For highly intermittently
occupied buildings, such as places of worship, purely
radiant heat emitters should be considered as these will
instantly result in occupants experiencing an increase in
radiant temperature.
Close control of temperature is often impractical in
industrial and warehouse buildings, in which temperature
variations of ±3 K may be acceptable. Also, in such
buildings the requirements of processes for temperature
control may take precedence over human comfort.
1.3.3.4
External design conditions
The external design temperature depends upon geographical
location, height above sea level, exposure and thermal
inertia of the building. The method recommended in
CIBSE Guide A is based on the thermal response
characteristics of buildings and the risk that design
temperatures are exceeded. The degree of risk may be
decided between designer and client, taking account of the
consequences for the building, its occupants and its
contents when design conditions are exceeded.
It should be noted that external design temperature is based
on air temperature near the ground. The temperature of
cloudless skies can be substantially lower than ground level
air temperature. As a result, external building surfaces that
can ‘see’ the sky can be subjected to appreciable longwave
radiation heat loss. This may be allowed for by increasing
the value of the radiant heat transfer coefficient (see CIBSE
Guide A, chapter 3) which will result in a higher U-value.
CIBSE Guide A, section 2.4, gives guidance on low
temperatures, including percentage exceedences, for 14 UK
locations. Guide A also provides percentage frequency of
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Operative temperature is a reliable method of specifying
the thermal environment in a room by the use of a single
temperature subject to certain limitations. These are
discussed fully in CIBSE Guide A, chapter 1, and briefly
discussed below in that they are influenced by type of
heating system and thermal characteristics of the room:
Asymmetric thermal radiation is a potential cause of
thermal discomfort. It typically arises from:
Design criteria
1-13
occurrence of combinations of low temperatures and high
wind speeds for the same locations.
It is both the mass in contact with the internal air and the
heat loss rate that play a dominant role in determining
whether a particular structure should be judged to be of low
or high thermal inertia. In critical cases, dynamic thermal
modelling should be undertaken.
The thermal inertia of a building may be represented in
terms of a thermal response factor, fr as described in Guide
A. This is defined as:
fr = { Σ (A Y)+Cv)}/{Σ (A U)+Cv) }
(1.3)
where Y is the thermal admittance value of each internal
surface (W·m–2·K–1), U is the unit thermal transmittance
value of each external element of the building fabric
(W·m–2·K–1), A is surface area of a building element (m2)
and Cv is the ventilation conductance (W·K–1).
A full definition and discussion of admittance is provided
in CIBSE Guide A. The response factor is also used to
determine what allowance should be made for intermittent
heating when sizing room heat emitters and is discussed in
section 1.5 below.
Buildings (or individual rooms) with a response factor ≤ 4
are considered to have a low thermal inertia (‘lightweight’)
whilst those with a response factor > 4 are considered to
have a high thermal intertia (‘heavyweight’).
Care should be taken when assessing thermal inertia.
Modern, well-insulated, buildings may have high response
factors whilst buildings with heavyweight constructions
may have high infiltration rates and so may have a low
response factor. Furthermore, heavyweight constructions
may be insulated from the room air by lightweight finishes
such as carpets or suspended ceilings.
1.3.4
Design criteria for other than
human comfort
Heating may be required for other than human comfort
and well-being. The following section provides some
guidance for design criteria for the protection of the
building fabric (condensation avoidance and frost
protection), and for highly specialised applications such as
agricultural buildings and food storage, animal facilities
and laboratories. These requirements are very diverse: it is
essential that the designer obtains a detailed brief from the
client.
Avoidance of condensation
The selection of both indoor and outdoor design conditions
relevant to the avoidance of condensation in buildings
differs from that for thermal comfort. Coincident outdoor
humidity and air temperatures are important, as are
ventilation rates and the rate at which moisture is liberated
indoors. Outdoor design conditions may not coincide with
the coldest month. Less extreme conditions will be less
appropriate to interstitial than surface condensation due to
thermal inertia of the structure. Details of how to select
design criteria to avoid condensation is given in CIBSE
Guide A.
Consideration should be given to operating heating systems
outside normal occupancy times should indoor temperatures
fall below levels at which condensation might occur.
Swimming pool halls are particularly susceptible to
condensation and are discussed in section 1.3.4.5.
1.3.4.2
Frost protection
Exceptionally low outdoor air temperatures can cause
damage to the building fabric and building services.
Buildings that are unoccupied for long periods of time are
particular vulnerable and consideration should be given to
operating the heating outside occupancy times so as to limit
the minimum temperatures in buildings.
Heating coils and filters in ventilation systems are
particularly susceptible to frost damage as these are exposed
to outdoor air, often on the roofs of buildings. Frost
protection measures should be incorporated that start up
heating systems serving such ventilation plant on detecting
low ambient temperatures. This might be a separate heating
system used only for frost protection. In all cases it is
advisable to log the operation of heating systems operating
under frost protection control to ensure that it is operating
correctly and is not consuming energy unnecessarily.
Design outdoor temperatures for the sizing of frost
protection systems will invariably be lower than those
selected for the thermal comfort of building occupants. A
risk assessment should be carried out to determine
appropriate design outdoor temperatures based on
frequency of occurrence of outdoor temperatures taking
into account local microclimate.
Table 1.5 Recommended temperature
ranges for some farmed animal housings
Animal species
Optimum
temperature
range / °C
Adult cattle
0–20
Calves
10–15
Pigs
5–25
Piglets:
— at birth
— after 2 days
35
28–33
Fattening pigs
11–22
Laying poultry
20–25
Broiler chickens
15–25
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The generally adopted external design temperature for
buildings with low thermal inertia (capacity) is that for
which only one day on average in each heating season has a
lower mean temperature. Similarly for buildings with high
thermal inertia the design temperature selected is that for
which only one two-day spell on average in each heating
season has a lower mean temperature. Table 1.5 in CIBSE
Guide A shows design temperatures derived on this basis
for various location in the UK. In the absence of more
localised information, data from the closest tabulated
location may be used, decreased by 0.6 K for every 100 m
by which the height above sea level of the site exceeds that
of the location in the Table. To determine design
temperatures based on levels of risk, see CIBSE Guide A,
chapter 2.
1.3.4.1
1-14
1.3.4.3
Heating
Horticulture facilities
Due to the high ventilation rates and very low thermal mass
of many horticultural buildings, outdoor design
temperatures may need to reflect more extreme winter
conditions. The risk, based on frequency of occurence data,
should be agreed with the client.
1.3.4.4
Animal facilities
Buildings housing animals, other than for medical research,
are covered by the Animal Welfare Act 2006 and the Welfare
of Farmed Animals Regulations 2007. DEFRA has
produced a number of Codes of Recommendations for
various farm animals including the requirements for
heating and ventilation of the buildings used to house
them. Table 1.5 sets out recommended temperature ranges
for some farmed animal housing.
1.3.4.5
Sports facilities
Sport halls will normally need to accommodate a wide
range of sports activities (e.g. indoor bowls and five-a-side
football). The very wide range of metabolic rates for these
different activities will require a correspondingly wide
range of indoor temperatures to be achieved to provide
reasonable levels of thermal comfort. Furthermore, higher
ventilation levels will be required with the more active
sports while some sports, e.g. badminton, will require low
air velocities, typically less than 0.1 m/s, in the active areas.
Swimming pool halls require high indoor air temperatures,
high humidities and low air velocities to provide thermal
comfort for pool users to limit the cooling effect arising
from evaporation of water from the skin. These high
temperatures and humidities combined with chlorine from
the pool water create a corrosive atmosphere. Consideration
therefore needs to be given to limiting condensation on the
pool hall structure through the control of ventilation.
Consideration must also be given in sports facilities to the
thermal comfort requirements of spectators which can be
very different to those engaged in sports.
Table 1.5 in CIBSE Guide A lists recommended operative
temperatures for some sports facilities. Further guidance is
available for sports and swimming pool halls in the UK in
Sport England design guides (Sport England, various
dates). A database of information provided for specific
sports by their governing bodies is also available through
Sport England. ASHRAE also provide details for a wide
range of recommended operative temperatures for sports
facilities (ASHRAE, 2011).
1.3.4.6
Laboratories and clean rooms
Laboratories and clean rooms may have very specific
requirements for the thermal environment including in
some instances very close control. Mechanical ventilation
systems also with exacting performance specifications are
The external design condition should reflect the risk and
consequences of the heating system failing to maintain the
required internal design conditions under extreme weather
conditions.
1.3.4.7
Industrial buildings with specialist
processes
As with laboratories, manufacturing processes may require
control of the thermal environment, including air velocity
and humidity, within specified limits. Infiltration rates can
be very high in industrial buildings due to large access
doors that may need to accommodate vehicles. Consideration
should be given to incorporation of air locks, air curtains or
other methods of preventing excessive infiltration.
1.3.4.8
Industrial spaces, warehouses and
other large halls
Warehouses are often only occupied for short durations and
heating systems are likely to be designed so as to provide a
minimum indoor temperature for the protection of the
building contents or the building fabric, including
avoidance of condensation.
With some industrial buildings, infiltration rates can be
very high and consideration should be given to incorporation
of air locks, air curtains etc. Radiant heating, including
localised, should be considered. Guidance on energy
efficient heating of large halls is provided by REHVA
Guidebook 15: Energy efficient heating and ventilation of large
halls (Kabele et al, 2011).
1.3.4.9
Design infiltration rates
Infiltration will appear as a heating load within the room to
be dealt with by the room heat emitter and will influence
the thermal inertia of the room. It is highly variable and
difficult to predict with accuracy. Infiltration heat loss may
constitute a large proportion of the total heat loss from a
perimeter room, particularly in well insulated buildings. It
is increasingly important therefore that a realistic estimate
of infiltration rate be made. In contrast, mechanical
ventilation is specified and accurately controlled. The
heating demand of any mechanical ventilation is usually
dealt with centrally. Design outdoor ventilation rates
should therefore differentiate between infiltration and
mechanical.
National building energy codes in many countries set
minimum standards of air tightness. In the UK this is
defined by the air permeability (m3·h–1/m2), determined by
physical tests and measured under controlled conditions
with the room/building mechanically pressurised to 50 Pa
above local atmospheric pressure. Details are given in
CIBSE TM23: Testing buildings for air leakage. The notional
building, used to generate the UK target emissions rate for
CO2 emissions, uses a permeability of 5 m3·h–1/m2 whilst
setting an upper limit of 10 m3·h–1/m2 for the design building
(see section 1.3.6).
Empirical values of infiltration are given in CIBSE Guide A
for a range of building types (including both those built
before and after the inclusion of minimum standards of air
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Design indoor temperature for horticultural buildings such
as greenhouses will vary with the species of plant being
grown and may vary with season. Minimum night time
temperatures are normally required. ASHRAE Handbook:
Fundamentals (ASHRAE 2013a) provide some recommended
night temperatures for a range of greenhouse crops.
usually required. It is essential that a full brief is obtained
from the client.
Design criteria
1-15
An empirical relationship between air permeability and
infiltration rate has been developed and is presented in
CIBSE TM23. Again, this should be used with caution. It
provides only an average infiltration rate for the whole
building, not individual rooms and is based on data for
houses and offices only.
Alternatively, estimates of infiltration rates in individual
rooms can be made using the methodology set out in CIBSE
Guide A; details of construction standards and areas
required and an assessment of pressure differences acting
across each facade of the building is made to determine
infiltration rate under design conditions.
1.3.5
Environmental performance
targets
It should be understood that the various national building
energy regulations set minimum standards for environmental
performance. When setting environmental targets for
individual projects consideration should be given to
exceeding these minimum standards for several potential
reasons:
——
this will provide some margin when demonstrating
compliance
minimum technical specifications are presented in the form
of a set of technical documents that can be regularly updated
without the need for primary legislation to amend the Act.
Since 1984, these documents have been updated several
times and will continue to be updated as a means of gradually
improving standards. In England these are in the form of
Approved Documents. The current list of Approved
Documents are set out in Table 1.6. A similar method is used
in the rest of the UK with Wales, Scotland and Northern
Ireland producing their own form of technical documents.
Approved Documents are written in such a way as to provide
a degree of flexibility in the design of buildings and building
services. By demonstrating compliance with an approved
document to the building control body (bcb), the design is
deemed to comply with the specific Building Regulations to
which it refers. However, the designer is permitted to
demonstrate compliance with the Building Regulations by
some other method that is acceptable to the bcb.
A design-stage submission demonstrating compliance must
be made to the bcb in order to obtain permission to
commence construction. Following completion, an ‘asbuilt’ submission is then required to obtain full Building
Regulations approval.
Part L is further divided into 4 approved documents
(DCLG, 2013):
——
Part L1A: Conservation of fuel and power (new
dwellings)
——
Part L1B : Conservation of fuel and power (existing
dwellings)
——
Part L2A: Conservation of fuel and power (new
buildings other than dwellings)
——
Part L2B : Conservation of fuel and power (existing
buildings other than dwellings)
This section will deal only with Parts L2A and L2B.
Table 1.6 Approved Documents (England) as of 2013
——
the client may have an environmental policy that
requires an improvement on prevailing standards
Approved Documents
Part
——
the building will achieve some degree of futureproofing (particularly important given that many
buildings may require energy certificates).
Structural safety
A
Fire Safety
B
Resistance to contaminants and moisture
C
In addition to national/local governmental regulations,
there are also a number of non-governmental codes that
look at the wider environmental impacts of buildings such
as BREEAM, LEED, the Australian Green Star and the
Abu Dhabi Estidama. Some governments and planning
authorities require that minimum targets be met under
these schemes.
Toxic substances
D
Resistance to sound
E
Ventilation
F
Sanitation, hot water safety and water efficiency
G
Drainage and waste disposal
H
As an example of governmental regulations and nongovernmental codes, the UK Part L 2013 Building
Regulations for England and BREEAM (UK) are discussed
in more detail below.
Heat producing appliances
J
Protection from falling, collision and impact
K
Conservation of fuel and power in buildings
L
Access to and use of buildings
M
Glazing safety
N
Electrical safety
P
Material and workmanship, Regulation 7
7
1.3.6
Part L Building Regulations
2013 (England)
The 1984 Building Act (updated in 2000) created the current
format of the Building Regulations in the UK where the
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
permeability in the UK regulations). Infiltration rates are
quoted in units of air change per hour. Such allowances for
infiltration rates should be used with caution. Firstly, air
change rate is a function of room/building volume whereas
infiltration rate (m3/s) is a function of the area of leakage on
the exposed facade and is not influenced by the volume of
the building. There is a risk of overestimating infiltration
rate using the air change rate method for rooms/buildings
with large volumes. Secondly, air permeability
measurements are based on air flowing from inside to
outside through the building fabric when the building is
pressurised above outdoors. In practice, air infiltration will
be a mix of infiltration and exfiltration resulting from
pressure differences across the fabric that vary with height
and orientation.
1-16
Heating
Parts L2A and B are supported by additional documents
including:
The non-domestic building services compliance guide
(NDBCSG) (DCLG, 2013e)
——
hvac guidance for achieving compliance with Part L of
the Building Regulations 2008 (TIMSA, 2008),
dealing specifically with insulation
——
National Calculation Methodology (NCM) modelling
guide (for buildings other than dwellings in England)
(BRE, 2014)
——
SBEM (simplified building energy model) software and
technical guide (BRE, 2014a)
——
Standard Assessment Procedure (SAP) (BRE, 2012)
——
Energy Efficiency and Historic Buildings (English
Heritage, 2012)
——
Item
Requirement
CO2 emissions
from regulated
services
ber of both the proposed and the as built
Low carbon
technology
The designer must consider the use of low and
zero carbon technology such as chp and
renewable energy based on an assessment of the
technical, environental and economic feasibility
Building envelope
thermal
performance
The area weighted average U-values of each
element of the external building fabric must not
exceed the maximum permitted values.
Building envelope
air tightness
The air permeability must not exceed the
maximum permitted value.
Solar gains
The building should incorporate passive means
to reduce solar gains.
Building services
controls
The control for HVAC systems must meet or
exceed the stated minimum provision as set out
in the NDBSCG.
Building services
energy efficiency
The energy efficiency of the plant and systems
must not be less than the minimum permitted
values as set out in the NDBSCG.
Energy metering
Adequate metering must be installed to allow at
least 90% of the estimated annual consumption
of each fuel to be assigned to the end-use
categories to aid energy management and
targeting.
Commissioning
and testing
Building services are commissioned and tested in
accordance with specified codes.
Energy
Performance
Certificate (EPC)
The building owner must be provided with an
EPC based on as-built data.
Building log book
The building owner must be provided with a log
book that complies with CIBSE TM31 and
includes the data used in the CO2 emissions and
calculations and the EPC report.
various British and European Standards.
Different requirements and minimum standards apply to
new-build and existing buildings and so are described
separately in the sections immediately below. Some current
minimum performance standards are presented here in
order to illustrate the workings of the regulations. These
are based on the standards that came into force in 2014. It is
expected that the practice of revising these minimum
standards upwards every three to five years will continue. It
is important to ensure that the correct approved documents
and supporting material is used.
1.3.6.1
Part L2A: Conservation of fuel and
power (new buildings other than
dwellings)
Part L2A applies to the design and installation of the
building envelope and regulated building services in most
new buildings. (Exemptions apply only where the building
is temporary, infrequently used or has a very low demand
for energy.) The regulated services include:
——
space heating
——
domestic hot water
——
cooling
——
mechanical ventilation
——
fixed lighting.
Table 1.7 gives an overview of the compliance requirements.
The CO2 emissions rate for the regulated services is given in
kgCO2 per year per m2 of total useful floor area. The rate for
the building (design stage as as-built), is known as the
building emissions rate (BER). The target rate is known as
the target emissions rate (TER). Both must be calculated by
a competent person using approved software and the
methodology set down in the NCM. Currently, the regulated
services exclude vertical transportation and services
installed for business purposes (e.g. computers, catering).
The CO2 emissions are related only to energy consumption
in use, excluding embodied. Greenhouse gases other than
CO2 are excluded.
The TER and BER are both calculated for the sum total of the
regulated building services combined. TERs for individual
services are not required. This is because of the complex
interaction between the various building services and built
building must not exceed the ter
form. For example, solar shading will reduce cooling energy
demand in summer but also reduce useful passive solar
heating in winter and so increase heating energy demand.
A more efficient lighting scheme will also lead to increased
demand for heating. It may also be impossible to entirely
separate energy used for say heating and cooling where a
combined system is used in an air conditioned building.
This allows for some design flexibility. However, minimum
performance standards for the separate elements of the
building fabric, each regulated service and its component
plant ensures that no part of the building or services can be
unduly inefficient. This prevents designers offsetting
excessive energy consumption that might arise from a poor
building fabric performance by means of low carbon
technology (though the TER is set sufficiently low as to
encourage the use of renewable). The performance of low
carbon technology is likely to deteriorate with age, more so
than building fabric thermal performance, or may not be
used as intended. Furthermore, it is easier to add such
technology in the future than improve the fabric.
In calculating the TER, the NCM creates a model of a
‘Notional Building’ that is the same size and shape and
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
Table 1.7 Requirements under Part L2A 2013
Design criteria
1-17
Table 1.8 Fabric thermal standards for the notional building and actual
building
Area weighted U-value (W/m2K) and
air permeability (m3/s m2 at 50 Pa)
Actual building
(maximum
permitted)*
(Table 5, DCLG
2013)
Roof
0.18
0.25
Exposed walls
0.26
0.35
Exposed floors
0.22
0.25
Building element
1.8
2.2
Entrance doors
2.2
3.5
Vehicle access doors
1.5
1.5
Air permeability
3.0
10.0
Windows and rooflights
†‡
* In addition to area weighted U-values, Part C of the Building
Regulations sets maximum U-values for any element so as to reduce the
risk of condensation.
† Calculated in the vertical position. Corrections should be for other
orientations.
‡ Where curtain walling is used, the limiting area-weighted U-value is
the same as for windows. However, the notional building will be based
on the use of conventional walls and windows.
usage as the building being designed (the ‘actual building’).
However, there are important differences between the
notional and actual building models:
——
The glazed area of the notional building is
determined by the activity of the space and may be
less than in the actual building. In particular, the
notional building does not use curtain walling but
is based on conventional, well-insulated walls with
punched windows.
For space heating provided by a boiler, the boiler efficiency
used is the boiler seasonal efficiency. This is defined as:
Boiler seasonal
0.81 η30 + 0.19 η100
efficiency (%) =
(1.4)
where η30 is the declared efficiency (%) at 30% of full rated
load and η100 the declared efficiency at 100% rated load.
This applies only to boilers with a duty of less than 400kW.
For boilers of duties greater than 400kW, the boiler seasonal
efficiency must be provided by the boiler manufacturer.
These efficiencies must have been determined by the
manufacturer in compliance with BS EN 15316. This is a
family of standards that deal with different types of boilers
for space heating and the production of dhw. It should be
noted that manufacturer’s test data is normally based on
the net calorific value (ncv) of the fuel while the NDBSCG
quotes minimum seasonal efficiencies in terms of gross
calorific value (gcv). These can be converted using factors
provided in the Standard Assessment Procedure (SAP)
(BRE, 2012).
Where multiple boilers are installed, the NDBSCG gives a
method for determining the overall boiler seasonal
efficiency using the part and full load efficiencies for each
boiler. Where a single boiler is used, the overall seasonal
boiler efficiency is the same as the seasonal boiler efficiency.
The NDBSCG gives minimum overall seasonal boiler
efficiencies for gas and lpg as a function of their output.
DHW generators
——
The notional building utilises better standards of
fabric thermal performance and air tightness than
the minimum standards required in the actual
building (see Table 1.8).
Boilers that produce only dhw are subject to different rules
for determining minimum acceptable efficiencies and are
dealt with in a separate chapter in the NDBSCG.
——
The actual building model incorporates models of
the regulated services as designed/installed in order
to determine the BER from the calculated hvac and
lighting loads. The notional building model, in
contrast, does not include any models of the
regulated services. Instead, a set of fixed factors are
used to convert the heating, cooling, ventilation
and lighting loads into the TER. These factors have
been calculated so that, in combination with the
improved fabric performance, the TER is on average
some 9% lower than the previous (2010) Part L TER.
Biomass boilers
As a result, the BER of a building design that just meets the
minimum standards of energy efficiency for the building
fabric and services will exceed the TER.
Minimum heating system efficiency
These minimum acceptable standards for heating system
efficiencies are set out in the Non-domestic Building Services
Compliance guide (NDBSCG) (DCLG, 2013e). Different
minimum standards apply, and different methods of
specifying these minimum standards, depending upon the
type of heat generator used. The NDBSCG also sets out
minimum control requirements for heating services.
The NDBSCG uses a different approach to determining
biomass boiler efficiency compared to gas, oil and lpg
boilers and sets different minimum standards.
Heat pumps
Heat pump minimum performance is defined in the
NDBSCG using both the coefficient of performance (cop)
and the seasonal coefficient of performance (scop) for heat
pumps with output of less than 12kW.
The cop should be determined using the test procedure
specified in the NDBSCG.
The NDBSCG gives values of minimum cop depending
upon the type of heat pump.
Other heat generators
The NDBSCG gives minimum efficiencies for other heat
generator types including chp and direct electric.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Notional building
(Table 3, DCLG
2013)
Boilers
1-18
Heating
1.3.6.2
Part L2B: Conservation of fuel and
power (existing buildings other than
dwellings)
There is no requirement for a BER/TER calculation to be
performed, although an EPC may be required. Table 1.9
provides an overview of the requirements.
It should be noted that minimum energy efficiency standards
for new/replaced items of the building envelope and regulated
services are somewhat different to those for new-build.
Where the building is enlarged or the size or extent of
regulated building services increased, consequential
improvements to the overall energy efficiency of the
building may be required. Consequential improvements
require that a technical and financial feasibility study is
carried out to determine suitability of range of improvements
including replacement of windows, upgrading of hvac and
lighting systems and controls, installation of low carbon
technology. Approved Document L2B gives guidance on
the measures that are likely to prove economically viable.
Historic and listed buildings
The definition of nearly nearly zero energy buildings is
defined only loosely in the Directive with individual
member states required to produce their own detailed
definition. The Directive does state however that a nearly
zero energy building should be one that has a low demand
for energy and and that this energy is provided mostly by
renewable energy technology in order that carbon emissions
are very nearly zero. This implies that the building fabric
thermal performance will be very good and the better it is
the less dependence on low and zero carbon technology.
At the time of writing, the English Government has
indicated a likely minimum fabric energy efficiency
standard (FEES) (ZCH, 2013) for new dwellings. This is in
the form of a calculated energy annual energy demand for
heating and cooling rather than limiting U-values and
permeability. This provides for more flexibility in the
design of new dwellings. The Government expects that in
order to achieve compliance, most designers will choose to
adopt higher fabric thermal efficiencies. Standards for nondomestic buildings are awaited but it is likely that these
will follow the same principle as domestic. The Zero
Carbon Hub has carried out a study of building fabric
thermal performance required to achieve FEES for various
dwelling types (ZCH, 2014). Table 1.10 lists the typical
U-values and permeabilities found to comply with FEES
together with current maximum permitted values.
Part L2B of the Building Regulations makes special
provision for work carried out on existing buildings that
are either:
Table 1.9 Requirements under Part L2B 2013
——
included in the Statutory List of Buildings of
special architectural of historic interest
Item
Requirement
——
are located within conservation areas
——
are listed within the Local Development Framework
as of special consideration
Thermal performance
of replaced elements of
building envelope
The U-values of each replaced/new
elements of the external building fabric
must not exceed the maximum permitted
values.
——
are located within national parks or other historic
areas
Building services
controls
——
are traditionally constructed with permeable fabric.
The control for replacement/new systems
must meet or exceed the stated minimum
provision
Building services
energy efficiency
The energy efficiency of replaced/new plant
and systems must not be less than the
minimum permitted values
Energy metering
Adequate metering must be installed to
allow the energy consumption of
replacement/new plant to be monitored
Consequential
improvements
Where the building has a total useful floor
area exceeding 1000 m2, and has an
extension or a fixed building service is
installed for the first time or increased in
capacity, improvements to the energy
efficiency of the building of at least 10% of
the cost of the primary work may be
required.
Commissioning and
testing
New and replacement building services are
commissioned and tested in accordance
with specified codes
Building log book
The building owner must be provided with
a log book that complies with CIBSE TM31
in respect of new and replaced plant.
Such buildings are not unconditionally exempt from the
regulations, rather they should be upgraded in accordance
with energy efficiency requirements but only in as far as
such improvements in energy efficiency do not result in
unacceptable changes to the appearance or character of the
building. Such work may require Listed Building Consent
and/or the consent of the local authority. Buildings
constructed before about 1945 may depend upon a
permeable fabric to prevent damage from damp. These may
require specialist measures to ensure that moisture is not
trapped as a consequence of energy efficiency measures.
Guidance on historical and listed buildings is given in a
second tier Part L document produced by English Heritage
(English Heritage, 2012).
1.3.6.3
‘Nearly zero energy’ fabric
performance
The recast Energy Performance of Buildings Directive (EU
2010) requires that all new (domestic and non-domestic)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Part L2B applies to the refurbishment of buildings
including the replacement of regulated systems or elements
of the building fabric. Exemptions include buildings less
than 50 m2 floor area, some listed buildings, buildings used
predominantly for worship, temporary buildings. Special
provisions apply to historic buildings (see below).
buildings are ‘nearly zero energy’ by the end of 2020 (or the
end of 2018 for public authority buildings. Member states
are required to produce a plan of action for achieving this
including transitional measures.
Design criteria
1-19
Table 1.10 Typical values of fabric thermal performance that comply
with FEES for dwellings
2013 limiting
U-value of external
walls
0.18 W/m2·K
0.30 W/m2·K
U-value of roof
0.13 W/m2·K
0.20 W/m2·K
U-value of external
floor
0.13 W/m2·K
0.25 W/m2·K
U-value of windows
1.4 W/m2·K
2.0 W/m2·K
Permeability
5 m3/m2/hr
10 m3/m2/hr
Thermal bridging
<0.05
<0.15
Oxides of nitrogen are produced during combustion
due to the presence of nitrogen in the combustion
air rather than the fuel. The amount of nitrogen
oxides produced is a function of the design of the
heat generator. Oxides of nitrogen are harmful to
health, lead to the formation of ozone and smog and
are powerful greenhouse gases. Boilers are classified
according to their emissions of NOx as set out in BS
EN ISO 21258 (BSI, 2010).
——
Sulphur dioxide results from the burning of sulphur
containing fuels (see section 1.6) and results in the
formation of acid rain.
——
Particulates result from the combustion of liquid
and solid fuels due to incomplete combustion. As
such, particulate emissions are a function of both
heat generator performance and fuel used. Other
pollutants include benzene and carbon monoxide.
Particulates and other air pollutants lead to reduced
air quality and are detrimental to health.
standards
Table 1.11 Passivhaus standards for dwellings.
Minimum standard
U-value of walls, floors and roofs
0.15 W/m2·K
U-values of windows
0.85 W/m2·K
Air tightness
0.6 air changes/h at 50 Pa
1.3.6.4
Passivhaus standard
Passivhaus also sets minimum standards for insulation and
air tightness of new dwellings (see Table 1.11). The
standards for insulation are higher than is currently
proposed for zero carbon dwellings. Airtightness is
measured by a similar method as that used in the UK but is
presented in terms of air change rate and so is not directly
comparable with air permeability. Passivhaus does not set
specific standards for non-dwellings but does suggest that
the same approach should be used.
1.3.7
Energy and CO2 emissions
benchmarks for existing
buildings
Although Part L Building Regulations do not set target
emission rates for refurbished buildings, it is possible to
produce a TER and to use this as the basis for a voluntary
benchmark. In addition:
——
CIBSE Guide F: Energy efficiency in buildings
contains benchmark energy consumption data for a
wide range of existing buildings in the UK and
detailed guidance on energy efficiency measures.
This includes ‘good practice’ energy consumption
data. However, this data is based on an increasingly
old building stock and so does not take recent
improvements in energy efficiency into account.
——
CIBSE TM46: Energy Benchmarks contains energy
and CO2 emissions benchmarks that were used in
the development of Display Energy Certificates.
1.3.8
NOx, SOx, particulates and
greenhouse gases other than
CO2
The Environment Act (1995) (see section 1.7.8.5) allows
local authorities to set limits on emissions from heat
generators including NOx, SOx and particulates:
Some gases, including methane (see section 1.6) and
fluorinated hydrocarbons (F-Gases) used as refrigerants in
heat pumps (see section 1.7), are powerful greenhouse gases.
The use of fluorinated hydrocarbons are regulated in
Europe by the F-Gas Regulations 2015 (see section 1.2).
CIBSE Guide L: Sustainability contains information on
pollution management.
1.3.9
Environmental assessment
schemes
There are a number of schemes for assessing a wide range of
environmental impacts of new and refurbished buildings.
BREEAM and LEED are available for worldwide application
and are described below for non-domestic buildings. Other
schemes include Green Star (Australia), HQE (France),
DGNB (Germany), BEAM (Hong Kong), Estidama (Abu
Dhabi). There are also schemes for specific types of
construction projects such as DREAM (defence related
buildings) and SKA Rating (fitting-out of buildings).
1.3.9.1
BREEAM
The Building Research Establishment Environmental
Assessment Method (BREEAM) is widely used in the UK
(and increasingly so throughout Europe and the rest of the
world), to certify a new or refurbished building’s overall
environmental performance. This is graded as Pass, Good,
Very Good, Excellent or Outstanding. Some planning
authorities require a minimum BREEAM rating, usually
Very Good or Excellent, to be achieved. Most UK
Government funded buildings also have minimum
BREEAM targets.
The current version of BREEAM, applicable to new nondomestic buildings constructed in the UK, awards points on
a sliding scale calculated against around 45 environmental
benchmarks. Those that may directly influence the choice
and design of heating systems are listed in Table 1.12.
Different criteria are likely to apply to refurbished buildings.
BREEAM criteria and scoring methodology are regularly
updated, typically every two or three years. Further
information is available on the BREEAM web site (www.
breeam.org).
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
FEES
——
1-20
Heating
Table 1.12 BREEAM 2014 benchmark criteria related to heating systems for new-build
% of maximum
possible score
Requirement for new-build
Thermal comfort
About 2%
Thermal modelling should be carried out to demonstrate that the CIBSE recommended thermal comfort
criteria can be achieved (or other more appropriate or onerous criteria where this exists). The heating
system to incorporate zone controls to allow for variations in heating demand arising from incidental heat
gains or proximity to external envelope. Appropriate controls to be provided to allow occupants to adjust set
point temperatures within zones.
Energy and CO2
emissions
About 8–10%
Using accredited National Calculation Methodology software, the annual energy demand, annual energy
consumption and annual CO2 emissions (TER/BER) for the Notional and Design building are compared. An
overall Energy Performance Ratio (EPR) is then calculated that takes into account statistical energy/CO2 data
for the particular building type. In this way, the EPR takes into account the energy efficiency of the building,
the energy efficiency of the regulated services (including the heating system) and the CO2 emissions arising
from the fuels used. A minimum score is required to achieve Excellent or Outstanding
NOx emissions
About 2%
Where heat generators result in NOx emissions of 100 mg/kW·h or less. Note that biomass is unlikely to
meet this target. Electric heating , including heat pumps, will not meet this target.
Use of low and
zero carbon
technologies
About 3%
Where low or zero carbon technologies are used (as recognised by the European Renewable Energy
Directive 2009) e.g. solar hot water heating, biomass, ground source heat pumps. Note that these may also be
eligible for the Renewable Heat Incentive payments (see section 1.2.9.1). A minimum score is required to
achieve 'excellent' or 'outstanding'.
1.3.9.2
LEED
The Leadership in Energy and Environmental Design
(LEED) rating system, is a tool created by the US Green
Building Council intended to measure building performance
against a national baseline. Although used predominantly
in North America, LEED is often requested by global
corporations for buildings in Europe and elsewhere.
1.4
Choice of system
1.4.1
General
A space heating system consists of:
——
energy source (fuel)
——
heat generator (boiler, heat pump etc.)
LEED certification is rated in four levels: Certified, Silver,
Gold and Platinum. In the United States, all federal
building projects must meet a LEED rating of Silver or
higher.
——
heat emitters (radiators, unit heaters etc.)
——
heat distribution medium (air, water etc.).
Evaluating the environmental performance of new
construction and existing building projects in five main
credit categories, projects have to document compliance
with required criteria in each category, as well as optional
credits in order to score enough points to achieve
certification. Projects need to achieve a minimum of 40 out
of 110 possible points in order to reach Certified level.
Selection of the most appropriate system will involve a
number of decisions considering each of the above, though
not necessarily in the order indicated. The factors to be
considered can be loosely grouped into two areas relating to
practical system installation and to performance and use
factors. Installation factors include:
——
fuel availability
Credits that may directly affect the choice and design of
heating systems for new-build only are listed below:
——
space required for fuel, heat generating plant and
heat distribution system
——
Energy and Atmosphere prerequisite 1:
Fundamental Commissioning of Building Energy
Systems
——
potential plant room locations
——
capital cost of installation
——
ease of installation
——
Energy and Atmosphere prerequisite 2:
Minimum Energy Performance
——
ease of replacement, especially of large plant
——
Energy and Atmosphere credit 1:
Optimizing Energy Performance
——
flexibility: any requirements for future change of
use or changes in fit-out.
——
Indoor Environmental Quality credit 6:
Controllability of Systems
——
Indoor Environmental Quality credit 7:
Thermal Comfort
Each of these is discussed in turn in this section.
Performance factors include:
——
running costs
——
environmental impact
——
ability to meet internal design conditions specified
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Benchmark
Choice of system
1-21
limits on exposed surface temperatures
——
zonal control
——
speed of response
——
ease of use by occupants.
above the saturation pressure of the water to prevent
flashing into steam, allowing a reasonable safety margin.
Typical values are shown in Table 1.13.
This section gives an overview of the type of heating
systems available together with their characteristics and
applications so as to assist in the selection of the most
appropriate system for a specific project. Subsequent
sections deal in more detail with fuels, heat generators and
the plant, equipment and method of heat distribution
associated with each principle type of heating system.
1.4.2
System classification
Both space heating and domestic hot water systems may be
classified as either decentralised or centralised.
In decentralised systems, heat generation occurs at the
point of use. Examples relevant to non-domestic
applications of space heating are:
——
electric heaters
——
gas-fired radiant heaters
——
direct fired unit heaters
——
unitary heat pumps.
Decentralised domestic hot water systems produce the hot
water at, or very close to, the point of use and often serve a
single sanitary fitting. These may be instantaneous or
include a small amount of storage. An example is the
electric point of use water heater commonly used to serve a
washbasin in commercial buildings.
Centralised space heating systems involve the generation of
heat from the fuel in a central plant, remote from the heat
emitters. This may include district heating. This heat is
then distributed to each heat emitter utilising one of several
heating mediums:
——
liquid water (hydronic)
——
steam
——
air.
Where air is used as a distribution medium, the temperature
to which the air can be heated is severely limited in order to
reduce stratification in the room and the burning of dust
particles. Consequently, such systems are commonly known
as 'warm air systems' with a maximum temperature of about
35 °C.
1.4.3
Operating temperatures
Choice of centralised or
decentralised systems
Centralised systems offer greater choices of fuel. In
particular they can make use of community/district heating,
chp, and renewable fuels such as biomass or waste heat.
Decentralised systems are generally limited to gas or
electricity, or electricity only if there are restrictions on
combustion equipment within the heated space. Centralised
systems will therefore offer greater scope for reducing CO2
emissions through choice of fuel. Plant duties will be
greater than for decentralised systems and so are more
likely to incorporate energy efficiency measures. However,
centralised systems may have extensive distribution pipe or
ductwork requiring energy intensive pumps/fans and
significant heat losses. These may outweigh the potential
energy/CO2 savings associated with the more energy
efficient central plant.
Decentralised systems have the advantage of requiring no
central plant room and very little distribution space.
Decentralised systems also provide standalone operation in
each heated space, useful where the occupancy of each space
differs. Energy consumption in individual spaces is more
easily metered with decentralised systems.
1.4.4
Similarly, centralised domestic hot water systems generate
hot water at a central point and distribute it through
pipework to multiple sanitary fittings. The system may or
may not include storage.
1.4.2.1
Steam systems operate at a temperature determined by its
pressure. This will vary throughout the distribution
pipework. Steam is usually generated at a minimum
pressure of 7 bar, equivalent to 170 °C.
Particular applications
The choice of heating system will depend upon a number of
factors as discussed above. Each project will have its own
unique set of requirements and it is essential that these be
considered for each project. It is not appropriate therefore
to recommend a heating system for a particular project.
However, general guidance can be given where the building
has certain specific characteristics, some of which are listed
below. Some building projects may encompass more than
one of these characteristics.
Hydronic systems may be further classified according to
their operating temperatures or pressures. The operating
pressure at all points in the system must be maintained
1.4.4.1
Table 1.13 Classification of hydronic heating systems by temperature
In some instances, such buildings may be exempt from
building energy codes. For example places of worship are
exempted from the Part L Building Regulations for England
due to heating being required for only a few hours each year.
Category
Low temperature
Temperature / °C
Approximate
minimum operating
pressure / bar
<90
1
Medium temperature
90–120
3
High temperature
>120
5
Infrequently occupied buildings
Very fast response heating systems should be employed to
minimise the time required, and therefore energy
consumed, for pre-heat. High temperature, directional
radiant heating systems will give the fastest response as
they can warm up the occupants directly without
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
1-22
Heating
Convective systems can provide a rapid warm up of the
room air, provided infiltration rates are not excessive, but
will not quickly increase the radiant temperature and may
lead to sensations of stuffiness (see section 1.3.3 on human
thermal comfort).
1.4.4.2
Poorly insulated and leaky buildings
Consideration should be given first of all to improving the
thermal properties and air tightness of the building to
reduce heating demand, with due regard to relevant
building regulations and life cycle costs.
Where fabric heat losses are still relatively high, surface
temperatures in winter may be sufficiently low to prevent
recommended operative temperatures being achieved in
perimeter zones. Radiant heating systems will help combat
these low room surface temperatures but will lead to
increased fabric heat loss. An assessment should be made of
heat emitters with different proportions of radiant and
convective heat output to achieve a compromise between
thermal comfort and running costs.
Highly radiant heat emitters will allow a lower air
temperature to be used to achieve the same operative
temperature and so lead to lower running costs for buildings
with high infiltration rates. However, care needs to be taken
to avoid draughts at the perimeter.
1.4.4.3
Listed and historical buildings
Listed buildings and other buildings of historical
importance can present a particular challenge as these may
have been originally designed with limited or no heating
systems. Furthermore, the building fabric is likely to have
very poor insulation properties and high permeability that
cannot be significantly improved without unacceptably
altering the building’s appearance. This is recognised
under Part L of the Building Regulations (England) and
the equivalent regulations for Wales, Scotland and Northern
Ireland. English Heritage has produced a guide to the
heating of such buildings (English Heritage, 2012)
including the upgrading of thermal insulation and
reduction in air permeability.
1.4.4.4
Highly insulated buildings
Modern, well-sealed buildings have increasingly lower
demand for space heating. High insulation levels mean that
even on a design day in winter, room surfaces are close to
room air temperature. Low temperature heating systems
such as underfloor should be considered. The lower
temperature means that heat pumps can be employed, or
possibly some form of heat reclaim. Boilers will operate in
condensing mode more frequently.
During occupation, incidental gains may be sufficient to
offset heat losses so good zonal control is required and the
heating system should have a fast response.
Care needs to be given to the sizing of heat emitters so as to
achieve a reasonable pre-heat time, particularly on a
Monday morning or following a holiday or a very cold
weekend. This may require a pre-heat margin (ratio of
emitter duty to design heat loss, known as plant ratio)
greater than has traditionally been used in the past.
Alternatively, longer pre-heat periods may need to be
specified. Section 1.5 discusses the pre-heat and plant ratios
in detail.
In highly insulated buildings, infiltration and ventilation
heat loss becomes increasingly significant. Extra care needs
to be taken to ensure adequate provision for infiltration and
ventilation has been included in heat loss calculations,
particularly in naturally ventilated buildings. Where
infiltration/ventilation heat loss dominates, radiant heating
systems will allow a lower air temperature to be set and so
result in reduced fuel consumption.
For further information see CIBSE TM29: HVAC strategies
for well-insulated airtight buildings.
1.4.4.5
High thermal mass buildings
When heated intermittently, the heating energy
consumption of buildings with high thermal mass is, in
theory, somewhat greater than for low thermal mass
buildings (of the same thermal resistance and permeability)
due to the higher mean internal temperature maintained by
the high thermal mass building. High thermal mass
buildings can make better utilisation of solar and other
incidental heat gains however, so that heating energy
consumption may be only marginally higher (or even
lower) than for buildings of low thermal mass. Such issues
are discussed in detail in CIBSE TM41: Degree days: theory
and application. High thermal mass buildings do require
larger heat emitters to achieve acceptably short pre-heat
times (see section 1.5) with consequential increase in capital
costs and space. Where occupancy times are relatively long
and frequent, it may be beneficial to design the heating
system to operate continuously during cold weather. A life
cycle analysis considering both fuel and capital costs should
be carried out to determine overall savings. CIBSE TM41
describes such an approach.
1.4.4.6
Rooms with high ceilings
Many buildings have rooms with high ceilings including
atria in commercial buildings, auditoria, sport halls,
industrial buildings and warehouses. High ceilings result
in stratification of room air with the air at the top being
significantly warmer than at floor level. This results in
increased heat loss through the fabric at high level without
any benefit to occupants’ comfort. Consideration should
therefore be given to the use of predominantly radiant
rather than convective heating or to underfloor heating
which is likely result in lower energy use. If convective
systems are used they should be configured such that
stratification is minimised by ensuring good mixing of the
room air between floor and ceiling.
The Federation of European Heating, Ventilation and Air
Conditioning Association (REHVA) has produced a
booklet that discusses how to design efficient heating
systems for large halls (REHVA, 2011). This includes an
analysis of a range of radiant heating systems and heated
floors.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
unnecessarily warming up the building fabric and in turn
the air within the heated space. Provided there is adequate
distance between emitter and occupants, acceptable levels
of thermal comfort can be achieved with relatively low
room air temperature.
Choice of system
1.4.5
1-23
Choice of fuel or energy source
——
natural gas
——
liquefied petroleum gas (lpg)
——
petroleum based oils
——
grid electricity
——
on-site renewable electricity
——
on-site solar hot water
——
biofuels (solid, liquid and gaseous)
——
waste heat
——
heat supplied from district heating
Health and safety issues are generally governed by
regulations (e.g. Gas Safety in the UK) but will impact on
the need for trained operating personnel at all stages of fuel
delivery, storage, combustion and removal and disposal of
any solid residue.
Table 1.14 shows indicative relative performance in each of
these categories. These do need to be used with caution
however as there will be variations depending upon
equipment used, site location and application.
Further details of individual fuels are given in section 1.6.
1.4.5.1
Biofuels are produced from organic matter and available in
gaseous (biogas), liquid (bio-oil) and solid form (biomass).
Biofuels are becoming increasingly popular due to their
lower CO2 emissions compared to fossil fuels and in some
cases lower cost, though capital costs of plant are higher.
There are some concerns with their overall sustainability,
particularly bio-oils (DEFRA 2012).
The following factors should be considered:
——
cost of fuel, operation and maintenance
——
cost of installation
——
eligibility for government-funded schemes
——
environmental impact
——
delivery and storage space required
——
requirement for flues/chimneys
——
health and safety issues.
Biofuels
Cost of fuel is likely to be volatile over the life of the
installation and consideration needs to be given to possible
future increases in fuel tariffs. Costs are likely to be
influenced by demand and availability, particularly with
biofuels. Some fuels and forms of heating will comply with
the Renewable Heat Incentive (see section 1.2) and attract
payments for heat generated.
Environmental issues will include emissions of greenhouse
gases and other pollutants. Limits may be imposed by
national or local regulations (see section 1.3). This will
influence the requirements for flues/chimneys. There may
be other environmental issues such as impact of fuel
Biogas is produced principally from the aerobic digestion
of crop and animal waste from agriculture and the organic
material in refuse. After removal of unwanted gases such as
carbon monoxide and carbon dioxide, it comprises mostly
of methane and can be used as a replacement for natural
gas.
Bio-oils are organic oils that have been produced from
crops or waste vegetable oils. These are intended as direct
replacement for petroleum oils though some modifications
may be required to equipment burning such fuels.
Biomass may comprise waste organic matter (such as straw)
from agriculture but is mostly commonly derived from
wood. Wood chips are derived from forestry waste or
coppicing that has been graded and dried to some extent,
typically to 35% moisture content by weight. Wood pellets
have much lower water content being manufactured from
wood waste from saw mills. Specially designed boilers are
Table 1.14 Indicative relative performance of fuels
Fuel
Installation
Maintenance
and operation
CO2
SO2
NOx(2)
Particulates
Delivery
storage
space
Flues
Fuel
Costs
Emissions
Health
and
safety
Natural gas
Low
Low
Low
Med
Low
Low
None
Low
Low
Low
lpg
Med
Low
Low
Med
Low
Low
None
Med
Low
Med
Oil
Med
Med
Med
High
Med
Med
Med
Med
High
Med
Grid
electricity
High
Low
Low
(1)
High
High
High
(1)
High
Low
None
Low
Biomass
(wood chip)
Low
High
High
Low(3)
Med(3)
High(3)
High(4)
High
Med
Med
Bio-gas
High
High
High
Low
Med(3)
Med(3)
Low(4)
High(5)
Low
Med
Bio-oils
Med
Med
Med
Med
Med
Med
Med
Med
Med
Med
(1)
(3)
(1)
Typical average emissions at power station. There are zero emissions at point of use
(2)
NOx emissions dependent also on design of heat generator
(3)
Highly dependent upon sourcing and processing of fuel
(4)
Particulates can be reduced through appropriate filtration of flue gases
(5)
If produced on site
(1)
(3)
(4)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The choice of energy source will be limited to what is
available on site or can be delivered to and stored on site.
These include:
extraction , production, transportation. Carbon dioxide
emission factors are discussed in section 1.4.5.5 below.
1-24
Heating
1.4.5.2
Renewable electricity
Heating is normally provided by means of low temperature
hot water or medium temperature hot water to heat
exchangers within in each building (known as ‘heat
interface units’). These heat exchangers directly replace
individual boilers. Each building then has its own heat
distribution system, heat emitters and controls. It is
essential that heat energy consumption is accurately
monitored for each user so that heating costs can be charged
according to energy consumed. Flat rate tariffs will
invariably lead to excessive consumption.
There are a number of existing district heating schemes,
particularly in London (GLA 2014), and consideration
should be given to connecting to these. It is important
however to appreciate any limitations that might be
imposed on the user with regard to maximum return water
temperatures and minimum heating loads that can affect
the tariffs charged, (see section 1.13).
Renewable electricity generated on site, usually by means
of photovoltaic cells or wind turbines, or non-fosil fuel
fired chp and fuel cells, offers the potential for zero
emissions of greenhouse gases and other air pollutants.
With the advent of feed-in tariffs (see section 1.2.10 well
designed installations may result in overall lifetime costs
being similar to those of conventional fuels. It is normal
practice to install grid-connected systems, allowing top-up
from the grid at times of high demand on site and export to
the grid during times of low demand.
For new developments where there is no district heating
scheme, consideration should be given to installing a
community heating scheme, possibly including chp. The
zero carbon building target due to come into force in 2020
for new-build, non-domestic buildings (earlier for public
buildings and domestic) is expected take into account the
carbon reductions arising from connecting existing
buildings to community heating.
Details of how to design photovoltaic installations are
contained in CIBSE TM25: Understanding building integrated
photovoltaics.
Guidance on the evaluation and implementation of
community heating may be found in the Carbon Trust’s
Good Practice Guide GPG377 (Carbon Trust, 2005).
1.4.5.3
Detailed guidance on the design and operation of district
heating may be found in the Technical Guide to District
Heating published by the Building Research Establishment
(Wiltshire, 2014) and CIBSE/ADE Heat networks: Code of
Practice for the UK (CIBSE, 2015).
Community and district heating
Community heating involves the provision of heating from
a central source to, e.g., all the apartments in an apartment
building or to several buildings on a site or nearby. District
heating is the provision of heating to many buildings over
a larger area and is normally run by a specialist company.
If available, consideration should be given to taking a
supply of heat from an existing district deating (dh)
scheme. Most dh schemes either make use of large-scale
chp or energy from waste plants or even surplus heat from
industrial processes.
Community and district heating offer the potential for both
reduced CO2 emissions and lower running costs compared
to individual heating systems. This arises as a result of the
larger and more diverse heat load and as a consequence:
——
plant operating at maximum efficiency for longer
periods
——
potential for incorporation of chp or waste heat
——
more energy-efficient plant
——
use of non-fossil fuels
——
use of lower cost fuels
——
reduction in maintenance costs
——
1.4.5.4
Waste heat
Waste heat is heat liberated by some process that would
normally be thrown away but which may offer the
opportunity to be reclaimed and usefully used. The higher
the temperature of the waste heat the better as this make
the thermal energy easier to recover; increases its energy
content and allows a wider application for its use. Common
examples of potential sources of waste heat in buildings are
given in Table 1.15, together with indicative temperature
ranges.
Table 1.15 Indicative temperatures of sources of waste heat
Source of waste heat
Indicative
temperature range
Flue gases arising from combustion associated
with industrial process
>100 °C
Heat rejection from refrigeration circuits
possible payments under the Renewable Heat
Incentive or Feed-in Tariffs
35–45 °C
(up to 60 °C from a
de-superheater)
Waste domestic hot water
35–40 °C
——
possible reduction in Climate Change Levy
Exhaust air ventilation
20–23 °C
——
possible contribution towards achievement of zero
carbon status.
Heat generation by photovoltaic arrays
20–40 °C
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
required usually with some means of cleaning the flue gases
before discharging to atmosphere to minimise particulate
emissions. Flue emissions are relatively high in NOx. Key
issues with biomass are the need to meet local clean air
regulations; the regular delivery of the fuel to site and its
storage, both requiring considerable space; relatively
sophisticated automated fuel handling systems; high
capital cost; the need for trained personnel. These are offset
by the relatively low cost of the fuel and the low CO2
emissions. For further information see Carbon Trust Good
Practice Guides (Carbon Trust 2009a, 2009b), the Royal
Commission report on biomass (RCEP 2004) and CIBSE
AM15: Biomass heating.
Choice of system
1-25
Table 1.16 Carbon emission factors for fuels used in NCM 2014
CO2 emissions
/ (kgCO2/kW·h)
Natural gas (mains or liquefied, lng)
0.216
Liquified petroleum gas (lpg)
0.241
Heating oil
0.298
Wood chips
0.016
Wood pellets
0.039
District/community heating: biomass
0.031
District/community heating: waste
0.047
Grid electricity
0.519
Electricity displaced from grid*
0.519
Heat is recovered using some form of gas or liquid heat
exchanger (heat recovery device). These consume fan or
pump energy and this needs to be taken into account when
assessing their performance. The effectiveness of any heat
exchanger is defined as
Rate at which heat is
recovered (kW) × 100%
Effectiveness = ———————————
Maximum possible
recovery rate (kW)
* Where electricity is generated on site, the NCM allows a credit of 0.519
kgCO2 for every kW·h of electricity generated, treated as ‘displaced grid
electricity’. In the case of chp, this credit for grid displaced electricity
can be deducted from the CO2 emissions attributable to the input fuel.
The balance can then be used to determine the effective CO2 emissions
factor for the useful heat energy recovered from the chp. Section 1.7.10
gives a methodology based on NCM 2014.
Where waste heat temperature is low, it may be used for
pre-heating with top-up provided from the heating system.
Common applications are the pre-heating of outdoor air by
the exhaust air stream and the pre-heating of domestic hot
water in hot water cylinders.
Alternatively, the temperature of the waste heat can be
raised by means of a heat pump, frequently used with
exhaust ventilation air. That is, the waste heat becomes the
heat source for the heat pump. The cop of the heat pump
can be considerably improved.
A second major consideration is the frequency with which
the waste heat is available and its timing in relation to the
buildings demand for heat. Where supply and demand do
not match, it may be appropriate to incorporate thermal
storage. When heat is available at 100 °C or more, it can be
used to generate cooling using absorption refrigeration
(1.5)
The effectiveness will depend upon the heat exchanger
surface areas and the heat transfer coefficients of the fluids.
The latter are related to fluid flow friction factors. To
increase the effectiveness of the heat exchanger either its
size, and therefore capital cost, must be increased or the
fan/pump power must be increased. The addition of fins to
the the air-side of the heat exchanger will result in both
increased capital cost and fan/pump power. In the extreme
case, the cost of (or CO2 emissions associated with) the
consumed electricity will exceed that of the thermal energy
recovered. The effectiveness of the heat exchanger should
be optimised to maximise the net saving.
Once a heat exchanger effectiveness is selected, payback
period should be determined or a life cycle analysis based
on cost and/or CO2 undertaken. This should include
estimates of down time and maintenance.
1.4.5.5
Carbon dioxide emission factors for
fuels
The current Part L Building Regulations (England) specify
the CO2 emission factors to be used in the National
Calculation Methodology (NCM) for non-domestic
buildings. These are based on those given in SAP (BRE,
2012), a selection of which is included in Table 1.16. Other
countries will use a different set of carbon dioxide emission
factors in their building codes.
Table 1.17 Comparison of heat generators
Type of heat generator
Typical flow
temperature / °C
Fuel
Typical seasonal
efficiency (1) / %
Capital cost (3) / £
Air to air heat pump
40
Electricity
250
Medium
Air to water heat pump
40
Electricity
350
Medium
Ground source heat pump
40
Electricity
350
High
High efficiency boiler, lthw
80
Gas, lpg, oil, bio-gas and bio-oil
85
Low
Condensing boiler, lthw
70
Gas, lpg, oil, bio-gas and bio-oil
90(2)
Low
Combined heat and power
90
Gas, lpg, oil, bio-gas and bio-oil
Not applicable
High
Solar thermal
80
Solar radiation
Not applicable
Medium
Biomass boiler, lthw
80
Wood chip, pellets
80
High
Steam boiler
170
Gas, oil, biofuels
85
High
m/hthw boiler
>90
Gas, oil, biofuels
85
High
(1)
Based on gross calorific value of fuel
(2)
Significantly higher seasonal efficiency possible when return water temperature is below 50 °C for extended periods of time
(3)
Includes all associated plant essential to heat generator but excludes cost of heating distribution system
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Fuel
plant rather than heating. This may allow the waste heat to
be used year-round.
1-26
1.4.6
Heating
Choice of heat generator
——
boilers (for hydronic systems)
——
steam boilers
——
combined heat and power
——
solar thermal
——
heat pumps
——
direct electric.
These are compared in Table 1.17 in terms of typical flow
temperatures, fuel used, seasonal efficiency and capital cost.
1.4.6.1
Boilers for hydronic systems
Boilers are available in a large range of sizes from a few tens
of kW to several MW output. They are by far the most
common form of heat generator due to their relative
simplicity and their compactness and low capital cost
per kW. They are available for a wide range of fuels (biofuels,
natural gas, lpg, light and heavy oils, electricity and dualfuels), with natural gas being by far the most popular. They
may be supplied with an integral burner or be designed to
accept a matched burner.
Boilers (other than electric) are essentially heat exchangers
for transferring heat from the flame (radiation) and flue
gases (convection) to water. As such the design of the heat
exchanger determines the type of boiler, in particular the
weight, water content and the design operating pressure.
Boilers are available with:
——
heat exchangers constructed from cast-iron sections
(sectional boilers)
——
copper, steel or stainless steel heat exchangers
surrounding the combustion chamber
——
a separate combustion chamber with tubes for flue
gases within a water-filled shell (shell boilers).
The first two types are also available in modular form with
outputs of typically of the order of 50–500 kW. Shell boilers
are produced with outputs typically from 300 kW to in
excess of 6 MW.
Modern boilers incorporate improvements in the design of
the heat exchangers to improve heat transfer rates and allow
recovery of latent heat of steam in the flue gas (both gas and
oil fired) by permitting condensation without corrosion;
gas-air modulation to improve combustion efficiency; and
improvements in control to minimise cycling. These have
lead to considerable improvements in efficiency, particularly
at part-load where flow and return water temperatures can
be reduced and condensation enhanced A high part-load
efficiency is important because boilers will operate mostly
at less than design load. Boilers are tested under both full
and part load conditions to measure efficiency from which
the boiler seasonal efficiency is calculated (see section
Boilers are available for low, medium and high temperature
systems. Medium and high pressure installations will
require trained personnel to operate the equipment and are
be subject to specific national safety regulations such as the
Pressure Systems Safety Regulations 2000 (HSE, 2000c) in
the UK. Boilers are operated at pressures that are maintained
in excess of the saturation pressure of water at their
operating temperature. How this pressurisation is effected
is a major design issue with hydronic heating systems.
Further details of boilers for hydronic systems are given in
section 1.7 and CIBSE AM14: Non-domestic hot water heating
systems. See also section 1.8 for details of the design of
hydronic heating systems.
1.4.6.2
Steam boilers
Shell and tube steam boilers generate steam which is
collects above the water line in the shell of the boiler. As
both liquid water and steam is in contact, the pressure in
the shell is equal to the saturation pressure corresponding
to the water/steam temperature. In steam generators, dry
superheated steam is produced.
Burner designs are similar to those used on boilers serving
hydronic systems and available for a range of fuels including
electric and dual-fuel.
Steam systems are normally operated at above 1 atmosphere
(i.e. above 100 °C) though they can be operated at sub
atmospheric pressures. Specific pressure safety regulations
apply (HSE, 2000c) and competent staff required to operate
them.
Further details of steam boilers and steam heating systems
are given in section 1.9 and CIBSE TM59: Design and
operation of modern steam systems, while CIBSE Guide G
provides information on water treatment and prevention of
corrosion in steam systems.
1.4.6.3
Combined heat and power
Part L of the Building Regulations (England) require that
consideration is given to low and zero carbon technology
such as chp. Some planning authorities, in particular the
Greater London Authority in the UK (GLA, 2011), also
require that consideration be given to the use of chp for
new large developments particularly when associated with
community or district heating.
All engines that produce work (e.g. electricity) from a heat
source (e.g. combustion of gas) will also produce heat as a
byproduct. In a power station, this heat is often rejected to
the atmosphere, river or sea. The ratio of thermal energy
rejected to electricity generated is typically between 0.9 and
2.0 and therefore represents a considerable loss in overall
efficiency. In a combined heat and power (chp) system, this
otherwise waste heat is used to meet some local heating
demand, e.g. space heating, dhw heating, heat input to
absorption chillers to provide cooling or process heat.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The choice of heat generator for space heating will depend
upon a number of factors including the availability of
appropriate fuel on site or the potential for fuel storage and
the type of heat emitter and distribution medium preferred.
A very significant factor in the choice will be any
requirement for either CO2 reduction and/or delivery of
renewable energy. Heat generators for space heating
include:
1.3.6.1). Improvements in boiler design have also lead to
reductions in NOx emissions, though such emissions for
biomass boilers remains relatively high due to nitrogen in
the fuel itself.
Choice of system
1-27
The case for using chp depends on requirements both for
heat and electricity, their diurnal and seasonal variability
and the extent to which they occur simultaneously. The
optimum chp plant capacity for a single building needs to
be determined by an economic assessment of a range of
plant sizes and in general will result in only part of the load
being met by chp, the rest being met by a boiler. Where
standby power generation is required to reduce dependency
of public supplies of electricity, it may be particularly
advantageous to install chp, though maintenance costs and
maintenance down-time will be greater which may
necessitate a second machine. CIBSE AM12: Small-scale
combined heat and power for buildings gives detailed guidance
on the application of chp in buildings.
1.4.6.4
Heat pumps
Heat pumps are devices that can absorb heat at a low
temperature (e.g., the outdoor air, the ground or a lake) and
emit it a higher temperature (typically about 40 °C) where it
can provide useful heating. In the process, some high-grade
energy must be supplied to the heat pump. This is most
commonly in the form of electricity (to a compressor),
though gas-fired (absorption) heat pumps are gaining in
popularity. They are refrigeration systems in which the
condenser provides the useful heat. The amount of heat
produced always exceeds the high-grade input.
The efficiency, referred to as the coefficient of performance
(cop) is defined as:
Useful heat output (kW)
cop = ——————————— × 100%
High grade input (kW)
(1.6)
Electric heat pumps have peak cops typically between
about 200% and 500% whilst gas-fired heat pumps have
peak cops of around 140%. Heat pumps therefore have an
efficiency advantage over direct electric heating or gas-fired
boilers.
The environmental advantage hinges on the CO2 emission
factor of the fuel used. For electricity drawn from the UK
grid, a seasonal cop of around 2 is required for an
electrically-driven heat pump to achieve lower emissions
than would be obtained from a gas condensing boiler.
However, as the electricity grid becomes decarbonised heat
pumps will provide increasingly low carbon sources of heat.
Gas-fired heat pumps, despite their relatively low cop, may
currently result in lower CO2 emissions per unit of useful
heat output (kgCO2/kW·h) than electrically driven units
due to the lower carbon emission factor for natural gas.
Air source heat pumps may be used to extract heat either
from outside air or from ventilation exhaust air. When
outside air is used as a heat source, the coefficient of
performance tends to decline as the air temperature drops.
There can also be problems with icing-up of the heat
exchanger where the outside air is of high humidity, which
is frequently the case in the UK. This requires periodic
defrosting, which is often achieved by temporary reversals
of the heat pump. Because of these factors, air-to-air heat
pumps have a relatively low seasonal cop (in the range of
2.0 to 2.5) when used for heating in a typical UK climate.
As cop declines with outside temperature, it is not economic
to size air source heat pumps for the coldest conditions, and
they often include electrical resistance coils for supple­
mentary heating (although this is no longer permitted
under the Microgeneration Certification Scheme).
Ground or water source heat pumps extract heat from the
ground or bodies of water, either at ambient temperature or
with temperature raised by the outflow of waste heat. They
have the advantage over air source heat pumps that their
heat source has much greater specific heat capacity and,
provided it has sufficient mass, varies much less with
outside temperature. Small ground source heat pumps
should have a seasonal cop of around 3.5 in a typical UK
climate. Details of ground source heat pumps are given in
CIBSE TM51.
Many heat pumps used for heating in commercial buildings
are reversible and can therefore provide cooling in summer
at no additional capital cost. Heat pumps are also capable of
delivering simultaneous heating and cooling using the
warm and cool sides of the heat pump. However, so as not
to encourage the installation of mechanical cooling, some
schemes such as the UK Renewable Heat Incentive (RHI)
(see section 1.2) may exclude heat pumps that are able to
cool.
1.4.6.5
Direct electric
Direct electric heating systems include individual air or
radiant heaters, storage heaters, and steam or hot water
(including immersion and electrode) boilers up to hundreds
of kW duty. Using electricity for heating can result in lower
capital costs but consideration needs to be given to
potentially higher running costs and greater environmental
impact particularly through higher CO2 emissions.
1.4.6.6
Solar collectors
Solar water heating collectors are widely used around the
world to provide domestic hot water, particularly where
sun­shine is plentiful and fuel is relatively expensive, but are
rarely used for space heating as solar radiation is at its
minimum during the heating season. In the UK climate, a
well designed domestic installation can typically provide up
to half the the annual hot water requirements, using either a
separate pre-heat storage cylinder or a cylinder with two
primary coils, one linked to the solar collector and the other
to a boiler. Although technically successful, the economics
of such systems to date have at best been marginal in the UK
when assessed against heat produced by a gas or oil boiler,
although this is likely to change with the introduction of the
RHI. Solar collectors are also widely used for heating
outdoor swimming pools in summer, for which they are
more likely to be cost effective. Further details of solar hot
water collectors are provided in section 1.7.7.5.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The most common form of chp currently used in buildings
are gas-fired spark ignition engines, oil fired diesel engines
and gas/oil fired turbines. Increasingly, diesel engines can
be run on bio-oils. Fuel cells are also available, ideally
running on hydrogen but able to run on a variety of
hydrocarbon gases. chp can offer environmental advantages
when compared to conventional heating systems and grid
electricity supply in terms of reduced total CO2 emissions
but may result in increased local emissions of air pollutants.
Although capital costs are high, running costs can lead to
reasonable payback periods. Both reductions in CO2 and
running costs depend upon the majority of the heat
generated being used.
1-28
Heating
1.4.7
Physical dimensions
Choice of heat emitters
——
radiators
——
low temperature radiant panels
——
high temperature radiant heaters
——
natural convectors
——
fan convectors/fan coils
——
active beams
——
underfloor heating.
Architectural
Choice of heat emitter will be influenced by the choice of
fuel and heat generator. However, heat emitters are the
main interface between occupants and the heating system
and in many instances the choice of heat emitter may drive
decisions on fuel and heat generator. In any event, the
choice of heat emitter will also include:
——
physical dimensions
——
architectural considerations
——
occupant thermal comfort
——
energy consumption
——
speed of response
——
exposed surface temperature
——
room noise levels
——
installation costs
——
maintenance requirements
——
life expectancy.
Architectural considerations include the appearance and
positioning of the heat emitter. Underfloor heating is
unobtrusive but will limit the use of floor surface coverings.
Low temperature radiant panels can be integrated within
the ceiling or walls as can fan coils. Radiators are available
in a number of styles and materials.
Occupant comfort and energy consumption
Convectors provide virtually 100% convective heating;
radiators, low temperature radiant panels and underfloor
heating provide a mix of convective and radiant heating
depending on the exposed surface temperature; high
temperature radiant heaters provide virtually 100% radiant
heat.
Convective systems will result in the room air temperature
exceeding the room mean radiant temperature. Radiant
systems will result in the opposite. The convective/radiant
split will therefore have an impact on both thermal comfort
(see section 1.3) and the heat loss rate from the room (see
Table 1.18 Comparison of heat emitters
Emitter type
Heating medium
Capital cost
per kW
output
Life
expectancy
Maintenance
requirements
Indicative heat
output rate
% radiant
output
Response times
Radiators
lthw (80–60 °C)
Low
20 years
Low
1–2 kW/m2 (face
area)
20–40
Medium
Low surface temp
radiators
lthw (60–40 °C)
Low/medium
20 years
Low
0.5–1 kW/m2
(face area)
20–45
Slow
Low temp radiant
panels
lthw (80–60 °C)
Low
20 years
Low
0.3 kW/m2 (face
area)
60–80
Medium
Low temp radiant
panel
Electric (70 °C
surface temp)
Low
20 years
Low
0.3 kW/m2 (face
area)
60–80
Medium
Natural convectors
lthw (80–60 °C)
Low
20 years
Low
0.1–0.5 kW/m
(linear length)
≈0
Medium
Fan convectors and
fan coil units
lthw (80–60 °C)
Low/medium
15 years
Medium
10–30 kW/m2
(grille area)
0
Fast
Underfloor heating
lthw (50–40 °C)
Medium
20 years
Low
50–80 W/m2
(active area)
50
Slow
Underfloor heating
Electric
Low/medium
20 years
Low
50–80 W/m2
(active area)
50
Slow
High temperature
radiant tube
Direct gas-fired
Low/medium
15 years
High
4 kW/m (linear)
≈ 100
Fast
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Fan convectors and fancoils will be the most compact.
Radiators will have a smaller face area than radiant panels
for the same surface temperature but have greater depth.
High temperature radiant panels can achieve high heat
outputs per unit area but are restricted in where they can be
located to avoid thermal discomfort. Underfloor heating is
limited by the low surface temperatures demanded to avoid
discomfort, resulting in heat output rates an order of
magnitude less than achieved by radiators. Heat output
rates from natural convectors vary considerably but are
typically less than half that from radiators.
Heat emitters available include:
Choice of system
Rooms with high floor to ceiling heights will suffer from
stratification with increased heat loss rates through the roof
and again a radiant system will be of benefit, including
underfloor heating. Conversely, where room heat loss is
dominated by the fabric, radiant systems will result in
higher room surface temperatures than would a convective
system and hence higher rates of heat loss and energy
consumption. However, this needs to be balanced against
the need for occupant thermal comfort.
Low room surface temperatures, a consequence of poorly
insulated fabric and in particular large areas of glazing, can
result in highly asymmetric radiant temperatures leading
to occupant discomfort. Radiant heat emitters located on
cold walls or beneath glazing will reduce this effect.
1.4.7.1
Radiators
Radiators are the most frequent choice of emitter. They are
available in a wide variety of shapes, sizes and output
ranges, making it possible to obtain a unit (or units) to
match the heat requirements of almost any room or zone.
Panel radiators are available in single/double and triple
panel configurations, with fins behind each panel to
increase heat output rate. They are normally constructed
from steel. Column radiators are available in cast iron or
aluminium.
Radiators operate with exposed surface temperatures well
above room temperature. The higher this temperature the
greater the heat output. Some applications, e.g. primary
schools, may limit the surface temperatures in the interest
of occupant safety.
Despite their name, the heat output from radiators is
predominantly by convection, particularly column and
multi-panel radiators with fins.
Further details on radiators for hydronic systems are
included in section 1.8.
Low temperature radiant panels
Speed of response
1.4.7.2
In buildings that are occupied intermittently, such as
churches, highly radiant systems can provide rapid and
highly localised warming of occupants without significantly
raising room air or surface temperatures. Underfloor
heating systems have very long response times and are best
suited to buildings with long and predictable occupancy
times. Where underfloor heating systems are installed, fan
convectors are sometimes provided for either top-up in
very cold weather or as fast response heating systems for
start-up.
Radiant panels comprise a flat metallic surface facing into
the room. The rear is bonded to a serpentine pipe carrying
hot water or an electric heating element and covered in
insulation. Such panels may be installed on a wall,
sometimes flush with wall surface; installed within the
plane of a false ceiling or simply suspended horizontally
from the ceiling, particularly in industrial buildings with
high ceilings.
Surface temperature
In some instances exposed surface temperatures of heat
emitters may be limited for the safety of occupants such as
in schools and healthcare facilties. The surface temperature
of heated floors must be limited to avoid thermal discomfort.
This will depend upon how long occupants’ feet are in
contact with the floor (i.e. sitting or walking).
Room noise levels
The heat output rate of fan convectors and fan coil units is
governed by the air velocity flowing over the heating coils.
Whilst very high heat output rates are achievable, some fan
coil units can lead to noise disturbance unless selected to
run on slow speed.
Table 1.18 gives a comparative performance for heat
emitters including installation costs, maintenance
requirements and life expectancy. This is indicative only
and will vary with manufacturer’s design. Heat emitters
served by steam will give similar heat output rates to lthw
for similar heat emitter surface temperatures.
The following paragraphs briefly describe each of the above
heat emitters. More detailed descriptions are contained in
later sections dealing with specific heating system types
(hydronic, steam, air and unitary).
Heat output is mostly radiant, particularly when mounted
horizontally at high level. Care needs to be taken to ensure
that the asymmetric radiant temperature at head height
does not exceed that required for thermal comfort (see
CIBSE Guide A).
Further details on radiator panels for hydronic systems are
included in section 1.8.
1.4.7.3
High temperature radiant heaters
High temperature radiant heaters produce virtually 100%
radiant heat output by virtue of their high surface
temperatures. The three common types are direct gas-fired
tubular heaters, direct gas-fired radiant plaque and electric
quartz lamps. All three are classified as unitary heaters (see
section 1.11)
Gas fired tubular heaters comprise a pressurised gas burner
and a steel tube through which the flue gases flow radiating
heat to the room before exiting via the flue to outdoors. The
radiant tube reaches surface temperatures of up to around
500 °C and is backed with a reflective panel to direct radiant
heat downwards into the room.
Plaque heaters utilise a ceramic element which is directly
heated by the gas flame, reaching temperatures of about
800 °C. The electric quartz heater comprises a lamp
contained within a quartz tube allowing the lamp filament
to operate at temperatures exceeding 2500 °C.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
section 1.5) the extent of which will depend upon the room
characteristics. Well insulated buildings with low
infiltration rates tend to have only small differences
between air and mean radiant temperatures and the choice
of heat emitter will have little impact on occupant comfort
or energy consumption. Where room heat loss is dominated
by infiltration, radiant heat emitters can achieve the
required operative temperature with lower air temperatures
and so lower heat loss rate and energy consumption.
1-29
1-30
Heating
Natural convectors
Wall-mounted natural convectors may be used instead of
radiators. These utilise finned pipes or electric heating
elements within an enclosure close to room temperature so
that there is negligible radiant heat transfer. They may also
be used where there is insufficient space for mounting
radiators, e.g. in base-board or trench heating configurations.
The output from natural convectors varies considerably
with design and manufacturer’s data for individual emitter
types should be used.
1.4.7.5
Fan convectors and fan coil units
Fan convectors and fan coil units produce high heat outputs
from compact units using forced air circulation. Their
output may be considered to be entirely convective and is
approximately proportional to temperature difference
between the heating element and the room air. Fan
convectors comprise a heating coil (lthw or electric), a fan
within a case and inlet and outlet grilles. Fan coil units
usually contain in addition a filter, a coil to provide cooling
and integral controls.
Where systems contain a mixture of natural and forced air
appliances, the different output characteristics of the two
types should be taken into account, particularly with regard
to zoning for control systems.
1.4.7.6
Active beams
Active beams comprise linear lthw coils within a case
located at ceiling level and connected to a ducted mechanical
ventilation system. The beams are usually provided with
chilled water to provide cooling and may integrate other
services such as lighting. Most modern, highly insulated
buildings where the steady-state heating load is generally
in the range 25–45 W/m2 are suited to the application of
heating from active beams. Typically hot water is supplied
at flow temperature of less than 40 °C to avoid excessive air
discharge temperatures from the beam resulting in
stratification.
A general rule of thumb is to use beams for heating,without
supplementary heating, only if the internal surface
temperature of the windows is above 14 °C . Their use with
full height glazing where the U-value is greater than
1.2–1.4 W/m2·K needs careful consideration. To avoid cold
downdraughts, it may be necessary to use low level
perimeter heating or trench heating to supplement the heat
output from active beams.
Active beams require the primary ventilation system to be
operating during preheating, therefore this should have a
facility to operate in full recirculation mode and at a boost
temperature to minimise the pre-heat period so as to operate
efficiently.
Underfloor heating
Underfloor heating uses the floor surface itself as a heat
emitter. Heat is provided in most cases by the circulation of
warm water as part of a hydronic system, involving
appropriately spaced pipes positioned beneath the floor
surface. The pipes may be embedded within the screed of a
solid floor or laid in a carefully controlled configuration
beneath a suspended floor surface. Insulation beneath the
heating elements is clearly very important for good control
of output and to avoid unnecessary heat loss.
Floor surface temperature is critical to comfort, as well as to
heat output. The optimum floor temperature range for
comfort lies between 21 and 28 °C depending on surface
material (see CIBSE Guide A, 2015), so systems are normally
designed to operate at no higher than 29 °C in occupied
areas. Higher temperatures are acceptable in bathrooms
and close to external walls with high heat loss, such as
beneath full-length windows. As surface temperature is
limited, flow temperatures of lthw are typically no more
than about 40 °C. This makes underfloor heating an ideal
application for heat pumps and condensing boilers.
In cases where heat demand is low, electric underfloor
heating is sometimes used. This can lead to reduced capital
costs but at the cost of increased running costs and CO2
emissions.
The heat emission characteristics of floor heating differ
considerably from those of radiators due to the much
smaller temperature difference between the heat emitter
surface and the room. As a result heat output from a heated
floor is almost directly proportional to the temperature
difference. A small increase in room temperature will result
in a significant drop in heat output rate leading to a degree
of self regulation. However, the high thermal mass of the
heated floor leads to a long lag time between adjusting heat
input and changes in floor temperature.
The limit on surface temperature may mean that floor
heating by itself cannot provide sufficient heat output. It is
not uncommon therefore to use floor heating to meet base
heat demand with a fast response emitter such as a fan
convector for top-up or extreme winter days. This is
particularly relevant in older buildings and where ‘spot
heaters’ are required in open plan circulation spaces.
Heat output from the floor is roughly 50% radiant and 50%
convective. Stratification of the room air temperature is
reduced compared to a radiator or convector (Brown, 2011),
2·4
Underfloor
Radiator
1·2
0·2
16
18
20
22
Room temperature / °C
24
Figure 1.4 Room temperature profiles for radiator and underfloor
heating.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1.4.7.4
1.4.7.7
Height above floor
High temperature radiant heaters need to be located at a
safe distance from occupants. They are particularly suited
to large halls, manufacturing facilities and retail units with
high ceilings.
Choice of system
1-31
Table 1.19 Characteristics of heat distribution media
Principal characteristics
Air
The main advantage of air is that no intermediate medium or heat exchanger is needed and there is no risk of water leak.
The main disadvantage is the large volume of air required and size of ductwork and high energy consumption by fans.
However, warm air heating can be combined with mechanical ventilation or air conditioning.
Low temperature hot
water (lthw)
Generally recognised as simple to install and safe in operation. Output limited by system temperatures but this may not
be a problem in modern, well insulated buildings.
Medium temperature hot
water (mthw)
Permits a greater difference between flow and return temperature than lthw so smaller pipework required for same heat
output. Requires more complex system pressurisation schemes. Unlikely to be cost effective except in very large
buildings.
High temperature hot
water (hthw)
Permits even greater flow/return temperature differences than mthw and so even smaller pipework. However, inherent
dangers require that all pipework is welded similar to steam systems. Unlikely to be appropriate except where heat must
be transferred over large distances.
Steam
Utilises latent rather than sensible heat so extremely high heat transfer carrying capacity. Can be designed to operate at a
wide range of pressures and temperatures. Usually used for large sites or when some site process requires steam.
Hot thermal fluids (oils)
Used for radiant heating in some industrial buildings as a alternative to steam where the thermal fluid also heats process
plant . Operates at atmospheric pressure and does not require water treatment.
as shown in Figure 1.4, leading to improved levels of thermal
comfort (subject to limiting floor surface temperature).
1.4.7.8
Location of heat emitters
As it is generally desirable to provide relatively uniform air
and radiant temperatures throughout a room or zone,
careful consideration should be given to the location of heat
emitters. They should normally be located close to any cold
surfaces, such as windows and poorly insulated external
walls. These may cause thermal discomfort due to cold
down draughts of air or by ‘radiation draught’. The heat
emitter will help to reduce radiant draught (see section 1.3)
by raising the surface temperature of the building fabric
itself through the heat emitter’s convective heat output
and/or by direct radiant heat output.
Where predominantly radiant heat emitters are used,
radiant asymmetry close to occupants should be checked.
High temperature radiant heat emitters must be located at
high level, usually several metres above head height.
Manufacturer’s advice should be sought.
1.4.8
Choice of distribution medium
The medium for distributing heat around the building
needs also to be considered, taking account of requirements
for heat emitters. Air and water are the commonest choices
but steam is still used in many existing buildings and
refrigerant fluids are used in heat pumps. For decentralised
heating systems, electricity is the most versatile medium
for distribution as it can be converted to heat at any
temperature required at any location. However,
consideration of primary energy, CO2 emissions and
running cost tend to militate against the use of electricity.
Gas and oil may also be distributed directly to individual
heaters.
The choice of distribution medium must take account of
the balance between radiant and convective output required.
When air is used for distribution, the opportunity for
radiant heat output is very limited but water and steam
systems can be designed to give output that is either
predominantly convective or with a significant radiative
component. However, when highly directed radiant output
is required then only infrared elements powered by
electricity, directly fired by gas heaters or steam heated
radiant tubes are applicable. The relative merits of various
distribution media are described briefly in Table 1.19.
Water and steam require small volume flow rates compared
to air. This results in pipework being much smaller for
hydronic and steam systems than the ductwork required by
air systems. In addition, the energy required to distribute
hot water or steam is very much less than for air, per unit of
heat energy delivered.
1.4.9
Choice of domestic hot water
system
Systems for the production of domestic hot water include:
——
hot water storage calorifiers (centralised or
decentralised)
——
plate heat exchangers with minimal storage
(centralised)
——
point of use hot water heaters (with or without
minimal storage capacity)
——
centralised hot water generators (with minimal
storage capacity).
Hot water within cylinders can be heated by the lthw,
mthw or steam from the same heat generator(s) that
provide the space heating, by electric immersion heater, a
combination of the two or by direct gas firing. They may be
centralised or decentralised.
Plate heat exchangers are increasingly used as these can
provide relatively high dhw flow rates without the need for
storage.
Point-of-use hot water heaters are mostly electric but
occasionally gas-fired, providing hot water usually to single
dhw draw-off points. Some heaters contain a small amount
of storage.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Medium
1-32
Heating
Centralised hot water generators are gas or oil fired with
relatively large heat output rates and providing high dhw
flow rates. They contain a minimal storage volume.
——
The decision to utilise centralised or decentralised systems,
whether to incorporate storage and whether to combine
with the space heating system depends upon:
This section will give only an overview of these issues and
procedures, concentrating on their application. It also
includes a discussion on the selection and sizing of heat
generators taking into account diversity of heating loads.
choice of space heating system
——
dhw demand
——
number and location of dhw draw-off points.
Where draw-off rates are highly intermittent and/or drawoff points widely distributed throughout the building, a
decentralised system will minimise heat losses from
pipework and water wastage at the taps. Local systems will
have much less distribution pipework and reduce waiting
time for hot water to reach taps. Where draw-off rates are
relatively small, point of use systems may be preferable. For
example in large office buildings, wash hand basins in toilet
blocks are frequently provided by small point of use electric
water heaters.
The room heat loss calculations presented here are based on
the CIBSE ‘Simple Model’, a model based on steady-state
heat transfer. A similar model is given in BS EN 12831 (BSI,
2003), though this assumes that room air and operative
temperature are equal in value. Both methodologies assume
little or no temperature variation throughout the room and
are based on continuous heating to which simple factors are
applied to allow for intermittent operation.
Where local ‘spot’ heating is required (e.g. using high
temperature radiant systems) or highly intermittent heating
used (e.g. in a building that may only be occupied
occasionally), these calculation methods should not be used:
readers are referred instead to CIBSE Guide A, chapter 5.
Storage systems allow a high dhw draw-off rate without the
need for high energy input rates. For example, a 120 L hot
water cylinder (with only a 6 kW rated lthw heat exchanger
or 3 kW electric immersion), is quite capable of delivering
20 L/min or more of hot water. An instantaneous system
would require in the order of 70 kW input.
1.5.2
Where high and frequent draw-off rates are required, e.g. a
hotel, centralised hot water generators are common. Where
chp is provided, the chp normally provides the dhw
through a storage system so as provide a year round and
relatively constant heating demand.
Figure 1.5 is a flow chart of the key steps in sizing heating
systems for buildings. Heating demands for room heating,
mechanical ventilation and dhw systems are carried out
separately. Note that the room heat emitter characteristics
(fraction radiant output) are required in order to determine
the room steady-state heat loss. Individual heat emitters,
ventilation heating equipment and dhw primary heaters
can then be sized, allowing for intermittent operation.
Diversity factors are then applied as appropriate and these
together with the operating profiles of each heat use are
used to determine the peak simultaneous load to be offset
by the heat generators. An allowance is then made for heat
losses from distribution systems. Finally, standby capacity
is incorporated and the number and duties of heat
generators determined.
The prevention of the growth of Legionella bacteria in dhw
systems is most important. Preventative measures are set
out in CIBSE TM13 and the Approved Code of Practice L8
(HSE, 2013b) produced by the Health and Safety Executive
(HSE, 2013). HSE also publish HSG274: Legionnaires'
Disease (HSE, 2014), part 2 of which deals with the control
of Legionella bacteria in hot and cold water systems.
1.5
1.5.1
Heating load calculations
and sizing methodology
General
This section discusses the issues related to the sizing of
heating systems (heat emitters and heat generators), and
presents a methodology for doing so in terms of achieving
desired room temperatures, ventilation air temperatures
and the production of domestic hot water (dhw).
Details of the building heat loss calculation methodology
are provided in CIBSE Guide A:
——
Guide A, chapter 3, gives details of the thermal
performance of building fabric including
calculation of U-values
——
Guide A, chapter 4, gives details of infiltration and
ventilation
Calculation principles
Heating systems cannot be sized until a heating system
type has been selected. In particular, the ratio of radiant to
convective heat output of the heat emitters affects room
heat loss rate.
1.5.3
Room design heating load
Room heat load is the peak heating demand placed on the
room heat emitter. It is equal to the sum of all the steady-state
heat losses from the room under design conditions with an
allowance made for intermittent operation where appropriate.
Heat losses from the room include both fabric and infiltration
and ventilation if applicable. Heat gains, e.g. from people or
machines, are usually ignored except in instances where such
gains will always exist when heating is required.
Infiltration is the uncontrolled ingress of outdoor air
through the fabric of the building arising from pressure
difference between inside and out caused by wind and
buoyancy (stack) forces. Infiltration will always contribute
to the room design heat loss.
Ventilation is the intentional introduction of outdoor air to
a room. It may be by natural means (natural ventilation)
through openings such as windows or vents, or it may
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
Guide A, chapter 5 sets out in detail the heat loss
equations to be used in determining room design
heat loss and room emitter duty.
Heating load calculations and sizing methodology
1-33
Figure 1.5 Heating system calculation
flowchart
Select space heating and DHW system type
Mechanical
ventilation heating
Domestic hot water
heating duty
Select design conditions
for room
Select design criteria for
mechanical ventilation
system/s
Select design criteria
for DHW
Select room heat emitter
characteristics
Select mechanical
ventilation system
Select DHW system
including storage
Calculate room steady
state heat loss
Calculate design steady-state
heat demand
Calculate design steady-state
heat demand
Calculate room heat
emitter duty for intermittent
operation
Mech vent heating duty =
steady state heat demand
DHW system heating duty
= steady state heat demand
Repeat procedure for
all rooms
Repeat for all other mech
ventilation systems
Repeat for all other
DHW systems
Calculate room fabric U
values and Infiltration/
ventilation rate
Analyse operating profiles of each heat demand and apply
diversity factors to sums of room heat emitter, ventilation
system and DHW system duties connected to
centralised heating system
Determine distribution heat losses
Summate distribution losses and diversified duties for
heat emitters, mech vent systems and DHW systems to
determine heat generator duty
Select standby capacity and number/duties of heat
generators
incorporate a fan (mechanical ventilation). Ventilation air
may or may not result in a room heat loss load depending
upon the temperature at which it enters the room.
For intermittently operated heating systems at start-up, the
room fabric will be cold and will need to absorb heat from
the room emitter before room design temperature can be
achieved in time for the start of the occupancy. This is
known as the pre-heat period. During the pre-heat period
the room heat load will always exceed the combined fabric
and infiltration steady-state heat loss.
In the case of mechanical ventilation, it is common practice
to heat the air prior to introduction to the room. Provided
that it is heated to room temperature, it will not appear as a
room heat loss or load on the room emitter. It will appear
instead as a load on the ventilation heating plant. Where it
is introduced below room temperature it will appear as a
room heat loss and, if introduced above room temperature,
a room heat gain. However, mechanical ventilation systems
are not normally operated during the pre-heat period as the
rooms are not yet occupied. In the case of natural ventilation,
it is difficult to preheat the outdoor air prior to introduction
to the room. In most cases therefore natural ventilation will
appear as a room heat loss and a load on the room heat
emitter. During pre-heat period, windows and vents will
normally be shut and infiltration only occurs.
However, the ventilation heat loss outside the pre-heat
period could be greater than the allowance made for preheat. This is particularly so in the case of natural ventilation.
Therefore, where there is a ventilation heat loss to the
room, the heat load during and following the pre-heat
period should be determined and the emitter should be
sized on the larger of the two.
1.5.3.1
Fabric heat loss
Fabric heat loss occurs by conduction of heat through those
parts of the structure exposed to the outside air or adjacent
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Room heat
emitter duty
1-34
Heating
to partially heated or unheated areas, often referred to as
the ‘building envelope’. The steady-state heat loss through
the external elements of the building to outdoors can be
calculated from:
(1.7a)
where Φf is the design steady-state heat loss through the
building fabric (W), U is the thermal transmittance of each
building element (W·m–2·K–1), and A is the area of that
element (m2), θei is the design indoor environmental
temperature (°C) and θao is the design outdoor temperature
(°C).
Where there is a heat loss to adjacent spaces this can be
incorporated into equation 1.7a by means of a modified
U-value U':
U' = U (θei – θ) / (θei – θao)
(1.7b)
where U is the actual U-value of the separating fabric
element and θ is the environmental temperature of the
adjacent space. Indoor environmental temperature is a
temperature index given by:
θei = θr / 3 + 2 θm / 3
(1.8)
where θr is the room air temperature and θm is the room
mean radiant temperature (see CIBSE Guide A for a
detailed discussion of environmental temperature).
An allowance may be required for back losses from heat
emitters and stratification of the room air. CIBSE Guide A,
chapter 5, gives recommendations.
1.5.3.2
Steady state heat loss due to infiltration can be represented
by:
(1.9)
where qinf is the infiltration rate (m3/s), ρ is the air density
(kg/m3), cp is the specific heat capacity of air at constant
pressure (J/kg·K), θai is the inside air temperature (°C) and
θao is the outside air temperature (°C).
By convention, the thermodynamic properties for the air
are taken at the internal conditions, for which the density
will be approximately 1.20 kg/m3 and the specific heat
capacity approximately 1000 J/kg·K. This leads to the
following simplifications:
Φinf = 1200 qinf (θai – θao)
The air permeability of a building can be measured by
pressurising the internal space and measuring the air flow
rate needed to maintain that pressure. This is the basis for
the air permeability test required of new buildings under
the current Building Regulations Part L (England) which
sets minimum standards of permeability. Methods of
estimating infiltration rates based on air permeability are
given in CIBSE Guide A and TM23.
For existing buildings that have not been recently pressure
tested, Table 1.20 gives recommended air infiltration rates.
These are applicable to single rooms or spaces and are
appropriate for the estimation of infiltration heat loss.
1.5.3.3
Infiltration heat loss
Φinf = qinf ρ cp (θai – θao)
Air infiltration is related to surface area of the building
envelope rather than building volume. As a result estimates
based on air change rate tend to exaggerate infiltration
losses for large buildings. At the same time, infiltration
heat loss is concentrated at the perimeter of the building so
that the use of average air change rates for the whole
building will underestimate infiltration heat losses in
perimeter rooms. Air infiltration rates based on air change
rates must therefore be used with care. Being driven by
wind and stack pressures, infiltration rate varies with wind
speed, wind direction and outdoor temperature and as is
therefore difficult to predict. Infiltration rates vary widely
from building to building depending on build quality and
location.
(1.10)
Where outdoor air is supplied as part of a natural or
mechanical ventilation scheme and this air is not heated to
room temperature prior to entering the room, it will appear
as a heating load on the room emitter. It is represented by a
similar equation as for infiltration:
Φv = qv ρ cp (θai – θvs)
(1.11)
where V is the volume of the room (m3). This then gives N
in the units of air changes per hour.
Combining equations 1.10 and 1.11 gives:
Φinf = 0.33 N V (θai – θao)
Design ventilation rates are discussed in chapter 2 of
Guide B.
The steady-state ventilation heat loss can be represented in
terms of a ventilation conductance:
Φv = Cvent* (θai – θvs)
(1.15)
where Cvent* (W/K) is the ventilation conductance.
This can be written in terms of the indoor/outdoor air
temperature difference for compatibility with that for
infiltration:
(1.12)
Φv = Cvent (θai – θao)
(1.16)
Cvent = Cvent* (θai – θvs) / (θai – θao)
(1.17)
where:
This is often written as:
Φinf = Cinf (θai – θao)
(1.14)
where qv is the ventilation rate (m3/s) and θvs is the
temperature at which the air enters the room (°C).
It is common practice to represent air infiltration rates in
terms of room air change rate, N defined by:
N = 3600 qinf / V
Ventilation heat loss
(1.13)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Φf = ∑ (U A)i (θei – θao)
where Cinf (W/K) is referred to as the infiltration
conductance.
Heating load calculations and sizing methodology
1-35
1.5.3.4
Building/room type
Air infiltration allowance air
changes / h
The total design steady-state heat loss rate for a room is
given by the summation of the fabric and infiltration and
any ventilation losses:
Art galleries and museums
1
Assembly and lecture halls
0.5
Banking halls
1 to 1.5
Bars
1
Canteens and dining rooms
1
Churches and chapels
0.5 to 1
Dining and banqueting halls
0.5
Exhibition halls
0.5
Factories:
—
up to 300 m3 volume
—
300 m3 to 3 000 m3
—
3000 m3 to 10 000 m3
—
over 10 000 m3
1.5 to 2.5
0.75 to 1.5
0.5 to 1.0
0.25 to 0.75
Fire stations
0.5 to 1
Gymnasia
0.75
Houses, flats and hostels:
—
living rooms
—
bedrooms
—
bed-sitting rooms
—
bathrooms
—
lavatories, cloakrooms
—
service rooms
—
staircases, corridors
—
entrance halls, foyers
—
public rooms
1
0.5
1
2
1.5
0.5
1.5
1.5
1
Hospitals:
—
corridors
—
offices
—
operating theatres
—
storerooms
—
wards and patient areas
—
waiting rooms
1
1
0.5
0.5
2
1
Hotels:
—
—
—
—
1
1
1.5
1.5
bedrooms
public rooms
corridors
foyers
Laboratories
1
Law courts
1
Libraries:
—
reading rooms
—
stack rooms
—
storerooms
0.5 to 0.7
0.5
0.25
Offices:
—
private
—
general
—
storerooms
1
1
0.5
Police cells
5
Restaurants, cafes
1
Schools, colleges:
—
classrooms
—
lecture rooms
—
studios
2
1
1
Sports pavilion changing rooms
1
Swimming pools:
—
changing rooms
—
pool hall
0.5
0.5
Warehouses:
—
working and packing areas
—
storage areas
0.5
0.2
Room total steady-state heat loss
Φt = ∑ (U A)i (θei – θao) + Cv (θai – θao)
(1.18)
where Cv is the sum of the infiltration conductance (Cinf)
and ventilation conductance (Cvent).
This ignores any heat gains within the room arising from
lighting, equipment or occupants. It is conventional to
ignore such heat gains as room heating is normally switched
on prior to occupancy. However, it may be appropriate to
take such gains into account if the room is continuously
occupied.
Equation 1.18 is based upon the room air and environmental
temperatures. However, the design room temperature is
normally specified in terms of operative temperature θc
with θei and θai being unknown. The equation can be
rewritten by introducing the factors F1cu and F2cu where:
F1cu = (θc – θao) / (θei – θao)
(1.19)
F2cu = (θc – θao) / (θai – θao)
(1.20)
φt = [F1cu ∑ (U A)i + F2cu Cv] (θc – θao)
(1.21)
and:
so that:
F1cu and F2cu are functions of the thermal properties of the
room and, significantly, the radiant fraction of the heat
emitter. They are given by the equations:
F1cu = 3 (Cv + 6 ∑ A) / B
(1.22)
F2cu = [ ∑ (A U) + 18 ∑ A] / B
(1.23)
where:
B = {∑ (A U) + 18 ∑ A +1.5 R [3Cv – ∑ (A U)]}
(1.24)
R is the fraction of heat output for the heat emitter by
radiation. Typical values of R are given in CIBSE Guide A,
chapter 5.
In many cases involving relatively modern well-insulated
buildings and heat emitters with moderate R values, neither
F1cu or F2cu varies greatly from unity and there will be
minimal difference between operative, environmental and
air temperature in the room. Given the uncertainty of the
value of Cv, in such cases it may be acceptable to approximate
equation 1.18 to:
Φt = [ ∑ (U A)i + Cv] (θc – θao )
(1.25)
However, in older, poorly insulated buildings, or where
there are high infiltration rates or predominantly radiant
heat emitters are proposed, F1cu and F2cu can differ
significantly from unity and equation 1.21 should be used.
Equations 1.21 through 1.24 demonstrate that the room
heat loss is influenced by the radiant heat output fraction of
the emitter. For rooms where [3 Cv – ∑ (A U)] is positive,
both F1cu and F2cu will be reduced by an increase in R. That
is, where infiltration heat loss is greater than one third of
the fabric heat loss, increasing the radiant component of
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Table 1.20 Recommended allowances for air infiltration for selected
building types
1-36
Heating
1.5.3.5
Pre-heat time and room emitter duty
The room heat loss rate given by equation 1.21 is applicable
only to idealised, steady-state conditions. In reality, heating
systems are invariably operated intermittently. During the
off period, the room fabric temperature will fall as heat
continues to flow from the external fabric surfaces to
outdoors and simultaneously from all the surfaces within
the room to the room itself where there is still an infiltration
heat loss. On start up, the room heat emitter will then need
to provide heat to the room at a greater rate than the steadystate heat loss in order to raise the temperature of all the
fabric within and surrounding the room. The greater the
heat output of the room emitter relative to the steady-state
room heat loss, the shorter the pre-heat time required to
bring the room back up to design temperature. The heat
output rate of the emitter will be greater at the start of the
pre-heat period, as the environmental temperature of the
room will be lower than at the design condition. However,
this increase is relatively small except for low temperature
heat emitters and is usually ignored.
The thermal response of the room is quantified by the
response factor, fr, given by:
fr = {∑ (A Y) + Cv}/{∑ (A U) + Cv}
(1.26)
1.5.3.6
Intermittent heating and choice of
plant size ratio
Intermittent heating permits a reduction in room
temperature while unoccupied with a consequent reduction
in fuel consumption. Further, the shorter the pre-heat
period, the greater the saving. This implies that the greater
the plant size ratio, the greater the economy in energy
consumption. However there are disadvantages in selecting
a higher plant ratio:
——
increased physical size and capital costs of emitters,
distribution system and heat generators
——
lower seasonal efficiency as the heating system will
run at less than full load, except during pre-heat
——
in modern, well insulated buildings, the value of
Σ (A Y) will tend to be very much greater than the
Σ (A U) value in equation 1.23. As a result, the
response factor fr and plant ratio F3 can be very
large.
There is therefore an optimum plant ratio based on total
life cycle costs. This requires an assessment of annual
energy and capital costs for different plant ratios using a
discounted cash flow method. Alternatively, a life-cycle
carbon emission assessment may be preferred.
However, estimating annual energy consumption is difficult
as the length of the pre-heat period is at best only known
under design conditions.
where Y is thermal admittance.
This is a form of non-steady-state U-value and represents
the ease with which heat can flow into a structure following
a change in surface temperature. Further details along with
values of Y for a range of typical building elements are
available in CIBSE Guide A, chapter 3. The room response
factor is the ratio of non-steady-state to steady-state heat
flow. It is important to note that the term ∑ (A Y) applies to
all surfaces within the room including internal walls etc.
whilst ∑ (AU) applies only to external walls etc. (i.e. through
which heat flows to outdoors or spaces at a temperature
below that of the room).
A more comprehensive approach, including economic
appraisal, is described in a paper (Day et al, 2001) which
proposes a method for calculating the pre-heat time
required, taking account of the plant capacity in relation to
the mean temperature of the whole daily cycle. It goes on to
optimise plant size by finding the minimum life cycle cost,
taking account of both capital and running costs. The paper
also reports conclusions reached from applying the model
to a large gas-fired system (750 kW), as follows:
——
The greater the thermal capacity of the building,
the smaller the optimal plant size ratio. In
determining the effective thermal capacity of the
building, as a general guide, the first 100 mm of the
inner fabric skin should be taken into account.
——
For the particular case studied, the optimum plant
size ratio was found to be 1.63 but the economic
savings which result from this choice do not vary
significantly for plant size ratios of ±10% of the
optimum.
——
Plant size ratios >2.0 are not justified for most
typical buildings.
——
Smaller plants have higher values of marginal
installed cost (£ per extra kW), so the optimum
plant size ratio will be lower.
The ratio of the duty of room heat emitter to room steadystate room heat loss, F3, can then be calculated by:
24 fr
F3 = ———————
H fr + (24 – H)
(1.27)
where H is the hours of plant operation (including preheat
time). F3 is referred to as the plant ratio.
The room heat emitter duty is then given by:
Φp = F3 Φt
(1.28)
It is important to note that while the value of H in equation
1.27 includes the pre-heat time, this equation cannot be
used to calculate the pre-heat time. To determine pre-heat
time requires a fully dynamic thermal model based on
discrete time steps such as finite difference. In using such a
model a time step must be used which is small compared to
the pre-heat time (typically in the order of about 5 minutes).
In general, it may be observed that, unless rapid warm-up is
essential, plant size ratio should be in the range 1.2 to 2.0
and the heating period extended during cold spells.
Optimum start control can ensure adequate pre-heat time
in cold weather.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
the heat emitter will reduce room heat loss and so reduce
heating energy consumption. Conversely, when the
infiltration heat loss is less than one third of that of the
fabric, convective heat emitters will result in lower heat
loss and energy consumption.
Heating load calculations and sizing methodology
Mechanical ventilation heat
loss
Heat demand due to ventilation that appears at the
ventilation heating coil, Φv (W), is calculated using the
same energy balances as for infiltration (equation 1.9):
Φv = 1200 qv (θa2 – θa1)
(1.29)
where θa2 is the air temperature required after the heating
coil (°C) and θa1 is the air temperature on to the heating coil
(°C) and qv is the mechanical ventilation rate (m3/s).
Where heat recovery is included, equation 1.29 can be
written in terms of the outdoor air temperature, θao (°C), the
temperature of exhaust air on to the heat recovery device,
θar (°C) and the effectiveness, E, of the heat recovery device:
Φv = 1200 qv {(θa2 – θao) – E (θar – θao)}
(1.30)
Manufacturer’s data for effectiveness should be used as
these can vary significantly. Guidance on calculating and
applying heat exchanger effectiveness is given in CIBSE
Guide C, chapter 3. Table 1.21 gives typical ranges.
The second tier document supporting Part L2 of The
Building Regulations 2010 England and Wales, the Nondomestic Building Services Compliance Guide (NDBSCG)
(DCLG, 2013e) specifies, for some types of heat recovery
devices, minimum values of dry heat recovery efficiency. It
is presumed that this is the same as effectiveness for sensible
only heat transfer.
1.5.5
Domestic hot water
The method of determining the heating demand of
domestic hot water systems is dependent upon whether the
system is instantaneous or incorporates storage. The
principle is set out below. Further details of dhw sizing are
given in section 1.11, in CIBSE Guide G and in BS 8558
(2011b).
Table 1.21 Typical effectiveness ranges for heat recovery devices
Device
Effectiveness
Air to air flat plate recuperator
0.4–0.6
Run-around coils
0.4–0.6
Thermal wheel (non-hygroscopic)
0.6–0.7
40
35
30
25
20
HTHW (125°C)
15
MTHW (100°C)
10
LTHW (75°C)
5
DHW (60°C)
0
0
50
100
150
200
250
External pipe diameter (uninsulated) / mm
300
Figure 1.6 Maximum permitted heat loss from dhw and heating pipes
for compliance with Part L 2013
1.5.5.1
Instantaneous systems
This includes systems than contain a small amount of
stored hot water, enough to meet demand for only a very
short time after which they function as instantaneous water
heaters.
The heating load, Φdhw (W) imposed on the heat source is
given by:
Φdhw = ṁ Cpw (θw2 – θw1)
(1.31)
where ṁ is the peak design mass flow rate of the dhw (kg/s),
Cpw is the specific heat of water (J/kg·K), θw2 is the required
flow temperature of the dhw and θw1 is the temperature of
the cold water entering the dhw heater (°C).
1.5.5.2
Storage systems
The general principle is to determine the heat input rate
required to heat up the storage volume within a specified
time period (the recovery time). The heating load, Φdhw (W)
imposed on the heat source is given by:
Φdhw = M Cpw (θws – θw1) / Trec
(1.32)
where M is the mass of the stored water (kg), θws is the
design storage temperature (°C) and Trec is the recovery
time (s).
Typical recovery time is 1 to 2 hours.
1.5.6
Distribution losses
Pipework and ductwork conveying hot water and warm air
will lose some heat between heat generator and point of use.
An allowance should therefore be made both in determining
the heat load imposed on the heat generator and in
determining fluid flow rates in the distribution system.
Building energy codes usually specify requirements for
insulation. As an example, Part L of the Building
Regulations for England 2013 sets minimum standards for
the insulation of such pipes and ducts in terms of maximum
permitted heat loss per unit length of pipe (W/m) as detailed
in the current Non-Domestic Building Services Guide
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The amount of mechanical ventilation provided is
determined according to how the building is to be used and
occupied. In most buildings, ventilation is provided at a
rate aimed at ensuring adequate air quality for building
occupants but in some industrial buildings it must be based
on matching process extract requirements. Mechanical
ventilation is controlled, the design amount known, and
the heat load readily calculated. Ventilation requirements
may be specified either in volume supply (litre·s–1) or in air
changes per hour (h–1). Recommended air supply rates for a
range of buildings and building uses are given in CIBSE
Guide A. More detailed guidance on ventilation is given in
chapter 2 of CIBSE Guide B.
45
Maximum permitted heat loss / W/m
1.5.4
1-37
1-38
Heating
(DCLG, 2013d). The maximum permitted heat loss
increases with pipe size and mean temperature of the hot
water. Figure 1.6 is a graphical representation of these
values.
For pipework, the total heat loss from both flow and return
sections should be included in determining distribution
losses unless it is expected that this could partly contribute
to the heating of the building.
At early design stage, it is usual to make some allowance for
heat losses from pipework as a percentage of the heating
capacity delivered by the pipework. Figure 1.7 shows the
heat loss for lthw pipework, insulated so as to just comply
with the NDBSCG, as a percentage of the heat carrying
capacity of the water. This uses the same standard conditions
as Figure 1.6 (75 °C mean water temperature for lthw with
the pipe suspended horizontally in still air at 15 °C), as this
is representative of typical installations within buildings. A
total (flow and return) pipe length of 100 m has been used
together with a flow/return water temperature difference of
20 °C This shows that for pipework smaller than about
50 mm external diameter, caution needs to be applied in
making percentage allowances for heat loss based on the
heat carrying capacity of the pipe.
In contrast to pipework, the NDBSCG gives a single value
of the maximum permissible heat loss from ductwork
regardless of the duct dimensions. This is 16.34 W/m2 based
on the surface area of the duct. Where the duct transports
chilled air during the cooling season, more onerous
standards apply. Minimum insulation thickness should be
determined using BS EN ISO 12241 (BSI, 2008) based on
an in-duct air temperature of 35 °C and the duct being
horizontal in still air at 15 °C. CIBSE Guide C, chapter 3,
also contains information on heat loss from ductwork.
Percentage heat losses in air distribution ductwork can be
considerably higher than with pipework systems due to the
larger surface areas of the ducts and the smaller heat
carrying capacity of air. Heat loss calculations should be
carried out.
Heat loss/heating capacity / %
25
20
100 m total (forward and return)
pipe length insulated to
NDBSCG 2013 LTHW at mean 75°C
20°C flow/return ∆T
horizontal pipe in still
air at 15°C
15
10
5
0
0
200
50
100
150
External pipe diameter (uninsulated) / mm
Figure 1.7 Ratio of heat loss to heat carrying capacity for insulated
lthw pipework
250
Heat generator peak heating
load
The peak simultaneous load imposed on the heat generators
will in general not be the sum of the individual heat emitter
duties, ventilation heating load, dhw heating load and
distribution losses. There will invariably be some diversity.
In addition, there may be a requirement for a design margin.
1.5.7.1
Load diversity
Whilst heat emitters must all be sized to include the local
infiltration heat loss, the total simultaneous infiltration
heating load that the central heat generator(s) has to meet
will be somewhat less as some of the perimeter rooms will
be experiencing exfiltration. At all times, the total building
infiltration rate must exactly balance the exfiltration rate.
Some exfiltration will occur through unheated areas such as
openings at the top of lift shafts or atria.
Prior to occupancy, mechanical ventilation systems may
not be switched on, operating at reduced volume flow rate
or on full recirculation. As a result, at times of design room
heat loss, the mechanical ventilation heat load may be
significantly reduced or not operating (during the pre-heat
period). Likewise, domestic hot water systems utilising
storage may be designed to be fully charged prior to room
heating being switched or not operated until after the preheat period (i.e. for a kitchen). The designer should
therefore make an assessment of what the peak simultaneous
heating load on the central heat generator(s) (and
distribution system) is likely to be. It should be borne in
mind however that at start up of the heating system, local
space heating controls in spaces that have cooled to below
their set point temperatures will call for maximum heating.
1.5.8
Design margins
Design margins are intended to make provision for
uncertainties inherent in determining peak heating
demand. CIBSE Research Paper 4: Engineering design
calculations and the use of design margins identified and
classified the source of such uncertainties. These are listed
in Table 1.22 below with common examples.
Where design margins are included to allow for such
uncertainties, these are often cumulative. In addition, plant
and equipment will inevitably be selected as ‘the next size
up’. This can result in unnecessary oversizing and as a
consequence:
——
increased capital costs
——
increased plant space requirements
——
reduced efficiency and increased running costs.
In their investigation of oversizing of hvac systems in
existing buildings in the UK, BSRIA (Crozier, 2000) found
that 80% of heating systems were oversized, some by as
much as 400% with oversizing by 50% to 100% common.
Design margins must therefore be used with caution.
Nevertheless, the designer must be able to deal appropriately
with the risk of heating systems not performing as required.
A key uncertainty in the determination of peak heating
demand is building performance, particularly for
refurbishment projects. This applies to both U-values and
infiltration rates.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The minimum thickness of insulation required to comply
should be determined using BS EN ISO 12241 (BSI, 2008)
assuming horizontal pipes in still air at 15 °C. Alternatively,
CIBSE Guide C, chapter 3, gives information on how to
determine heat loss rates from insulated pipes.
1.5.7
Energy sources
1-39
where design margins are added, they should be
clearly identified within the calculations
——
where appropriate, design margins should be
approved by the client
——
avoid cumulative design margins
U-values
——
specify assumptions made as part of the design
Infiltration rates
——
specify the operating design limits of the design.
Source of uncertainty
Examples
Design criteria
Required indoor temperature
Design outdoor temperature
dhw demand
Building performance
Calculation methods
Calculation of U-values
Pre-heat margin
1.5.9
Pipework resistance
Equipment performance
Heat output rates of boilers and emitters
Pipe and ductwork insulation standard
achieved
Commissioning tolerances
Accuracy of measurement
Commissioning to –0%, +10% of design
Provision for duct leakage
Future requirements
Changes in use of building
Extensions of building
Deterioration over time
Reduction in boiler heat output rate
Increased pipework resistance due to
scaling
Increase in infiltration rate of building
Custom
Policy or tradition of adding safety
margin
Choice of number and duties of
heat generators
In non-domestic buildings, it is common practice to install
multiple heat generators. It is also common practice to
provide additional capacity to allow for breakdown or
maintenance. For example, a 600 kW heating load could be
provided by 4 identical boilers of 200 kW (33%) each. In the
event of one boiler being out of service, full design heating
load would still be available. (The distribution system need
only be sized for 600 kW however.) This is often referred to
as ‘N+1’.
The choice of how much, if any, additional capacity to
install depends upon the estimated risk of sufficient heat
not being available and the seriousness of the consequences.
In reality, the calculated peak heating demand rarely occurs
as:
——
for most of the time, actual outdoors temperatures
are greater than the design value
——
there are likely to be some internal heat gains
In many cases when dealing with existing buildings, there
will be little or no documentary evidence of U-values. The
actual U-values will depend upon the standard of
construction and this can vary throughout the building. In
particular, insulation may be partially missing or damaged.
——
design margins may have been incorporated.
Infiltration is highly dependent upon build quality.
Uncertainties in determining infiltration rates are discussed
in section 1.5.3.2.
The consequences of failing to achieve indoor design
temperature will depend upon the building use and the
degree of underheating. Where the heating system is
designed predominantly to provide thermal comfort, a
short term fall of 1 or 2 K in the room temperature is
probably acceptable. This may not be the case where
industrial or other processes are involved.
Ideally, physical testing should be carried out to determine
U-values with thermal imaging used to identify problem
areas. Air infiltration rates can be estimated from pressure
testing, though this will yield only an average for the whole
building and will not identify areas where high infiltration
occurs. The designer should make clearly identified
provisions for higher U-values and infiltration within the
heat loss calculations where these are suspected rather than
as a design margin tagged on at the end.
As a result, for most of the heating season full heating
capacity will not be required to achieve indoor design
temperature.
Where there are uncertainties regarding design criteria and
future requirements, these should be agreed with the client
and the implications made clear in terms of risk and
increased costs. The agreed design criteria should then be
used to determine heating demand.
Multiple heat generators, properly controlled, can lead to
improvements in overall seasonal efficiency. In some cases,
it may be advantageous to install different sizes of heat
generator, particularly if the heating load during summer
or weekends perhaps is very small so that the smallest of the
heat generators can meet demand at such times. The
installation and control of heat generators in hydronic
heating systems is discussed in section 1.8. The choice of
how many heat generators and of what output ideally
requires a careful assessment by the designer of risk, capital
costs and energy savings.
The designer should clearly state the need for adequate
maintenance and water treatment to prevent significant
deterioration in heating system performance.
1.6
Energy sources
CIBSE made the following recommendations in their
research paper:
1.6.1
General
——
Energy sources for heating can be broadly categorised as
follows:
design margins should not be added unless there
are valid design reasons
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
Table 1.22 Classification and examples of uncertainties in design
1-40
Heating
gaseous, liquid and solid fuels
——
electricity
——
solar
——
ambient thermal energy.
These categories can be further broken down into the
primary resource, i.e. whether from fossil or renewable
sources. The inclusion of ambient thermal energy above is
for completeness and includes both air and ground sources
which, in general, require the temperature to be raised
through the use of heat pumps for practical use in buildings.
Higher temperature geothermal heat sources exist but these
are used primarily to generate electricity (particularly in
North America, New Zealand, Italy, Iceland, Mexico, the
Philippines, Indonesia and Japan) rather than heating and
will not be discussed here.
1.6.2
Factors affecting choice of
energy source
The choice of energy source for heating will depend upon a
number of factors:
——
availability
——
cost
——
environmental impacts
——
site conditions and constraints.
1.6.2.1
Availability
Ability to access a particular energy source is clearly the
primary consideration. For example, while much of the UK
is served by a national gas network, there are some areas
that are too remote for economic connection or where
capacity to deliver is low. However, where connection to the
grid is possible, natural gas is normally the fuel of choice
for heating on grounds of both cost and environmental
impact either as the principal energy source or as a
supplement to chp or low-carbon/renewable energy.
Where liquid or solid fuel is being considered it is important
to identify local supply chains. This is particularly
important where local markets are not mature, e.g. with
some biofuels (solid or liquid). Some consideration also
needs to be given to the long term security of supply. For
example, the UK now imports around 50% of its gas
consumption.
1.6.2.2
Energy prices
The price of fuel remains a very important factor affecting
fuel choice and a strong determinant of life-cycle cost.
Current and historical prices throughout Europe are
available from EuroStat (EC, 2014) For the UK, current
energy prices and recent price trends are published
quarterly online by the Department of Energy and Climate
Change. Separate tables are given for domestic and
industrial prices. Electricity prices are typically around
three times that of natural gas. The lower conversion
efficiency for gas into useful heat compared to electricity
reduces this gap a little, but the relative costs show why gas
has been strongly favoured over electricity. Electric heat
pumps can in some cases overturn the price advantage of
gas provided they return a relatively high seasonal cop.
Energy prices can be volatile. For example, between the
first quarter of 2013 and the last quarter of 2015, the average
price paid for electricity increased from 8.9 to 10.1 p/kWh
while for gas the price fell from 3.0 to 2.5 p/kWh (for an
average size consumer) (DECC 2016). Attempting to guess
forward prices is clearly difficult. This can be particularly
important when considering combined heat and power, as
the relative difference between gas and electricity prices —
often referred to as the spark gap or spark spread — have
profound effects on the economics of these schemes.
1.6.2.3
Environmental impact
The use of energy affects the environment through the
upstream activities associated with production, conversion
and delivery of the energy source to site and at the point of
use. There is also the embodied energy of the heating
system to consider along with the extraction, processing
and depletion of natural resources used in its construction.
See section 1.3 and CIBSE Guide L: Sustainability. Fuel
combustion will have detrimental impacts on local air
quality (NOx, SOx and particulates) and globally on
greenhouse gas concentrations in the atmosphere, which is
widely recognised as a likely cause of climate change. As
heating accounts for around three-quarters of all energy
used in buildings and more than a third of all final energy
use in the UK, it is a very significant contributor to the
total environmental impact from energy use. Some fuels are
themselves greenhouse gases and will contribute to climate
change through leakage to the atmosphere. In recognition
of this, it is usual to specify for such fuels both the CO2
(equivalent) emissions factor for a fuel when combusted
and also its global warming potential (gwp) of the fuel
when released into the atmosphere. The gwp is calculated
relative to that of CO2.
The Digest of UK Energy Statistics (DUKES) (DECC,
2014a) identifies the main sources of CO2 emissions arising
from fuel combustion as:
——
32% from power stations
——
17% from industry
——
25.5% from transport
——
16% from the domestic sector.
This reveals the high CO2 emissions associated with
electricity, which should be taken into account when
considering its final use. Section 1.4.5.5 gives the CO2
emission factors for a number of fuels in terms of kW·h of
useful energy produced as used in Part L 2013 Building
Regulations (England). Electricity obtained from the
public supply has an emission factor of about two and a half
times that of natural gas.
Petroleum oils can create severe pollution of ground and
surface water that is difficult to remove. As a consequence
of this, there are strict regulations in many countries
regarding the storage of petroleum oils and minimisation
of water pollution risk.
1.6.2.4
Site conditions and constraints
In remote areas, the absence of mains gas normally leaves a
choice between oil, liquified petroleum gas (lpg), and solid
fuel (usually biomass in the form of logs, wood chip or
pellets), all of which require significant space for storage
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
Energy sources
1-41
Gaseous fuels
1.6.3.1
Petroleum Gases
The main gaseous fuels used to heat buildings in most
European countries are natural gas and liquefied petroleum
gases (lpg). Natural gas consists predominantly of methane
(CH4). lpg is the generic term for propane (C3H8), butane
(C4H10) or mixtures of the two. Propane and butane are
produced during the distillation of crude oil or in some
cases recovered from natural gas wells.
Natural gas is supplied to users by extensive pipework
distribution systems in many developed countries including
much of Europe, North America and Australia. The UK is
one of the largest users of natural gas in Europe with some
76% of the energy used for space heating and dhw in 2010
being produced by natural gas (DECC, 2013). The calorific
value (cv) of natural gas varies slightly depending upon its
source. A typical value of cv and other key properties are
given in Table 1.23. Methane has a boiling point of –161 °C
at atmospheric pressure and cannot be liquefied without
cooling. Liquefied natural gas (lng) is transported by ship
at about –162 °C and 1 bar pressure to terminals where it
can be stored and injected into the natural gas grid in the
gaseous state as required. The density in the liquefied state
is about 600 times greater than the gas at room temperature
and pressure. Alternatively, for transport by road or rail,
natural gas can be compressed (CNG) at ambient temperature
to about 250 bar when its density increases by a factor of
about 100. It is not economically feasible to store lng or
cng on site for heating purposes.
Propane and butane are gases at room temperature and
pressure but, in contrast to methane, can be liquefied by
pressurisation alone at room temperature. The ratio of the
volume of the gas to liquid phase is around 600:1. This
makes transportation and storage very convenient as both
1600
Saturation vapour pressure / kPa
1.6.3
1800
Propane
1400
Butane
1200
1000
800
600
400
200
0
-60
-40
-20
0
20
Temperature / °C
40
60
Figure 1.8 Vapour pressure of propane and butane
fuels can be stored in relatively small pressurised vessels at
ambient temperature. The vessels will contain both liquid
and vapour phases at saturated temperature and pressure.
That is, the pressure will be determined solely by the
temperature. Figure 1.8 shows the saturation vapour
pressure of butane and propane as a function of temperature
(NIST, 2014). At 20 °C, propane has a vapour pressure of
about 850 kPa (7.5 barG) compared to about 200 kPa
(1.0 barG) for butane. Storage vessels for propane therefore
need to withstand significantly higher pressures than those
for butane.
The pressure in the storage vessel is used to deliver the fuel
to the heat generating plant. To do this requires that the
pressure is greater than atmospheric by a sufficient margin
to overcome the pressure losses through the supply pipe
and valves and still maintain an adequate operating pressure
at the burner. Butane’s saturated vapour pressure falls to
atmospheric pressure at about 0 °C so that it can be used
only when the storage vessel and pipework are maintained
above this temperature. In comparison, propane will
maintain a pressure above atmospheric down to about
–40 °C. Due to its high flammability and explosion risk,
storage vessels are located outdoors. In climates where
winter outdoor temperatures sometimes fall below 0 °C,
butane is not suitable as a heating fuel. In the UK and other
Table 1.23 Key physical properties (typical) of petroleum gases
Property
Natural gas (methane)
Commercial propane
Commercial butane
Chemical formula (of main constituent)
CH4
C3H8
C4H10
Density relative to air (15 °C, 1 atm.)
0.60
1.5 (vapour)
2.0 (vapour)
Density relative to water (15 °C)
—
0.512 (liquid)
0.575 (liquid)
Boiling point (at 1 bar) (°C)
–161
–42
–2
Gross calorific value (MJ/kg)
55.5
50.4
49.5
3
Gross calorific value (MJ/m ) (15 °C, 1 atm.)
38.7
95
122
Latent heat of vaporisation (MJ/kg) (at normal boiling
point)
—
0.43
0.39
Stoichiometric ratio of air to gas by volume
9.7
23
30
Carbon emissions factor (kgCO2eq/kW·h)
0.206
0.229
0.249
Greenhouse warming potential
56
≈3
≈3
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
and access for delivery. Solid fuels have greater maintenance
requirements than oil or lpg. lpg may be cleanest and
most convenient but is generally significantly more
expensive than heating oil. Although expensive, electricity
may be the best choice where heating requirements are very
small, especially if used to power a heat pump.
1-42
Heating
The key properties of propane and butane are shown in
Table 1.23. Butane and Propane are supplied in the UK to
BS 4250 (BSI, 2014).
The quantity of air required for combustion of lpg is up to
about three times greater than that for natural gas. This has
implications for the sizing of combustion ventilation.
Further, as lpg in the vapour phase is heavier than air, no
part of an lpg installation should be located below ground
level within buildings.
Natural gas and lpg are odourless: a stenching agent is
added for safety reasons.
1.6.3.2
Landfill gas and bio-methane
Landfill gas is collected from wells inserted in land-fill
sites, often complementing measures to prevent hazards
arising from the escape of gas. It typically consists of
between 40 and 60% methane by volume with the remainder
mostly carbon dioxide and traces of many other gases. The
calorific value of landfill gas is in the range 15 to 25 MJ·m–3,
depending on its methane content.
Landfill gas is mostly used without processing, other than
the removal of moisture and dust. Because of its low
calorific value it is relatively expensive to transport and is
most suitable for heat generation when it can be produced
close to a heat load, which favoured its early exploitation
for brick kilns adjacent to clay pits used for land fill. In
other cases, it is used to generate electricity from gas
turbines or reciprocating engines. The life expectancy of
gas production from landfill sites is typically 15 to 30 years.
Bio-methane is produced from the anaerobic digestion of
organic wastes including food waste, agricultural residue
and sewage sludge. Some of the gas produced is used to
maintain optimum temperature for the digestion process.
Table 1.24 Heating oils
Class
Common UK
name
Applications
C2
Kerosene
Domestic heating
D
Gas oil
Larger domestic and commercial heating
E
Light fuel oil
Larger commercial and industrial heating
F
Medium fuel oil
Large commercial and industrial heating
G
Heavy fuel oil
Very large industrial heating
It is economic in many cases to use the raw gas in combined
heat and power generation.
Bio-methane gas can be cleaned and injected into the
natural gas grid. In some instances, a small amount of
propane may be added to achieve the required calorific
value. Injection of bio-methane into the gas grid is
supported under the Renewable Heat Incentive scheme
(see section 1.2).
1.6.3.3
Hydrogen
Hydrogen is not freely occurring on the Earth, and generally
has to be obtained either by chemical extraction from
complex hydrocarbons (from petroleum products), or by
splitting water into its component parts – typically by using
electrolysis. Both are energy intensive processes, and
hydrogen is more usually considered to be an energy vector
that can be used for energy storage and transmission, for
use in specialised end-use applications. These can be in
adapted combustion processes (burners or internal
combustion engines), or to produce electricity and heat
from a fuel cell. Hydrogen has a gross calorific value of
11.85 MJ·m–3 (at 15 °C) and flame temperature (in air) of
around 2000 °C.
The advantages of hydrogen are that upon combustion, or
combination with oxygen in a fuel cell, the main discharge
component is water vapour. High temperature combustion
can also give rise to NOx as with other fuels, and if the
source of the hydrogen is from hydrocarbons there will be
associated carbon emissions from its production. It is
possible to produce hydrogen locally by the reforming of
methane (natural gas) or methanol. However, there will be
local associated CO2 emissions.
There are opportunities to use hydrogen as a storage
medium for intermittent renewable electricity generation
to improve supply and demand matching. However, this is
expensive as it requires electrolysers, fuel cells and complex
control strategies.
The disadvantages of hydrogen are that it is highly reactive,
and that it has associated difficulties with storage and
transportation, as discussed in section 1.6.8.3. It is perceived
as a dangerous fuel, although this can be unjustified as leaks
are quickly dispersed.
1.6.4
Liquid fuels
1.6.4.1
Petroleum fuel oils
BS 2869 (BSI, 2010) contains specifications for various
classes of liquid fuels designated by the letters A2, C1, C2,
D, E, F and G. The fuels commonly used for heating are
Table 1.25 Key physical properties of petroleum heating oils
Property
Class C2
Class D
Class E
Class F
Class G
Density at 15 °C (kg·m–3) (BS 2869)
803
850
940
970
980
Kinematic viscosity (mm2·s–1) at 40 °C (BS 2869)
1.0–2.0
1.5–5.0
—
—
—
Kinematic viscosity (mm2·s–1) at 100 °C (BS 2869)
—
—
<8.2
<20.0
<40.0
Maximum sulphur content by mass(%) (BS 2869)
0.1
0.1
1.0
1.0
1.0
Pour point (°C)
—
—
–6
24
30
46.4
45.5
42.5
41.8
42.7
Gross calorific value (MJ·L )
–1
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
countries with such climates, lpg supplied as a heating fuel
comprises virtually 100% propane. In countries with
warmer winters, lpg for heating may include a significant
fraction of butane. This mix may vary with the season of
the year reflecting outdoor temperatures.
Energy sources
1-43
important disadvantage with such fuels that the designer
should be aware of.
The key properties of these fuels are shown in Table 1.25.
Further information can be found in CIBSE Guide C and
BS 2869 (BSI, 2010). BS 5410 Parts 1 and 2 (BSI, 2014 and
2013) give advice on choice of grade of petroleum oil for
space heating and hot water production.
1.6.5
Solid fuels
1.6.5.1
Coal
The viscosity of the oils increase from C2 to G, as do the
pour points (the temperature below which the oil will flow
adequately and the temperature at which they need to be
maintained). The more viscous oils require more
sophisticated plant and maintenance. At the same time, the
cost of fuels decrease from C2 to G. Hence, the heavier oils
are used in larger installations.
The calorific values are not specified in BS 2869. The
figures in Table 1.25 are typical only and will vary with
supplier. For more accurate values, the supplier should be
contacted. Suppliers often quote net rather than gross
calorific values.
1.6.4.2
Liquid biofuels
The European Renewable Energy Directive 2009 (EU,
2009b) sets a targets for 20% of energy to be produced from
renewables by 2020, sets out guidance for the support by
member states of biofuels and requires each member state
to produce an action plan. The UK has produced an action
plan that encourages the use of biofuels in heating, subject
to sustainability criteria being met. Gas oil may now
contain up to 7% by volume of biofuel (fatty acid methyl
esters, FAME) derived from used cooking oil, animal fats or
plants and seeds grown specifically for making fuel.
The use of fame in liquid fuels creates overall carbon
reductions in life-cycle analysis of emissions when
compared to the use of traditional mineral (fossil) fuels
such as kerosene. This is as a result of the carbon absorbed
by the plant or animal during its life cycle, offsetting carbon
emissions produced during the combustion process. Bioliquids produced from used cooking oil methyl esters
(ucome) offer greater life-cycle green house gas savings, as
ucome is considered to be a waste at the point that it is
converted to a fuel.
fame for heating purposes should meet the requirements of
BS EN 14214 (BSI, 2012). Fuel must also be produced
under strict quality assurance systems to achieve consistent
quality and properties of the fuel. fame to BS EN 14214 can
either be used as part of a blend with mineral fuels such as
kerosene or gas oil or used in its pure form as a 100% bioliquid fuel. Bio-liquid blends should conform to OFTEC
(Oil Firing Technical Association) industry blend standard
PrOPS 24 (OFTEC, 2010a, b, c).
Where bio-liquids are to be used, advice should be taken
from equipment manufacturers to ensure material
compatibility as the properties of the fuel can differ to that
of mineral fuels. Where blends contain a high proportion of
bio-liquid, heated tanks and trace heating on the oil supply
line may be required.
Most liquid biofuels have a limited storage times before the
oil degrades and/or forms a sediment. Storage life can be
increased with the use of additives. Nevertheless, this is an
Coal is classified according to its chemical composition and
graded according to size. CIBSE Guide C gives the
properties of numerous varieties of coal, including moisture,
ash and sulphur content. Gross calorific value ranges from
24 to 34 MJ·kg–1.
1.6.5.2
Waste
Municipal waste may be burnt unprocessed, with heat
extracted or electricity generated as part of the incineration
process. Alternatively it may be used to produce refusederived fuel pellets, which may be used to fire some types of
boiler plant. It has a calorific value about two thirds of that
of coal and produces around 50% more ash.
1.6.5.3
Wood
Wood fuels are of interest because their use can result in a
net decrease in greenhouse gas emissions. Forestry waste
results from the normal processes of forestry management,
which has the principal objective of maximising the value
of the timber crop. Thinning and harvesting leave residues,
consisting of branches and tree tops which have no value as
timber and, if not used for fuel, would be discarded. Waste
wood is also available from industrial sources, particularly
from saw-milling and furniture making. Its use as a fuel has
a net benefit in greenhouse gas emissions, both by avoiding
the need to burn a fossil fuel and by avoiding the production
of methane that would result from decomposition on the
forest floor or in landfill.
Wood fuel may be produced by growing arable coppice
specifically for fuel production. The carbon dioxide released
on combustion will have been sequestered during growth
and there is no net contribution to CO2 emissions.
Notwithstanding its environmental advantages, wood is a
low quality fuel, with a calorific value of around 19 MJ·kg–1
(5.2 kW·h·kg–1) when dry and only around 10 MJ·kg–1
(2.8 kW·h·kg–1) at the typical moisture content (55%) when
harvested.
Straw is also used as a fuel, particularly since the phasing
out of straw-burning on fields in the early 1990s. It is burnt
in high temperature boilers and used to supply heat and hot
water, usually on a fairly small scale.
CIBSE AM15: Biomass heating gives details of all aspects of
biomass heating.
1.6.6
Electricity
Electricity is the most versatile form in which energy is
delivered and may serve almost any end-use of energy,
including those for which fuels are consumed directly.
However, the high quality and versatility of electricity must
be seen in the context of its high cost, which reflects the
high primary energy input to electricity generation. It can
typically be used in point of use resistance heaters, to heat
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
listed in Table 1.24 along with typical applications. It
should be noted that different classification letters and
common names are used outside the UK.
1-44
Heating
emissions from all sources (Digest of Environmental
Statistics) (DEFRA, 2014).
The Digest of UK Energy Statistics (DUKES) (DECC,
2016) shows that the generation mix for electricity in the
UK has changed radically since 1997 though during this
time the total amount of electrical energy supplied has
changed little. In 1997 45% of electricity production was by
oil and coal, 27% by gas, 27% by nuclear and only 1% by
renewables. The “dash for gas” saw the fraction of electricity
produced by gas increase to a peak in 2010 of 47%. In 2014,
gas has fallen back to 30%, nuclear declined to 18% and
renewables climbed to 13% with coal still providing nearly
39%.
Power stations other than nuclear produce oxides of
nitrogen (NOx). Currently, the average NOx emissions for
UK grid electricity is estimated to be about 750 mg/kW·h
(BRE, 2011).
The change in mix of fuels used for electrical generation in
the UK has resulted in changes in the average carbon
emissions factor for grid electricity. Of particular interest is
how they are likely to change in the future. The Building
Research Establishment has advised the UK Government
that by 2025, the carbon emissions factor for grid electricity
(to be used in Part L calculations) is likely to be around
one-half of its present (2013) value of 0.52 kg/CO2(eq)/kWh
(BRE,2011). This is predominantly due to the expected
increase in renewable energy and reduction in coal-fired
power generation (BRE, 2012).
Gas produces negligible emissions of sulphur dioxide to the
atmosphere, and reduced concentrations of other
atmospheric pollutants. As a result, UK sulphur dioxide
emissions from power stations have declined by 88% since
1990, contributing to a greater than 84% reduction in UK
1.6.6.1
Low carbon electricity generation
The mix of very low carbon sources (nuclear and renewables)
on the grid and from decentralised sources are set to grow
in Europe. Under the European Union Renewable Energy
Directive (EU, 2009a) all member states were required to
submit national reports demonstrating their plans for
achieving their renewable energy targets for 2020. The UK
target is for a 30% share of renewable electricity by 2020, as
part of the need to meet 15% of all energy use by that year
(DECC, 2011). This is clearly a very demanding target, but
a range of financial support measures have been put in place
to encourage a larger market share from renewables.
Since 2008 the UK government has been keen to encourage
new nuclear power stations to be built. While this may be
longer term, new nuclear is seen as an important part of the
strategy to reduce UK CO2 emissions, while reducing
dependency on imports of gas.
Together new renewables and new nuclear could make a
strong contribution to a decarbonised electricity supply
network, which may encourage future uptake of electricity
as an energy source for heating.
Figure 1.9 Distribution of solar irradiation across Europe (reproduced courtesy of GHI Solar map © 2014 GeoModel Solar)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
water in central plant (resistance or electrode boilers), and
increasingly (and more efficiently) with heat pumps.
Energy sources
1.6.7
Solar source
The radiation energy arriving from the sun to the top of the
Earth’s atmosphere is approximately 1367 W/m2 (± 3.5%).
However, the amount that reaches the surface at any point
depends on the solar-to-surface geometry (orientation,
surface tilt etc.), the location, time of day, and atmospheric
conditions. The latter in particular makes assessment of
total solar irradiation (the energy received over a specified
time period) difficult to predict. CIBSE Guide A sets out
the calculation methodology for determining direct (beam)
and diffuse irradiance (in W/m2) for differing geometries
and times of the day and year for any latitude. Direct and
diffuse radiation added together give the global radiation,
all of which can be absorbed by a solar collector.
What is more useful for the solar system designer is a
knowledge of typical solar irradiation (i.e. energy per unit
area) over stated time periods, as this can be used, together
with collector system efficiencies, to make assessments of
how much solar energy can be made use of. CIBSE Guide J
provides a table of mean monthly beam and diffuse solar
irradiation (in W·h/m2) for different orientations and
surface tilt angles, for a limited number of UK locations.
Data is also given for design beam and diffuse daily and
hourly irradiance (in W/m2), which are useful when sizing
solar storage systems.
1.6.8
Handling and storage of fuels
and regulations
1.6.8.1
Natural gas
Europe contains an extensive natural gas network. The
network in the UK comprises distribution pipework
operating at a cascade of pressures, defined as follows:
——
high pressure (above 7 bar).
——
intermediate pressure (above 2 bar and up to 7 bar)
——
medium pressure (above 75 mbar up to and
including 2 bar)
——
low pressure (not exceeding 75 mbar).
The incoming supply to a building or development is likely
to be either medium pressure or low pressure depending on
the proximity of the development to suitable distribution
mains and building gas demand. The pressure is then
metered and reduced to the required supply pressure in the
building (usually between 21–75 mbar).
It is important to have early negotiations with the gas
provider because the normal ‘metering pressure’ provided
to a development is usually 21 mbar (2100 Pa) irrespective
of the incoming pressure. This may be acceptable, but there
are instances where a higher pressure is required at boilers
or where a high pressure drop in the distribution system
cannot be avoided. If the supply to the building is medium
pressure the gas provider will normally be able to provide
low pressure at up to 75 mbar after the meter, which is
acceptable provided the gas distribution system in the
building is strength and tightness tested accordingly.
In the UK it should be noted that the declared metering
pressure is not guaranteed after the meter. For example, for
a 21 mbar metering pressure the ‘peak level operating
pressure’ (plop) is normally 25 mbar and the ‘lowest
operating pressure’ (lop) is normally 18 mbar. Therefore
the designer should assume that the gas pressure after the
meter may fall to 18 mbar and ensure that boilers or other
appliance can still operate should the gas pressure drop to
17 mbar at the inlet to the gas control valve. (This assumes
a design pressure drop of 1 mbar between the meter and
appliance to comply with the relevant codes of practice
described in section 1.6.8.4).
Where possible increasing the metering pressure is
generally preferable to installing gas boosters which are a
potential point of failure, can be difficult to commission
and require regular maintenance. If a gas booster is required
however it is recommended to install the unit as close as
possible to the appliance it serves. If a booster is installed
after the meter with a significant run of downstream
pipework before the appliance this can cause major pressure
instability resulting in nuisance tripping of the booster and
boilers and possible meter pressure problems.
The gas supplier must be consulted before fitting a pressure
booster, which must include protection against disturbance
to the gas supply or damage to the meter by excessive
suction or pressure. This is normally achieved using a low
pressure cut-off switch and a non-return valve on the gas
supply side of the booster and a pressure relief bypass
around the compressor.
In the UK if a booster is located in a meter / regulator room
the associated electrical equipment should be at least
suitable for use in a zone 2 area (refer to BS EN 60079-15:
2010). A risk assessment must be performed to ensure
compliance with Dangerous Substances and Explosive
Atmosphere Regulations (DSEAR) (HSE, 2002b).
1.6.8.2
Propane (LPG)
As propane is stored in pressure vessels in the form of a
liquid/vapour mixture (see 1.6.3.1), the pressure will be a
function of temperature. At 15 °C the pressure is about
800 kPa (7 barg). Tanks are cylindrical and made of steel to
BS EN 12542 (BSI, 2010). The outlet pipe is connected at
the top of the tank so that only vapour enters the delivery
pipework. Two stages of pressure reduction are provided
between the vessel and the intake point at the building in
order to control the pressure:
——
First-stage regulator: normally fitted to the vapour
connection on the storage vessel and reduces the
pressure to approximately 1.75 kPa (0.75 barg).
This must incorporate an overpressure shut-off.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
In the UK solar heat is generally confined to the supply of
domestic hot water and swimming pools. This is generally
supplemented with a fossil fuel back up to ensure continuous
supply when solar energy is insufficient to meet demand. A
well designed system can provide some 50% of annual dhw
demand. More southerly located European States can
achieve higher fractions and may be subject to building
energy codes that makes solar collectors mandatory for
new-build. The economic or carbon effectiveness of a solar
thermal installation will depend on the amount of solar
radiation received at a site, as well as the type of thermal
conversion technology employed. Figure 1.9 shows the
distribution of annual solar irradiation across Europe.
1-45
1-46
——
Heating
Regulators should comply with BS EN 16129 (BSI, 2013). It
is common practice for the fuel supply company to own the
storage vessel and all pipework and fittings up to the 1st
stage regulator with the building owner owning the
remainder of the installation.
An lpg installation will normally be classified as a ‘pressure
system’ in accordance with the Pressure Systems Safety
Regulations (HSE, 2000c) for those sections operating at
a) Above ground tank installation
Valve hood
Tank
Regulator
Concrete
hard-standing
Service
pipework
Warning mesh
200 mm below
surface
b) Below ground tank installation
Appropriate
backfill, e.g.
excavated earth
with stones and
sharp objects
removed
Marker
pegs
Surface
cover
above 0.5 barG (that is up to the second-stage regulator).
The installation user will have responsibilities under these
regulations.
Siting of tanks
Tanks must be sited outdoors, either above ground or
buried as shown in Figure 1.10. The siting of the storage
tank(s) requires careful consideration both in terms of
access by the lpg delivery vehicle and safety. The UKLPG
trade association produce a code of practice on the siting of
lpg tanks (UKLPG 2012) and safety provisions. Calor Gas
Ltd produce a useful guidance note (Calor, 2011). The
delivery vehicle should be able to park within 25 m of the
storage tank. Drains, gullies and cellar hatches close to
tanks should be protected from gas entry.
When buried, it is traditional to back fill using the excavated
earth provided this is stone free. A protective and warning
grid should be installed above the tank. The tank must be
fixed down to the foundation to prevent the tank lifting due
to buoyancy forces. It must not be located under any paths
or roadways nor should any structure be placed above it
including decking.
Tanks above ground should be enclosed in a securely fenced
compound if the site is accessible by the public. Where
there is a risk of damage from vehicles, a protective barrier
should be provided. As the pressure within the tank is
determined by the temperature of its contents, care must be
taken to reduce excessive heat gains to the tank. Tanks are
painted in a light colour to reflect solar irradiation. For the
purposes of reducing fire spread, there are minimum
separation distances between tanks, between tanks and the
property boundary and between the tanks and any
buildings. These distances increase with the storage
volumes and can be reduced by means of fire walls. Figure
1.11 gives minimum separation distances. The area around
the tank must be kept clear of debris and any other
combustible material including vegetation. Fire fighting
appliances (hose reel and dry powder extinguishers) should
be provided within the compound for storage volumes
exceeding 2000 L. It is essential that adequate ventilation is
provided so that in the event of a leak, the vaporised fuel
will quickly disperse. Fences should preferably be open
mesh.
Sizing of tanks
This is based on:
Service
pipework
Concrete
base
Anchor straps
Sacrificial anode
(providing corrosion
protection)
Figure 1.10 Typical above and below ground installation of lpg tanks
(reproduced by kind permission of Calor Gas Ltd)
——
consumption rate
——
period between deliveries
——
the maximum draw-off rate required.
The total storage volume will be determined by the
consumption rate and the planned interval between
deliveries. The number of tanks is likely to be determined
by the sizes of tank available. However, in some instances it
may be determined by the maximum flow rate of propane,
that is the peak heating demand, that can be achieved.
When the fuel vapour is drawn off from the tank, the liquid
fuel boils to replace it. The latent heat of vaporisation is
absorbed from the liquid so reducing its temperature. In
turn, heat will flow into the tank from its surroundings.
The tank and its contents will therefore be cooler than its
surroundings. Its temperature will depend upon the rate at
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Second-stage regulator: fitted after the first-stage
regulator. This can either be at the vessel or on the
wall of the building and reduces the pressure from
1.75 kPa (0.75 barG) to the working pressure of
37 mbarG and includes an over-pressure shut-off
valve, an under-pressure shut-off valve and a limited
relief valve. The relief valve is designed to
accommodate small changes in pressure due to
changes in temperature and so prevent nuisance
tripping of the over-pressure valve.
Energy sources
1-47
With no firewall
With firewall
Perimeter
Property
perimeter
A
A
C
C
A
1-1·5 m
A
A
A
B
Building
Propane
capacity /
litres
Minimum safety distances
from buildings, boundary,
property line or fixed
ignition source / m:
No firewall No firewall
A
B
3
1·5
1200
3
1·5
2000
7·5
4
3400
7·5
4
4000
7·5
4
8000
15
7·5
24,000
* the height of the vessel if greater
Firewall
Building
Maximum
number
of tanks
in group
Distance
between
tanks / m
5
3
6
6
3
6
1
1
1
1
1
1·5
which it can extract heat from its surroundings. Under
steady-state conditions the heat flow rate into the tank will
equal the rate of latent heat removal by the drawn off
vapour. This will be a function of the temperature of the
surroundings and the surface area of the tank. The ratio of
tank surface area to volume will decrease with increase in
tank diameter so that for a given storage volume, two small
tanks will provide a higher maximum draw-off rate than a
single large tank.
For tanks buried in soil of low thermal conductivity,
freezing of the soil can occur as propane will remain above
atmospheric pressure, and so continue to be drawn off, well
below 0 °C. The maximum draw-off rate for a buried tank is
typically only half that of a surface tank.
Gas distribution pipework must comply with the relevant
parts of the Gas Safety (Installation and Use) Regulations
(1998) and associated ACOP L56 (HSE, 2011). The relevant
British Standards for installation of gas pipework are BS
EN 1775 (BSI, 2007). Gas pipework for buildings, maximum
operating pressure less than or equal to 5 bar – Functional
recommendations and BS 6891 (BSI, 2015) for domestic
premises. Compliance with these is generally achieved by
designing in accordance with the IGEM/UP/2 (Institute of
Gas Engineers and Managers) (IGEM, 2014a).
In particular, pipes within buildings must:
be protected from failure caused by movement
when installed in walls and floors
Height of
firewall* / m
1·4
1·5
2
2
Height of vessel
Height of vessel
Figure 1.11 Minimum fire
separation distances for external,
above ground lpg storage vessels
from 2500 to 9000 L (reproduced
by kind permission of Calor Gas
Ltd)
(b)
not be installed within the cavity of a cavity wall
(c)
not be installed under the foundations of a building
or a wall
(d)
not be installed in an unventilated shaft, duct or
void
(e)
take the shortest practicable route through a solid
structure and be enclosed in a gas-tight sleeve
(f)
be electrically bonded, including
bonding during modification.
(g)
not pass through or along a protected shaft
containing a stair or lift unless the following
conditions apply:
Gas distribution pipework
(a)
A
(h)
temporary
——
the operating pressure does not exceed
75 mbar
——
the pipe material is carbon or stainless steel
and the joints are screwed or welded
be protected by lightning conductors when exposed
at high level (e.g. on a roof).
For external gas pipework for low and medium pressure (up
to 2 bar) the preferred material is medium density
polyethylene (mdpe) which comes in either straight lengths
or in coils. This is installed 600 mm below a footway or
750 mm below a verge or 900 mm below a carriageway.
Where the gas pipe enters the building from below floor
level the mdpe pipe should be installed within a prefabricated steel sleeve. Galvanised steel or copper is used for
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
A
A
1-48
Heating
Table 1.26 Gas operating pressures and maximum pressure drop
between primary meter and appliance (reproduced with kind permission
of IGEM)
Operating pressure
(OP)
Natural gas
≤ 25 mbar
1.0 mbar
> 25 mbar
10% OP
Propane*
Butane*
Maximum design pressure
drop at design flow
≤ 42 mbar
2.5 mbar
> 42 mbar
10% OP
≤ 33 mbar
2.5 mbar
> 33 mbar
10% OP
* The pressure for propane and butane drop is measured from the
outlet of the pressure regulator on the bulk storage tank/cylinder to the
plant manual isolation valve.
f = 0.0044 [ 1+43.5 d–1 ]
(1.34)
For other gases the friction factor can be obtained from:
indoor pipework with galvanised steel recommended for
larger pipework. Steel pipes should comply with BS EN
10255 (BSI, 2004) and copper pipes with BS EN 1057 (BSI,
2006).
f = fsp / e2
(1.35)
where fsp is the smooth pipe friction factor and e is the
efficiency factor (0.86 for steel, 0.97 for pe pipe).
For un-boosted natural gas installations and lpg
installations, the gas flows entirely due to its static pressure:
the pipework must be sized so as to achieve the design flow
rate with a pressure drop not exceeding the available
pressure allowing for the supply pressure required at the
appliance.
The smooth pipe fricton factor is given by:
Table 1.26 lists the maximum pressure drops between
primary meter and appliance recommended by the Institute
of Gas Engineers and Managers in their document IGEM/
UP2 (IGEM, 2008).
where Re is the Reynolds number. In UK the Reynolds
number may be taken as follows.
CIBSE Guide C provides a methodology and data to enable
pipes to be sized so as not to exceed the recommended
pressure drop. Guide C also includes spreadsheets that will
carry out the calculations. These spreadsheets may be used
for any fluid including natural gas and LPG provided that
the pressure drop does not exceed 10% of the inlet static
pressure (that is, the fluid can be treated as incompressible).
Alternatively, IGEM/UP/2 provides a table of approximate
volume flow rate of gas in straight horizontal pipes with a
1 mbar pressure drop and a maximum operating pressure of
75 mbar. For other conditions, equations 1.33a and 1.33b
(taken from UP/2) can be used, provided the drop in
pressure along the pipe does not exceed 10% of the inlet
pressure.
For maximum operating pressure less than 75 mbar:
Q= 57.1 × 10 -5 [ pd 5 (sLf)-1 ] 0.5
(1.33a)
For operating pressure between 75 mbar and 5 bar:

(
)
 0.5
Q = 12.7 ×10  P1 − P2 (sLf ) 
−3
2
2
−1
(1.33b)
where Q is gas flow rate (m3/hr), p is gas pressure drop due to
friction (mbar), P1 is upstream gas pressure (bar), P2 is
downstream gas pressure (bar), d is internal pipe diameter
(mm), s is the density of gas relative to air (dimensionless), L
is length of pipe (m) and f is the Fanning friction factor
(dimensionless).
Equation 1.33a is derived from the Darcy equation for the
flow of an incompressible fluid in a straight circular
conduit. It should be noted that equations 1.33a and 1.33b
fsp = (14.7519 + 3.565X + 0.0362X2)–2
(1.36)
X is a factor given by:
X = log10 Re – 5
(1.37)
For natural gas:
Re = 25 043 × Q / d
(1.37a)
For lpg:
Re = 83 955 × Q / d
(1.37b)
The pressure loss caused by bends, valves, secondary meters
and all other fittings must be added to that in the straight
sections of pipe and may be a significant fraction of the
total. CIBSE Guide C contains values of velocity pressure
loss factors for a wide range and size of pipework fittings
whilst IGEM/UP2 lists typical values of equivalent length.
For valves and meters it is recommended that manufacturer’s
data is used as these can result in particularly high pressure
losses.
IGEM/UP/2 also gives the equation below to calculate the
effect of altitude in high rise buildings. Lighter than air
(e.g. natural gas) will show an increase in pressure, whereas
heavier than air gas (e.g. propane) will show an increase.
h = 0.123 (1 – s) H
(1.38)
where h is the pressure change due to altitude (mbar), H is
the altitude change (m) and s is the density of gas relative to
air (dimensionless).
1.6.8.3
Hydrogen
The properties of hydrogen give rise to a number of
challenges its transportation and storage. It is highly
reactive has a very low density (and low energy density),
and has low viscosity. These properties impact on options
for storage, the types of materials that can be used, and
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Gas type
use the Fanning friction factor (f) rather than the friction
factor (λ) used by CIBSE and most other organisations. The
Fanning friction factor is exactly one quarter the value of
the CIBSE friction factor, (f = λ / 4). The friction factor is
a function of pipe surface roughness and Reynolds number
(Re) and may be determined by the Poiseuille (Re < 2000)
or the Haaland (Re > 3000) equation, or by means of the
Moody Diagram, (see CIBSE Guide C). However, as the
Reynolds number is itself a function of volume flow rate,
equation 1.33 cannot be solved directly but requires
iteration. IGEM/UP2 avoids the need for such iteration by
providing an approximate solution for Fanning friction
factor which is a function of pipe diameter only. For natural
gas:
Energy sources
1-49
considerations for safety. A detailed description of these
properties can be found in ISO Technical Report 15916
2004 Basic considerations for the safety of hydrogen systems
(ISO, 2004).
The other important consideration for hydrogen storage
and transportation is that of materials. Hydrogen can react
with carbon in steels and other alloys to cause embrittlement
and failure under stress. Materials for use in hydrogen
cylinders are dealt with in a range of standards: BS EN
12245 (BSI, 2009); BS EN 12257 (BSI, 2002); BS EN ISO
11119-1 (2012); BS EN ISO 11114-4 (2016). Hydrogen’s
small molecular size and low viscosity means that it can
permeate most substances and also will leak much more
readily through joints or cracks. Storage vessels, cylinders
and pipelines are therefore often composite structures that
can safely contain the hydrogen without long term material
degradation.
Further information on the storage and handling of
hydrogen can be found in the HSE Research Report RR769
(HSE, 2010) and the BCGA Code of Practice CP33 (BCGA,
2012) on the bulk storage of gaseous hydrogen.
1.6.8.4
Gas industry legislation, standards
and industry governing bodies
In the UK, all combustion installations using gas (including
natural gas and lpg) must comply with the Gas Safety
(Installation and Use) Regulations (GSIUR) (HMSO, 1998)
which cover the safe installation of gas fittings, appliances
and flues and also require that installation work be
undertaken by a person approved by the HSE. The GSIUR
control all aspects of the installation, maintenance and use
of systems burning gas. The text of the Regulations and
guidance on how to comply with them are contained in
Health and Safety Executive (HSE) Approved Code of
In the UK, the Institution of Gas Engineers and Managers
(IGEM) produce a series of standards that prescribe best
practices for various work activities for the gas industry.
The standards are drafted by expert panels representing a
cross-section of the relevant parts of the gas industry and
are trusted gas industry standards used to assist in
compliance with national legislation and official approved
codes of practice and guidance. Regulatory bodies such as
Health and Safety Executive (HSE), Office of the Gas and
Electricity Markets (Ofgem) and Gas Safe Register
contribute to the drafting process. IGEM standards relate
to aspects of numerous regulations including the GSIUR,
some of these are listed in Table 1.27(b). along with guidance
documents and codes of practice. Generally designs that
comply with the IGEM standards will comply with current
UK legislation.
The IGEM standards are divided into the following
subjects:
——
transmission and distribution (TD series)
——
gas measurement (GM series)
——
gas generally (G series)
——
gas utilisation (UP series)
——
gas legislation (GL series)
——
safety (SR series).
Ofgem is a forum for a wide range of professionals within
the gas industry to share industry knowledge. In addition
protecting consumers they determine strategy, set policy
priorities, take decisions on a range of matters, including
price controls and enforcement. They also provide a source
of gas industry news and technical information and publish
a number of Standards which engineers are expected to
work to when carrying out gas work. The Authority’s
powers are provided for under the Gas Act (HMSO, 1995).
Gas Safe Register replaced CORGI as the gas registration
body in the UK in 2009 and ensures that all gas engineers
on the register are gas safe and qualified to work with gas.
Gas Safe Register is the official gas registration body for the
United Kingdom appointed by the relevant Health and
Table 1.27(a) Some UK Regulations and guidance related to natural gas and lpg
Regulation
Associated guidance / Approved codes of practice
Pipelines Safety Regulations (HMSO, 1996)
L82: A guide to the Pipelines Safety Regulations L82 (HSE, 1996)
Pressure Systems Safety Regulations (HMSO, 2000)
L122: Safety of Pressure systems L122 (HSE 2000)
Gas Safety (Management) Regulations (HMSO, 1996a)
L80: A guide to Gas Safety Management Regulations L80 (HSE, 1996a)
Gas Safety (Installation and Use) Regulations (HMSO, 1998)
L56: Approved Code of Practice L56 (HSE, 2013a)
Dangerous Substances and Explosive Atmospheres
Regulations (DSEAR) (HMSO, 2002)
L138: Approved Code of practice L138 (HSE, 2013b)
Reporting of Injuries, Diseases and Dangerous Occurrences
Regulations (RIDDOR) (HMSO, 2013)
Reporting Accidents and Incidents at Work (HSE, 2013c)
Construction Health and Safety Manual Vol 1 Sect. 6
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
There are three forms of storage: gaseous, liquid or metal
hydride. Because of its low energy density it has to be stored
at high pressure in its gaseous form — around 170 bar for
stationary applications, and up to 700 bar for vehicle use.
Storage in liquid form has a higher energy density, but
requires cooling to below –250 °C, with an associated energy
expenditure (30%) and safety issues of very low temperature
substances. An alternative is storage within metal hydrides,
which can have a higher volumetric energy density than
liquid hydrogen (HSL, 2006). However, hydrides require
high temperatures to release the hydrogen, with the losses
that this incurs.
Practice L56: Safety in the installation and use of gas systems
and appliances (HSE, 2013b). See Table 1.27(a) for other key
guidance. The detailed guidance in this document applies
principally to small appliances but similar requirements
apply generally.
1-50
Heating
Table 1.27(b) The key IGEM standards
Title
Notes / key points
UP/1
Strength testing, tightness testing
and direct purging of
installations. This gives standard
procedures for installation and
testing / commissioning
Building services should be familiar with the test pressures / acceptable pressure drops and leakage
rates associated with the procedures for strength and tightness testing and purging of pipework. For
a maximum operating pressure (mop) of <100 mbar the strength test pressure is 2.5 × mop.
UP/2
Installation pipework on
industrial and commercial
premises
This is the key document for the design, installation, operation and maintenance of natural gas or
lpg gas pipework both internally and buried. It covers: pipe types and sizing; valve types and
selection; gas pressures; locations for test and purge points; pipe materials and jointing methods;
pipework routes and ventilation of ducts, voids and risers in buildings; the installation of gas
boosters.
Table 1.28 is reproduced from UP/2 and gives maximum design pressure drops between meter and
appliance for different operating pressures and gas types.
If a gas pipework system is designed and installed in accordance with IGEM/UP/2 this should satisfy
the requirements of DSEAR.
UP/3
Gas fuelled spark ignition and
dual fuel engines
Gives procedures for design installation and testing of spark ignition and compression ignition
engines of both the reciprocating and rotary type, either stationary or portable, for both continuous
and standby duties. Covers design of pipework, air supplies for both combustion and cooling /
ventilation and exhaust systems
UP/10
Installation of flued gas
appliances in industrial and
commercial premises
The standard covers the installation of a range of flued gas appliances including hot water and steam
boilers of net heat input exceeding 70 kW with no upper limit and hot water boilers of gross heat
input not exceeding 2 MW, thermal fluid heaters warm air heaters radiant heaters.
It gives:
UP/11
Gas installations for educational
establishments
•
Recommended combustion air and ventilation air requirements for both naturally and
mechanically ventilated plant rooms; detailed guidance on flue design of chimneys and flues
including fan diluted type (flue dilution systems).
•
Recommendations for the installation of gas and biomass / bio-fuel appliance in the same
facility providing a calculation method for chimney discharge heights in order to show
compliance with Local Air Quality Management Technical Guidelines LAQM. TG(09).
This Standard sets down the minimum requirements for safety in educational establishments, for
architects, designers, science health and safety advisers, in the operation of their systems and
equipment. It covers the design, installation, operation, and maintenance of gas pipework, systems
and appliances used for teaching purposes in educational establishments, including schools; colleges;
universities and training facilities including automatic isolation valves (AIVs) and other safety
features for educational establishments.
Reference should be made to IGE/UP/10 for central heating and domestic hot water plant
installations.
GM/6
Non-domestic meter installations
for ‘standard designs’ with meters
of badged capacity exceeding
6 m3/h and not exceeding
1076 m3/h with maximum
operating pressure (upstream) not
exceeding 75 mbar. Where a
standard design is not
appropriate, IGE/GM/8 should be
used.
This gives standard designs for a metering pressure of 21 mbar.
Note: The designer should check with the gas provider what the lowest meter installation outlet
pressure will be for a 21 mbar metering pressure, this is normally 18 mbar. If the pressure drop
between the meter and the appliance is 1 mbar in accordance with UP/2 this may cause problems if
the appliance requires more than 17 mbar.
GM/8
Non-domestic meter
installations. Flow rate exceeding
6 m3/h and inlet pressure not
exceeding 38 bar. This is divided
into 5 parts covering: design;
locations, housings and
compounds; fabrication,
installation, testing and
commissioning; operation and
maintenance and notices and
labels
This standard is for non standard designs and is useful when the building services designer has
responsibility for design of non standard meter / governor compounds within developments.
G/5
Gas in flats and other multidwelling buildings
These procedures summarise best practice for the design, installation, operation and maintenance of
gas installations for flats and other multi-dwelling buildings.
The procedures define core areas of safety which need to be considered including ventilation of pipes
and pipework, location of gas meters, types of termination and entry of network pipelines,
particularly with respect to ventilation and maintenance.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Standard
Energy sources
Safety Authority for each area, they produce regular
technical bulletins and guidance documents.
There are a number of guidance documents specific to lpg
installations.
For small storage installations it is possible to show
compliance by following the guidance given in Approved
Document J of the Building Regulations (England) (DCLG,
2010).
UKLPG is the trade association for the lpg industry in the
UK and represents all the major lpg companies in the UK.
They seek to promote the safe use of lpg and set the
technical and safety standards for the industry. They
produce a series of Codes of Practice covering all aspects of
the installation and bulk storage including:
——
Code of Practice 1 (4 parts) covers bulk lpg storage
at fixed installations (UKLPG, 2013, 2012, 2012a,
2013a)
——
Code of Practice 22 covers the design, installation
and testing of lpg pipework systems, lpg Piping
System Design and Installation (UKLPG, 2011)
——
BS 6891 (2015) provides guidance and
recommendations for the installation of lpg supply
systems in domestic premises and small commercial
premises for pipework sizes up to 35 mm.
Hydrogen
Hydrogen falls under a range of safety regulations, includng
The Dangerous Substances and Explosive Atmospheres
Regulations (DSEAR) (HMSO, 2002), and the Control of
Accident and Major Hazard (COMAH) Regulations
(HMSO, 2009). Under the Notification of Installations
Handling Hazardous Substances (NIHHS) Regulations
2002 (HSE, 2002a), the HSE needs to be notified of any
storage facility greater than 2 tonnes.
1.6.8.5
Oil
The storage and handling of petroleum-derived fuels and
some bio-oils represents a significant potential source of
pollution to groundwater. Further, although the storage of
such oils are not seen as necessarily a source of fire, it is
essential due to their combustibility that they are protected
from the spread of fire. Exposure to such oils poses health
risks. As such, the design, installation, operation,
maintenance and removal of oil storage installations are
heavily regulated. Minimum requirements for fire
segregation and protection apply. Storage tanks require
some form of secondary containment should a leak in the
primary storage tank occur and particular care needs to be
taken in the method of filling the tanks. In the UK,
pollution of groundwater is a criminal offence, as it is in
other parts of Europe, and may result in imprisonment. A
summary of the key UK regulations, codes of practice and
guidance is given at the end of this section.
Siting of tanks
Tanks may be installed within or on a building or outdoors
and separated from the building it serves. The latter may be
partly or wholly buried in the ground. Where practicable,
outdoor locations are preferred as this reduces fire risk
within the building with a consequential reduction in the
level of fire protection required. In the UK, different
regulations apply to tanks installed above ground to those
wholly are partly buried: tanks within basements are
treated as above ground provided the base and sides are
accessible for inspection.
Where an oil storage tank is to be buried, it must be
specifically designed and constructed to withstand pressures
acting upon it from the outside. Before specifying
underground storage facilities planning permission may be
required and advice should be sought from the regional
Environment Agency.
Where an oil storage tank is to be installed outdoors above
ground or within or on a building, it must comprise of a
primary tank located within a secondary container (bund).
Where supplies are to be delivered by road, consideration
must be given to access for vehicles. Where there is
restricted access, tanks can be fitted with an extended fill
pipes. This would typically take the form of a steel pipe
connected to the tank and routed to an accessible position.
The pipe should incorporate a drip tray at the fill point.
Such an arrangement will need to include measures to
reduce the risk of overfilling the remote tank.
Storage tank construction and bunding
Tanks are typically manufactured from polyethylene (BS
EN 13341) (BSI, 2005) or steel (BS 799-5) (BSI, 2010).
For above ground installation it is increasingly common to
use an integrally bunded tank. This consists of a single skin
primary tank within a larger tank. Figure 1.12 shows a fully
closed, integrally bunded tank constructed from (a) steel
and (b) plastic. Partially open arrangements also exist where
the secondary tank is partially open to the atmosphere.
OFTEC (Oil Firing Technical Association) has produced
standards OFS T200 and T100 for integrally bunded oil
storage tanks constructed from steel and plastic respectively
(OFTEC, 1999, 2001). Alternatively, the primary tank can
be sited with a specially constructed bund (Figure 1.13).
Guidance on the construction of bunds is provided by
CIRIA/Environment Agencies (see Table 1.31). The
capacity of the bund in both cases must be at least 110% of
the primary tank capacity. Where there is to be more than
one primary oil storage tank within a constructed bund, the
bund must be sized to hold 110% of the largest tank’s
capacity, or 25% of the total capacity, whichever is the
greater. When sited outdoors, open or partially open bunds
are not recommended as these are likely to collect rainwater
and debris which will reduce its effective volume and will
require safe disposal of such material that may be
contaminated with oil.
For below ground installation, in the UK, such tanks are
required to be of the double-skinned type and incorporate
leak detection monitors with automatic alarms and
constructed to BS EN 12285-1 (BSI, 2003).
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
LPG
1-51
1-52
Heating
Tank and bund
overfill alarm panel
Contents
gauge
Access
hatch
Overfill
cut-out valve
Dip
point
Suction draw
off to services
Inspection
hatch
Louvred
vent panel
Vent
100 mm
100 mm
Fill
Removable
inner tank
Cabinet with
drip tray
Drain
Bund
Base
Removable
inner tank
Figure 1.12(a) Integrally bunded oil storage tank constructed of steel (reproduced by kind permission of CIRIA)
Tank and bund
overfill alarm
panel
Contents
gauge
Removable
bund cover
Lockable
inspection
hatch
Top draw off
(preferable
where ever
possible)
Internal
vapour
vent
Fill point cap and
overfill cut out
Inner
primary
tank
Vent to
atmosphere
Sludge
removable
flap
Gravity feed
draw off
(optional)
check valve
required if
installed
Base
Figure 1.12(b) Integrally bunded oil storage tank constructed of plastic (reproduced by kind permission of CIRIA)
Vent to
BS799: Part 5
Dial
gauge
Non-return
valve
Gate
valve
Oil
Bund/secondary
containment
Fill
point
Isolation
valve
Figure 1.13 Storage tank housed with a specially constructed bund.
(Reproduced by kind permission of OFTEC)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Removable
roof
Energy sources
1-53
Table 1.28 Summary of BS 5410 in relation to pipework serving oil heating installations
BS 5410-2 (2013)
Pipes should be:
Pipes should:
•
constructed of steel or copper when installed within a building or above
ground externally
•
take the shortest practicable route with minimal
directional changes
•
protected from damage by ensuring it is appropriately routed and supported
with suitable clips affixed to permanent (masonry) structures
•
avoid the use of elbows or square tees
•
be protected from corrosion.
•
sleeved where it passes through the walls of buildings
•
electrically bonded in accordance with BS 7671.
Screwed joints should be made using soft setting petroleum resistant compounds
in conjunction with ptfe tape.
Steel pipes and fittings should comply with the requirements
for medium or heavy tubes as specified in BS EN 10255.
Copper pipe should be jointed with compression fittings which are either Type A
with inserts or Type B flared.
Copper pipe should comply with BS EN 1057. Fittings should
be of the compression type which are either Type A with inserts
or Type B flared.
Galvanised pipes and fittings and soft soldered joints should not be used.
Where oil supply pipe systems are required to be located externally below ground,
they should:
•
be suitably protected against the risk of accidental damage
•
not contain any joints
•
where constructed of steel be protected against corrosion
•
where constructed of copper be of the plastic coated type.
Where oil supply pipe systems are required to be located below
ground, they should be laid in concrete ducts which are fully
accessible by way of removable covers.
Approved types of plastic pipe can be used externally below ground only.
Where joints are buried, permanent access must be provided to them.
Oil tank bases
Suitable base materials include:
The need to provide suitable bases and supports for
domestic oil storage tanks both of steel and plastic
construction is of paramount importance for reasons of
both safety and environmental protection. If an oil storage
tank is inadequately supported the oil storage tank itself
can be weakened leading to the eventual failure of the oil
storage tank and escape of the stored fuel.
(a)
concrete at least 100 mm thick
(b)
paving stones at least 42 mm thick laid to give an
imperforate base
(c)
stonework at least 42 mm thick, laid to give an
imperforate base.
Guidance on supports for steel and plastic tanks of up to
3500 litres is covered by BS 5410-1 (BSI, 2014). Guidance
on supports for steel and plastic tanks above 3500 L should
be sought from BS 5410-2 (BSI, 2013) and the tank
manufacturer.
Fire separation
To satisfy fire protection requirements, the overall size of
the base should be larger than the oil storage tank and any
integral oil storage tank bund, so that when the oil storage
tank is installed, the base has a clear projection of a
minimum of 300 mm around the extremity of the oil storage
tank as shown in Figure 1.14.
Base
300 mm
To prevent the spread of fire from adjacent buildings to
storage tanks sited above ground outdoors, a minimum
separation should be achieved. BS 5410-1 gives guidance on
tanks of up to 3500 L capacity and BS 5410-2 gives guidance
on tanks of more than 3500 L capacity, see Figure 1.15.
Recommended separation depends upon the fire rating of
adjacent boundary walls. Where such separation cannot be
achieved, fire screens should be constructed.
Storage tanks located within or on the roof of a building
should be housed within a fire-rated chamber that houses
the tank(s) and associated oil supply system only and is
provided with appropriate and adequate means of
ventilation. Guidance is provided in BS 5410-2. Tanks
should preferably be located at a low level as the whole of
the structure supporting the tank chamber should be fire
rated to at least the standard of the chamber.
Temperatures for storage of liquid fuels
300 mm
Oil storage tank
300 mm
300 mm
Figure 1.14 Minimum clearance required around oil storage tanks
(Reproduced by kind permission from OFTEC)
The viscosity of fuel oil decreases with an increase in
temperature. Fuel oils of classes E to H require heating to
reduce the viscosity sufficiently to allow the fuel to flow. It
is usual to maintain the oil tank contents at the temperatures
shown in column 2 of Table 1.29 with additional heating
provided to the outflow as column 3. Heating to the tank
may be provided by hot water coils or by electric immersion
heaters with outflow pipes insulated and provided with
trace heating.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
BS 5410-1 (1997)
1-54
Heating
BS 5410-2 (for more than 3500L)
Boundary with less than 30 minutes fire resistance
Boundary with less than 120 minutes fire resistance
At least 760 mm
At least 6000 mm
At least 1800 mm
At least 6000 mm
Building wall with less than 30 minutes fire resistance
Building wall with less than 120 minutes fire resistance
Boundary with less than 30 minutes fire resistance
Boundary with less than 120 minutes fire resistance
With firewalls
Less than 760 mm
Firewall
Less than 6000 mm
Firewall
300 mm
300 mm
900 mm
900 mm
300 mm
300 mm
900 mm
900 mm
Firewall
Less than 1800 mm
Building wall with less than 30 minutes fire resistance
Firewall
Less than 6000 mm
Building wall with less than 120 minutes fire resistance
Wall with at least 120 minutes fire resistance
1860 mm - 6000 m
opening with at least
60 minutes fire resistance
At least 1800 mm
300 mm
900 mm
Firewall
Firewall
With openings
Wall with at least 30 minutes fire resistance
Figure 1.15 Fire separation
requirements for aboveground external oil storage
tanks (Reproduced by kind
permission of OFTEC)
Less than
1800 mm
Less than
1800 mm
Wall with at least 30 minutes fire resistance
Wall with at least 120 minutes fire resistance
Class C fuels do not require heating. Class D fuels may
benefit from being maintained above about 5 °C. Where
bio-oils are being used, manufacturer’s advice should be
sought.
Table 1.29 Storage and outflow temperatures for petroleum fuel oils
Pipework
All oil pipework, fittings and equipment should be installed
in such a manner so as to minimise the risk of leakage and
be adequately protected from fire. Oil supply systems
serving heating appliances of up to 45 kW output should
comply with BS 5410-1 and above 45 kW with BS 5410-2.
The current standards are summarised and compared in
Table 1.30.
Supply pipework systems
Single pipe delivery is suitable for class C and D fuels. Class
E fuel oil should be supplied from a circulating ring main,
with further preheating of the fuel within the burner before
feeding to the atomiser. Class F and G oils require an
outflow heater to raise the oil to pumping temperature and
trace heating applied to the ring main pipework and other
components.
Where it is intended that there will be more than one oil
fired appliance supplied from a single oil storage tank, it is
recommended that wherever possible separate oil supply
Class
Minimum temperature / °C
Storage
Outflow
E
10
10
F
25
30
G
40
50
H
45
55
pipes are run from a manifold at the tank. Where it is
necessary to connect multiple oil storage tanks together, the
oil supply system should be fitted with a non-return valve
in each supply from each tank before connecting them
together to achieve a combined system.
Gravity
Some oil storage tanks are installed with bottom outlet
draw off, and are often installed on raised piers to facilitate
a gravity feed to the appliance.
It is important that the minimum head of pressure as
specified by the appliance manufacturer is achieved to allow
for the correct operation of the equipment supplied and to
prevent the risk of fuel starvation. Fuel starvation can cause
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
With no firewalls
Approved document J (for up to 3500L)
Energy sources
1-55
damage to working components such as fuel pumps etc.
and prevent the safe operation of vaporising appliances and
equipment. It is equally important that the maximum head
of pressure as specified by the appliance manufacturer is
not exceeded as this can result in malfunction and/or
permanent damage occurring: a pressure-reducing valve in
the oil supply pipe between the tank and appliance may be
required.
Guidance on oil handling systems incorporating a transfer
pump is covered in BS 5410-2.
Sub-gravity
1.6.8.6
Where a tank serves a pressure jet appliance, it is
advantageous to specify a tank of the top outlet type as there
are no perforations or connections below the highest level
of the oil in the tank. Therefore, in the event of a leak at the
supply pipe connection it eliminates risk of oil escaping
under gravity pressure. This also has the advantage that
repairs can be made to tank connections without the need
to extract and store the tanks contents.
Storage
A top outlet tank can be used with pressure jet appliances
with the aid of either a de-aerator or two-pipe system.
Those serving vaporising appliances will require an oil
lifter.
For sizing sub-gravity oil supply pipes, OFTEC Technical
Book 3: Domestic and Commercial Requirements For Oil
Storage and Supply Equipment (OFTEC, 2010b) provides
charts for appliances using a de-aerator system.
Pumped ring main
Pumped ring main oil supply systems are typically specified
in commercial environments where installations consist of
multiple oil fired appliances. Figure 1.16(a) shows a typical
gravity pumped ring main system, Figure 1.16(b) shows a
typical sub-gravity pumped ring main system.
There are a number of regulations and codes of practice
relating to oil storage and distribution. Those applicable to
the UK are set out in Table 1.30.
Coal
Solid fuel is normally delivered by road vehicle and
unloaded by tipping or by conveyer. Access for delivery
should be designed to suit the type of delivery vehicle
expected, taking account of turning circle and space for
tipper operation.
CIBSE Guide C gives the bulk density and specific volume
of various types of coal, which may be used to design storage
capacity. A minimum capacity equivalent to at least 100
hours operation at full output is recommended. The usable
capacity of a bunker depends upon the methods by which
fuel is delivered and extracted from the bunker and may be
less than the nominal volume. Rectangular bunkers with
flat bases are difficult to empty completely without manual
trimming. Bunkers with hopper bottoms empty completely
but require vehicle access at a high level if they are to be
filled by tipper. Bunker bases should be designed to suit the
method of coal extraction, avoiding dead volumes that fail
to leave the bunker. Low friction linings for outlet chutes
may assist free flow and aid extraction.
Bunkers should be covered by grid screens, which are sized
to prevent the entry of large objects that could damage the
coal extraction equipment. A 100 mm grid is usual, strong
enough to support the weight of operators or, if necessary,
vehicles.
Safety
Hazards can arise from spontaneous combustion and
explosions caused by dust or methane. Monitoring of
2 3
Outside
Outside
Inside
8
1
Inside
7
9
4 5 6
1
10
2 3
7
4 5 6
1 Oil storage tank
2 Isolating valve
3 Oil filter (strainer type)
4 Service valve
5 Oil filter
8
9
6 Remote-sensing fire valve
7 Fire valve sensor
(fixed above burner)
8 Service valve
9 Appliance
Figure 1.16(a) Typical gravity oil supply system. (Reproduced by kind
permission of OFTEC)
1 Oil storage tank
2 Isolating valve
3 Anti-syphon valve
4 Service valve
5 Oil filter
6 Remote-sensing fire valve
7 Fire valve sensor
(fixed above burner)
8 De-aeration device
(incorporating non-return valve)
9 Burner fuel pump
(set for two pipe operation)
10 Wall mounted appliance
Figure 1.16(b) Typical single sub-gravity oil supply system with the pipe
connection through the top of the tank incorporating an external
de-aeration device. (Reproduced by kind permission of OFTEC)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
When specifying a top outlet tank it should be noted that
an anti syphon valve should be incorporated into the oil
supply pipe assembly, and calibrated, to prevent free
flowing oil escaping from the tank under syphonic action if
the oil supply pipe is ruptured.
Regulations, codes and guidance
1-56
Heating
Back pressure relief valve
Figure 1.17(a) Gravity pumped ring
main (Reproduced by kind
permission of OFTEC)
Transfer
pump
Discharge
port
Pressure
gauge
Main tank on
gravity feed
Isolating valve
Pressure
reducing valve
Remote-acting
system fire valve
Remote-acting
fire valve
Filter
Off-takes to burners
Filter
Back pressure relief valve
Discharge
port
Figure 1.17(b) Sub-gravity pumped
ring main (Reproduced by kind
permission of OFTEC)
Pressure
gauge
Transfer
pump
Isolating valve
Pressure
reducing valve
Return
line
Remote-acting
system fire valve
Remote-acting
fire valve
Filter
Filter
Isolating
valve
Suction
line
Non-return/
foot valve
Main tank on
suction lift
carbon monoxide levels and minimising storage volume
during the summer shut-down period can help to avoid
spontaneous combustion. Dust and gas explosions arise
within certain concentrations, which may be monitored
and controlled. Specialist advice should be sought on the
prevention of explosions in solid fuel storage.
Ash extraction and disposal
Fully automatic ash removal is available on some boilers
but on others ash and clinker must be removed by hand.
Various methods are available for ash handling, including
screw and vibratory conveyors and vacuum systems.
Fuel handling
Screw conveyors and elevators are used to raise coal to
mechanical stoker hoppers for small boilers; overhead
monorail, skip hoist and pneumatic handlers are also used.
For large boiler plant, chain-and-bucket and belt-andbucket elevators are used, as are belt, drag-link and screw
conveyors.
For large plant, the ash may be sold directly for use as a
construction material. If it is to be used for block making, it
should meet the requirements described in BS EN 13055-1
(2002). For smaller plant, ash is likely to removed as part of
the general waste removal service, after which it may be
disposed of in land fill or supplied to the construction
industry.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Isolating
valve
Energy sources
1-57
Table 1.30 UK Regulations, codes of practice and guidance on oil-storage tanks
Requirement/Guidance related to oil storage and distribution
Scope
EC Directives on Dangerous
Substances (76/464/EEC) and
Groundwater (80/68/EEC)
Cover the avoidance of water pollution from a range of substances
including oil.
All member states required to enact
national regulations so as to prevent water
pollution from such substances.
Control of Pollution (oil storage)
Regulations 2001(England)
Applies to :
Water Environment (oil storage)
Regulations 2006 (Scotland)
In compliance with EC directives above, sets minimum
requirements for the design, construction, installation and
maintenance of oil storage tanks so as to prevent leakage or
spillage that might result in the contamination of water sources.
Control of Pollution (oil storage)
Regulations 2010 (Northern
Ireland))
These are commonly referred to as the Oil Storage Regulations
(OSR). There are some differences in scope for England, Scotland
and Northern Ireland.
There are currently no OSRs for Wales. The Welsh Environment
Agency See PPG2 below.
Groundwater Regulations 1998
and 2009
All oils,
All storage ≥200 L capacity
All building types served by installation
(other than a dwelling if tank ≤3500L
capacity)
Tanks sited indoors or outdoors (Scotland
and Northern Ireland) or indoors (England)
Excludes tanks located wholly
underground (unless within a building
(Scotland)
Implementation of European Directives above :sets minimum
requirements for preventing pollution of groundwater in England,
Wales, Scotland and Northern Ireland through release of
hazardous substances including hydrocarbons.
Applies to storage of petroleum
hydrocarbons either partly or wholly
buried below ground.
Part B Building Regulations
(England) 2013
Minimum standards of fire safety in buildings
Oil storage tanks within buildings
constitute a special hazard. Approved
Document B2 2006 gives requirements for
non-domestic buildings and apartment
buildings.
Part J Building Regulations 2013
(England)
Requirement J6: all oil storage tanks and the connecting pipework
to be installed so as to limit risk of fire spread.
Covers class C2, D and liquid biofuels.
(Similar building regulations
apply in Wales, Scotland and
Northern Ireland)
Requirement J7: oil storage tanks and connecting pipework to be
so constructed so as to prevent escape of oil and possible pollution
of ground and water courses. Includes requirements for bunds.
Makes specific reference to BS 5410-1 (2014) as means of
demonstrating partial compliance.
BS 5410-2: 2013
Code of Practice for oil firing
– Part 2: Installations of over
45 kW output capacity for
spaceheating, hot water and steam
supply services
Environment Agency
Provides detailed guidance on the design, installation,
commissioning and maintenance of oil installations for heating
including storage tanks and connecting pipework
Excludes lpg.
J6 applicable to all oil storage tanks >90 L
capacity but no upper limit on tank size
and no limit on type of building served.
J7 applies only to storage tanks ≤3500 L
capacity installed outdoors (but not below
ground) serving dwellings
Applicable to installations >45 kW rated
output.
Petroleum fuel oils
Oil storage tanks of any capacity
associated with such systems
Gives guidance on compliance with Control of Pollution (oil
storage) regulations 2002 for England and equivalent regulations
for Scotland and Northern Ireland. Applies also to Wales
Applicable to all oil storage tanks above
ground but intended specifically to
domestic and small installation
Environment Agency Prevention
of Pollution Guidance PPG27
2007: Installation,
decommissioning and removal of
underground storage tanks
Gives guidance to avoid pollution from underground tanks and
associated pipework
Applicable to all types of oil
CIRIA C535: Above ground
proprietary prefabricated oil
storage tank systems, 2002
Describes prefabricated above ground oil storage tanks and gives
recommendations
Covers tanks up to 14000 m3
Applicable to all types of oil
CIRIA R163 Construction of
Bunds for Oil Storage Tanks
This document provides guidance for the construction of masonry
and concrete bunds for oil storage tanks
Masonry bund guidance applies to tanks
up to 3.5 m wide and 1200 mm high
Prevention of Pollution Guidance
PPG2 2010: Above ground oil
storage tanks
Concrete bund guidance applies to tanks
up to 3.5 m wide and 900 mm high
BS 799-5: 2010 Oil burning
equipment – Part 5: Carbon steel
oil storage tanks – Specification
Gives guidance on the requirements for the construction and
testing of static carbon steel tanks
Applies to above ground single skin:
Service tanks
Primary storage tanks with a capacity of
≥90 L capacity with a maximum height of
10 m
For the storage of liquid fuel conforming
to BS 2869 and fame to BS EN 14214
table continues....
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Regulation/Code of Practice
1-58
Heating
Table 1.30 (continued) UK Regulations, codes of practice and guidance on oil-storage tanks
Requirement/Guidance related to oil storage and distribution
Scope
BS EN 13341: 2005 + A1: 2011 –
Static thermoplastic tanks
Gives guidance on the construction and testing of static blow
moulded and rotationally moulded polyethylene tanks
Applies to above ground single blow
moulded and rotationally moulded
polyethylene tanks for the storage of
domestic heating oil, kerosene and diesel
fuels
It is only applicable to such tanks with a
capacity from 400 L to 10 000 L
OFTEC Standard – OFS T200
– Steel oil storage tanks and tank
bunds
Gives guidance on the construction and testing of static carbon
steel tanks with or without bunding
Applies to above ground tanks for the
storage of liquid fuel conforming to BS
2869, FAME to BS EN 14214, lubricating
oils and waste oils
Bunds with a capacity >90 L
OFTEC Standard – OFS T100
– Polyethylene oil storage tanks
and tank bunds
Gives guidance on the construction and testing of static
rotationally moulded polyethylene tanks and tank bunds
Applies to above ground rotationally
moulded polyethylene tanks for the
storage of Classes A2, C1, C2 and D fuel
oil to BS 2869
It is only applicable to such tanks and tank
bunds with a capacity from 400 L to
10 000 L
1.6.8.7
Biomass
Biomass is defined as ‘recently living solid organic matter’
and may be used as a fuel in place of burning fossil fuels to
generate heat for space heating and hot water preparation.
Although a range of biomass fuels are burnt to generate
heat, within the UK’s built environment the fuels in use are
predominantly wood derived. Further classification is by
moisture content and degree of processing, ranging from
wood pellets to wood chips.
Wood pellets
Wood pellets are a manufactured wood fuel, typically
having a moisture content of about 10% and of diameter
6 mm or 8 mm (±1 mm) depending on the die the sawdust
has been forced through. Wood pellet energy values is
typically 4.8 kW·h/kg. Many wood pellet boilers can be
fuelled with either size of wood pellet but as the burn
characteristics are significantly different it is best practice
to have the boiler re-commissioned following a change to
maintain optimum efficiency and avoid any local
overheating issues. Care should be taken to avoid damage to
pellets in transport, delivery, storage and in transit from the
store to the boiler. Specifying wood fuel with good durability
from a quality wood fuel supplier helps to minimise damage
but the largest factor is often the design of the pneumatic
delivery pipes, the fuel store and the auger arrangement.
Wood pellet fuel that becomes damp/wet will expand
causing blockages and even physical damage to fuel store,
the boiler or the auger components.
aldehydes and CO). Wetter fuel has a higher density but
lower net heat content. 3.5 kW·h/kg is the typical useful
heat available from dry wood chip but this is reduced by
approximately 1 kW·h/kg for each additional 20% of
moisture present. Wood fuel is typically purchased by
weight and/or volume. With variable moisture content,
particularly with wood chip, arranging to pay per unit of
heat delivered may be more advantageous.
Fuel specification
Wood fuel specification allows control of fuel quality over
time. BS EN 17225 series (BSI, 2014) Solid biofuels. Fuel
specifications and classes is applicable to non-industrial uses
of wood fuels particularly for fuelling biomass boilers. The
most relevant sections are:
——
Part 1: General requirements
——
Part 2: Graded wood pellets
——
Part 4: Graded wood chips.
Specifiable factors are: origin, diameter, moisture content,
ash, mechanical durability, fines, additives, net calorific
value, bulk density, nitrogen, sulphur, chlorine, arsenic,
cadmium, chromium, copper, lead, mercury, nickel, zinc.
For wood chip bulk density, chip size and moisture content
are the most important factors.
Wood fuel stores
Wood chips
Due to the different physical characteristics of wood pellet
and wood chip, the fuel stores have designs reflecting the
two fuel types.
Wood chips can vary from fuel chipped from well air-dried
logs with a moisture content of about 30% to fuel chipped
from recently harvested wood that may have a moisture
content of 50%. This will affect the energy available in the
fuel, the density of the fuel, the ease with which it can be
ignited and burnt, the most suitable design of biomass
boiler, the level of microbial activity (composting) occurring
within the fuel store, the rate of growth of moulds, gaseous
emissions from the stored fuel (particularly terpenes,
Wood pellet stores can take advantage of the ability of the
pellets to flow. Delivery is usually via an air blower mounted
on the delivery vehicle. The profiled interior of a built
store, angled at about 45°, guides the fuel to the extraction
auger. Flexible bag and glass reinforced plastic (GRP) silos
have pre-engineered profiles to guide pellets to a fuel transit
auger. Block built stores have profiling typically made from
plywood. A rubber impact absorbing mat hung from the
pellet store ceiling helps to minimise the damage to pellets
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Regulation/Code of Practice
Heat generators
Wood chip stores tend to be significantly larger due to the
lower energy density. Wood chips tends to lock together
inhibiting flow. A swinging arm driven by the extraction
auger can agitate a circular plane above a flat bottomed fuel
store and encourage chip to move across and into the
extraction auger. For larger stores (and larger chip sizes) a
walking floor arrangement can be used to move chips
towards the auger. Delivery is typically via tipping directly
into the store, requiring the store entrance to be at a lower
level than the delivery vehicle. Weather proofing of the lid
or door closing the store entrance is important.
For both wood pellet and wood chip the sizing, design and
arrangement of the fuel store suitably close to the biomass
boiler is critical to reliable operation. This is best achieved
by using specialist advice during the design stage as once
installed it is difficult to improve on an overly long auger
route, tight radius auger /fuel delivery pipe path,
inconvenient fuel store lid, a store that is undersized
compared to the fuel usage rate and the preferred fuel
delivery volume/frequency or a store with inconvenient
delivery vehicle access.
Safety
Biomass systems present certain unique safety issues:
(a) Explosion risk
There are 3 areas that may present a risk of explosion:
——
——
Within the delivery/vent pipes and within the fuel store:
Particularly during pneumatic delivery of wood
pellets combustible fuel dust can reach high
concentrations suspended within the air in the
store. To minimise the combustion risk any
potential ignition sources (motors, switches etc.)
should be located outside the store. As a static
electrical charge can build up during fuel delivery,
all conducting components should be earthed and
the fuel delivery and vent pipes should be made
from an electrically conductive material. If the store
cannot be illuminated via windows from outside
the store then any light or level sensor within the
store should be ATEX 94/9/EC ‘explosion proof ’
rated and comply with BS EN 1127 (2011).
In the plant room: especially during pellet delivery
fuel dust should be controlled so as to not cause a
hazard or nuisance in the plant room. Built fuel
stores should be lined with ply wood and the joints
sealed. Flexible bag fuel stores should be made of a
material capable of retaining the fuel dust. Fuel
delivery and vent pipes should have sealed joints
and the vent pipe fitted with a vent sock outside the
building. This is all the more important as it is
impossible to remove all ignition sources from the
plant room.
——
Within the flue/chimney: potentially, unburned fuel
particles may be carried through the boiler to mix
with hot particles or gases in the flue or chimney.
The boiler design should prevent this happening
and using a draft diverter with integral explosion
relief, and indicator, reduces the damage should any
explosion occur. Thought should be given to the
direction of the discharge of the hot gases should
the relief valve ever be activated.
(b) Fire risk
There are two areas where risk of fire must be considered:
——
The fuel store contains a large quantity of fuel that
may cause damage to a building and to health if
allowed to burn in an uncontrolled manner. The
auto ignition point for wood fuel is about 300 ºC.
Avoidance of ignition sources and using heatintumescent seals where augers pass through walls
and on door seals helps to limit the fire risk.
——
Burn back from biomass boiler combustion is
typically controlled by adjusting the fuel delivery
quantity and rate, controlling the air flow to be
from the auger towards the burner, using thermally
triggered water dousing in case of excess temperature
in the feed auger and using fire resistant valves to
control of the fuel feed.
(c) Enclosed space
For either wood pellets or wood chips the fuel store typically
forms an enclosed space and so thought must be given to
providing sufficient ventilation prior to and during any
period where maintenance or operation personnel may
need to enter the store. Ladders may need to be built into
underground stores and entrance may require oxygen level
checking and using teams of more than one. Further details
of the properties of wood fuels and the design of storage
facilities are provided in CIBSE AM15: Biomass heating.
1.7
Heat generators
This section describes the wide range of heat generators
that are available to the heating systems designer together
with the principles of combustion, regulations to limit air
pollution arising from combustion and the design of flues
and chimneys.
1.7.1
Choice of heat source
Table 1.31 lists a number of types of heat generator together
with an indication of their comparative characteristics.
If available, consideration should be given to taking a
supply of heat from an existing district heating (dh)
scheme. Heat supplied in this way may be commercially
attractive and may also have significantly lower
environmental impact than using boilers or even on-site
chp. Most dh schemes either make use of large-scale chp
or energy from waste plants or even surplus heat from
industrial processes. The low CO2 content of heat from
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
and the store wall upon delivery. Wood pellets having a
higher energy density can lead to more compact fuel stores.
Consideration should be given to keeping the wood pellets
dry as pellets that have become wet, due to accident,
flooding or vandalism, expand and degrade. Wet pellets
may block handling equipment or damage the store/
building. Constructing part of the store to relieve the stress
of expansion and providing access for manual removal of
wet fuel may be required. Explosion relief may also be
considered, particularly for block built pellet stores where
the consequences of an explosion would be more severe.
1-59
1-60
Heating
Table 1.31 Comparative characteristics of various types of heat generators
Type of heat generator
Typical flow or leaving
temperature range / °C
CO2 emissions
NOX emissions
Capital cost
/ (gCO2/kW·h)
/ (g/kW·h)
/£
35-55
Low
High
Medium
Ground source heat pump
(electric)
35-55(1)
Low(3)
High(3)
High
Solar thermal
50-80
Very low
Very low
Medium
50-80
Medium
Low
Low
80-90
Medium
Low
Low
Biomass boiler
80-90
Very low
Medium
High
Combined heat and power (gas
spark ignition engine)
80-90
Low(2)
Low(2)
High
m/hthw boiler (4)
90-120
Medium
Medium
High
Steam boiler
100-170
Medium
Medium
High
35 – 2500 (quartz lamps)
High(3)
High(3)
Low
Condensing boiler (4)
High efficiency boiler
(4)
(4)
Direct electric heating
(3)
(3)
(1)
Higher temperatures can be achieved by either utilising the superheated refrigerant gas leaving the compressor though the quantity of heat is limited
or by using CO2 as a refrigerant though this requires a low temperature heat source.
This is based on the total energy (heat and electricity) generated and offsetting some the CO2 and NOX emissions against those saved by displacing
grid electricity. UK data for average CO2 and NOX emissions from power stations has been used (DECC, 2012).
(2)
(3)
Based on use of UK grid electricity (see note 2).
(4)
Assumes oil or gas-fired.
such sources can contribute significantly to achieving an
environmental target for a building.
Where a supply of dh is available, connection to the dh
main may be either direct, or indirect via a heat exchanger.
Direct connection is normally used in small heat
distribution systems where heat is distributed at
temperatures not exceeding 90 °C, e.g. using heat from a
chp unit based on an internal combustion engine. For
indirect connection, the role of the lead boiler is effectively
assumed by a plate heat exchanger. This allows the
distribution system within the building to be run at a
temperature and pressure suitable for the building rather
than for the dh supply.
The heating distribution system controls and heat emitters
in the building can be similar to those used with a boiler
however designs should adopt variable volume control and
achieve as low return temperatures as possible as this will
benefit the dh operation. With low return temperatures the
dh system can provide lower volumes and hence use smaller
pipes and heat losses from the return pipe will also be lower.
It is recommended that the dh company should be allowed
to review and comment on the design of the connection
method and the heating system.
Further information on connecting to heating networks is
provided in section 1.13. Detailed guidance on the design
and operation of DH may be found in the Technical Guide to
District Heating published by the Building Research
Establishment (Wiltshire, 2014).
1.7.2
Boilers
Boilers are available in a large range of types and sizes and
almost all non-domestic hydronic and steam heating
systems rely on one or more boilers. Boiler efficiency has
improved markedly over the past two decades. Technical
developments have included:
——
the use of new materials
——
increased heat exchanger surface areas
——
reduced water content
——
exploitation of the condensing principle
——
gas-air modulation
efficiency
——
modularisation to optimise system sizing.
to
improve
combustion
These developments have resulted in considerable
improvements in performance at part load, with
considerable benefit to seasonal efficiency.
1.7.2.1
Boiler selection
The following factors need to be taken into account in
selecting a boiler for a particular application:
——
output in relation to calculated system requirements
(see section 1.5)
——
efficiency, particularly at part load
——
hydraulic pressure at which the boiler must operate
——
system operating temperature: it is particularly
important that return water be maintained above
the minimum recommended by the manufacturer
for non-condensing oil-fired boilers to avoid
corrosion from acid condensation in the flue system
——
flue gas conditions, to comply with emission
requirements
——
corrosion and water treatment, taking account of
the specific recommendations of the boiler
manufacturer
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Air source heat pump (electric)
(1)
Heat generators
acoustic considerations, taking account of noise
both inside and outside the boiler room
——
floor temperature beneath the boiler: the
temperature of a concrete floor should not be
allowed to exceed 65 °C; this should not occur
where the base of the boiler is water cooled, but may
otherwise require a refractory hearth under the
boiler
——
space in the boiler house, especially with regard to
access for maintenance
——
access for initial installation and subsequent
replacement.
Boiler types
1.7.2.2
There are a number of distinct types of boiler, characterised
by the materials used in their construction which has
impacts on:
——
return water temperature restrictions and ability to
recover latent heat from flue gases
——
minimum water flow rates
——
water treatment
——
working pressures
——
range of duties available.
Condensing boilers
The flue gases produced include water vapour. This has a
high energy content. Condensing boilers are designed to
condense the water vapour and so recover the latent heat. A
drain to remove the condensate is therefore necessary.
Condensation is achieved by either extending the size of the
heat exchanger (or providing an additional heat exchanger),
and reducing the design return water temperature so that
part of the heat exchanger surface on the gas-side falls below
the dew point temperature of the flue gas. Condensation
commences with return water temperatures of about 55 ºC
with the condensation rate increasing as the return water
temperature is depressed. At the same time, the increased
heat exchanger surface and lower return water temperature
also increases the sensible heat recovered. As a result,
condensing boilers have efficiencies some 10 percentage
points higher than non-condensing boilers when operating
with low return water temperatures.
The relatively cool combustion gases lack buoyancy and it
is usual to have additional fan power, either as a separate
fan in the flue or increased fan-burner pressure, to assist the
evacuation of flue gases. However this creates a positive
pressure in the flue, so flue components must have joints
between components that are both pressure tight and water
tight, to retain condensate in the flue.
Some manufacturers of condensing boilers provide two
return water connections, one for return water at subcondensing temperatures and one for above-condensing
temperatures. A single return is simplest, however, mixing
the return flows from low temperature, compensated and
constant temperature circuits usually results in a return
temperature that precludes condensing. With as little as
10% of the total return water mass flow rate being cool, a
boiler may deliver full condensing efficiency even without
reducing its flow temperature. In this way maximum use
can be made of low temperature water returning from low
temperature circuits rather than mixing it with warmer
water prior to entering the boiler(s).
The materials used to construct the heat exchanger must be
able to withstand the slightly acidic condensate; stainless
steel is frequently used for these heat exchangers as is
aluminium. However, aluminium on the water side may
require specialised water treatment. Institution of Gas
Engineers and Managers publication IGEM UP/10 (IGEM
2014b) gives detailed advice on the use of stainless steel
flues and plastic condensate pipes. The acidity is caused by
both the presence of carbon dioxide in the flue gas and, in
the case of sulphur-bearing fuels, sulphur dioxide. The
latter gives rise to a sulphurous/sulphuric acid which is
very corrosive. Advice should be sought on discharging the
condensate to drain. Natural gas is very low in sulphur and
the majority of new gas boilers are now of the condensing
type. Oil-fired condensing boilers are available but should
be used only with very low sulphur content fuel oils.
Non-condensing boilers require that the return water
temperature is maintained sufficiently high so that the heat
exchanger remains above the dew-point of the flue gas.
Manufacturer’s guidance should be followed to ensure that
they are installed and operated so that condensation does
not occur which could lead to corrosion. Particular care
needs to be taken when starting such boilers from cold.
Cast iron sectional boilers
Boilers of this type are constructed out of sections joined by
barrel nipples, with the number of sections selected to
produce the required output. They are normally operated at
pressures below 350 kPa and have outputs of up to 1500 kW.
Where access is limited, the boiler may be delivered in
sections and assembled on site. It is important that water
flow be maintained at all times to meet the manufacturer’s
recommendations, including a period after shut-down to
disperse residual heat. Return water is usually required to
be above the flue gas dew point to avoid condensation
forming and increasing the corrosion rate unacceptably.
Boilers of this type are covered by BS 779 (BSI, 1989).
Low carbon steel sectional boilers
These are similar to cast iron boilers except that their
sections are made of steel. Similar recommendations apply.
Welded steel and reverse flame boilers
Welded steel and reverse flame boilers are fabricated from
steel plate. The combustion chamber is pressurised and a
‘blind’ rear end reverses the burner discharge back over the
flame, in counter-flow. The gases then pass through a
circumferential ring of fire tubes around the combustion
chamber. This arrangement achieves high efficiency and
compactness. They are typically designed for a maximum
working pressure of 450 kPa (3.5 barG) but can be designed
to operate at up to 1 MPa (9 barG), with outputs between
100 kW and 3 MW. Boilers of this type are covered by BS
855 (BSI, 1990). Some designs allow low temperature
returns, without condensing, but most require protection
from return temperatures lower than the flue gas dew point.
Steel shell and fire-tube boilers
Steel shell and fire-tube boilers consist of a steel shell and a
furnace tube connected to the rear combustion chamber,
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
1-61
1-62
Heating
Modular boilers
Gas boilers rely on various different types of burner, as
follows.
1.7.3.1
Modular boilers are packaged installations comprising a
relatively large number of small, individual boilers of
typically about 50–200 kW, available with on/off or
modulating control. These are supplied with an integral
sequential control system that detects the heat load on the
system and fires up individual boilers into the circuit as
required. The result is that a very high turndown is possible
with the load on each boiler firing remaining high even
when the system load is low, leading to improved operating
efficiency. Non-condensing variants should be protected
from cool return water. minimum flow rates are often
required.
Low water content boilers
Low water content boilers have compact heat exchangers
designed for maximum surface area. Common materials for
heat exchangers include aluminium, copper and stain­less
steel. Both natural and forced draught combustion types
are available. Low water content boilers offer rapid heat-up
and high efficiency coupled with compact size and low
weight.
Good water circulation through the heat exchanger is
essential during boiler operation and a means of flow
sensing is usually required, interlocked with the burner.
The water-side pressure drop is generally much greater
than for high water-content boilers. Combined with the
need to maintain a relatively high minimum water flow
rate, this will lead to increased pump energy consumption.
Life expectancy is usually significantly shorter than for cast
iron or steel boilers with larger combustion chambers and
higher water content.
1.7.3
designers and installers. Guidance on installation is
provided in IGEM UP/10 (IGEM 2014b), and BS 6644
which also include information on ventilation and flues for
appliances with a net output above 70 kW.
Gas-fired boilers
Gas boilers are available in a large range of types and sizes
for use with both natural gas and liquefied petroleum gas
(lpg). The properties of both types of gas are described in
section 1.6. Modern appliances are designed and
manufactured in compliance with European standards. BS
EN 15502-1, BS EN 15502-2-1 and BS EN 15502-2-2
specifies the common requirements and test methods
concerning construction, safety, fitness for purpose, and use
of energy, as well as the classification and marking of gasfired central heating boilers up to 1000 kW output. Under
European gas safety legislation, all new appliances must
display a CE mark of conformity; to install appliances not
having the CE mark or to modify appliances displaying the
mark may be unlawful. Strict requirements for gas safety
apply similarly to forced draught and natural draught
burners.
Appliance standards deal not only with construction but
also cover efficiency and emissions to the atmosphere.
However, standards cannot easily cover the quality of the
installation, which is the responsibility of competent
Forced draught burners
Typically of the nozzle mix type burner in which gas and air
are supplied separately right up to the burner head, where
mixing then takes place. The air is supplied by means of a
centrifugal fan, the gas by means of the gas pressure. In
some instances, the fan provides just sufficient pressure to
overcome the pressure drop through the burner (sometimes
referred to as fan-assisted). In others the fan will also
overcome the air resistance through the boiler. This has an
impact on the pressure relative to atmosphere at the flue
connection and subsequently flue design.
The effectiveness of the combustion process relies on the
design of the mixing head and the pressure of the air and
gas at the head, particularly in achieving low emissions of
nitrogen oxides (NOX) and carbon monoxide (CO). Most
burners are made to comply with BS EN 676: 2003+A2 2008
(BSI, 2003). It is rare today to see a burner with a separate
pilot since most start at a low fire condition at the main
burner. Air proving is essential with a ‘no-air’ check being
made before the fan starts, to check that the proving switch/
transistor is operational. The combustion system is
normally purged with up to 5 volumes of air in order to
remove any traces of gas or remaining products of
combustion prior to firing up. The gas safety train to the
main burner supply incorporates a low inlet pressure
switch, a pressure regulator and two high quality safety
shut off valves. Above 1200 kW there is a requirement for
either a valve seat condition proving system or a double
block and vent valve position proving.
Forced draught (or pressure jet) burners are available with
on/off, high/low or fully modulating control. High/low is
typically 100–50% of maximum output. Modulating
burners traditionally offer modulation between 100% and
50% of maximum output. This 2:1 turndown ratio has been
extended to 4:1 or even greater and can be an advantage on
condensing boilers but will lead to cooler flue gases at low
fire and for steel or cast iron boilers can lead to back end
corrosion. In this case burners may have to be operated
with limited turndown to avoid reducing the service life of
the boiler. Additionally the CO emissions are significantly
higher at low fire.
Some larger burners require higher gas pressures than are
available from the gas supply system. In such cases, a gas
pressure booster may be required, which is typically
provided by a simple centrifugal fan. Overall safety
requirements are covered by IGEM UP/2 (IGEM, 2014a);
they include a stainless steel flexible pipe either side of each
booster and a pressure switch to cut off the booster at low
line pressure.
It is possible for forced draught burners to operate in dual
fuel mode, using an additional nozzle for oil firing. Larger
types of dual fuel burner may incorporate a rotary or
spinning cup to atomise the oil but many simply rely on
high oil pressures at the atomiser.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
from which convection tubes are taken to provide two-pass
or three-pass operation. Boilers of this type are suitable for
pressures up to 1 MPa and are available with outputs up to
12 MW and are often used for steam applications. The
relevant standards are the BS EN 12953 series (2003). Backend protection from cool return water temperatures are
normally required.
Heat generators
Pre-mix burners
Pre-mix burners tend to be packaged with a particular
boiler and are available across a more limited range of up to
1 MW maximum output. More extensive turndown ratios
are available at up to 5:1 as part of a condensing boiler.
Wider turndown ratios are possible but limited by increased
CO emissions at very low fire as shown in Figure 1.18.
1.7.3.3
Natural draught (atmospheric)
burners
Atmospheric burners have the advantage of being simple
and less costly to manufacture leading to large numbers
currently being in use. The incoming gas at the injector
induces combustion air with which it mixes before reaching
the head. The amount of air induced is typically 40 to 50%
of what is required and the remainder is drawn in by the
draught created by the combustion process itself. Because
of its slow and staged mixing, the flame envelope is larger
and requires a larger combustion chamber than forced
draught and pre-mix burners. Boiler heat exchangers also
need to be of low resistance on the flue gas side limiting the
maximum efficiency possible. Adding a fan in the flue can
allow secondary heat exchangers to be added but since the
advent of pre-mix burners and with increasing minimum
efficiency and lower emission requirements, they are
infrequently installed.
1.7.3.4
80
70
60
50
40
30
20
10
0
NO(x)
CO
10
0
1.7.4
Oil fired boilers
1.7.4.1
Installation
Guidance on the installation requirements for boilers of
45 kW and above, including flue termination and
combustion and ventilation air supply, is given in BS 5410-2
(2013). In particular, where boilers are to be installed in a
building they should be sited within a fire rated boiler
room. The boiler room should house the boiler and
associated boiler plant only.
40
50
60
Output / %
70
80
90
100
Burners for oil boilers almost always rely on atomisation,
which is carried out mechanically. Oil of various grades is
used for firing: gas oil (Class D) is most frequently used in
commercial heating installations though kerosene (Class
C2) is used for most oil-fired condensing boilers; fuel oil
(Classes E, F and G) is used in some large installations.
There are two principle types of atomising burner, both of
which are forced draught and may need acoustic attenuation.
Guidance on the installation requirements for oil fired
boilers is provided by OFTEC:
——
for (pressure jet burners up to 70 kW): OFTEC
Technical Book 2: Domestic and Light Commercial
Servicing and Commissioning’ (OFTEC, 2010a)
——
for pressure jet burners less than 2 MW: OFTEC
Technical Book 7: Commercial Servicing and
Commissioning (OFTEC, 2010c)
——
for oil boilers up to and above 45 kW output:
OFTEC Technical Book 4: Oil Fired Appliance and
System Installation Requirements (OFTEC, 2012c).
1.7.4.2
Pressure jet burners
These are the most frequently used. They consist of a fan to
provide combustion air and to mix it with atomised droplets
of oil produced by a nozzle fed at a high pressure from a fuel
pump. Since effective atomisation depends on the flow of
oil to the nozzle, the turndown ratio is limited to about 3:1
though multiple nozzles can be installed. Turndown ratios
in excess of 4:1 are available but minimum flue gas
temperatures to avoid corrosion often restrict their
application.
1.7.4.3
In the UK, Guidance on the installation requirements for
boilers up to 45 kW output for Class C2 and Class D oil,
including flue termination and combustion and ventilation
air supply, is given in Approved Document J 2010 of the
Building Regulations (England) (DCLG 2010) .
30
Figure 1.18 Variation of CO and NOx emissions with turndown for
pre-mix burners
Pulse combustion
Air is induced into the combustion system by means of the
Helmholtz effect. The rapid forward flow of the exploding
combustion products within a strong chamber leaves a
shock wave behind that induces the gas and air required for
the next pulse, which ignites automatically. The cycle
continues until the gas supply is turned off. Pulse
combustion operates at high pressure and enables very
small heat exchangers and flues to be used. Pulse combustion
boilers are more common in North America than Europe.
20
Rotary burners
These are used only on very large boilers of the welded shell
type, where fuel heavier than Grade D is burned.
Atomisation is achieved by centrifugal action as oil is fed to
a rotating cup, which throws droplets into an air stream
produced by the primary combustion air fan. A secondary
combustion air fan enables the burner to operate over a
wide turndown range, which may be up to 5:1. This type of
burner can be readily adapted for dual fuel (gas/oil)
operation.
1.7.4.4
Part load operation
Single stage (on/off) burners operate with a single heat
output. They would be equipped with a single oil nozzle,
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
These differ from forced draught burners principally in
that the air for combustion is mixed with the gas before it
reaches the burner head. They produce very short intense
flames that can work in very compact combustion chambers
and, due to lower excess air levels, can achieve higher
efficiencies.
NOx / CO in / mg/m3
1.7.3.2
1-63
1-64
Heating
one oil pressure setting and a fixed air setting. The burner
would be controlled via the appliance control thermostat.
Three stage burners operate with three levels of heat output.
These burners would be equipped with three nozzles and
usually operate with a single oil pressure setting. The
combustion air may be controlled by an electrical air
damper motor, a hydraulic ram or a combination of the two
devices. The control of these burners can be by means of
individual thermostats or by a specifically designed control
system. The burner requires to be commissioned at each of
the three stages. This type of burner would more commonly
be found in process applications where a more precise
control of the heat input to the appliance is necessary.
A modulating burner does not operate at fixed heat outputs
as with one, two and three stage burners, but it can operate
anywhere between a set minimum and maximum level.
The burner would commonly be equipped with a single
nozzle (spillback nozzle) with the pump set to give suitable
atomising pressure and the nozzle delivery controlled via
the setting of the return pressure from the nozzle.
1.7.4.5
Blue flame
Historically, oil boilers using pressure jet burners have
commonly used burners that produce a bright yellow flame.
As a result of Government commitment to lower greenhouse
gases, oil pressure jet burners that use ‘blue flame’ technology
have been developed and introduced to lower the level of
pollutant gases emitted from the appliance during the
combustion process. They operate in a similar manner as
yellow flame burners with the main difference being the
configuration of the combustion head and the presence of an
ionisation probe as opposed to a photoelectric cell.
1.7.5
Solid fuel boilers (general)
Solid fuel burners are less flexible in use than those for
gaseous or liquid fuels and consideration must be given at
an early stage to arrangements for the storage and handling
of fuel, the removal of ash and grit, flue gas cleaning and
operation and maintenance of the boiler house. Also, it is
necessary to design the system to ensure that heat can be
safely dissipated when the boiler is shut down or the load
sharply reduced. For detailed information on biomass
boilers see CIBSE AM15: Biomass heating (CIBSE, 2014).
There are a number of burner designs used for boilers
serving heating systems in buildings as described below.
1.7.5.1
Underfeed stokers
Most commonly used for sectional and fabricated steel
boilers operating at outputs up to 1.5 MW. The fuel is
supplied through a tube using an auger, regulated to match
1.7.5.2
Coking stokers
Used with shell boilers rated at up to 4.5 MW. A ram pushes
fuel from a hopper into the boiler, where there is partial
distillation of the volatile components of the coal. The fuel
then travels forward into a moving grate where combustion
is completed, relying on induced draught.
1.7.5.3
Chain grate stokers
Used in large shell boilers, with outputs of up to 10 MW. An
endless chain grate feeds fuel continuously into the boiler
furnace, where combustion takes place with either forced or
induced air supply.
1.7.6
Solid fuel boilers (biomass)
1.7.6.1
Selection
This is primarily dictated by the expected heat demand and
the intended fuel type. Although some biomass boilers can
modulate, they are inherently most efficient and lowest
polluting when run at high output with as few stops and
starts as possible. As well as a building’s peak heat load,
knowledge of its expected base/summer load and daily load
profile should be sought. Wood pellet and wood chip up to
about 30% moisture content can be burnt in similar
underfed stoker boiler designs. Higher wood chip moisture
content necessitates increased drying space and time within
the boiler and so is better burned in moving grate design
boilers.
Buffer vessels are often used to reduce boiler cycling as well
as to avoid over temperature situations where the load
reduces faster than the turndown rate of the boiler.
Thermal stores are used to extend run hours by load
shifting, their size being optimised on the energy storage
required, the cost of the vessel, the space available, the
reduction in biomass boiler peak output and the load
profile. It is normal to select a biomass boiler to run as the
lead heat generator and to have gas or oil fired boilers
provide peak load and back-up heat with a lower space
requirement and installed cost per kW than biomass
boilers.
1.7.6.2
Emissions
These include particulates, NOx and SOx. Modern biomass
boilers with control of combustion air have reduced
particulate output. Where boilers require particulate
abatement, cyclonic separators can be fitted to the flue to
remove particulates to PM10 size. For further reduction
ceramic filters are used to remove all particulates above the
size PM2.5. NOx is minimised by control of the combustion
temperature, dwell time and excess air. SOx is minimised by
using fuels low in sulphur. Ash is a benign by-product of
biomass combustion as long as the fuel does not contain
contaminants such as paint, plastics or glue. Biomass
boilers must comply with the Clean Air Act and local
planning requirements (see section 1.7.7)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Two stage (high/low) burners operate with two levels of heat
output. They would be equipped with either two nozzles
and a single oil pressure setting or with one nozzle and two
pressure settings. The two levels of combustion air would
be controlled by either an electric air damper motor or by
hydraulic ram. The operation of the burner requires a
second control thermostat to allow the burner to move from
one stage to the other. The burner requires to be
commissioned at the first and second stage. This type of
burner is commonly used in commercial applications.
the requirements of the furnace, and combustion air is
controlled by fans. Fuel types and grades may be restricted.
Heat generators
1-65
Waste gases
Waste gases
2nd
pass
Flue
Furnace
tube
2nd
pass
Flue
1st
pass
Furnace
tube
1st
pass
Figure 1.19(a) Two pass dry back shell boiler (courtesy of Spirax Sarco
Ltd)
Figure 1.19(b) Two pass wet back shell boiler (courtesy of Spirax Sarco
Ltd)
1.7.6.3
chamber is contained entirely within the boiler, which
improves the efficiency of heat transfer. Modern packaged
boilers commonly use three passes to achieve high efficiency
and compact dimensions. Shell boilers are covered by
BS EN 12953 series (2003).
Maintenance
Although modern boilers include features such as assisted
ignition, assisted ash removal and automated heat exchanger
cleaning, they still require regular attention and the
emptying of ash bins. The frequency of maintenance
requirements is affected greatly by the quality of the wood
fuel used and the number of stop/start cycles performed.
1.7.6.4
Heat meters
In the UK heat meters are a requirement for any renewable
energy installation in non-domestic properties under
Building Regulations.
They are also required to prove on-site renewable heat
generation under the Renewable Heat Incentive (RHI) (see
section 1.2.9). The Building and Engineering Services
Association has published a Good Practice Guide to heat
metering (BESA, 2012a) that is intended to assist in meeting
the requirements of the RHI. CIBSE have also produced
TM39: Building energy metering.
1.7.7
Water tube boilers differ from shell boilers in that the heat
source surrounds tubes circulating the boiler water, see
Figure 1.20. They are able to operate at higher pressures
than shell boilers because the tube diameters are much
lower than those of the shell of a shell boiler, with
corresponding reductions in the hoop stress. As shell
boilers are limited in practice to pressures below 27 bar
(gauge), or a steam temperature of 340 °C, water tube types
tend to be used for applications requiring high pressure,
high temperature or very large steam output. Water tube
boilers are available in smaller sizes but offer no advantage
over shell boilers for most commercial and industrial
applications involving heating. Water tube boilers are
covered by BS EN 12952 (2012)..
Coil type steam generators are a form of low water-content
boiler, similar to a flash steam boiler, see Figure 1.21. The
Steam boilers
Steam
Boiler or
steam drum
A steam boiler differs from a water circuit boiler in that it
produces a phase change from water to steam, which
introduces additional requirements for the control of both
the pressure and the water level within the boiler. Also, as a
pressurised vessel containing water and steam at above
100 °C, it requires greater attention to the maintenance of
safety in operation.
Steam boilers may be broadly classified into three types:
——
shell (or fire tube) boilers (these can be horizontal
or lower water content compact vertical types)
——
water tube boilers
——
coil type steam generators.
Shell boilers operate by passing heated gases through tubes
in the boiler. Figures 1.19(a) and 1.19(b) show typical shell
boiler configurations for ‘two-pass’ operation, in which the
heated gases from the furnace are reversed to flow through
the boiler for a second pass to extract more heat. In the ‘dry
back’ configuration, the flow is reversed by a refractory
lined chamber; in the ‘wet back’ version the reversal
Water
Heat
Riser
Downcomer
Lower,
water drum
or mud drum
Figure 1.20 Water tube boiler (courtesy of Spirax Sarco Ltd)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Dry back
reversal
chamber
Wet back
reversal
chamber
1-66
Heating
15 bar(g)
120
5 bar(g)
10 bar(g)
100
80
0 bar(g)
60
40
20
0
80
85
90
95
100
105
Percentage of 'from and at' rating / %
110
Figure 1.22 From and at variation with feedwater temperature
(reproduced courtesy of Spirax Sarco)
equivalent to 627 W. In practice, boilers are operated under
a range of conditions and the steam output under different
conditions may be calculated using steam tables or
estimated from the graph shown in Figure 1.22. ‘From and
at’ ratings are widely used by manufacturers of shell boilers.
Some manufacturers give boiler ratings in kW. Steam
output may be calculated from the difference between the
specific enthalpy of the feed-water and the steam at the
required pressure.
Figure 1.21 Coil type steam generator (reproduced courtesy of Babcock
Wanson)
usual construction is a spiral coil of water-tube, arranged
vertically as a single, or monotube, coil. Circulation is oncethrough and pumped under pressure, as a forced-circulation
boiler. They have narrow-tube construction, without any
large-diameter drums or tanks which means that they are
safe from the effects of explosion as they do not contain a
pressure vessel. The pump is of the positive displacement
type and the flowrate is adjustable according to the quantity
of steam required at that time and the burner output is
throttled to maintain a constant working temperature. The
burner output required varies according to the quantity of
water being evaporated. They are very compact (they can be
half the footprint of a shell boiler) can produce dry saturated
steam from a cold start in a few minutes and have low
surface heat losses.
1.7.7.1
Steam boiler rating methods
Steam boiler output depends on operating conditions and
is rated in three ways:
——
‘from and at’ rating
——
kW rating
——
boiler horse power (bohp).
The ‘from and at’ rating is based on the amount of steam (in
kg) at 100 °C and atmospheric pressure that the boiler can
generate in 1 hour from a feed water temperature of 100 °C.
Under these conditions, each kilogram of water requires
2258 kJ of heat per hour to convert it to steam, which is
‘Boiler horse power’ tends to be used only in the USA,
Australia and New Zealand and should not be confused
with the imperial unit of power, which is approximately
746 W. In Australia and the USA, 1 bohp is defined as the
power required to evaporate 34.5 pounds of water per hour
at 212 °F at atmospheric pressure. This is essentially the
same form of definition as the ‘from and at’ rating, except
that it is based on 34.5 lb instead on 1 kg; 1 bohp is
equivalent to 15.51 kg/hour. In New Zealand, bohp is
defined in terms of the heat transfer area of the boiler.
1.7.7.2
Efficiency
Steam boiler efficiency depends upon the design of the
boiler and the conditions under which it is operated. Some
boilers incorporate an ‘economiser’, which consists of an
additional heat exchanger using exhaust gases to preheat
the feed-water before it is returned to the boiler. However,
economisers may not be used on boilers with on-off level
controls. Efficiency in steam systems also depends on
minimising heat losses from the boiler feed tank (which
should be well insulated to prevent heat losses), minimising
blow-down losses, which can be done by the use of RO
water as noted in section 1.9.3.3 and recovering as much
heat as possible from the blow-down.
1.7.7.3
Safety devices
Steam boilers must be fitted with appropriate safety devices.
In the UK, these are currently covered by BS 759-1 (BSI,
1984). Each boiler must have a name plate, with a serial
number and model number which uniquely identifies it
and its manufacturer and gives details of various tests to
which it has been subjected. It must also be fitted with a
safety valve to protect it from overpressure and the risk of
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Feedwater temperature / °C
150
140
Heat generators
1-67
Glass
Steam
cock
Water level
Protector
shields
Drain
cock
Water
cock
Figure 1.24 Typical steam boiler gauge glass (courtesy of Spirax Sarco)
explosion; in the UK, BS 6759 (1994) covers safety valves
for steam boilers. Safety valves are also covered by BS EN
12953 (2003).
A typical safety valve is shown in Figure 1.23. Safety valves
must be capable of discharging the full ‘from and at 100 °C’
capacity of the boiler within 110% of the design boiler
pressure and be set at no higher than the design pressure.
At least one safety valve is required for all boilers; boilers
rated at more than 3700 kg·h–1 are required to have two
single safety valves or one double safety valve. The discharge
pipe from the safety valve must have no obstructions and be
drained at the base to ensure that condensate cannot
accumulate. Each boiler must also be fitted with a stop
valve (or crown valve) to isolate it from the plant it serves.
This should always be fully open or fully closed, and should
not be used as a throttling valve.
Other safety equipment required by steam boilers includes:
——
a feed check valve to prevent return flow from the
boiler when the feed pump is not operating
——
a bottom blow-down valve, which may be manual
or automatic in operation
——
a pressure indicator, which may be a simple
Bourdon gauge with a dial of least 150 mm in
diameter
——
a gauge glass to show the level of water in the boiler
(see Figure 1.24). In the UK, gauge glasses should
comply with BS 3463 (BSI, 1975).
Combustion of fuels
1.7.8.1
General principles
Combustion is the rapid oxidation of a fuel (normally
hydrocarbon based) to release heat (an exothermic reaction)
and that yields various products. In an ideal reaction all of
the molecules of carbon and hydrogen in the fuel combine
with oxygen, which requires perfect mixing of fuel and
oxygen in a combustion chamber. The oxygen is normally
supplied by air from the surrounding atmosphere, which
contains only about 23% oxygen (by mass) with the rest
being (predominantly) nitrogen. This creates a challenge
for the design of burners to ensure a good air/fuel mix to
ensure complete combustion. This design will clearly vary
on whether the fuel is solid (such as coal or wood), liquid or
gaseous. For each fuel type, burners need to ensure good
mixing and suitable residence times to ensure correct
conditions are maintained for efficient operation. It is
possible to ensure complete combustion by supplying air in
excess of the ideal (or stoichiometric) amounts. However,
excess air dilutes the combustion products, reducing their
temperature, and allowing more heat to be carried away
from the process.
Heat is transferred from the combustion process to the
working fluid of the heating system (i.e. the primary water
flow) in two ways. The first is by radiant heat exchange
between the flame and the surfaces of the heat exchanger.
The second, and most important, is the convective exchange
between the hot products of combustion and the surface of
the heat exchanger. Thus burner/heat exchanger
configurations are highly important features of good boiler
designs, and will strongly influence overall efficiency of a
boiler. In addition good maintenance of the burner is
essential to ensure high efficiency is maintained over the
life of the plant.
The products of combustion need to be carried away from
the boiler and exhausted safely. In the case of the ideal
combustion of methane (CH4) we get
CH4 + 2O2 (+xN2) → CO2 + 2H2O (+xN2) (1.39)
where x is equal to approximately 7.5.
Where incomplete combustion occurs there is a danger that
the products will contain a proportion of the highly
poisonous carbon monoxide (CO), which will be both
highly inefficient and dangerous to health. This is why
there are stringent requirements for flues and chimneys to
safely disperse any toxic gases. The presence of carbon
deposits can be a very good telltale sign that poor combustion
is occurring — but this will also indicate the problem has
been going on for some time.
The presence of water vapour is also a challenge to flues.
Where the temperature in a flue falls below the dew point of
the water vapour this will condense. This condensate is
acidic and therefore consideration needs to be given to the
materials of the flueways.
Of course the condensing of flue gases also can be
advantageous, as about half of the heat in the flue gases is in
the form of latent heat of water. This can be recovered in
specially designed condensing boilers with suitable heat
exchangers and used in conjunction with heating systems
operating at low temperatures.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Figure 1.23 Typical steam boiler
safety valve (courtesy of Spirax
Sarco)
1.7.8
1-68
Heating
It is therefore important to know the constituent of a
particular fuel in order to understand its characteristics
during combustion, and the requirements for efficient and
safe operation of the plant. International agreements have
been reached regarding the emissions of sulphur dioxide
and other pollutants with most countries adopting national
regulations as discussed in the next section.
1.7.8.2
Combustion of gases
Combustion of gaseous fuels is the easiest to design for as
the fuel and air mix easily. Burners are relatively easy to
maintain, and may take the form of natural or forced
draught types. The available turndown depends upon
burner type (see section 1.7.3).
1.7.8.3
Liquid fuels
Liquid fuels must be converted to fine mist sprays in order
to obtain good mixing with supply air. This generally takes
place in pressure jet burners with forced air, which will
require higher level of maintenance to ensure jets do not
become blocked. Liquid fuels are generally more complex
hydrocarbons, but they may also contain other impurities
that may give unwanted products of combustion.
1.7.8.4
1.7.8.5
Most industrialised countries have adopted legislation,
such as a clean air act, to limit environmental pollution
from the combustion of fuels. Europe has produced a
number of directives that have resulted in member states
introducing or enhancing legislation that requires:
——
sulphur content of liquid fuels to be reduced
——
targets set for air quality
——
monitoring and reporting of air quality
——
limits set on emissions of various pollutants from
large combustion plant
The Sulphur Content of Liquid Fuels (SCLF) Directive
(EU 1999) has set targets for the reduction of sulphur
dioxide emissions from the combustion of heavy fuel oil
and gas oil. It limits the sulphur content in gas oil to 0.1%
and for heavy fuel oil to 1% by mass and requires member
states to monitor and report on sulphur content in such
fuels.
1.7.8.6
UK Regulations and Codes
In the UK several different strands of legislation are
relevant to combustion fuels and the design of flues and
chimneys, depending on the rated duty of the plant they
serve, the fuels used and where they are located.
Sulphur Content of Liquid Fuels (England and Wales) Regulations
2007
The European Directive is implemented in England and
Wales, through the Sulphur Content of Liquid Fuels
(England and Wales) Regulations 2007 (HMSO, 2007). In
2009, DEFRA reported that the average sulphur content of
heavy fuel oil was approximately 0.9% and that for gas oil
approximately 0.04% (DEFRA, 2012a).
The Building Regulations
Part J of the Building Regulations (England and Wales)
applies to all chimneys and flues, irrespective of the type of
building, or the capacity of the appliance they serve. It
includes the following requirements:
——
that sufficient combustion air is supplied for proper
operation of flues
——
that combustion products are not hazardous to
health
——
that no damage is caused by heat or fire to the fabric
of the building.
Solid fuels
Solid fuels present the biggest challenge to efficient
combustion. To ensure complete combustion the fuel must
remain in the chamber for a sufficient amount of time (the
residence time), which will in turn depend on the size of
the solid particle. Some large coal plant use pulverised fuel
that is almost a powder, but this is not common in central
heating boilers. Wood fuel (biomass) is the most common
solid fuel for modern heating applications, and the size of
particles may vary. There is better uniformity in wood
pellets than wood chips. This variation in particle size and
calorific value can lead to temperature variations throughout
the combustion chamber. This is particularly a problem in
mixed fuel combustion such as waste incineration. Uneven
temperature distribution can lead to high levels of NOx
production where hot spots occur.
Environmental legislation related to
combustion of fuels
Similar requirements are contained in Building Standards
(Scotland) Regulations (2011) and the Building Regulations
(Northern Ireland).
Approved Document J (DCLG, 2013) gives guidance on
how to satisfy the requirements of Part J. It also makes clear
that although Part J applies to all heat producing appliances,
the guidance in the Approved Document itself deals mainly
with domestic installations. Accordingly, the specific
guidance it contains is limited to solid fuel installations of
up to 50 kW rated output, gas installations of up to 70 kW
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Other unwanted combustion products may occur that can
be detrimental to health and the environment. At high
temperatures a small amount of the nitrogen present in the
combustion air will be oxidised to form various oxides of
nitrogen (NOx) which are both poisonous and have high
global warming potential. When the source of the nitrogen
is the combustion air only, the production of NOx can be
moderated or eliminated by good control of the combustion
process to ensure these temperatures are not reached in the
combustion chamber. When the fuel itself contains
nitrogen, for example biomass, reducing NOx emissions is
more difficult. Some fuels contain sulphur (certain fuel
oils, and some imported gases) which lead to acidic SOx
emissions that rapidly corrode flues and lead to acid rain.
The presence of complex organic molecules that contain
chlorine (e.g. plastics in municipal solid waste) can give rise
to highly poisonous dioxins and furans.
Heat generators
net (77.7 kW gross) rated input and oil installations of up to
45 kW rated heat output. The guidance includes:
the positioning of flues in relation to boundaries
and openings
——
protection from heat for persons likely to come into
contact with flues
——
the diameter of flues required for different types of
appliances
——
materials from which flues and chimneys may be
constructed
——
how chimneys may be lined to serve gas fired
appliances.
Environmental Protection Act
The Environmental Protection Act 1990 (HMSO, 1990)
gives powers to local authorities to control pollution from
industrial and other processes, which includes the
generation of heat and power. Large-scale (‘Part A’)
processes, with an output exceeding 50 MW, are subject to
control by the Environment Agency. Local authorities
control smaller scale (‘Part B’) processes, which may include
large boilers and chp units. One of the many requirements
is for the use of ‘best available techniques not entailing
excessive cost’ (‘ batneec’) to meet limits on levels of
contaminants in flue discharges.
The Environment Act
The Environment Act 1995 (HMSO, 1995a) includes
provisions for ‘local air quality management’ and sets air
quality standards for seven key urban pollutants: nitrogen
dioxide, carbon monoxide, sulphur dioxide, PM10 and PM2.5
particles, benzene, 1,3-butadene and lead. An area where
any of the standards are likely to be exceeded must be
designated as an ‘air quality management area’ and action
taken to reduce levels. This can lead to additional
restrictions on development in those areas.
Clean Air Act
Part 1 of the Clean Air Act 1993 (HMSO, 1993) prohibits
the emission of ‘dark smoke’, including emission from a
chimney of any building. Part 2 empowers the Secretary of
State to prescribe limits on the rates of emission of grit and
dust from the chimneys of furnaces, including boilers and
other heating appliances. The clean air act in relation to
chimney heights is discussed in section 1.7.12.
Some types of plant require additional considerations to
meet the requirements of the Environmental Protection
Act; reference should be made to HMIP Guidance Note
D1: Guidelines for Discharge Stack Heights for Polluting
Emissions (HMSO, 1993). CIBSE TM21 provides guidance
on minimising pollution at air intakes, including the
contribution made by chimneys and flues. For natural gas
and lpg, guidance may also be obtained from BS 6644
(2011) for boilers between 70 kW and 1.8 MW and Gas
appliances in industrial and commercial premises IGEM UP/10
(IGEM, 2014b).
LACORS and biomass boilers
Local Authorities in the UK are required by law to assess
air quality. Should air quality standards in any areas not be
met then they must designate those as Air Quality
Management Areas (AQMA). In such areas, planning
permission will only be given for developments that do not
adversely affect air quality. This includes the installation of
biomass boilers. Local Government Regulation (formerly
LACORS – Local Authority Coordinators of Regulatory
Services) have produced guidance for Local Authorities on
the potential air pollution from biomass boilers and what
measures should be incorporated in the design and
operation of such installations to reduce air pollution
(EPUK, 2009).
The burning of clean (virgin) wood products produces
oxides of nitrogen and sulphur as well as particulates, all of
which contribute to air pollution. Whilst the levels are
likely to be lower than with oil or coal, they will be higher
than for natural gas. As AQMAs are invariably within builtup areas, the guidance has the effect of favouring the
installation of biomass boilers in rural rather than urban
areas where there is likely to be a natural gas supply. The
burning of waste wood requires special attention and
permission as it may release highly toxic compounds.
Biomass boiler installations of below 20 MW (heat input)
are regulated under the Clean Air Act 1993 (Clean Air
Order 1981 for North Ireland), regulated by the local
authority (see above). In areas designated as Smoke Control
Areas, biomass boilers of less than 45 kW (heat input) must
be an ‘exempt appliance’. Where the boiler is designed to
burn more than 45.4kg per hour of biomass, then the
chimney height will need to be approved by the local
authority.
When making a planning application that includes a
biomass boiler of less than 20 MW, the local authority is
likely to request the following information:
——
details of the boiler(s)
——
description of the boiler flue/chimney
——
details of fuel to be used
——
building height and distance from other buildings
——
maintenance arrangements
——
description of fuel storage and fuel delivery.
Biomass boilers exceeding 20 MW require a permit to
operate under the Integrated Pollution Prevention and
Control system (Defra, 2011).
1.7.9
Heat pumps
1.7.9.1
General principles
In the refrigeration cycle, a liquid refrigerant evaporates at
low pressure and temperature. In the process it absorbs heat
from its surroundings (via the evaporator coil). The
pressure of the vapour is then increased and allowed to
condense at this higher pressure (and associated saturation
temperature) by rejecting heat to its surroundings (via the
condensing coil). In order to drive the cycle, some form of
high grade energy needs to be supplied so as to raise the
pressure of the refrigerant. This can be electrical or
mechanical energy (work) into a compressor or heat into an
absorber. Figure 1.25 shows the general arrangement. The
amount of heat rejected at the condenser is equal to the sum
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
1-69
1-70
Heating
of that absorbed at the evaporator and the energy absorbed
by the compressor/generator.
——
The temperature required by the heat load should
be as low as practicable so as to increase the cop.
Heat pumps are therefore best suited to low
temperature heating systems such as underfloor
heating and air systems.
——
The temperature of the heat source should be as
high as possible. During winter, space heating loads
increase but most heat source temperatures fall. As
a consequence, the cop tends to be lower during
colder months. An average cop over the heating
season, known as the seasonal cop (scop) or
seasonal performance factor (spf), is therefore used
to define performance based on the heating season.
The spf value includes all the relevant ancillaries
associated with the particular building, whereas
scop is associated with the heat pump unit itself.
Part L of the Building Regulations (England)
specifies both minimum values of cop at standard
conditions and minimum spf (see section 1.3).
——
In Europe, stated cop of a heat pump is referenced
to standard part load conditions (BS EN 14511)
(2013) and is determined from tests. The
methodology for determining the cop at a number
of standard conditions to an scop is defined in BS
EN 14825 (BSI, 2016).
The heat absorbed at the evaporator may come from a
variety of external sources such as outdoor air, lakes, rivers,
aquifers and the ground or waste heat sources such as
exhaust air in ventilation systems.
Heat pumps can be installed so as to provide simultaneous
heating and cooling. Alternatively, they can be designed in
such a way that the roles of the condenser and evaporator
can be changed over as required to provide either heating
or cooling. These latter are referred to as reversible heat
pumps. (Note that the UK Renewable Heat Incentive rules
exclude the use of air to air reversible heat pumps.)
Rather than using the term ‘efficiency’, the thermal
performance of the heat pump is referred to as the coefficient
of performance (cop) defined as:
cop = Φout / Pinput
(1.40)
Where Φout is the rate of heat output at condenser (W) and
Pinput is the power absorbed by the compressor/generator
(W).
The cop is strongly affected by the temperature difference
between the evaporating temperature and the condensing
temperature, reducing as the temperature difference
increases. The ideal cop can be expressed in the form:
(Tcond)
copc = ­­­ ——————
Tcond ­– Tevap
(1.41)
Where Tcond and Tevap are the absolute (Kelvin) temperatures
of the refrigerant at the condenser and evaporator
respectively and copc is known as the Carnot efficiency.
The value of the Carnot efficiency depends only on the
temperature levels between source and sink. The ratio of
the actual cop to the ideal value (cop/copc) is known as the
system efficiency index (sei).
The following points should be noted when selecting a heat
pump:
——
The temperatures in equation 1.41 are normally
those of the source and sink. The temperature of
the refrigerant in the condenser will need to be
above that of the heat load in order for heat to flow
into the heat load. Similarly, the temperature of the
refrigerant in the evaporator will need to be below
that of the heat source in order for heat to flow into
the evaporator. Both evaporator and condenser heat
1.7.9.2
Types of heat pump
There are two principle types of heat pump, characterised
by the refrigeration cycle used:
——
vapour compression cycle
——
absorption cycle.
Vapour compression cycle
These are by far the most common types of heat pump and
are available in a very wide range of sizes. Electricity is used
to drive a refrigerant compressor. This compresses the
refrigerant and pumps the refrigerant around the circuit,
maintaining the low and high pressure regions in the
evaporator and condenser respectively. Relatively high
cops can be achieved. Figure 1.26 shows how the cop varies
with condensing and evaporating temperature. The upper
and lower limits for each evaporating temperature
correspond to system efficiency index (sei) values (see
previous section) of 0.3 and 0.7, typical of vapour
compression heat pumps.
It should be noted that cop shown is based only on the
electricity used at the compressor. Electrical energy will
also be consumed by any fans and pumps associated with
the heat pump and the power for certain ancillaries must be
included when the cop is quoted in accordance with BS EN
14511 (2013).
Heat output may be regulated by simple on-off control of
the compressor or by speed regulation, usually using an
inverter. As with all refrigeration cycles, there is a minimum
time period required before a heat pump can be restarted.
With on/off control, consideration should be given to
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heat pumps are refrigeration systems where the heat
rejected from the condenser is the useful output rather than
the cooling at the evaporator. The heat output rate at the
condenser is greater than the rate at which energy is input
at the compressor/generator, and the performance is
expressed as the ratio of these terms, defined as coefficient
of performance (cop). This gives heat pumps an important
advantage over other forms of heating. Although nonrenewable energy may be used to drive the compressor/
generator, renewable heat is extracted from the source and
in some instances, this qualifies for support under such
schemes as the UK Renewable Heat Incentive (see section
1.2.10).
exchangers should therefore have high effectiveness
so as to minimise the difference between refrigerant
evaporating and condensing temperatures and
increase the cop.
Heat generators
1-71
Heat rejection
20
Refrigerant evaporating
temperature 0°C
18
16
Condensor
COP
12
10
8
Pressure
reducing
device
Compressor
Energy
input
6
4
2
0
Evaporator
Heat input
Figure 1.25 The refrigerant cycle principal for cooling and/or heating
incorporating a thermal buffer tank in hydronic systems
served by heat pumps.
The refrigerants used are typically fluorinated hydrocarbons
(hfc). Smaller systems may use propane or butane. hfcs
are non-toxic and non-inflammable but are subject to the
F-Gas Regulations. Propane and butane are flammable and
so usually restricted to smaller systems.
The maximum condensing temperature that can be
efficiently achieved with this type of heat pump is 50–55 °C.
For dhw in commercial applications temperatures may
need to be raised periodically to a higher value to comply
with Legionella regulations (see section 1.12.4.2). Some heat
pumps are designed and have control systems to permit
this, either by operating the system at higher condensing
conditions, or by using an electric back up heater. Where
heat at more than about 55 °C is consistently required,
consideration should be given to:
——
using the heat pump only for lower temperature
space heating requirements (such as underfloor
heating) and providing an alternative means of
producing higher temperature heat for dhw (e.g.
chp)
——
installing the heat pump in series with an alternative
heat source (usually a condensing boiler) such that
the latter will operate only when the heat pump
cannot achieve the desired flow temperature (see
section 1.8 for further details)
——
utilise the heat pump for the preheating of the cold
fill to the dhw
——
use a CO2 vapour compression or gas-fired
absorption heat pump as described below.
CO2 transcritical cycle
This heat pump uses R744 (CO2) as a refrigerant. It follows
the same general principles as the conventional vapour
compression cycle except that the refrigerant does not
condense in the condensing coil. It operates above its
critical temperature in the high pressure side of the plant.
The term ‘gas cooler’ is therefore used instead of ‘condenser’.
0
40
50
10
20
30
Refrigerant condensing temperature / °C
60
Figure 1.26 Range of cops that can be expected for an electric vapour
compression heat pump.
The gas cooler reduces the gas temperature from around
70 °C to a much lower level and can achieve a cop of
approximately 4 when raising the temperature of supply
water at relatively low temperature to high temperature.
The main disadvantage of the CO2 heat is its very high
operating pressure (up to about 130 bar) resulting in higher
capital costs than a conventional vapour compression heat
pump.
Gas engine vapour compression cycle
In this type of heat pump, the refrigerant compressor is
driven by a gas-fired spark ignition engine. hfc refrigerants
are normally used. High temperature heat reclaimed from
the engine exhaust supplements the lower temperature heat
from the condenser. This is particularly advantageous
where a high temperature is required, especially in winter
when evaporating temperatures will be low and cop will
fall.
As the definition of cop includes only the heat produced at
the condenser, a gas utilisation efficiency (gue) is often
quoted instead. In this case, the gue includes both the heat
reclaimed from the engine and that produced at the
condenser. A gue of 1.3 is typical.
Gas-fired absorption cycle
In the absorption cycle, the compressor is replaced by a
generator and absorber as shown in Figure 1.27. Heat is
applied to the generator that contains a solution of
refrigerant dissolved in a liquid solvent. In the gas-fired
absorption heat pump, the refrigerant is usually ammonia
and the solvent is water. A gas burner heats up the ammonia
solution in the generator, liberating high pressure ammonia
that passes through to the condenser. The weak solution of
ammonia in water is returned to the absorber (Dossat,
2001). The cold, low pressure vapour returning from the
evaporator is absorbed by water in the absorber. This is
then pumped to the generator.
Flow temperatures from the condenser of about 40 °C are
typical with a cop of around 1.4. For flow temperatures of
65 °C, the cop falls to close 1.0. As with the gas engine heat
pump, the term gue is often used in place of cop.
Ammonia is toxic. Consequently, such units are normally
located outdoors.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Refrigerant evaporating
temperature –10°C
14
1-72
Heating
Heat rejection
Generator
Pressure
reducing
device
Ground source heat pumps (gshp) utilise secondary coils in
the ground through a water/glycol mixture is circulated,
transferring low temperature heat from the ground to the
evaporator. Two types of secondary coil are used:
——
horizontal coil: buried at a depth of about 1.5 m deep,
usually in the form of a small diameter flexible
polyethylene tubes laid to form a helical coil
——
vertical coil: rigid polyethylene pipework installed
within grouted vertical boreholes or integrated
within the structural piles of the building.
Pump
Evaporator
Absorber
Heat absorption
Heat rejection
Figure 1.27 Absorption cycle
1.7.9.3
winter cops significantly higher than an ashp using
outdoor air.
Heat sources
Air source heat pumps
Air source heat pumps (ashp) utilise an air/refrigerant
evaporator coil to extract heat directly from the air stream
provided by a fan. The coil is finned on the airside. The coil
may be placed outdoors using the atmosphere as heat source
or within a ventilation exhaust air duct.
When outside air is used as a heat source, the cop falls as
the air temperature drops. There can also be problems with
icing of the heat exchanger where the outside air is of high
relative humidity, which is frequently the case in the UK
during winter months. This requires periodic defrosting,
which is often achieved by temporary reversals of the heat
pump and which reduces the seasonal performance factor
(spf) (see section 1.7.9.1) of the heat pump. However,
properly designed, ashps can achieve a spf of around 2.5
when used for heating in a typical UK climate (electric
vapour compression). It is not usually economic to size
ashps for the coldest outdoor conditions, and they are often
used in conjunction with supplementary heating (as
discussed above). With outdoor air source heat pumps, it is
common practice to install both the evaporator coil and
compressor outdoors in a single unit. This is a simple and
inexpensive way of installing a heat pump and the sound
generating components, can be sited away from noise
sensitive areas.
Utilising warm exhaust ventilation air can lead to very high
cops. It is common practice to utilise heat pumps in
swimming pool hall ventilation systems where the exhaust
air is typically around 30 °C and about 70% saturated and
both the appreciable sensible and latent heat in the exhaust
air stream can be recovered.
Ground source heat pumps
In winter, the temperature of the ground is usually above
outside air temperature. Below about 2 m depth, the ground
temperature is reasonably constant throughout the year
and close to the mean annual outdoor air temperature.
Utilising the ground as a heat source can therefore yield
The former method has the lower capital cost but is limited
by the area of ground available. It is common practice to
utilise car parking areas. In both cases the length of
pipework required is determined by the soil conditions.
CIBSE TM51: Ground source heat pumps and BSRIA BG
7/2009 (Brown, 2009) contain guidance on the sizing of
ground heat exchangers.
During the course of the heating season, the ground
temperature will fall as heat is extracted. This will result in
a reduction in cop and in the case of inadequate coil length
could result in freezing of the ground. Alternatively,
consideration could be given to using to heat pump to
provide cooling outside the heating season. The ground
will then behave as a large thermal storage system and
improve overall performance of the heat pump. Properly
designed and installed, heating-only gshps can achieve a
spf of 3.5 or higher.
Water source heat pumps
Aquifers, rivers, canals and lakes offer the potential for heat
sources with good heat transfer characteristics.
Aquifers are natural underground reservoirs, usually
associated with chalk strata. The water in the aquifer is
typically at around 12–15 °C and at a depth of 50–150 m. A
pair of wells are normally sunk, with a diameter of around
400 mm. One well is used to abstract the water, bringing it
to the surface by means of a submerged pump located close
to the bottom of the well. The second well is used to return
the water to the aquifer. In the UK, permission is required
from the Environment Agency to abstract water from
aquifers. Permission is subject to any possible impact on
other users in the vicinity. In all cases, strict conditions
apply to the temperature and condition of the water
returned. Constructing wells involves a high cost and risk:
the amount of water that can be abstracted from a pair of
wells is notoriously difficult to predict with certainty and
may reduce in time. It is essential therefore that a competent
hydrogeologist is consulted.
Using surface water such as rivers, canals and lakes is
technically simpler than using aquifers. Water is usually
piped from the source to the heat pump and returned. Due
regard needs to be given to filtration and the prevention of
fouling. At the same time, the ecology of the water source
must be protected. Permission will be required from the
Environmental Agency who will normally impose
limitations on the amount of heat that can be extracted and
the temperature difference between the flow and return.
Refer to CP2: Surface water source heat pumps: Code of Practice
for the UK (CIBSE, 2016).
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Condensor
High
temperature
heat input
Heat generators
1-73
Outside
Inside
Figure 1.28 Air to air heat pump
arrangements (reproduced by kind
permission of BSRIA, BG7/2009)
–
Single split
Outside
Inside
–
Multi-split
+
Outside
Through the
wall or window
unit
Inside
+
Outside
–
Inside
–
+
Outside
Inside
+
Outside
Inside
+
Passive
heat exchanger
–
1.7.9.4
Heat pump arrangements
Heat pumps are often categorised by the final heating
source and heat distribution medium used as shown in
Table 1.33.
Category
Heat source
Typical heating
distribution
medium
Notes
Air to air
Outdoor or
exhaust
ventilation
air
Room air (fan
coil unit) or
ducted
ventilation
supply air
Available in a
number of
configurations
including heat
recovery (see
below)
Air to water
Outdoor or
exhaust
ventilation
air
lthw heating
Often used as an
alternative to a
boiler in
hydronic
systems
Water to air
Ground,
aquifer or
surface
water
Mechanical
ventilation
supply air
Likely to be
more
appropriate
where high
ventilation
heating load or
used for both
heating and
cooling
Water to
water
Ground,
aquifer or
surface
water
lthw heating
Often used as an
alternative to a
boiler in
hydronic
systems
Figure 1.28 shows a number of common arrangements.
Alternatively, a ‘through the wall’ arrangement utilises a
single unit with the evaporator section on the outside of the
wall and the room fan coil section on the inside of the wall.
This avoids the need for refrigeration pipework.
Heat recovery
or exhaust air
heat pump
Table 1.33 Heat pump categories
Air to air
The single split system has an external unit (often housing
the evaporator and compressor) and a single indoor fan coil
unit that houses the condensing coil. The two units are
connected by refrigerant pipework. In the multi-split unit,
several indoor fan coil units are connected to the refrigerant
circuit.
Indoor air to
air heat pump
with fresh air
supply
circuit
The whole unit may also be sited indoors where it may if
required provide fresh air ventilation with heat recovery.
1.7.9.5
Environmental considerations
The use of any type of heat pump will generally result in a
reduction in CO2 emissions compared other forms of
heating utilising fossil fuels or electricity. However, electric
vapour compression heat pumps will result in NOx and SOx
emissions at the power station and there is the risk of
leakage to atmosphere of refrigerants that are themselves
powerful greenhouse gases.
circuit
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
+
1-74
Heating
CO2 emissions
For any type of heat generator the CO2 emissions C (kg CO2)
in a time period 't' are given by
Section 1.6 gives details of SOx emissions arising from the
generation of grid electricity in the UK and a number of
other fuels.
F-Gases
Eh
C = ——— Cfuel
scop
(1.42)
where Cfuel is the carbon emission factor for the fuel
(kgCO2/kW·h), Eh is the heat output from the heat generator
(kW·h) during time period t and scop is the average cop of
the heat generator during time period t (see 1.7.9.1). When
assessing heating systems, it is usual to take the time period
as the whole heating season.
Equation 1.42 can be used to derive an expression for the
minimum scop of a heat pump required so that its CO2
emissions during a heating season will be less than those
from an alternative heat generator:
scophp > scopalt × Chp / Calt
(1.43)
where scophp and scopalt are the seasonal coefficients of
performance of the heat pump and alternative generator
and Chp and Calt are the carbon emissions factor of the fuels
used by the heat pump and alternative heat generator.
Using the CO2 emissions factors specified in the
current Building Regulations (England) (DCLG, 2013),
C (gas) = 0.216 kgCO2/kW·h and C (grid electricity) =
0.519 kgCO2/kW·h, gives the following results:
——
for a vapour compression heat pumps using
electricity drawn from the UK grid, an scop of 2.07
or higher is required to achieve lower CO2 emissions
than would be emitted from a gas fired condensing
boiler with a seasonal efficiency of 86%
——
for a gas-fired absorption heat pump or a gas-engine
vapour compression heat pump, an scop of 0.86 or
higher is required to achieve lower CO2 emissions
than would be emitted from a gas fired condensing
boiler with a seasonal efficiency of 86%.
NOx and SOx emissions
Although an electric vapour compression heat pump
releases no local NOx or SOx emissions, consideration
should be given to such emissions at the power station.
Table 1.34 gwp values (100 year time horizon) of some common
refrigerants (Defra, 2011a)
Refrigerant
gwp
R134a
3800
R410A
1725
R290, R600a (propane, butane)
3
R717 (ammonia)
0
R744 (CO2)
1
Many vapour compression heat pumps use fluorinated
hydrocarbons (hfc) as a refrigerant. These are powerful
greenhouse gases. These can escape to the atmosphere due
to leakage or during charging. In Europe, there are
regulations governing their use — the F-Gas Regulations
(see section 1.2). Guidance on their use and handling is
provided in the UK by DEFRA in information sheet RAC7
(DEFRA 2011a). Table 1.34 lists the global warming
potential (gwp) of refrigerants commonly used in heat
pumps.
1.7.9.6
Further information
Detailed guidance on ground source heat pumps is given in
CIBSE TM51. BSRIA has produced a design guide for heat
pumps (Brown, 2009) that includes sizing, selection,
installation and commissioning information as well as
integration with other heating systems. The Carbon Trust
has produced a booklet based on 28 case studies of actual
installation of ground source heat pumps in the UK (Carbon
Trust, 2011). BS EN 15450 (2011) gives general information
on the design of heating systems using heat pumps.
1.7.10
Combined heat and power
Combined heat and power (chp) involves plant for
producing electricity that also produces useful heat. These
may be loosely grouped according to the electrical output
as:
——
micro-chp: up to about 5 kWe
——
small-scale
2000 kWe
——
large-scale chp: above about 2000 kWe.
chp:
between about 5 kWe and
Micro-chp is intended for individual homes. Large-scale
chp is used in industrial applications and large district
heating schemes. The following relates to small-scale chp,
suitable for non-domestic buildings, community and small
district heating schemes.
The advantage of chp is that, provided the heat can be
usefully employed, overall CO2 emissions and running
costs can be significantly reduced in comparison to taking
electricity from the grid and producing heat from
conventional heat generators.
1.7.10.1
Types of CHP
Table 1.35 provides a overview of the most common forms
of small-scale chp. The ranges given are typical only.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
In comparison to most other forms of heating, heat pumps
result in a reduction in energy consumption at the point of
use (metered energy) due to their efficiency (cop) as in all
cases this is greater than 100%.
BREEAM (BRE, 2014) (see section 1.3.9 above) uses a
figure of 617 mgNOx per metered kW·h of grid electricity
consumed. This is in comparison to a modern gas-fired
boiler of less than about 100 mgNOx/kW·h of heat produced.
Heat generators
1-75
Engine
exhaust gases
Gas
Control panel
Hot
process
water
supply
Generator
Engine
Exhaust heat
exchanger
Cool process
water return
Engine heat
exchanger
Gas spark ignition (GSI) engine CHP
The exhaust will provide the highest temperature for heat
recovery followed by the engine jacket and the oil cooler.
This is by far the most common form of chp, usually
purchased as a complete package. The smaller units are
often based on road vehicle engines. A typical packaged
plant is shown in Figure 1.29. This includes the engine,
generator, heat exchangers and controls.
Fig:
2.1
Heat can be
recovered
from the engine exhaust, the cooling
jacket to the engine block and the oil cooler. The fraction of
total heat reclaimed from non-turbo-charged engines is
typically about:
——
exhaust: 50%
——
engine jacket: 42%
——
oil cooler: 8%
Figure 1.29 Typical packaged gsi engine
chp plant
Turbo-charged engines are fitted with after coolers. These
produce around 7% of the total heat available. However,
this heat is at too low a temperature to be reclaimed. Instead,
this low grade heat is rejected via a separate circuit to the
atmosphere using a dry air cooler.
In many of the smaller packaged chp units, the three heat
exchangers serving the exhaust, engine jacket and oil cooler
are connected in series to produce lthw at a flow
temperature of about 90 °C. The return water temperature
must be sufficiently low at all times to ensure adequate
cooling of the oil and engine block. This is typically about
55 °C. Where there is a demand for both high and low grade
heat, it is advantageous to recover the heat from the exhaust
gases separately.
Table 1.35 Main forms of small-scale chp
Type
Typical fuel
Electrical
duty (kW)
Spark ignition internal
combustion engine driving an
electrical generator
Natural gas
Compression ignition internal
combustion engine driving
electrical generator
Diesel
Gas turbine driving electrical
generator
Natural gas or
bio-gas
100–2000
Organic Rankine cycle turbine
driving electrical generator
Biomass
200–2000
14
85
5
1.4–4.2
lthw
Fuel cell (pafc) (with methane
reformer)
Natural gas
400
38
80
1.1
0.01
lthw
(2)
Typical
electrical
efficiency (%) (1)
Typical total
efficiency (%)
Typical heat
to power
ratio
NOx
emissions
(g/kW·he) (3)
Heat output
medium
3–500
25–30
75–85
1.5–2
2
lthw
500–2000
30–38
75–85
1–1.5
2
lthw
100–2000
35–40
65–75
0.5–1.5
2–7
lthw
(1)
Bio-diesel
mthw
Bio-oils
20–25
60–70
1.5–3
0.2
lthw
mthw
Steam
Bio-gas
(1)
Based on gross calorific value of the fuel, plant operating at full load
(2)
Can be run on bio-gas.
(3)
NOx emissions may be reduced by means of catalytic converters on the exhaust
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Engine
exhaust
1-76
Heating
Where it cannot be guaranteed that the return water
temperature will always be low enough to meet the
operational requirements of the engine, then the engine
will need to be shut down at low heating load. Alternatively,
heat rejection will need to be provided. gsi chp can
turndown to about 50% load when the electrical efficiency
of the chp will reduce to about 90% of full load efficiency.
chp is usually restricted to using waste heat. More recently
The gsi engine generates very high levels of noise and
vibration. The plant is mounted on a heavy base
incorporating anti-vibration mountings. In addition to
providing silencers on the exhaust, the plant is often placed
in an acoustic chamber. Plant is normally located within
basement areas due to its weight, noise and vibration.
Fuel cells combine hydrogen with oxygen within an
electrical cell to produce DC electricity. The waste product
is water. As hydrogen is not normally available, it is
produced as required from natural gas (methane, CH4)
using a reformer. The reformer uses energy and produces
CO2 in the process. Even so, a high electrical efficiency
(based on the calorific value of the methane) can still be
achieved. Other than pumps for circulating the lthw,
there are no moving parts and so these machines are very
quiet. Although there are CO2 emissions from the reformer,
NOx emissions are close to zero.
The efficiency of a ci engine is greater than that of a gsi
engine, resulting in higher electrical efficiencies though for
larger engines, this difference is relatively small. However,
ci engines can maintain a near constant efficiency down to
about 50% load. At the same time, both the amount and
temperature of heat recoverable from ci engines tends to be
less so that overall efficiency is lower than for gsi engine
chp. NOx emissions of ci engines are noticeably higher.
These can be reduced by the use of catalytic converters but
electrical efficiency reduces slightly.
As with the gsi chp, this type of chp is heavy and requires
noise and vibration attenuation.
The great advantage of the ci engine is that it can be
designed so as to be able to run on a range of liquid fuels,
including bio-oils, which can be stored on site. This means
that ci engine chp has the potential to double up as standby
power. Considerable interest is being shown in ci engine
chp that run on bio-oils as this can lead to very large
reductions in overall CO2 emissions. However, the
manufacturer must be consulted and will normally set
constraints on the maximum percentage mix of biofuel and
the specification of the fuel used.
The arrangements for recovering and rejecting heat are
similar to those for gsi chp. Maximum lthw flow
temperature is about 80 °C unless heat from the exhaust is
recovered separately.
Gas turbine CHP
This has not been used widely due to the high cost and
specialist maintenance requirements of gas turbines.
Further, until recently, small-scale turbines were not
commercially available. The overall electrical efficiency is
poor compared to gsi engine chp and part load efficiency
falls quickly. The advantage is that higher temperature heat
can be recovered than with gsi and ci engines. Furthermore,
heat rejection circuits are not required.
Gas turbines are lighter than gsi and ci engines but are
equally as noisy.
Noise and vibration attenuation is required.
Fuel cells
There are several types of fuel cell. The most commonly
used to date for small-scale CHP in buildings is the
phosphoric acid fuel cell (pafc), though even this has few
installations, mostly in North America and Korea. The
most commonly used is a 400 kWe unit. Approximately
two-thirds of the heat reclaimed is low grade at about 60 °C
but requires a very low temperature return that may not be
practicable for many heating applications. The remaining
one-third is available at up to about 120 °C.
As there are no moving parts within the fuel cell,
maintenance requirements are kept to a minimum.
However, the fuel cell stack has to be replaced after about 10
years. This is may constitute up to 50% of the capital cost of
the fuel cell.
1.7.10.2
The conventional method of providing heating by boilers
and electricity supplied from the national grid can be
relatively inefficient compared to the use of chp where
electricity and heat are generated together on-site.
It is convenient to be able to compare the heat efficiency of
chp with that of a heat-only generator such as a boiler or
heat pump. This can be done by deducting from the total
energy input to the chp the primary energy that would
need to be consumed by the grid power station to produce
the same amount of electricity at the point of use as the
chp. CIBSE AM12 shows how the following equation for
chp heat equivalent heat efficiency can be derived:
Table 1.36 Values of X and Y to use in equation 1.45c
Fuel used
Rated electrical
output range / MW
X
Y
Natural gas
≤1
249
115
>1 to ≤10
195
115
>10 to ≤25
191
115
≤1
275
120
>1 to ≤25
191
120
≤1
285
120
>1 to ≤25
251
120
Rankine cycle turbine CHP
This is the same thermodynamic cycle used for the steam
turbine. By utilising an organic fluid, often a refrigerant,
rather than water, the cycle can operate at a lower
temperature and pressure so reducing capital costs of the
plant. Electrical efficiency is very poor and so this type of
CHP efficiency
Liquid biofuel
Biogas
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Compression ignition (CI) engine CHP
is has been fired using heat from biomass boilers.
Heat generators
chp equivalent heat efficiency
1-77
=
ηth / (1 – ηe / ηgrid)
(1.44)
This equivalent efficiency can be compared directly to that
of an alternative heat generator to assess primary energy
savings from the use of CHP.
It is important in this comparison that all efficiencies are
quoted based on gross (higher) calorific value.
1.7.10.3
The Combined Heat and Power
Quality Assurance scheme
ηe = electrical energy produced in one year / fuel input energy in year
Eout / Fin
(1.45a)
ηt = useful thermal energy produced in one year / fuel input energy in year
=
Hout / Fin
Quality index (qi) = X ηe + Y ηt
(1.45b)
(1.45c)
where the fuel input is based on the higher calorific value.
It should be noted that some chp manufacturers quote fuel
input duty using the lower calorific value (lcv or net).
The factors X and Y depend upon the fuel used and the
duty of the chp. Table 1.36 gives a sample of these values
(CHPQA, 2013a).
In order to qualify for enhanced capital allowances,
reductions in climate change levy, or the Part L 2013
Building Regulations (England) (DCLG, 2013), qi >105
and he > 0.2.
1.7.10.4
M = Fin Cfuel – Eout Cgde
Determining carbon dioxide emissions
The National Calculation Methodology (BRE, 2014),
provides a procedure to determine the CO2 emissions
attributable to the useful heat produced by a chp system for
the purposes of Part L Building Regulations. Over a period
of one year:
M = (H P / E + H P / (R E)) Cfuel – (H P / R) Cgde
(1.46)
(1.47)
The equivalent CO2 emissions factor for the useful heat
produced by the chp, Cheat in kgCO2/kW·h is then given by:
Cheat = M / Hout
1.7.10.5
In the UK the Combined Heat and Power Quality Assurance
Scheme (CHPQA, 2013) is designed to assess the quality of
the efficiency of CHP installations. The CHPQA Quality
Index is a measure of the energy efficiency and
environmental performance of a CHP system. It is used to
test compliance with the Enhanced Capital Allowance
scheme and the Climate Change Levy scheme (see section
1.2) and Part L 2013 of the Building Regulations (England).
It is determined as a function of the mean annual electrical
efficiency, ηe, and the mean annual heating efficiency ηt:
=
E is the sum of the mean annual electrical and heating
efficiencies: E = ηe + ηt. Equation 1.46 can be can be
shown to be equivalent to:
(1.48)
Applications to buildings
chp will only deliver a financial and environmental saving
when some or all of the heat produced can be used. Given
the relatively high capital cost of chp, it is usual to size the
chp on a base heating load — that is a heating load that will
exist for the majority of the building occupied time. This
heating base load may be less than half of the peak heating
load. The chp is then operated as the lead heat generator
with additional heat generators coming on line only when
the heating demand cannot be met in full by the chp.
Examples of integrating chp with other heat generators can
be seen in section 1.8. This approach maximises the hours
run of the chp and avoids installing excessively large and
expensive chp plant. It is generally recommended that the
chp should be sized such that it will run at least 3500 hours
per year (full load equivalent) so as to maximise investment.
Buildings with a high domestic hot water use and with
extended occupancy times such as hotels, hospitals, leisure
centres and communal residential heating schemes are
generally good candidates for chp.
However, the optimum size of chp will vary with building
type and the type of chp and chp fuel being used.
Furthermore, chp may be chosen because of its
environmental rather than financial advantages. Adhering
rigidly to a target of at least 3500 full load hours per year
may result in chp being ruled out prematurely at design
stage. It is preferable to carry out a detailed optimisation
study in which different sizes of chp are assessed over a
whole year. In order to do this, detailed heat load profiles
will be required together with a model of how the chp will
be operated, its minimum turndown and its full and part
load efficiencies. To increase the time that the chp operates
at full load, when chp is most efficient, thermal storage can
be incorporated.
As chp will produce both heat and electricity, consideration
needs to be given to how the electrical output will be used.
Where the electrical output exceeds the building’s
requirements it may be possible to export this electricity.
1.7.10.6
Further information on CHP in
buildings
CIBSE Applications Manual AM12: Small-scale combined
heat and power for buildings, gives detailed guidance on the
feasibility, design and implementation of chp in buildings.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
where ηth is the chp thermal efficiency (= useful heat
output rate (kW)/ fuel input rate (kW)), ηe is the chp
electrical efficiency (= rate of electricity generation (kW)/
fuel input rate (kW)) and ηgrid is the efficiency of the
electricity grid at the point in the network where the chp is
generating.
where M is annual CO2 emissions attributable to heat
produced by chp (kgCO2), H is the annual heat demand of
the building (kW·h), P is the fraction of heat demand
provided by chp such that H P = Hout, R is the annual heat
to power ratio = Hout/Eout, Cfuel is the carbon emissions
factor for the fuel used by the chp (kgCO2/kW·h) and Cgde is
the carbon emissions factor for grid displaced electricity
(kgCO2/kW·h).
1-78
Heating
area) but quickly falls below that of the ETC as the
temperature difference between collector and ambient
increases. Secondly, the ETC efficiency deteriorates less
than that of the FPC as the incident solar intensity decreases.
1.7.11
A flue is defined as the conduit that transports the products
of combustion from the combustion chamber to the
atmosphere while a chimney is the wall of the flue and whole
of the supporting structure (see BS EN 15287-1 (2007)).
Solar water heating collectors
Solar water heating collectors are widely used around the
world to provide domestic hot water, particularly where
sunshine is plentiful and fuel is relatively expensive. In the
UK, they are increasingly used for new-build and
refurbishment projects as they can provide significant
reductions in greenhouse gas emissions.
Modern solar collectors used for water heating are either of
the flat panel type or the evacuated tube type. The former
consists of a glazed box, insulated on the back and
containing a heat exchanger surface which absorbs much of
the solar irradiation transmitted through the glazing. Heat
is mostly lost to the surrounding by convection from the
surface of the glazing. In an evacuated tube collector, the
solar heat absorbing surface is contained within a glass tube
from which air has been expelled to create a vacuum. Heat
loss by convection from the surface of the collector to its
surroundings is therefore greatly reduced. The disadvantage
of the evacuated tube collector (ETC) compared to the flat
plat collector (FPC) is, apart from increased cost, a reduction
in solar heat absorbing surface.
The heat loss from the collector surface depends upon the
temperature difference between the solar collector and
ambient. This in turn depends upon the intensity of the
incident solar irradiation. Efficiency is therefore usually
stated as a function of both temperature difference and
irradiation intensity. Figure 1.30 shows the efficiency of
two modern solar collectors – a flat panel and an evacuated
tube collector. These values have been determined from
standard test data published by the manufacturer. The solid
lines represent efficiency with an incident irradiance of
1000 W/m2 (representative of peak intensity in southern
England) and the dotted lines at 500 W/m2. This
demonstrates firstly that the FPC will have a higher
efficiency than that of the ETC when the panel temperature
is close to ambient (due to its greater solar absorbing surface
0.8
0.7
1.7.12
1.7.12.1
(1)
— where height is defined as the vertical distance
above the ground or adjoining or adjacent
buildings
(2)
Incident solar intensity
1000 W/m2
500 W/m2
Panel efficiency
0.2
0.1
0
0
20
40
60
80
Temperature difference / ºC
Figure 1.30 Comparison of flat plate and evacuated tube collector efficiency
to create sufficient draught (stack) so as to generate
the design flow rate of flue gases to ensure safe and
efficient operation of the heat generator:
— where height is the vertical distance above the
heat generator. It is related to the average
temperature of the flue gases relative to that of
outdoor air and the resistance to flow afforded
by the flue
— in some instances, all of the draught may be
provided by the chimney or it may be
supplemented mechanically by means of a
forced draught burner or a fan within the flue.
To this end, the flue and chimney should:
——
terminate clear of building structures
——
incorporate sufficient insulation so as to minimise
heat losses through the walls of the chimney and
thereby increase the draught produced; reduce the
incidence of condensation within the flue that
would otherwise lead to corrosion; eliminate risk of
igniting building structure
——
be made of materials that are impervious to flue
gases and can resist corrosion
——
create minimal resistance to the flow of the flue
gases
——
operate effectively over the full range of loads
experienced by the combustion plant.
0.5
0.3
to disperse the products of combustion into the
atmosphere without causing excessive local
pollution or fire risk:
— the position is governed by regulations and/or
codes of practice subject to adequate discharge
velocity of the flue gases into the atmosphere
0.6
Flat panel
Principles of flue design
Flues serve two essential purposes both of which set limits
on the maximum height of the flue outlet:
Evacuated tube
0.4
Chimneys and flues
A variety of documents, suitable for different types of fuel
and size of installation, are available that provide advice on
the design of flues and chimneys in the UK. These are
listed in Table 1.37. Note that these do not apply to
incinerators.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The chp association has produced GPG234 Guide to
community heating and chp (CHPA, 2002). The Carbon
Trust has produced Introducing combined heat and power
(Carbon Trust, 2012). The Environmental Protection
Agency of the USA has produced a detailed report on chp
technologies (US EPA, 2008).
Heat generators
1-79
Table 1.37 Guidance available on the design of flues/chimneys
Range
Guidance document (UK)
Notes
All fuels
Up to 45 kW output
(solid fuel and oil)
Approved Document J: Combustion appliances and fuel
storage systems
Up to 70 kW net input
(gas )
Building Regulations (England) 2013
Gives guidance on the design, sizing and
installation of flues and chimneys including
positioning of flue outlets. Intended for domestic
applications but appropriate to small commercial
applications.
All fossil fuels
Between 150 kW and
150 MW gross input
1956 Clean Air Act Memorandum: Chimney Heights,
3rd edition (HMSO, 1993)
Deals with determining flue heights and positions
and efflux velocities.
Natural gas and
Greater than 70 kW net
input
IGEM/UP/10, 4th edition.
Detailed guidance, with examples, on all aspects of
design, sizing and installation of flues and
chimneys including chimney positions and heights.
For installations greater than 150 kW gross input is
based on the methodology of the Clean Air Act
Memorandum.
lpg
Installation of flued gas appliances in commercial and
industrial buildings (IGEM, 2014b)
Also includes some information on biomass
installations when used alongside gas.
lpg
70 kW to 1.8 MW net
input
BS 6644: Specification for the installation and maintenance
of hot water boilers (2011)
Guidance on design, sizing and installation of flues
and chimneys including height and position.
Oil
Up to 45 kW output
BS 5410 -1: Code of practice for oil firing: Part 1:
Installations up to 45 kW output capacity for space heating
and hot water supply purposes (2013)
Guidance on design, sizing and installation of flues
and chimneys including height and position.
Oil
Above 45 kW output
BS EN 5410 -2: Code of practice for oil firing: Part 2:
Space heating, hot water and steam services for commercial
and industrial premises (2013)
Guidance on design, sizing and installation of flues
and chimneys but limited guidance on flue outlet
positions.
Biomass
Less than 20 MW
Biomass and air quality guidance for local authorities,
(England and Wales) (EPUK, 2009)
General guidance
Natural gas and
CIBSE AM15: Biomass heating
1.7.12.2
Clean Air Act Memorandum (UK)
The 1956 Clean Air Act Memorandum: Chimney Heights
(Third Edition) (HMSO, 1981), provides a methodology for
determining the minimum height of chimneys required to
achieve adequate dispersion. It was developed to ensure
adequate dispersion of gaseous pollutants, specifically
sulphur dioxide and oxides of nitrogen. Although the 3rd
edition was produced prior to the 1993 Act, it is not part of
the Act and remains current.
The memorandum provides two distinct methodologies for
determining chimney heights: one for installations burning
fuels with an appreciable sulphur content (greater than or
equal to 0.04%) and one for fuels below 0.04% (referred to as
very low sulphur, vls). vls fuels are essentially natural gas
and lpg, though as a result of the European Directive to
reduce the sulphur content of liquid fuels (EU, 1999), some
kerosenes can now meet this limit. In the case of fuels with
appreciable sulphur contents, that is where sulphur dioxide
is the principal pollutant, an allowance for the nature of the
local development (ranging from rural to large city centre)
must be made. A higher level of local pollution from the
chimney will be acceptable in a lightly developed area than
in city areas where there is already likely to be relatively
high levels air pollution. As such, minimum required
chimney heights will be greater in more developed areas.
For fuels of both appreciable and low sulphur content, an
‘uncorrected’ chimney height is first calculated. An
adjustment is then made to take into account the heights of
any buildings in the vicinity, including the building
housing the heat generator. Except in the case of some vls
installations employing fan dilution (see section 1.7.12.10),
the flue outlet must be:
——
at least 3 m above the level of any adjacent areas to
which there is 'general access' (e.g. roof areas) or
openable windows
——
never less than the height of any part of an attached
building that is within a radius of 5 times the
uncorrected chimney height.
It does not define what is meant by the term general access.
It is presumed to refer to areas where building users other
than building maintenance personnel have access.
There are limitations to the methodologies in the
memorandum:
——
it does not ensure that adequate draught or efflux
velocity will be achieved
——
it is a relatively simple approach that cannot take
into account local topography such as hills. In such
instances more complex dispersion models will be
required
——
it is intended for installations where the principle
pollutants are sulphur dioxide or oxides of nitrogen.
It is not appropriate for emissions from incinerators
nor does it address dispersion of particulate matter.
1.7.12.3
Efflux velocities
A high efflux velocity will ensure that under most conditions
that there is no down-washing of the flue gases. For natural
draught systems up to 2.2 MW gross input, an efflux
velocity of at least 6 m/s is recommended at full load. Where
there is a component of mechanical draught provided,
higher efflux velocities of at least 7.5 m/s are recommended
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Fuel
1-80
Heating
for 2.2 MW (at full load) rising to 15 m/s at 135 MW
(HMSO, 1981).
1.7.12.4
Flue gas volume flow rate
The mass flow rate of the products of combustion can be
calculated from the stoichiometric equation, making
allowance for the amount of excess air used to ensure
complete combustion. The volume flow rate can then be
determined corresponding to the temperature of the flue
gases. However, it is normal practice to seek guidance from
the heat generator manufacturer or utilise design charts
based on fuel type and heating duty. CIBSE Guide C
contains charts for solid, liquid and gaseous fuels. IGE/
UP/10 (IGEM, 2014b) contains data for gas-fired
installations including flue dilution systems (see section
1.7.12.10).
Flue draught
1.7.12.5
A natural draught flue produces suction at its base by virtue
of the difference in the density between the column of hot
gas within the flue and the outside air. This can be expressed
by the formula:
∆ pd / H = ( ρa – ρg) g
(1.49)
where ∆ pd is the pressure difference between top and
bottom of flue (Pa), H is the height of the flue (m), ρa is the
density of ambient air (kg·m–3), ρg is the mean density of
flue gases (kg·m–3) and g is the acceleration due to gravity
(m·s–2).
1.7.12.6
Flue sizing and resistance to flow
Guidance on sizing of ducted systems and the determination
of resistance to flow of gases in ducts is given in CIBSE
Guide C. This includes pressure loss factors for a range of
fittings. IGEM/UP/10 gives data specific to flues. In order
to minimise the resistance to flow and so minimise the
height of flue outlet to generate the required draught the
designer should:
——
position the boilers as close as possible to the
chimney to limit friction (and heat losses) in the
connecting flue system
——
avoid all short radius 90° bends in flue systems
——
avoid abrupt section changes and use trans­
for­
mation sections with 15° included angles
——
arrange the entry section to slope at 45° or more to
the horizontal
——
avoid protrusion of the flues beyond the inner face
of the chimney or main flue connection
——
utilise materials with a smooth internal surface
finish
——
make flues circular or square and avoid aspect
(width to depth) ratios greater than 1.5:1
——
slope flues up towards the chimney where possible
——
provide clean-out doors at each bend in the flues, at
the chimney base, and adjacent to fans and dampers
to aid maintenance
——
avoid long ‘dead’ chimney pockets under the flue
entry points, which are corrosion zones, and can
cause harmonic pulsation problems.
500
1.7.12.7
450
400
Temperature / °C
Flue gases have a dew-point temperature below which
water vapour condenses. With sulphur bearing fuels, a
second (acid) dew-point occurs at a higher temperature
which depends on the type of fuel, amount of excess air,
sulphur content and combustion intensity. The sulphur in
the fuel is oxidised to SO2 during the combustion process
and a proportion of this is oxidised further to SO3, with
subsequent formation of sulphuric acid.
Summer
temperature
(20°C)
350
300
250
Winter
temperature
(0°C)
200
150
100
50
0
0
1
2
3
4
5
6
7
Draught per metre of chimney height / Pa
Figure 1.31 Flue draught at summer and winter temperatures
Flue corrosion and acid smut
formation
8
9
The peak rate of corrosion tends to occur around 30–40 °C
below the acid dew-point and a dramatic increase in
corrosion rate occurs below the water dew-point. Acid dewpoints generally lie in the range 115–140 °C for the type of
boiler plant used for heating but depend upon excess air
used, flame temperature, sulphur content etc. A significant
depression in acid dew point temperature occurs where
fuels have less than 0.5% sulphur content. It can also be
reduced or eliminated by stoichiometric combustion
conditions that can only be approached on very large plants.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
These velocities would lead to excessive frictional losses if
maintained within the flue. It is therefore normal practice
to design for flue gas velocities in the flue of around half the
efflux velocities and to incorporate a reducing section at the
flue outlet to accelerate the flue gases to the recommended
values. The flue diameter should not be less than the flue
outlet diameter of the heat generator.
The draught produced by a flue is proportional to its
vertical height and the temperature of the gas within it.
Figure 1.31 shows the draught available for typical winter
and summer ambient conditions at various flue gas
temperatures. This gross draught is available to provide the
energy required to move the flue gases through the
particular boiler, flue and chimney system.
Heat generators
1-81
Where flues are oversized, or where more than one boiler is
used with one flue/chimney, the inner chimney surface
temperatures may fall below acid dew-point conditions,
even with insulation applied. To avoid these problems, it is
strongly recommended to install one flue/chimney per
boiler, correctly sized for maximum practicable full load
flue gas.
Chimney outlets should not be positioned such that air
inlets into the building are on the leeward side of the
chimney for the prevailing wind direction. Generally
internal chimneys have less heat dissipation than freestanding units but where external chimneys are used they
should, where possible, be positioned on the leeward side of
the building or site, considering the prevailing wind
direction.
Heat loss from flues and chimneys
1.7.12.8
To enable the correct chimney construction to be selected it
is necessary to predict the minimum internal surface
temperature likely to be obtained at the chimney terminal
under all loads. An approximate value may be obtained
using the following method. It should be noted that average
values are used for some parameters and that radiation from
the gases to the chimney is ignored in order to simplify
calculations.
The rate of heat loss from the chimney or flue is given by:
Φc = U A (θg – θao )
The approximate overall thermal transmittance is given by:
l2
1
1
1
l1
— = — + — + — + .... —
U
ho l1 l2
hi
where ho is the external film coefficient (W·m–2·K–1), l1 etc. is
the thickness of chimney layer 1 etc. (m), l1 etc. is the
thermal conductivity of chimney layer 1 etc. (W·m–1·K–1)
and hi is the internal film coefficient (W·m–2·K–1).
Equation 1.51 should be used only where the overall
thickness of the flue/chimney, including insulation, is small
compared to its diameter. Where this is not the case, an
equation appropriate to heat transfer through cylindrical
structures should be used as described in CIBSE Guide C.
Typical values of film coefficients ho and hi are given in
Figure 1.32. These are based on standard expressions for
convective heat transfer rate by natural convection (hi) and
forced convection (ho). See CIBSE Guide C.
The heat loss rate may also be deduced from:
Φc = qm cp (θg1 – θg2)
Alternatively, the volume flow rate of flue gases (m3/s) may
be used in conjunction with the specific heat capacity. The
specific heat is usually taken to be 1.22 kJ·m–3·K–1 at 200 °C.
For thermal equilibrium, equations 1.50 and 1.52 must give
the same heat loss, so they may be equated, i.e:
U A ( θg – θao ) = qm cp ( θg1 – θg2 )
θg = ( θg1 + θg2 ) / 2
150
15
120
100
65
40
10
5
0
0
0·25
0·5
0·75
1
1·25
1·5
Chimney external diameter / m
Figure 1.32 External and internal film coefficients
1·75
2
(1.54)
100
35
150
Internal film coefficient / W·m–2·K–1
175
Temperature difference between external
metal temperature and surrounding air / K
External film coefficient / W·m–2·K–1
20
(1.53)
where θg is given by:
25
200
(1.52)
where qm is the mass flow rate of gases (kg·s–1), cp is the
specific heat capacity at constant pressure of waste gases
(J·kg–1·K–1), θg1 is the temperature of gases entering the
bottom of the chimney (°C) and θg2 is the temperature of
gases leaving the top of the chimney (°C).
(1.50)
where Φc is the heat loss rate (W), U is the overall thermal
transmittance (W·m–2·K–1), A is the surface area (m2), θg is
(1.51)
30
200
250
350
25
20
15
10
5
0
0
3
6
9
12
15
Flue gas velocity / m·s–1
18
21
Waste gas temperature / °C
The flue should be insulated so as to maintain internal
surface temperatures. Even so, at start-up, some
condensation will occur.
the mean waste gas temperature (°C) and θao is the outside
air temperature (°C).
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
A smut is an agglomeration of carbon particles resulting
from a combination of stack solids and low temperature
corrosion products. If the inner surface of any flue/chimney
falls below the acid dew point temperature of the waste gases,
an acidic film forms on the surface. Stack solids adhere to
this film and build up into loose layers, which are dislodged
and ejected from the chimney as the firing rates change.
1-82
Heating
hi (θg2 – θsi) = U ( θg2 – θao )
(1.55)
where θsi is the temperature of the inside surface of chimney
(°C).
IGE/UP/10 also gives charts for determining flue heat loss
rate.
1.7.12.10
Draught production
Natural draught systems
In these systems, all the draught is created by the stack
effect. A draught diverter is usually fitted in the flue next to
the boiler outlet to maintain correct combustion conditions
under all firing conditions. Flue gas velocities must be
relatively low in order to reduce system resistances to a
practical level, resulting in large diameter flues. As a result,
systems that rely entirely on natural draught are becoming
less common, particularly with the increased use of
condensing boilers that result in relatively cool flue gases.
Forced draught systems
1.7.12.9
Draught stabilisers
The admission of cold air into the flue/chimney system
reduces the flue gas temperatures and hence the available
natural draught. Draught stabilisers deliberately introduce
cold air to regulate the draught by this means. The use of
draught stabilisers is not recommended when high sulphur
fuels are used, as reduced flue gas temperature also produces
corrosion and acid smut emissions.
Dampers for draught regulation should be fitted with safety
interlocks to prevent firing against a closed damper. With
high chimneys the damper should be arranged to close
when the firing equipment is off-load, to isolate the boiler
and limit cold air ingress to the system. This limits the
cooling effect on the internal flue and chimney system, and
the corrosion mechanism within the boiler gas-side heating
surfaces.
Dilution air inlet
In forced draught systems including pre-mix burners and
pressure jet burners, the firing equipment is fitted with a
fan to provide the necessary combustion air and to overcome
the burner resistance and the boiler resistance to gas flow.
The chimney draught required in these cases has to
overcome less overall resistance than in the natural draught
case and flue gas velocities can often be increased for a
given chimney height. Forced draught is typically used
with oil- or gas-fired packaged steel shell or cast iron
sectional boilers or cabinet warm-air heaters.
Induced draught systems
A fan may be fitted at the boiler outlet to cater not only for
the resistance of the firing equipment and the boiler but
also, in certain instances, of the flue when burning at
maximum rating. Draught is then not dependent upon
buoyancy conditions and higher gas velocities can be
Discharge
Fan
Combustion
air inlet
Ventilation
air outlet
Air flow
switch
Boilers
Figure 1.33 Fan dilution system
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
If the temperature of the waste gases entering the chimney
or duct is known or estimated, the temperature of the gases
leaving the chimney may be determined from equation
1.53. The minimum surface temperature may then be
established from:
Heat generators
achieved and controlled if required to maintain draught.
Such systems are useful with condensing boilers but are not
recommended for biomass systems where a failure in the
fan would result in a build up of an explosive gas mixture.
A forced draught fan is fitted to provide all combustion air
and overcome the resistance of air registers, or fuel bed. An
induced draught fan is fitted at the boiler outlet to take the
hot gases and overcome resistance of the boiler and the
flues and chimney system. It is usual to fit a draught
controller which, by damper control on the fans, maintains
the balanced ‘zero’ condition in the combustion chamber.
Examples of this type are found in most coal-fired boilers
fitted with chain grate stokers and oil- and gas-fired water
tube boilers. Due to the fan power employed high velocities
can be used in the flue system, which again is not dependent
upon chimney height. Generally such an arrangement is
only applicable to larger installations.
Fan dilution systems
By mechanically diluting the flue gases prior to discharge
to atmosphere, the requirements of the clean air act can be
met without the need to terminate the flue above roof level.
The Clean Air Act Memorandum makes exceptions for the
use of a fan dilution system as shown in Figure 1.33
provided that:
——
the gross heat input is less than 6 MW
——
a very low sulphur fuel is used
——
the efflux velocity is adequate
——
The minimum efflux velocity is given by
Vmin = 75 / F
where F is the dilution ratio expressed as the total volume
flow rate discharged (m3/s) divided by the stoichiometric
volume flow rate (m3/s) when Vmin is then given in m/s.
The flue outlet position is still subject to certain restrictions
relative to height above ground, nearby windows and
ventilation opening and adjacent buildings.
Fan dilution is normally used where natural draught flues
are not practical. Ideally, the air inlet and discharge louvres
should be positioned on the same wall or face of the
building. Shielding is recommended if the louvres are
likely to be subjected to strong wind forces. A damper or
butterfly valve is fitted near the dilution air inlet to balance
the installation. Protected metal sheet can be used for
ducting as flue temperatures with this system are low,
typically 65 °C. Where an appliance operates in condensing
mode the systems and any dampers should be preferably
from 316 grade stainless steel.
Further detailed guidance on the design of fan dilution
systems can be found in IGEM UP/10.
Balanced/room sealed flues
Balanced flues are used mainly for small gas-fired appliances
but may also be used for low sulphur content oil, e.g.
kerosene. The appliance is of a room-sealed construction
and is sited adjacent to an outside wall. The air for
combustion is drawn from outside and the products of
combustion are discharged using a common balanced flue
terminal. The close proximity of air inlet and combustion
products outlet makes the balanced flue terminal relatively
insensitive to wind conditions and location. The balanced
flue means that cold combustion air need not enter the
room containing the boiler and is preheated to some extent
by the outgoing flue gases.
Fan assistance can be used to reduce the size of the flue
assembly and allow the appliance to be sited away from an
external wall.
1.7.12.11
Chimney construction
Chimney linings
Chimneys should have internal surfaces that:
——
have sufficient thermal insulation to maintain inner
skin temperatures above the acid dew-point during
normal running operations
——
are chemically resistant to acids and flue gas
deposits generally
——
resist absorption of moisture and its re-evaporation
——
can withstand fairly rapid internal gas temperature
changes
——
have low thermal capacity to limit heat up time
——
can be installed,
economically.
inspected
and
replaced
Flexible steel liners may be used to line existing chimneys
but Building Regulations (England) do not permit their use
in new masonry chimneys.
Stainless steel
Stainless steel chimneys are available with either singleskin construction or with a twin wall in diameters up to
600 mm. Twin wall types may have either an air gap or
insulation and are applied to most new boiler installations.
For condensing boilers, oil fired boilers and biomass boilers
the inner is usually fully welded. Condensing boiler systems
should have proprietary joint seals to ensure the complete
flue is moisture and pressure resistant. These chimneys are
manufactured in accordance with BS EN 1856 (2009) and
certified in accordance with the test methodology in BS EN
1859 (2009).
Steel
Steel chimneys are either of single or multi-flue
construction, the outer windshield being designed to cater
for the required wind pressures under either guyed or selfsupporting design conditions. The structural requirements
are covered by BS EN 1993-3-2: 2006.
With single flue construction a simple method of insulation
consists of applying externally a cladding of 1.6 mm
polished aluminium sheet located 6 mm from the outer
mild steel chimney surface by means of heat resisting
spacers at 1.2 m intervals. This provides a 6 mm stagnant
air space for insulation, assisted by the reflectivity of the
polished aluminium.
With high sulphur fuels and chimneys having a gas volume
turndown of more than 2.5 times with modulating or twoposition firing equipment, this insulation is insufficient for
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Balanced draught systems
1-83
1-84
Heating
A similar mild steel multiple flue system can be installed
within a concrete structural outer shell, again providing
facilities for subsequent replacement.
Brick
Brick flues/chimneys should always be lined internally. For
solid or liquid fuels the lining may be gunned solid
insulation refractory or diatomaceous earth type insulation.
The insulation standard should not be less than the
equivalent of 115 mm thickness of diatomaceous earth for
flue gas temperatures up to 315 °C.
Where flue gas conditions dictate (e.g. low temperature,
high sulphur and moisture) an acid resisting brick inner
lining, backed by a lining of insulation material, can be
used. Careful attention must be paid to the lining con­struc­
tion and the type of jointing mortar used to prevent flue
gases leaking through behind the lining and setting up
corrosive conditions.
The effect of pressurised operation on these linings is
questioned and for general operation such chimneys should
be operated under suction or balanced draught conditions.
They must be carefully designed by a com­petent structural
engineer who is aware of the combined physical/chemical
effects involved.
Concrete construction
Similar comments to those on brick construction apply, but
the insulation thickness should generally not be less than
the equivalent of 150 mm diatomaceous earth in order to
limit the interface concrete temperature to a maximum of
50 °C under normal boiler plant operating conditions.
Ventilated chimneys
Here a ventilated air space is situated between the inner
lining and outer chimney shell. The construction should
not be used in general with high sulphur fuels due to the
cooling effect created and the consequent danger of acid
dew-point and acid smut emission.
1.7.13
Corrosion in boilers, flues and
chimneys
On the combustion side, the most common cause of
corrosion, in both heat generators and chimneys is due to
acidic condensation formed in the presence of water vapour
and oxides of sulphur following combustion of fuels
containing sulphur although oxides of nitrogen (originating
from the fuel or the combustion air itself) are becoming
more significant as ever lower sulphur content fuels are
used.
The acid dew-point varies with the type of acid and its
concentration. Further cooling of the gases to the water
dew-point may produce corrosive effects even more serious
than those produced at higher (i.e. more concentrated) acid
dew-points. During normal operation it is unlikely that the
water dew-point (about 38 °C) will be reached but this may
occur for intermittently operated plant. When the system
operation is such that the water circulating temperatures
can fall to 38 °C, condensation is inevitable.
In boilers that are shut down, flue deposits become damp
because of their hygroscopic nature and produce acid
sulphates which are likely to cause corrosion. Acid corrosion
is less likely to occur with coal rather than residual fuel oils,
for the following reasons:
——
the hydrogen content of coal is lower than that of
other fuels, therefore the amount of water vapour
produced during combustion is also lower
——
the small amounts of fly ash in the flue gases tend to
absorb free SO2.
The combination of less water vapour and lower levels of
SO2 means that lower gas temperatures may be used, result­
ing in a corresponding gain in plant efficiency. On large,
well operated and maintained plant, the production of SO2
may be minimised by controlling the excess oxygen in the
combustion zone. However, precise control is necessary
and this is unlikely to be achieved on small plants.
1.7.13.2
Prevention of flue corrosion
To minimise the risk of corrosion, the following points
should be noted.
——
sufficient insulation should be provided to maintain
inner skin temperature above the acid dew-point
during normal operation.
——
the flue or chimney lining should be chemically
resistant to acids and flue gas deposits.
——
the flue gas velocity must be sufficiently high to
prevent precipitation of acids and deposits on
internal flue linings.
——
avoid abrupt changes of direction in flue and stack.
The flue connection from the boiler should rise to the stack
and be kept as short as possible.
Corrosion can occur on the combustion side and on the
water side.
During boiler shut-down in the summer months, all
surfaces should be cleaned of all partially burned fuel and
ash, and dampers should be left open to ensure that air is
drawn through the boiler. Lime washing of all accessible
surfaces may be beneficial and, where good air circulation
can be obtained, trays of a moisture absorbing material,
such as quicklime, should be provided.
On the water-side, water quality and the metals used within
the whole system have a large influence.
Flues for condensing applications should be designed to
drain high levels of condensate to drain points, should be
1.7.13.1
Mechanisms of corrosion
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
chimney heights above 10–12 m. A mineral wool insulation
at least 50 mm thick should be sub­stituted for the 6 mm air
space. With multi-flue con­
struc­
tion the inner flues are
placed within a windshield structurally calculated for wind
pressures etc. as before. The internal flues are either
insulated with mineral wool, or the whole space around the
flues filled with a loose insulation that can be pumped into
place. Thermal expansion problems must be considered in
the design and provision made for replacing any one flue at
a future date.
Hydronic systems
manufactured from materials that will not corrode in the
presence of acidic condensate (notably stainless steel or
polymers) and should have joints that are both gas and
liquid tight.
Prevention of combustion side
corrosion of boilers
Condensing boilers control the corrosion rate by having
suitable materials (stainless steel, aluminium or polymers)
in contact with the cool combustion gases and condensate.
Heat exchangers are often designed such that the flow of
condensate helps to clean the heat exchanger surface aiding
efficiency and reducing the chance of hot spots or local
corrosion pits forming.
For non-condensing boilers the system should be designed
so that the average boiler water temperature does not fall
below about 56 °C — above the water dew-point. For details
of control of boiler systems, see CIBSE Guide H. Under no
circumstances should the boiler thermostat be used as a
control thermostat to reduce the flow temperature in a
heating system.
Low temperature hot water boiler corrosion usually occurs
at the smoke box prior to the flue connection, and is often
referred to as ‘back-end corrosion’. Maintaining the return
water temperature above 56 °C can provide protection from
this type of corrosion along with keeping flue gases above
140 °C (often limiting achievable burner turndown). At
start-up, a thermostatically controlled bypass between the
flow and return connections can be used to blend a small
proportion of hot flow water with cooler return water.
Circulation is achieved either by a small shunt pump or by
connecting the flow end of the bypass pipe to the primary
pump discharge and controlling the flow/return blend
through a three-port valve. In each case, the bypass is
isolated automatically when the system return temperature
reaches the pre-set minimum.
1.7.13.4
Prevention of water side corrosion
Designers of heating or cooling systems must consider
appropriate water treatment to ensure continuation of
efficient performance and avoidance of corrosion that could
reduce the service life of system components.
Knowledge of all of the materials in contact with the system
water and the temperature ranges likely to be experienced
is required to allow proper selection. As seen in the Hydronic
system design appendix of this Chapter, pipework tends to be
a combination of plastic or copper at small diameters and
steel for larger diameters. Additionally brass and copper are
often used in valves and components of heat emitters.
In heating systems the widest temperature range, and hence
the greatest potential for corrosion, is often across the heat
exchanger of the heat generator. In the case of boilers the
materials in contact with the system water are most
frequently carbon steel, cast iron, copper, stainless steel or
aluminium alloy.
Where the heat exchanger material is the same or similar to
the materials used for the distribution and heat emitter
system then the chemical treatment added to the fill water
is straight forward. Where the heat exchanger material is
different from the materials in the system (notably
aluminium alloys) extra care should be taken to ensure that
the water treatment chemicals are suitable.
Heat generators with low water content can be more
susceptible to scaling and corrosion making material and
water treatment choice critical.
It is sensible to provide bypasses so that cleaning chemicals
and flushing water does not have to travel through heat
generators and heat emitters unless so required.
When system water is lost it is often replaced with untreated
mains supplied make up water. This has the effect of
diluting the chemical water treatment and decreasing its
effectiveness at reducing corrosion. Diluted water treatment
can have a greater effect for aluminium alloy heat exchangers
making them less suitable for large systems that are unlikely
to receive prompt re-treatment and especially for replacing
old boilers of different materials of construction where it
may be difficult to flush out debris from earlier corrosion.
Make up water usually introduces additional dissolved
oxygen that can accelerate corrosion rates. Leaks should be
fixed and the water treatment corrected as soon as possible.
Steam systems invariably lose some water through
evaporation to atmosphere and so have a requirement for
regular make-up water.
BS 2486 (1997) gives recommendations for the treatment of
water and make up water for non-domestic heat generators
including steam boilers. BSRIA have produced guides
which give requirements for cleaning of pipework systems
prior to commissioning (BSRIA, 2012a) and water treatment
during the use of heating systems (BSRIA, 2013).
1.8
Hydronic systems
1.8.1
General
Hydronic heating systems utilise hot water to transport
thermal energy from the heat generator/source to the heat
emitters/loads by means of pipework. The water may be
distributed at a temperature below or above 100 °C but
remains at all times in the liquid phase*. This section
discusses the design principles including choice of flow and
return temperature, the general arrangement of hydronic
systems, the integration and control of heat generators and
the types and performance of heat emitters. Details of
materials used, pipe and pump sizing, heat loss from
pipework, variable temperature and volume flow control,
methods of pressurisation, provision for thermal expansion,
dirt and air removal and commissioning are all dealt with
in the Hydronic system design appendix of this chapter.
Heat sources for hydronic systems include:
——
boilers
——
heat pumps
——
chp
——
district heating
——
waste heat recovery
——
solar thermal collectors.
* In North America, the term ‘hydronic’ includes steam.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1.7.13.3
1-85
1-86
Heating
Table 1.38 Categorisation of hydronic heating systems by temperature
Minimum
pressure required
/ kPa absolute
Low temperature
<90
130
Medium temperature
90–120
130–340
High temperature
>120
>340
In addition, this section discusses the how the hydronic
system, including its controls, should be designed to ensure
that:
12
120
10
100
8
80
6
60
4
40
2
20
0
0
200
0
50
100
Temperature / °C
150
Saturationn pressure / m water
Saturationn pressure / bar (absolute)
Flow water
temperature
/ °C
Figure 1.34 Saturation pressure of water
——
water reaches each emitter at the necessary flow rate
and temperature to satisfy both peak and part-load
heating demands
——
heat generators operate safely and in a stable
manner
——
energy consumption by heat generators and pumps
is minimised.
Other issues relevant to hydronic heating systems are
covered elsewhere in Guide B1, including:
——
the choice of heat generators and emitters (section
1.4)
——
details of heat generators and water treatment
requirements (section 1.7)
——
dhw selection and sizing (section 1.12).
Heat emitters include:
The Hydronic system design appendix of this Chapter
discusses how the distribution pipework and associated
equipment should be designed to ensure that:
——
radiators
——
heat losses from pipework are minimised
——
natural and forced convectors
——
——
radiant panels
the pressures around the system are maintained
above saturation pressure to prevent boiling
——
fan coil units
——
——
active beams
water and pipework expansion/contraction is
accommodated
——
heater batteries
——
setting to work and commissioning can be properly
carried out
——
underfloor heating
——
——
calorifiers/heat exchangers providing dhw.
the system incorporates adequate equipment to
purge air from the water, filter out sludge and inject
dosing chemicals.
Hydronic systems will often contain more than one type of
heat generator/source and emitter. This great flexibility has
resulted in hydronic heating systems being by far the most
common in many countries.
Figure 1.34 shows the saturation pressure of water across a
range of temperatures commonly used in hydronic heating
systems both in bar (absolute) and the (approximate) head
of water required to produce that pressure. The saturation
curve is exponential resulting in a substantial increase in
saturation pressure for only a small increase in water
temperature. The pressure within the system must be
maintained above the saturation pressure in order to
prevent boiling of the water with a reasonable margin of
safety, usually equivalent to 17 K. As a result, hydronic
heating systems are rarely designed to operate above about
130 °C.
Hydronic heating systems are characterised by the design
flow temperature and minimum static pressure required as
set out in Table 1.38.
By far the largest number of hydronic heating systems are
designed as low temperature hot water (lthw). This section
will therefore concentrate on lthw systems. Even so, this
1.8.2
Choice of flow and return
water temperatures
Many existing hydronic lthw heating systems were
designed for flow and return temperatures of 82/71 °C*.
Recently, 80 °C and 60 °C respectively have tended to be
used. The optimum values of flow and return temperatures
for hydronic heating systems depend upon several factors
as discussed in Table 1.39. Each system will have its own
particular optimum. Further, flow temperatures to space
heating emitters can be reduced as outdoor temperatures
rise above design value.
Integration of different types of heat emitters and generators
in a hydronic system must take into account these different
temperature requirements. Different heat emitter types
may benefit from being installed in separate circuits to
allow different flow temperatures to be used. Where hthw
and mthw are used, the higher temperature is often
reserved for the main distribution pipework with heat
exchangers provided to allow a step down in temperature
* These temperatures are the equivalent of the 180 °F and 160 °F
temperatures historically used.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Category
still leaves a wide range of practicable water flow
temperatures to choose from. The choice of flow and return
water temperatures is a major consideration in the design of
hydronic systems and is discussed below.
Hydronic systems
1-87
Table 1.39 Factors affecting choice of system temperatures
Comments
Heat emitter type
Radiators, convectors and fan coils can operate well over a wide range of temperatures. Underfloor heating usually needs to
be limited to a flow temperature of about 45 °C.
dhw requirements
dhw requires storage temperatures of at least 60 °C to minimise the proliferation of Legionella pneumophilia and other
bacteria (see section 1.12). Flow temperatures in the hydronic systems will generally need to be in excess of 70 °C to
economically achieve this. Consideration should be given to providing a separate and dedicated heat generator for dhw.
Surface
temperatures
Where susceptible occupants, such as the very young, elderly or infirm, can touch heat emitters, or exposed pipework, as
might occur in infant schools and hospitals, the surface temperature must not be so high as to risk skin burns. Surface
temperatures are normally limited in such applications to about 43 °C (Building Bulletin 87: Guidelines for Environmental
Design in Schools, 2003 and Health Guidance Note: 'Safe' hot water and surface temperatures, 1998).
Heat emitter size
The heat output from any heat emitter is related to the difference between its surface temperature and the temperature of its
surroundings. The surface temperature of the emitter will be approximately equal to the average of the hot water flow and
return temperatures. The greater the surface temperature, the smaller the heat emitter needed to achieve a given heat output.
Minimum heat emitter size will be achieved with a high flow and return water temperature.
Pipe and pump
sizing
The heat carrying capacity of the hot water is directly proportional to the temperature drop between flow and return. A
greater drop will allow a smaller water flow rate to be used with consequential reductions in pipe sizes, pump duty and pump
energy consumption. For very large heating systems, particularly site or district heating schemes, the capital costs of
pipework together with pump energy cost are considerable. mthw and hthw give the opportunity for significantly greater
temperature drops, typically more than 40 K, and therefore greater heat carrying capacity without the need for very large
pipework and high pumping costs.
System pressure
The minimum static pressure at any point within the heating system must be maintained above the saturation pressure (with
a safety margin) as discussed in section 1.8.1. The higher the flow temperature the higher the static pressure required. hthw
and mthw have more demanding requirements for pressurisation and expansion/contraction and all plant, equipment,
pipework and joints need to be rated for these elevated pressures.
Use of heat pumps
Most types of heat pumps, when operating efficiently, produce relatively low flow temperatures of around 45 °C. Higher flow
temperatures may be possible with a loss in efficiency. As a result heat pumps are normally used in conjunction with emitters
designed for low flow temperatures, such as underfloor heating, with return water temperatures of around 30 °C.
Use of condensing
boilers
Condensing boilers require a return water temperature below about 55 °C to operate in condensing mode. Below 55 °C both
latent and sensible heat recovery will increase significantly as the return water temperature reduces. Flow temperature needs
to be selected that will achieve these low return water temperatures. Consideration should be given to operating the system
at a higher flow temperature at peak heating demand and reducing flow temperature as the heating demand falls.
Use of chp
Combined heat and power may also require relatively low return water temperatures, perhaps as low as 55 °C, to ensure
adequate cooling of the engine. As chp is normally used with top-up heating provided by boilers, care needs to be taken to
ensure these low return water temperatures are achieved. Operators of district/community heating schemes that incorporate
chp may penalise users by increasing tariffs if return water temperatures exceed specified limits.
Heat
generators
Primary
circuit
Interface
Heat
emitters
——
secondary circuit(s): containing the heat emitter(s)
Secondary
circuit(s)
——
interface: hydraulic connection between primary
and secondary circuit(s).
Figure 1.35 Schematic representation of a hydronic heating system
(and pressure) to lthw to serve heat emitters. This retains
the advantage of reduced pipe mains sizes without the need
to specify emitters and emitter circuits rated at high
pressure.
1.8.3
General arrangement of LTHW
systems
A hydronic heating system, regardless of size or complexity,
can be thought of as comprising three sections as shown
schematically in Figure 1.35:
——
primary circuit: containing the heat generator(s)
The design of all three must be considered carefully to
ensure compatibility as each of the three sections will
influence the operation of the others. Inappropriate design
will result in unnecessary energy consumption and possibly
control instability. There is no ‘one solution’ – optimum
design of all three sections will depend upon the aims of the
designer, the characteristics of the heat generators and heat
emitters and the building heat demand. In particular, the
control method(s) adopted for the heat emitters will affect
how both the return water flow rate and return water
temperature varies with heating demand and have a major
impact on the behaviour of the heat generators. As such it
is advantageous to start by considering the options available
for the secondary circuit(s).
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Factor
1-88
Heating
Heat emitter
Heat emitter
Heat emitter
Flow pipe
Heat emitter
Figure 1.36 Schematic arrangement of a two-pipe, pumped circuit
Heat
emitter
Heat
emitter
Figure 1.37 Two- and three-port control of heat emitter
Controller
Outdoor
temperature
A
Figure 1.38 Principle of flow temperature compensation
1
2
3
Figure 1.39 Simplified schematic primary circuit showing multiple heat
generators connected in parallel.
1.8.4
1.8.4.1
Two- and three-port control
Figure 1.37 shows arrangements of two- and three-port
valves used to control heat output from heat emitters.
C
B
Heat
generators
The pump must provide sufficient pressure to achieve the
design flow rate in the index circuit. This means that each
sub-circuit and heat emitter must be provided with some
form of flow regulation valve to allow the system to be
balanced. As sub-circuits/heat emitters nearer the pump
will be subjected to higher static pressure differentials, the
designer should ensure that the control valves selected can
operate effectively (that is with sufficient authority) and
take steps if necessary to limit the pressure differentials.
For further details see the Hydronic system design appendix
of this Chapter, CIBSE Guide H, CIBSE KS7, CIBSE
Commissioning Code W: Water distribution systems and
BSRIA’s Commissioning water systems (Parsloe, 2010).
Secondary circuit(s)
In the past, ‘one-pipe’ hydronic heating systems were
common. A single pipe was used both to supply water to
and remove water from each heat emitter. A pump was used
to circulate water through the pipe but circulation through
the emitters was by natural convection. Many such systems
are still in operation. Major disadvantages of this design
were that the temperature of water entering emitters
decreased in the direction of flow so reducing heat output
and only heat emitters with a low resistance to flow could
be used (typically radiators). Earlier systems depended
upon natural convection to provide water circulation in all
parts of the so-called ‘gravity’ system. Due to lack of control
and poor performance, modern design now uses only
pumped, two-pipe flow systems, to which discussion in this
section is restricted. Information on one-pipe and gravity
systems can be found in some early textbooks on heating
(e.g. Kut, 1968).
The heat output rate of any heating circuit will be given by
equation 1.56(a):
ΦL = mL Cp ( θf,L – θr,L)
(1.56(a))
Where ΦL is the heat output rate (kW), mL is the water flow
rate through the heating circuit, (kg/s), Cp is the specific
heat of water (kJ/kg·K), θf,L is the water temperature
entering the heating circuit (°C) and θr,L is the common
outlet water temperature from the heating circuit (°C).
With three-port control as the heating demand falls a
constant overall flow rate between flow and return pipes
will be maintained by increasing the flow through the
bypass and decreasing the flow through the emitter. From
equation 1.56(a) it is seen that this will result in an increase
in θr,L.
Two-port control (e.g. a thermostatic radiator valve), in
comparison, will reduce the water flow rate through the
emitter as the sensed heating demand falls. However, the
fractional reduction in water flow rate needed is significantly
greater than the fractional reduction in heat output required
due to the thermal characteristics of hydronic heat emitters
(see section 1.8.6.7). As a result, θr,L will decrease.
In summary, on a reduction in heating demand:
——
three-port control will result in no change in water
flow rate and a rise in return water temperature.
——
two-port control will result in a reduction in both
water flow rate and return water temperature.
Variable water flow rate control
Where two-port control valves are used on heat emitters,
the static pressure in the flow pipe will increase on falling
heat demand as the flow rate reduces. This will require the
valve to close further and make it more difficult for the
valve to maintain acceptable control. By controlling the
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Return pipe
In the two-pipe pumped arrangement, shown diagram­
matically in Figure 1.36, a separate flow and return pipe is
provided and each heat emitter is connected between the
two to form a series of parallel circuits. Water flow through
the pipes and the heat emitters is due to the pressure
differential produced by the pump. Each heat emitter
receives water at virtually the same temperature, provided
that flow pipework is adequately insulated.
Hydronic systems
Compensated flow temperature control
As outdoor temperature increases so space and ventilation
heating demand falls. It is therefore common practice to
adjust the flow temperature set point downwards in
response to higher outdoor temperature. This is known as
outdoor temperature compensation. Figure 1.38 shows how
compensated flow temperature control is achieved. This
includes a fixed bypass parallel to the controlled bypass to
port B. This fixed bypass is only required where the
maximum flow temperature to the heating circuit must
always be less than that available (at Port A). (Temperature
compensation of heat generator set-point flow temperature
is dealt with in section 1.8.5, ‘Primary circuit’.)
A mixing valve is installed upstream of the circulation
pump supplying the emitter circuit, allowing cooler return
water at port B to mix with the higher temperature flow
from the primary circuit at port A to produce the desired
flow temperature at port C.
Radiators, radiant panels and natural convectors are best
controlled using local zone/emitter controls combined with
flow temperature regulation. The flow temperature to the
circuit is modulated downwards as the outdoor temperature
rises above the design condition leaving the zone/emitter
control valve to respond to changing local incidental gains.
This reduces the range over which the local/emitter valve
needs to operate allowing a relatively inexpensive zone
control valve, such as a thermostatic radiator valve, to be
used.
Multiple heating circuits
1.8.4.2
In many applications more than one heating circuit will be
installed for the following reasons:
——
——
different areas of the building are to be heated by
different types of heat emitter requiring different
flow temperatures and/or means of heat output
control (two-port or three-port)
different areas of the building may have different
occupancy times. (It is a requirement for newbuild, non-domestic buildings over 150 m2 floor
area under Part L Building Regulations 2013
(England) (DCLG, 2013d) that time-controlled
zones be provided.)
Each secondary circuit is connected (in parallel) via some
form of header. This header will form the interface between
secondary circuits and the primary circuit. Its design will
be influenced by the type of heat generator(s) installed in
the primary circuit and is discussed in section 1.8.6.
1.8.5
Primary circuit
The primary circuit contains the heat generator(s). The
design of the primary circuit together with the control of
the heat generator(s) (and any associated pumps) should
ensure the following requirements are met:
——
the total heat output from the heat generators
matches the heating demand in the secondary
circuit(s) reasonably closely
——
the system is stable under all conditions
——
energy
consumption/carbon
minimised
——
thermal stress and acid corrosion of the heat
generator(s) is minimised.
emissions
are
Heat generators normally include integral controls, one
function of which is to maintain the set-point leaving water
temperature. This may be achieved by modulating the heat
output rate or by on/off or high/low/off switching. In the
case of modulating control, there is a minimum turn-down
ratio. (Section 1.7 describes typical methods of modulation
and turn-down ratios for a range of heat generators.) The
set point leaving water temperature of each heat generator
may be fixed or may be compensated to outdoor temperature
by means of an external master controller.
The heat output rate of a heat generator in a hydronic
system is given by equation 1.56(b):
· C (θ –θ )
Φ =m
(1.56(b))
hg
hg
p
f,hg
r,hg
where Φhg is the heat output rate of the heat generator (up
· is the water flow
to its maximum rated output) (kW), m
hg
rate through the heat generator, (kg/s), Cp is the specific
heat of water (kJ/kg·K), θf,hg is the water temperature leaving
the heat generator (°C) and θr,hg is the inlet water temperature
(°C).
Under steady-state conditions, the heat output rate from
the heat generator(s) (equation 1.56(b)) will equal the heat
demand rate of the heating circuits (equation 1.56(a)) and
all water temperatures and flow rates will remain constant.
Should the heat demand change, there will be a temporary
imbalance between the heat output rate and demand in
both the primary and secondary circuits. The control
system should be able to respond to this change and adjust
the heat output rate of the heat generator(s) accordingly.
Most non-domestic heating systems incorporate more than
one heat generator. In addition to providing a degree of
standby capacity (see section 1.5.9), this can result in
improved overall system efficiency provided that the heat
generators are both appropriately controlled and
appropriately hydraulically connected.
There are two quite distinct cases where multiple heat
generators are installed:
——
all the heat generators are identical (both in type
and in duty)
——
there is a mix of heat generator (types or duties)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
pump so as to reduce the static pressure and reduce the flow
rate, the two-port valve will perform better and there will
be a saving in pump energy consumption. Alternatively, a
differential pressure control valve (DPCV) may be installed
across a sub-circuit to maintain the design static pressure
difference as the two port control valves open and close.
Two-port pressure independent control valves (PICV) are
also available for individual heat emitters. Such valves can
improve the performance of two-port control but as they
work by increasing flow resistance they will not necessarily
result in pump energy savings. The use of DPCVs and PICVS
together with the control of pump output is discussed in
the Hydronic system design appendix of this Chapter, CIBSE
KS7 and BSRIA BG12: Energy efficient pumping systems – A
design guide (Parsloe, 2011).
1-89
Heating
100
arrangement is installed, each will also be subjected to the
same water flow rate. (Alternatively, each heat generator
may be provided with its own pump rather than using a
common pump.) As can be seen from equation 1.56(b), each
generator that is firing will provide the same heat output,
regardless of the heating demand, provided only that each
heat generator has the same outlet water temperature setpoint.
Efficiency (gross) / %
98
96
Load
94
20%
92
In terms of control, it is possible to operate all the heat
generators simultaneously (i.e. in unison) or to start each in
some prescribed sequence. The method selected should
take into account the efficiency/load characteristics of the
heat generator.
50%
90
75%
88
100%
86
0
10
20
30
40
50
60
Inlet water temperature / °C
70
80
Figure 1.40 Efficiency of a gas-fired condensing boiler as function of load
and inlet water temperature (reproduced courtesy of Hoval Ltd)
(a) Unison control
% full duty
100
67
rs
ato
at
He
33
0
er
gen
33
1,2
d3
an
67
100
% of full load demand
(a) Step control
100
% full duty
1
1,2 and 3
66
1 and 2
50
0
33
67
100
% of full load demand
Figure 1.41 Unison control (a) and step control (b) of three identical heat
generators connected in parallel
and the manner in which these are hydraulically connected
and controlled is quite different. This is discussed below
for each case.
1.8.5.1
Using identical heat generators
Figure 1.39 shows a simplified schematic where a number
of identical heat generators are connected in parallel within
the primary circuit with a pump in the common return
header. Valves have been omitted for clarity. Three identical
heat generators are shown though any number can be
accommodated. Each heat generator experiences the same
inlet water temperature. If the reverse return pipework
Unison control
Many heat generators show an increased efficiency at part
load, for example heat pumps and condensing boilers. An
example of the efficiency/load characteristic of a condensing
boiler is given in Figure 1.40. In such cases, the overall
efficiency of the group of heat generators would be improved
by operating them in unison. Figure 1.41(a) shows the
response of three such heat generators operating between
zero and 100% heating demand. It is assumed that each
generator has the same set-point flow temperature and has
been sized to provide one-third of the peak heating demand,
(ignoring any stand-by capacity) and can turn down to zero
output. In practice, minimum turn-down restrictions
apply: this is discussed at the end of this section. Being
connected in parallel, the output of each heat generator will
be identical at all times, as shown above. In this way, all of
the heat generators will be operating at the smallest fraction
of full load which combined will just match the heating
demand and therefore achieve maximum efficiency.
If the water flow rate in the primary circuit is maintained
constant, as the heat output falls so the difference between
the flow/return water temperature in the primary circuit
will also fall (equation 1.56(b)). If the flow water temperature
is maintained constant, the return water temperature to the
heat generators will increase. For condensing boilers this
will result in a drop in efficiency (see Figure 1.40), negating
the increase in efficiency due to operating at part load.
Alternatively, the water flow rate in the primary circuit
could be allowed to reduce as the heating demand falls so as
to lower the inlet water temperature to the heat generators.
The choice of whether to maintain a constant water flow
rate in the primary circuit or to allow it to vary with heating
demand depends upon the requirements of the heat
generator. Some heat generators, particularly those with
low water content, require a minimum water flow rate. The
manufacturer’s advice on minimum water flow rate should
be sought. How the water flow rate and return water
temperature vary with heating demand will be affected by
the design of the interface between the primary and
secondary circuits. This is critical to the operation of the
heating system and is discussed in section 1.8.6.
The flow water temperature set point for each heat generator
may be directly reset using flow temperature compensation
(as described in section 1.8.4). If the flow temperature set
point is sufficiently depressed with falling heating demand,
the return water temperature will decrease, adding to the
efficiency gains arising from operating at part load. Flow
temperature compensation is usually achieved by
scheduling flow temperature set point to outdoor
temperature and may be applied to both the heat generators
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1-90
Hydronic systems
and any variable temperature secondary circuits, subject to
minimum flow temperature requirements.
Step control
With step control, should water continue to flow through
those heat generators that are not firing, flow temperature
dilution will occur. The temperature of the water leaving a
heat generator that is not firing will be the same as the
return water temperature. The temperature of the water in
the common flow header will therefore vary depending
upon the number of heat generators firing: only with all
generators firing will the mixed flow temperature equal the
set point. Further, when a heat generator is switched off due
to a fall in heat demand, there may be a large and rapid drop
in common flow temperature which could be interpreted as
a rise in heat demand. The opposite will occur on firing up
a heat generator. This could lead to control instability.
The flow temperature dilution effect can be reduced by
shutting down the flow through a heat generator when it is
not operating by means of isolating valves or, where it has a
dedicated water pump, by switching off the pump. However,
problems have sometimes been experienced with this
approach, in part because it results in a variable water flow
rate around the primary circuit.
Minimum turn-down
In the above example of unison control, if the heat
generators selected have a minimum turn-down of 33% full
load output, then whenever the heat demand is less than
33% all three heat generators will switch on and off
simultaneously in an attempt to match the demand. In
comparison, with the step control example this will not
occur until the heating demand falls to 11%. Frequent on/
off operation of any heat generator will result in reduced
efficiency and increased thermal stress. In modern, wellinsulated buildings heating demand is frequently only a
very small fraction of peak demand due to internal and
solar gains (with peak demand occurring only at start-up on
a winter’s morning). In such cases, an assessment should be
made of the frequency and duration of low heating demands
and the feasibility of either using a larger number of smaller
heat generators or of changing from unison to step control
below a predetermined heating demand. Alternatively, use
could be made of a buffer vessel as discussed in section
1.8.6.
1.8.5.2
Use of mixed heat generators
Heat generators of different duties
In some cases it may be worthwhile installing heat
generators of the same type but of different duties. For
example, during marginal weather or when only some of
the heating zones are required to operate, the maximum
likely heating demand may be very much smaller than the
design heat load. At such times, the smaller heat generator
might be specified as the lead machine with the installation
operated using step control.
Heat generators of different types
A common example of utilising a mix of different heat
generator types arises when renewable/low carbon heating
is provided. Low carbon heat generators have a high capital
cost relative to conventional fossil-fuelled heat generators.
As a result, the former are usually sized at less than full
design heat demand and supplemented by the latter. The
low carbon heat generator is then invariably required to be
the lead generator, providing as much of the annual heating
energy demand as possible, with the fossil-fuelled
generator(s) operating as a top-up when required. To
maximise the fraction of heat provided by the low carbon
fuel, the fossil-fuelled heat generator should only be
permitted to fire when the low carbon heat generator is
operating at full duty. At such times, the fossil-fuelled
boiler should modulate its output while the low carbon heat
generator remains at full duty. However, if all of the heat
generators are connected in parallel then each heat
generator firing will operate at the same percentage output
regardless of the form of control used, (as discussed in
1.8.5.1). This will not maximise the fraction of heat demand
provided by the low carbon heat generator. In such cases,
series arrangements should be used.
Preferred arrangements for mixing low carbon heat
generators (including biomass boilers, heat pumps and
CHP) with fossil-fuelled top-up boilers are discussed in
more detail in section 1.8.8.
1.8.5.3
Measuring heating demand
When using the unison control strategy for a group of
identical boilers, there is no requirement to measure the
heating demand for control purposes (though it may be
required for monitoring of energy consumption). Each heat
generator will automatically try to maintain the leaving
flow water temperature at the set point. Any change in
return water temperature will automatically result in the
same change in heat output by all of the heat generators.
In contrast, in order to determine whether to start or stop
the next heat generator using step sequence control, the
controller requires a signal. This signal needs to be a welldefined and stable function of the total heating demand.
Ideally, heat demand would be measured by means of a heat
meter. This measures both flow and return water
temperature and integrates the difference with water flow
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Some heat generators, such as some non-condensing
boilers, achieve their maximum efficiency at full load. In
this case it could be beneficial to control them in sequence,
only starting another heat generator when those already
firing are already at full duty yet unable to meet demand.
This is often referred to as ‘step control’. Figure 1.41(b)
shows three identical heat generators (each sized at onethird peak demand) controlled in such a way. (As before,
standby capacity and minimum turn-down has been
ignored.) With a 33% heat demand, one heat generator will
be operating at full output. Should the demand increase, a
second heat generator will be started. However, being
connected in parallel, each will provide the same output so
that the output from the first heat generator will fall. For
example, at 50% heating demand, two heat generators will
be firing at 75% full load duty. The heat generators firing
will only be at 100% output at 33%, 66% and 100% heating
demand. At all other demands, they will be operating at
part load. Nevertheless, the second heat generator in the
sequence will never operate at less than 50% duty and the
third never at less than 67% duty. This may be acceptable.
This inability to operate some heat generators at full output
whilst others operate at part load is an inherent consequence
of the parallel connection.
1-91
1-92
Heating
flow rate through the circuit will be constant and a fall in
heat demand will result in a proportional rise in return
water temperature.
(a)
1
2
3
Primary circuit
(b)
Heat
generators
1
2
3
Primary circuit
Figure 1.42 (a) Common primary circuit pump and (b) individual
primary circuit pumps
Heat
generators
1
2
3
Figure 1.43 Principle of primary circuit with individual pumps and back
end protection on each heat generator
rate as in equation 1.56(a). The need to bring another heat
generator online (or shut one down) can then be determined
in a simple manner. The location of the measuring points
do need to be carefully chosen to ensure that the heat
demand measured is the total demand on the system under
all operating conditions. The location of heat meters is
discussed in the Hydronic system design appendix of this
chapter.
Heat metering is still relatively new and expensive. A
common alternative is to use the flow water temperature or
return water temperature as a proxy for heat demand. As
can be seen from equation 1.56(a), the heat demand is a
function of water flow rate, entering water temperature and
leaving water temperature. Using any one of these three
properties as an indicator of heat load requires that the
other two remain constant. As discussed in section 1.8.3.1,
for a secondary circuit utilising three-port control, water
1.8.5.4
Reducing thermal stress and
corrosion
All heat generators will experience some degree of thermal
stress when operating, particular during the warming-up
and cooling-down. Rapid cycling on and off will reduce the
generator’s life and should be avoided. On shutting down,
the controls must ensure water continues to flow through
the heat generator for a minimum specified time period to
dissipate stored heat and prevent overheating. This is
particularly so in the case of biomass boilers as the
combustion chamber will still contain burning fuel.
When firing, heat generators require a minimum water flow
rate to prevent local overheating. This is particularly the
case with low water content boilers which have a relatively
low thermal capacitance. The manufacturer of the heat
generator(s) should be consulted. As the total water flow
rate in the secondary circuit(s) is usually variable, the
primary circuit should incorporate its own water pump(s).
A single pump (duty and standby) may be installed in the
common return header to the heat generators, as shown in
Figure 1.42(a) or each heat generator may be provided with
its own, dedicated water pump as shown in Figure 1.42(b).
This later arrangement may also be employed to help
reduce flow temperature dilution when using step control
of the heat generators though, as the pump will need to run
for a minimum time after the heat generator shuts down,
there will still be temporary flow dilution following shutdown.
Where boilers (particularly older boilers and biomass
boilers) require a minimum return water temperature to
prevent corrosion arising from acidic flue gas condensation
on the heat exchanger, it is usual to provide each boiler with
its own pump complete with a recirculating circuit as
shown in Figure 1.43 This is referred to as back-end
protection. On firing up, the flow from each boiler is directed
back to the inlet of the boiler until the inlet water
temperature exceeds the minimum required.
1.8.5.5
Pump energy consumption
Pump input power is directly proportional to the product of
the water flow rate and pressure drop around the circuit.
The water-side resistance of heat generators varies, by as
much as two orders of magnitude, depending upon the type
of heat generator selected and the design flow/return
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heat
generators
However, with secondary circuits using two-port control, a
fall in heating demand will result in a fall in return water
temperature. Where return water temperature is used for
control purposes, this should therefore be measured in the
primary circuit and the primary circuit water flow rate
should be constant. Likewise, where flow temperature in
the primary circuit is to be used as a proxy for heating
demand, the primary water flow rate should be constant.
When flow temperature compensation of heat generators is
used a change in the primary return temperature could be a
result of either a change in heating demand or a change in
flow temperature set-point. Using primary return water
temperature alone as a proxy for heat demand can then lead
to instability.
Hydronic systems
1-93
S
Secondary circuits
Constant temperature flow
1
2
3
Variable temperature flow
Variable temperature return
Primary circuits
R
Automatic
air valve
Primary
flow
Secondary
flow
Vertical
header
Primary
return
X
Secondary
return
Sludge
trap
Drain cock
Figure 1.45 Details of low-loss header arrangement
temperature difference. Lower water content boilers will
tend to have higher resistance. Even so, input power to
primary circuit pumps is likely to be considerably less than
1% of the rated heat output rate of the heat generator. As a
result, even though pump run times might be significantly
longer than heat generator run times and will tend to run at
a constant duty, annual energy consumption by the primary
circuit pump(s)is unlikely to be greater than about 1% of
that of the heat generators. However, when relatively low
carbon fuels are used to generate the heat, the percentage of
CO2 attributable to the pumps electrical consumption could
become significant, particularly when designing low carbon
buildings or in meeting increasingly stringent building
energy codes. Consideration should therefore be given to
the method by which the water flow rate in the primary is
achieved and controlled, the hydraulic resistance of the
heat generators and the manufacturer’s specified minimum
water flow rates.
1.8.6
Interface between primary and
secondary circuits
The interface between the secondary circuits and the
primary circuit should achieve the following objectives:
(1)
Prevent the operation of the pump/control valves in
one of the secondary circuits affecting the water
flow rate in any other secondary circuit.
(2)
Ensure the safe and stable control of the heat
generators.
This interface is in the form of either a single or a twin header
which should be selected to suit the characteristics of the
heat generators. A number of solutions are discussed below.
Constant temperature return
1.8.6.1
Figure 1.44 Principle of
primary-secondary circuits
separation by a low loss header
Single, low loss header
Figure 1.44 shows a typical, simplified arrangement using a
single, low loss header into which primary flow and return
headers and all secondary flow and return circuits are
connected. The primary circuit shows a common circulation
pump but individual pumps could be provided to each heat
generator.
In order to achieve objective 1, the total pressure throughout
the length of the header should be very nearly constant. This
requires that the diameter of the header should be sized for a
very low pressure loss. A maximum velocity of 0.15 m/s at
full heating load is recommended (Palmer, 2015). An
immediate consequence of the low flow velocity is the
potential for sludge and debris to collect in the header. For
this reason, a low loss header should be mounted vertically
where possible with a sludge trap and drain cock at the
bottom. Figure 1.45 shows the arrangement of the low loss
header. (For convenience, only one pair of connections to
secondary circuit is indicated.) The lowest connection point
on the header should be above the level at which sludge
collects. As air will collect in the top of the header, an
automatic air valve should also be provided. Where space
constraints dictate that the low loss header be installed
horizontally, an automatic air vent should be installed
together with end flanges that can be removed for inspection
and cleaning, particularly during pre-commissioning stage.
This arrangement will in addition ensure that the water
flow rate in the primary circuit is independent of the water
flow rates in the secondary circuits, so meeting objective 2
where the heat generators used require a minimum water
flow rate, for example low water content boilers.
To maintain a constant, neutral pressure in the low loss
header, the pressurisation system should be connected
directly to the header and all pumps, in both primary and
secondary circuits, should be arranged with their suction
inlets towards the header.
Some boiler manufacturers give recommended dimensions
of the low loss header, based on the full heating load and
flow/return water temperature difference. These
recommendations vary somewhat between manufacturers,
who should be consulted. For example, one manufacturer
gives the distance between the secondary flow and return
connections (distance X in Figure 1.45) as a function of
total heating duty (Strebel, undated). An analysis of this
data (based on a 20 °C temperature difference) suggests an
allowance of 260 mm plus an additional 1.0 mm for every
1 kW of heating duty up to a total of 600 kW and for more
than 850 kW an allowance of 660 mm plus an additional
0.35 mm for every kW duty.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heat
generators
1-94
Heating
S
Secondary circuits
Constant temperature flow
1
2
3
Variable temperature flow
Low temperature return
High temperature return
R
S
Figure 1.46 Primary and secondary
circuits with separate flow and
return headers
Secondary circuits
Constant temperature flow
Heat
generators
1
2
3
Variable temperature flow
Low temperature return
High temperature return
Figure 1.47 Primary and secondary
circuits with low and high
temperature return headers
Secondary circuits
S
Constant temperature flow
Heat
generators
1
2
Variable temperature flow
Buffer
vessel
3
Variable temperature return
Primary circuit
Constant temperature return
R
Secondary circuits
S
Low loss
header
Heat
generators
1
2
Figure 1.48(a) Buffer vessel utilised
as low loss header
Constant temperature flow
Variable temperature flow
3
Variable temperature return
Primary circuit
Buffer
vessel
R
Using this manufacturer’s guidelines and a recommended
maximum water velocity of 0.15 m/s, a low loss header
serving a heating installation with a maximum duty of
1500 kW and designed for a flow/return temperature
difference of 20 °C, would have a (nominal) diameter of
400 mm and the distance between the secondary circuit
connections would be 1200 mm.
Constant temperature return
Figure 1.48(b) Buffer vessel in
common return
Primary/secondary flow rates
If the flow water temperature to each of the secondary
circuits is to be the same as that in the primary circuit,
then, referring to Figure 1.44, the direction of flow in the
low loss header must always be in the direction of S to R. In
order to ensure this, the flow rate in the primary circuit
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heat
generators
Hydronic systems
In Figure 1.44, if the primary flow rate exceeds the sum of
the secondary flow rates, the temperature differential across
the primary circuit will be less than across the secondary if
the heat output is to match heat demand. As both primary
and secondary circuits will be receiving water at the same
temperature, (S), the temperature of the water in the
common primary return (R) will be greater than that
returning from the secondary circuits, (qr,hg > qr,L in
equations 1.56 (a) and (b)). This may affect the efficiency of
the heat generators installed: for example the COP of heat
pumps may improve whilst the efficiency of condensing
boilers will be reduced, (see Figure 1.41).
1.8.6.2
Separate flow and return headers
Where the heat generators have a relatively high water
content and do not require a minimum water flow rate (as
discussed above), separate flow and return headers can
provide the interface between the primary and secondary
circuits shown in Figure 1.46 and the secondary pumps are
used to provide flow through the heat generators eliminating
the need for primary pumps. The flow rate through the
primary circuit will then vary.
With constant volume secondary circuits the return water
temperature will rise with a fall in heating demand whilst
with variable volume circuits it will decrease (see section
1.8.4.1). As a result, the common return water temperature
at R may increase or decrease as the heating demand falls.
Where, the majority of the heating demand is served by
variable volume circuits, the water temperature at R will
fall with reducing heat demand. For condensing boilers,
this will result in increased condensation and a
corresponding improved efficiency.
Where step control is used, the arrangement will still suffer
from flow temperature dilution. Temperature dilution can
be reduced by isolating heat generators when not firing
(with the same proviso as before regarding ensuring a water
flow through the heat generator prior to firing and for a
minimum time after firing has ceased). As the flow rate,
flow temperature and return temperature in the primary
circuit are all varying, a heat meter should be used to
provide a signal for step control.
High and low temperature return headers
Some condensing boilers are provided with two return
water connections, one for low and one for high temperature
return water as discussed in section 1.7. This increases the
condensation rate and therefore the boiler efficiency.
Where some of the secondary circuits are designed to
operate at lower flow and return temperatures than others,
there is advantage in providing two separate return headers,
one serving the low temperature secondary circuits and the
other the remainder. This is as shown in Figure 1.47.
1.8.6.3
Buffer vessels
Buffer vessels are used:
——
to reduce excessively frequent starts of heat
generators which both reduces system efficiency
and leads to premature plant failure
——
to prevent overheating of heat generators (such as
biomass boilers) on shut down.
Recommended arrangements for some specific heat
generators are given in section 1.8.8.
In some cases the buffer vessel may be used in place of a
low-loss header and will contain connections for the
primary flow and return and individual connections for the
flow and return secondary circuits as shown in Figure
1.48(a).
The heat generator(s) are controlled by one or more
temperature sensors in the buffer vessel. Multiple sensors
allow for stratification. When the heat output rate of the
primary circuit exceeds the heat demand of the secondary
circuits, the temperature of the water in the buffer vessel
will increase as it absorbs the excess heat until the upper
temperature limit is reached at which point the heat
generator(s) are switched off. The buffer vessel will then
continue to supply heat to the secondary circuits until the
temperature within the vessel falls below its lower limit and
switches the heat generator(s) back on.
As an alternative, the buffer vessel may be used placed in
the common return of the primary circuit (Figure 1.48(b))
in which case a low loss supply header is still required.
The need for and sizing of buffer vessels should be discussed
with the manufacturer of the heat generators.
Buffer vessels should not be confused with thermal stores
which are also discussed in section 1.8.8.
1.8.7
General arrangement of MTHW
and HTHW systems
Heat generators capable of producing MTHW (90–120 °C)
and HTHW (>120 °C) are limited to boilers and some large
capacity CHP. The essential difference between such
generators and their counterparts used to produce LTHW is
that they must be able to operate safely at the elevated
pressures required to prevent boiling. These system must
comply with the Pressure Systems Safety Regulations,
2000, (HSE, 2014 – AcoP 122 Safety of Pressure Systems,
HSE, 2014). System pressurisation is achieved by
pressurisation units using pumps (MTHW) or nitrogen
cushions (MTHW and HTHW). These are described in the
Hydronic system design appendix of this Chapter.
and HTHW offer greater flow/return water
temperature differences than can be achieved using LTHW,
significantly reducing water flow rates leading to reductions
in pipe sizes and pumping energy. MTHW and HTHW
systems are therefore usually employed where there is
substantial pipework distribution such as in large sites (e.g.
district hospitals) and district heating. Even so, the higher
MTHW
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
must always be equal to or greater than the total flow rate
into the secondary circuits. If not, some flow reversal will
occur in the header with cooler water from the secondary
returns flowing upwards in the header and mixing with hot
water from the primary flow. This is likely to occur if the
heat generators are controlled in sequence and the water
flow through those heat generators not firing is stopped in
order to prevent flow temperature dilution in the common
flow header. The flow rate in the primary circuit will then
reduce in steps as the heating demand falls. When controlled
in unison, the flow rate through the primary circuit will be
constant.
1-95
1-96
Heating
Output from renewable
heat generator
Time of day
kW demand/output
Output from
thermal store
Output from renewable
heat generator
Time of day
Figure 1.49 Top-up by (a) fossil-fuelled boiler (above) and (b) use of
storage vessel (below)
capital costs associated with plant and pipework being
specified for these higher pressures may outweigh such
savings. Further, generating and distributing hot water at
these elevated temperatures and pressures increases the risk
of serious injury to building users and operators which may
be unacceptable to the client. As a result, MTHW and
especially HTHW systems are far less commonly installed
than LTHW.
In MTHW and HTHW a primary/secondary circuit
arrangement is employed as for LTHW. However, it is not
uncommon to step-down the flow temperature and pressure
of final circuits serving heat emitters and calorifiers by
means of heat exchangers. In this way, such final circuits
can be designed and operated as conventional LTHW
secondary circuits. This maintains the advantage offered by
MTHW and HTHW in the sizing of the main distribution
pipework.
1.8.8
Integration of renewable/low
carbon heat generators
This section covers the integration of heating technologies
that either use renewable energy sources (e.g. biomass
boilers) or are categorised as low carbon technologies (e.g.
CHP and heat pumps) within a heating system. The general
principles of the design and control of hydronic systems,
including the heat generators in the primary circuit, is
discussed in sections 1.8.4 to 1.8.6. However, the use of
renewable/low carbon heat generators raises specific issues,
not least because they are often used in combination with:
——
top-up fossil-fuelled boilers
——
thermal storage
——
a mixture of the two.
The capital cost of renewable and low carbon heat generators
(£/kW) is generally significantly greater than for fossil-
fuelled boilers as a result of mass production of the latter.
When dealing with heating of buildings, peak heat demand
occurs only infrequently, especially in modern, wellinsulated buildings. It is therefore advantageous to select
renewable/low carbon heat generator(s) with a rated heat
output (kW) less than that of peak demand and to utilise
relatively inexpensive gas or oil-fired boilers to provide topup (and back-up) only when needed. This is illustrated in
Figure 1.49(a).
In order to achieve this, the renewable/low carbon heat
generator must operate as the lead heat generator and the
fossil-fuelled boiler only operate when the former is
operating at full load. As discussed in section 1.8.5, this will
not occur if the heat generators are installed in parallel.
Thermal storage vessels are used with a wide range of
renewable/low carbon heat generators. These allow the
renewable/low carbon heat generator to be run when there
is little or no heat demand. At such times the heat is stored
in the thermal store and used to provide the top-up when
heat demand exceeds the output of the heat generator(s).
This is illustrated in Figure 1.49(b). In this way, the
renewable/low carbon heat generator can provide all of the
heat energy and yet be sized well below the peak heat
demand.
Alternatively, the renewable/low carbon heat generator(s)
can be installed in combination with both a thermal store
and a top-up fossil-fuelled boiler, allowing the size of
thermal store to be reduced whilst still allowing the
renewable/low carbon heat generator to provide most of the
heating.
The proportion of heat produced by the renewable/low
carbon heat generator will not only determine the carbon
emission savings achieved, which may be of crucial
importance in meeting local or national regulations, but
can also significantly improve payments under the
Renewable Heat Incentive (RHI).
Table 1.40 outlines some key features of each heat generator/
heat source that will influence how they are integrated into
a hydronic system. A good understanding of these should
enable the building services engineer to make the correct
decisions. There are many ways of integrating the different
types of heat generators into hydronic heating systems.
This section will provide only an overview. Further details
are available in other CIBSE and BSRIA guidance
documents:
——
CIBSE AM12: Small-scale combined heat and power
for buildings (2013)
——
CIBSE AM15: Biomass heating (2014)
——
CIBSE TM51: Ground source heat pumps (2013)
——
CIBSE AM14: Non-domestic hot water heating systems
(2010)
——
BSRIA BG7/2009: Heat pumps – A guidance document
for designers (Brown, 2009)
——
BSRIA BG2/2007: CHP for existing buildings –
Guidance on design and installation (Teekarum, 2007).
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
kW demand/output
Output from fossil fuel
heat generator
Hydronic systems
1-97
Table 1.40 Key features relevant to the combining of different types of heat generator
Features
Non-condensing
gas or oil boiler
Return water temperature should not be allowed to fall below about 65 °C so as to prevent condensation at the back
end of the boiler. Back-end protection loop required (see Figure 1.43)
Minimum flue gas temperature of 140 °C to avoid back end corrosion, can limit practical turndown.
Flow temperature up to about 85 °C to provide design temperature differential up to about 20K.
Condensing boiler
To maximise efficiency return water temperature should be as near to 30 °C as possible. Manufacturers often quote
peak output/efficiency at 50/30 °C flow and return water temperatures. Flow temperature often directly weather
compensated.
Typical design temperature differential across the boiler of 20K to minimise pumping energy.
Normally operated using unison control to maximise boiler efficiency (see section 1.8.3.2)
Biomass boiler
Return water temperature must be maintained above 60 °C (wood pellet boilers) or 75 °C (woodchip boilers subject to
moisture content of fuel). Boilers should therefore incorporate a back-end protection loop (see Figure 1.44). Some
boilers utilise a dynamic return temperature control.
Need to be combined with a buffer vessel to dissipate heat on shut-down or, preferably, a thermal store.
Heat pump
(ground source)
Work most efficiently with low flow temperature (35–50 °C) therefore compatible with condensing boilers and weather
compensated heating distribution systems. Generally not suitable for DHW storage (except as pre-heat).
Small differential between flow and return temperature (e.g. 5 to 7K) needed to both maximise heat pump COP and
minimise emitter size.
Heat pump
(air source)
Most of the features of ground source type also apply to air source. In addition:
Combined heat and
power (CHP)
——
air source heat pumps have lowest CoP and heat output at design winter conditions – when maximum heat
output required
——
defrost cycle of air source pumps will be required during coldest weather.
Small-scale CHP produces flow water temperatures in the range of 80–90 °C; large-scale CHP more likely to produce
steam or MTHW.
Most CHP units designed on constant water flow rate with a 20 K differential.
Turndown limited to 25–50% of maximum continuous rating.
Performance often best when operated at maximum temperature into a 4-port thermal store, with a flow mixing valve
to reduce flow temperature to that required, to minimise heat ‘dumped’ to atmosphere and enable unit to run at full
load.
Heat from district
heating system
Flow temperature will depend on central plant but often 90 °C at peak load. The flow temperature is often scheduled
to the outdoor temperature.
Penalties are commonly applied to consumers if return water temperature from the consumers plant rises above a
maximum (e.g. 50 °C).
Plate heat exchangers normally used to hydraulically separate the district heating and customer’s plant.
Solar thermal
Flow temperatures can vary greatly depending upon weather conditions. Evacuated tubes can achieve temperatures in
excess of 90 °C in summer. Conversely, in winter, flow temperatures may be only about 30 °C.
More usually directly integrated with domestic hot water system but can be connected to a thermal store together with
other heat generators.
Maximum output occurs in summer when demand may be reduced and system may experience thermal stress.
1.8.8.1
Heat pumps
The main reasons why heat pumps are often installed with
conventional heat sources in a hydronic heating system
include:
——
the heat pump may not have sufficient capacity for
all weather conditions
——
the heat pump flow temperature may be too low for
some of the heating circuits
——
in existing buildings conventional heat sources
may be retained to cover plant breakdown
——
the combination may result in overall lower carbon
emissions than a heat pump alone
——
the heat pump has been sized to provide only a
fraction of the heating energy in order to meet
carbon emissions targets.
In buildings, the maximum peak space heating load rarely
occurs: sizing the heat pump(s) to meet the peak load would
result in a large and expensive machine(s). It should also be
noted that the available output from an air source heat
pump will decline as the outside temperature decreases,
further increasing the size of the required heat pump. In
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heat generator/source
1-98
Heating
Low loss header
Heat
pump
Heating
circuits
NC
Figure 1.50 Heat pumps connected
in parallel and with a top-up fossil
fuel boiler in series
Low loss header
Fossil
fuel
boiler
Heating
circuits
CHP
NC
Figure 1.51 CHP and fossil fuel
boiler in series
Low loss header
Fossil
fuel
boiler
4 port
thermal Low loss header or
store thermal store/buffer
CHP
Condensing
boiler
Heat
pump
the case of a ground source heat pump sizing the system on
‘design conditions’ could potentially grossly oversize the
ground collector. It would also mean unnecessarily high
capital costs in systems where this is dominated by the cost
Heat
of the ground
collector. Undersizing the ground collector
pump
however will result in the heat pump being unable to meet
thermal comfort conditions without back-up heating and
that the circulation temperature
ground
is lowered,
Low in
lossthe
header
or
which may lead to heat extraction
becoming
unsustainable.
thermal
store/buffer
A suitably sized thermal store will usually overcome these
issues.
Condensing
Heating
circuits
Figure 1.52 CHP and thermal store
with fossil fuel boiler in series
Sometimes a supplementary heat source is incorporated in
the system to cover the few days of very cold weather. In
retrofit projects this can be an existing boiler but in new
systems however it could be a small condensing boiler or a
direct electric source particularly if there is a source of
renewable electricity available on the site. It should be
noted, however, that BS EN 15450: Heating systems in
buildings – design of heat pump heating systems (2007) advises
that incorporating an additional back-up heater be selected
such that the energy supplied by the back-up system is
reduced to a minimum (e.g. less than 5 % of the total energy
supplied by the heat pump if the energy source of the back-
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heat
pump
Fossil
fuel
boiler
Hydronic systems
up heater is not renewable). A rule of thumb is to restrict
the use of non-renewable supplementary heat sources to no
more than 10% of the annual heating energy requirement.
The key requirement for optimum heat pump performance
is that it should always be the lead heating appliance and
operate at maximum possible output with the lowest
possible temperature lift. The simplest way to achieve this
is for the heat pump to be connected in series with the
conventional heat generator output. This will allow the
heat pump to act as lead heat generator (as discussed in
1.8.5) and allow the relatively low flow water temperature
from the heat pump to be boosted. Condensing boilers are
convenient for this as their efficiency is improved at low
inlet temperatures. The design of the heat distribution
system and heat emitters should be based on as low a flow
temperature as economically feasible without oversizing
heat emitters. In addition:
——
the distribution system controls resulting in as low
a return temperature to the heat pump as possible
(i.e. using two-port controls at emitters)
——
the differential between flow and return being
smaller than that for a condensing boiler circuit.
This will increase pumping power but with variable
volume controls this would be minimised
——
flow temperature weather compensated directly at
the heat pump and boiler. (The flow temperature
could incorporate a boost function at very low
ambient temperatures).
Figure 1.50 shows an example for hydronic integration of
heat pumps and a condensing boiler. The heat pumps are
connected in parallel to each other and the condensing
boiler in series with the heat pumps.
Under low load conditions the boiler is isolated (with a
control interlock to prevent firing) to avoid pumping
through the boiler and therefore minimise standing losses.
Under high load conditions the boiler modulates in series
with the heat pump to maintain the flow temperature in the
header. The flow and return temperatures should be
selected to suit the heat pump and heating load. An
appropriate return temperature is about 40 °C.
High temperature heat pumps are available to provide
water at 60 °C for domestic hot water production (including
those using CO2, see section 1.7) but it may be more cost
effective to use a low temperature heat pump to pre-heat the
cold water feed to the DHW and utilise another form of
heating for domestic hot water, such as a condensing gas
fired water heater. (Often heat rejection from air
conditioning plant can also be used to supplement DHW
heating by the inclusion of de-superheaters in the
refrigeration circuits.)
Due to the operating temperatures of heat pump systems,
they are not readily integrated with systems that need to
operate at high temperatures such as biomass boilers, CHP
units or non-condensing boilers in constant temperature
LTHW systems. They can however be integrated with solar
thermal particularly for domestic hot water heating to
reduce domestic hot water loads on the heat pump.
Alternatively solar thermal can be used to increase the heat
source fluid temperature / recharge the ground in a ground
source system.
Buffer vessels
Constant speed heat pumps are prone to premature failure
and reduced COP as a result of frequent on/off cycling.
Manufacturers usually recommend installation of a buffer
vessel with such heat pumps. A recent study commissioned
for Department of Energy and Climate Change (Kiwa,
2013) found that a buffer vessel could reduce cycling and
reduce energy consumption. See Section 1.8.8.3 below for
further details on buffer vessels and thermal stores. Variable
speed heat pumps may not require buffer vessels.
Manufacturer’s guidance should be sought.
1.8.8.2
Combined heat and power (CHP)
When CHP is installed in combination with a fossil-fuelled
boiler, the preferred solution is to connect the heat
generators in series as shown in Figure 1.51. (Note that this
figure is intended to illustrate the principle only and does
not show all valves.) With this arrangement the CHP can
operate as the lead heat generator. The fossil fuel boiler can
be controlled by means of heat meters or the flow water
temperature to the low loss header so that it will operate
only when the CHP cannot meet the heating demand but
still allow the CHP to run at full load.
The primary pump will ensure a flow of cooling water
through the CHP at all times. The fossil-fuelled boiler pump
runs only when the boiler is required to operate and should
be selected to ensure that the flow rate through the CHP is
not unduly affected. The normally closed bypass is provided
so that the fossil fuel boiler can act as a back-up when the
CHP is offline for maintenance.
The addition of a thermal store will allow the CHP to run for
extended periods at full load and provide up to 100% of the
heat energy without increasing the size of the CHP plant. A
preferred arrangement is shown in Figure 1.52. There may
be a need to ensure that the water temperature entering the
CHP unit never exceeds that recommended by the
manufacturer.
The CHP operation is controlled by the water temperature
in the thermal store and will run until the store is fully
charged irrespective of heating demand from the heating
circuits. The set point of the water in the store can be set
higher than the design flow water temperature to the
heating circuits so that the thermal capacity of the store is
maximised. The three-port mixing valve on the outlet from
the top of the store can then blend the flow water temperature
to the design value. The fossil fuel boiler can be controlled
as in Figure 1.51.
Determining the storage volume of the thermal store is a
complex optimisation process for which computerised
simulation models have been developed. For example, for a
community heating scheme a thermal store could be sized
to meet a full day’s domestic hot water demand. The
selection of storage temperature can be considered
independently from the distribution temperature as part of
the optimisation of the storage system. The lower the
design return temperature, the better viability can be
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The supplementary heat source should not be used to
reduce the pre-heating period. This is particularly
important in modern well insulated buildings where the
heating demand can be very small once the building is
occupied with much of the annual heating energy being
consumed prior to occupancy.
1-99
1-100
Heating
Low loss header
4 port
thermal
store
Biomass
boiler
Heating
circuits
Figure 1.53 Biomass boiler and thermal
store with fossil fuel boiler in parallel
Low loss header
Fossil
fuel
boiler
4 port
thermal Low loss header or
store thermal store/buffer
Biomass
boiler
Heating
circuits
Condensing
boiler
Figure 1.54 Biomass boiler and thermal
store with fossil fuel boiler in series
Heat
pump
achieved for the store as the energy storage capacity will be
higher for a given volume and stratification in the store can
be utilised, as described in the following section for biomass
boilers. The Carbon
Trust’s Biomass Decision Support Tool
Heat
can be used pump
to produce a load profile for a district heating
network and an estimate of the thermal storage required for
a CHP unit on a network (Carbon Trust, 2015).
Low loss header or
thermal store/buffer
Biomass boilers
1.8.8.3
CIBSE AM15: Biomass heating (2014) describes in detail
how biomass boilers can be integrated into low temperature
hot water Condensing
systems with fossil fuelled boilers and the systems
controlled. Itboiler
also addresses the integration of both buffer
vessels and thermal stores.
AM15 clearly differentiates between buffer vessels and
thermal stores as follows:
——
——
incorporates the functions of a buffer vessel within
it. A thermal store also provides start and stop
signals to automatic ignition boilers to ensure
efficient and stable boiler operation.
This section is restricted to the use of thermal stores with
biomass together with the integration of fossil-fuelled
boiler(s) that will together allow the rated output of the
relatively expensive biomass boiler to be minimised but
still provide most of the heat demand throughout the year.
Figures 1.53 and 1.54 show the preferred configurations for
a biomass boiler in parallel and series with a fossil fuelled
boiler utilising a 4 port stratified thermal store.
Some of the key features of these configurations are:
——
If the thermal store has sufficient capacity, the
biomass boiler is able to operate continuously
within its modulating range charging the thermal
store. This improves the biomass boiler efficiency
by preventing the boiler from switching off or
dropping into slumber mode.
——
The thermal store helps to ensure that as much heat
as possible is generated from the biomass boiler and
stored for future use.
——
A biomass boiler rated at significantly below the
peak load can, together with the thermal store, meet
Heat
pump
Buffer
vessel: Used to improve biomass system
efficiency by capturing residual heat from a biomass
boiler on shut-down, to provide start and stop
signals to automatic ignition boilers to ensure
efficient
Heatand stable boiler operation.
pump
Thermal store: used to enable a relatively small boiler
to provide a large proportion of the annual energy
demand from biomass. Typically a thermal store is
much larger than a buffer vessel and often
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Fossil
fuel
boiler
Hydronic systems
1-101
Table 1.41 Effects of finishes and architectural features on radiator
performance (taken from AM14, Table 4.11)
Effect
Ordinary paint or
enamel
No effect, irrespective of colour.
Metallic paint such
as aluminium
Reduces radiant output by 50% or more and
overall output by between 10 and 25%.
Bronze
Emission may be substantially restored by
applying two coats of clear varnish.
Open fronted
recess
Reduces output by 10%.
Encasement with
front grille
Reduces output by 20% or more, depending on
design.
Radiator shelf
Reduces output by 10%.
Fresh air inlet at
rear with baffle at
front
May increase output by up to 10%. This increase
should not be taken into account when sizing
radiator but should be allowed for in pipe and
boiler sizing. A damper should always be fitted.
Distance of
radiator from wall
A minimum distance of 25 mm is recommended.
Below this emission may be reduced due to
restriction of air-flow.
Height of radiator
above floor
Little effect above a height of 100 mm. If
radiators are mounted at high level, output will
depend on temperature at that level and
stratification may be increased.
peak loads without the need for fossil fuelled boiler
support. The fossil fuelled boiler tends to act as a
back-up rather than a top-up.
——
——
——
The disadvantage of parallel connection of heat
generators (whereby all heat generators operating
do so at the same load fraction), as discussed in
section 1.8.5, is partially if not wholly overcome as
the fossil-fuelled boiler rarely operates.
The ability to operate the biomass system at a
higher temperature than the load circuits, or fossil
fuelled boiler, to allow greater energy storage with
the thermal store or, conversely, a smaller thermal
store for a given energy storage requirement.
The thermal store itself acts as a hydraulic separator
ensuring that the biomass heat boiler is hydraulically
isolated from other equipment. This ensures it
cannot be influenced by either the fossil fuelled
boiler or the load circuits.
——
To ensure forward flow along the low loss header at
all times the flow rate of the thermal store pump
must be 10% greater than that of the load pumps.
These pumps should have variable speed drives.
(Typical control strategies can be found in AM15.)
——
On constant temperature load circuits, control of
variable speed load pumps could be based on the
temperature difference across the load to maintain
the design load temperature difference under all
operating conditions. This would minimise the
flow rate in the load circuit which then allows the
thermal store pump to operate at a corresponding
flow rate.
In the parallel arrangement, a condensing fossilfuelled boiler may be used and will condense
whenever the return water temperature from the
header falls below about 55 °C. In the series
arrangement, there is no advantage in using a
condensing boiler.
1.8.9
Heat output rate of heat
emitters
The heat output rate of heat emitters used in hydronic
systems is a function of:
——
mean water temperature in the emitter
——
temperature of the adjacent air and surroundings
——
heat emitter surface area
——
heat emitter thermal characteristics.
These can be described in terms of fundamental heat
transfer theory. However, in reality, the geometry of most
heat emitters is complex with heat emitters being selected
from test data obtained under standardised conditions.
1.8.9.1
Fundamental heat transfer
All heat emitters exchange heat by a combination of
convection, radiation and conduction though conduction is
usually negligible and ignored. In the case of some types of
convector, heat exchange by radiation is usually sufficiently
small to ignore. However, all emitters, including so-called
radiators and radiant heaters, include some convection:
only in very high temperature radiant systems does the
radiant heat transfer exceed the convective. An overview
only of the fundamentals is provided here. CIBSE Guide C
contains more detail.
Convection
The heat exchange rate by convection only, Φc (W), can be
represented by the equation:
Φc = A hc (θm – θai)
(1.57)
where A is the surface area of the heat emitter in contact
with the air (m2), hc is the air-side mean convective heat
transfer coefficient averaged over the area A (W/m2·K), θm is
the mean surface temperature averaged over the area A (°C)
and θai is the air temperature within the room (°C).
The surface area A can be difficult to measure as it will
often include fins and dimpled surfaces.
The heat transfer coefficient on the water-side is normally
orders of magnitude greater than on the air side so that the
surface temperature at any point is close to that of the water.
As a result, θm is often taken as being equal to the average of
the inlet and outlet water temperatures, though this is not
always the case particularly at low flow rates in radiators
when it can be much nearer to the outlet water temperature
due to mixing within the radiator.
The air-side convective coefficient is highly dependent
upon the local air velocity. In otherwise still air, heat
transfer is by natural convection driven by the changes in
buoyancy of the air.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Feature
——
1-102
Heating
Typically for natural convection:
hc = k ( θm – θai )
(1.58)
p
For forced convection, the value of hc must be determined
from equations typically of the form:
hc = k (Re)
(1.59)
q
where Re is the Reynolds number, a function that includes
the local air velocity; the exponent q varies between about
0.5 for laminar flow and 0.8 for turbulent flow.
CIBSE Guide C, chapter 3, includes equations for free and
forced convection over flat plates and tubes.
Radiation
The fundamental equation for radiant heat exchange Φr
(W) by radiation only is:
Φr = A F σ (θ 4m – θ 4room )
(1.60)
where A is the surface area of the heat emitter that can 'see'
the room (m2), F is the view factor between the heat emitter
and the room (dimensionless), θ m is the mean surface
temperature of the emitter averaged over surface area A
(K), θ room is the radiant temperature of the room as seen by
the heat emitter (K) and σ is the Stefan-Boltzmann constant
(5.67 × 10–8 W/m2·K–4).
The view factor for a heat emitter in a room where the room
dimensions are large compared to that of the emitter and
where the room surfaces have typical emissivities close to
unity is approximately equal to the emissivity of the
emitter, ε.
The radiant temperature of the room as seen by the emitter
is normally very nearly equal to the room mean radiant
temperature θ r (see CIBSE Guide A). Then equation 1.60
can be approximated with good accuracy in most cases to:
Φr = A ε σ (θ 4m – θ 4r )
(1.61)
Equation 1.61 can be further approximated to an accuracy
of about 2% by a linear function of temperature difference,
similar to that of the convective equation:
Φc = A hr (θ m – θ r)
(1.62(a))
where hr is the radiant heat transfer coefficient averaged
over area A (W/m2K).
It can be shown that for the temperature ranges typical of
most heat emitters, a useful approximation is:
hr = 4 ε σ θ 3
Manufacturers are obliged to quote the nominal output of
the emitter under a standard method for testing as specified
in BS EN 442-2 (2014).
The standard emission is under conditions of ‘excess
temperature’ of 50 K, i.e:
∆θ = (θ m – θ ai) = 50
(1.63)
where ∆θ is the excess temperature (K), θ m is the mean
water temperature within the emitter (°C) and θ ai is the
temperature of the surrounding air (°C).
The test conditions require that the surrounding mean
radiant temperature does not differ significantly from the
surrounding air temperature. They also require that the
inlet and outlet temperatures should be 75 °C and 65 °C
respectively in surroundings at 20 °C. The designer is not
obliged to adhere to these temperatures.
The ‘water-side’ of the heat exchange is given by:
Φ = qm cp (θ 1 – θ 2)
(1.64)
where Φ is the heat emission (W), qm is the mass flow rate
(kg·s–1), cp is the specific heat capacity of water (J·kg–1·K–1),
θ 1 is the inlet temperature (°C) and θ 2 is the outlet tem­
perature (°C).
The ‘air-side’ of the heat exchange is given by:
Φ = Km ∆ θ n
(1.65)
where Km is a constant for a given height and design of
emitter and n is an index.
The effects of architectural features and surface finish on
radiator output are summarised in Table 1.41. In general, it
may be observed that heat output is reduced when airflow is
restricted, such as by placing a shelf immediately above a
radiator, or by an enclosure. It is also reduced by surface
finishes with low emissivity, such as metallic paints or
plating.
Radiator output is also affected by the form of connection
to the system pipework. Testing is commonly done with top
and bottom opposite end (TBOE) connections. Other forms
of connection produce reduced outputs which may be
Chilled water in
Chilled water out
(1.62(a))
where θ = (θ m + θ r) / 2.
1.8.9.2
Radiators and natural convectors
Both radiators and convectors emit heat by a mixture of
radiation and convection. Even for a ‘radiator’, the
convective component may be well over half the total heat
emission when fins are included either behind or between
panels. The radiant heat output of a natural convector will
Hot water in
Hot water out
Figure 1.55 Active chilled beam diagrammatic showing chilled and hot
water coils
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
where the exponent p is typically 0.25 for laminar flow and
0.33 for turbulent flow; the constant of proportionality k
will depend upon the geometry, orientation and dimensions
be determined by the surface temperature of the casing and
is generally very small as the case is close to room
temperature.
Hydronic systems
1-103
recirculation facility is incorporated into the central air
handling plant. Fan energy in addition to pump energy will
also be required to drive the beams, whereas with radiators
only pump energy is required.
The characteristics of fan coil heaters are described in BS
4856 (BSI, 1972, 1997) which gives test methods for heat
output and air movement with and without attached
ducting, and for noise levels without attached ducting. The
heat output from fan coil heaters is approximately linear
with the difference between system temperature and room
air temperature, corresponding to n = 1.0 in equation 1.65.
The height of a space is also a determining factor in the
suitability of using active beams for heating. Most
manufacturers use a ceiling height of approximately
2700 mm, this is important because a height greater than
this may lead to poor distribution of the conditioned air
and bigger temperature gradients, resulting in stratification.
1.8.9.3
applying
factors
obtained
Fan coil heaters
The output from fan coil units is generally more sensitive
to airflow problems than to water circulation and this
should be borne in mind both at the design stage and when
investigating problems. Other practical difficulties with fan
coil units can arise from the use of copper tubing in their
fabrication, which can lead to corrosion if traces of sulphides
remain following manufacture.
1.8.9.4
Active beams
Most modern highly insulated buildings where the steadystate heating load is generally in the range 35–45 W/m2 are
suited to the application of heating from active beams as the
heat output is generally limited to 150 W/m length of beam.
Outputs higher than this can result in the secondary air
being too warm to mix properly with the room air, which
may cause stratification of warm air at ceiling level and
unacceptable cool temperatures at floor level.
Buildings with a high proportion of glazing or older
buildings which may have particularly leaky façades may
require some form of perimeter heating to offset cold
downdraughts or cold radiant effects from the glazing and
therefore providing heating from active beams may be not
be appropriate.
A general rule of thumb is to use beams for perimeter
heating only if the internal surface temperature of the
windows is above 14 °C or the glazing height is not more
than 1.5 m.
A typical method of providing heating and cooling from an
active chilled beam is by dividing the number of passes on
the coil between the heating and cooling elements, as shown
in Figure 1.55. For example, for a coil with 8 passes across
the length of the beam, 6 would be used in cooling mode
and the remaining 2 would be used for heating.
Typically hot water is supplied at a flow temperature of
around 40 °C with a return temperature of about 30 °C
(dependant on manufacturer). These low temperatures are
required in order to limit the air off-coil temperature to
around 30 °C and so reduce the risk of stratification of the
room air. These low temperatures are well suited to the use
of condensing boilers or ground source heat pumps. The
heat output rate from the heating coils are usually relatively
small so that consideration should be given to on/off rather
than modulating control.
To operate correctly in heating mode the beams will need
the ventilation (primary air) to be operating. Therefore to
pre-heat the building before occupancy will therefore result
in heating the fresh air, which, on a winter’s day will be
very inefficient (even with heat recovery) unless a
The testing and rating of active chilled beams is covered by
BS EN 15116 (2008) and further guidance can be found in
the Chilled Beam and Ceiling Association (HEVAC/FETA)
publication An introduction to chilled beams and ceilings
(CBCA, 2012).
1.8.9.5
Underfloor heating
The floor surface itself is used as a heat emitter and heat is
supplied by the circulation of water as part of a hydronic
system, through appropriately spaced pipes positioned
beneath the floor surface. Details of underfloor heating are
contained in the BSRIA publication Underfloor heating and
cooling (Brown, 2011).
Much of the equipment required for floor heating systems
is the same as that used for other hydronic heating systems.
However, the heat emitting floor surfaces require careful
design to produce the required surface temperatures and
heat output. Surface temperature should not exceed 29 °C
in general or 35 °C for peripheral areas, which are defined
in BS EN 1264 (2008) as ‘generally an area of 1 m maximum
in width along exterior walls’ and ‘not an occupied area’.
Where occupants are seated, so that their feet are in constant
contact with the floor, a maximum surface temperature of
25 °C is preferable.
BS EN 1264-1 gives the heat output available from the floor
surface as:
φ = 8.92 ( θ fm – θ i ) 1.1
(1.66)
where φ is the heat output per unit area of floor (W·m–2), θ fm
is the average floor surface temperature (°C) and θ i is the
room operative temperature (°C).
The limitation on surface temperature leads to a
corresponding limitation on heat output. For a room
temperature of 20 °C, the maximum heat output rate is
100 W·m–2 in general and 175 W·m–2 at the periphery.
The floor surface temperature is affected by the spacing
between pipes and the flow water temperature. It is also
affected by floor construction, floor covering and the depth
of the pipes beneath the floor surface. In practice, systems
are usually designed to operate at flow temperatures of
between 40 and 50 °C, with a temperature drop of between
5 and 10 K across the system. Higher flow temperatures are
likely to cause cracking of the screed. The overall design of
floor heating systems should be undertaken in accordance
with BS EN 1264.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
from
corrected for by
manufacturers.
1-104
Variation of heat emitter output with
system water temperature
Natural convectors and radiators
BS EN 442-2 (2014) obliges the manufacturer to test the
radiator at excess temperatures ∆θ = 30 K, 50 K and 60 K
so as to determine the value of n. Thus if the test conditions
are not precisely those specified, the experimental readings
can be adjusted to correspond to the nominal conditions.
The manufacturer is not obliged to publish the value of n
but some manufacturers give data for both ∆T = 50 K and
∆T = 60 K. From such data it would be possible to deduce
the value of n using:
ln (φ60 / φ50)
n = —————­—
ln (60 / 50)
(1.67)
where φ60 is the heat emission at 60 °C (W) and φ50 is the
heat emission at 50 °C (W).
A value of n = 1.24 has been obtained from the quoted
outputs of one manufacturer, but values of up to 1.33 may
be encountered.
Then for any value of ∆θ, the output can be determined from:
φ = φ50 ( ∆θ / 50 ) n
(1.68)
Forced convectors
Where manufacturer’s data are not available then use can
be made of the heat exchanger effectiveness theory. This
theory states that the effectiveness of a heat exchanger
remains constant when the fluid temperatures may change,
provided that the fluid mass flow rates remain constant.
CIBSE Guide C, chapter 3, provides details of how to
calculate the effectiveness of a heat exchanger. This can be
done at design conditions and then used to calculate the
heat output rate at different water inlet temperatures.
1.8.9.7
Variation of heat output rate with
flow rate
As with water temperature, the impact of changing the
water flow rate will depend upon whether the heat emitter
utilises natural or forced convection. Once again, the
following do not apply to under floor heating or any other
heat emitter with high thermal mass.
Natural convectors and radiators
Although a lower flow rate might cause a slight decrease in
the water-side convection coefficient, this small increase in
resistance is small in comparison with the overall resistance.
Thus it is reasonable to consider that the overall heat
transfer coefficient will remain constant. A reduction in the
mass flow rate of the water has a greater effect on the mean
water temperature and it is this that affects the heat
emission.
100
(t1 – t2) = 10 K
80
(t1 – t2) = 20 K
60
40
20
0
0
20
40
60
80
100
120
Relative flow / %
Figure 1.56 Variation of heat output of radiator with water flow rate
One way of reducing emitter output and reducing pump
power consumption is to reduce the pump speed, and hence
the mass flow. The effect is considered here, assuming that
the flow temperature θ 1 remains constant. The mathematics
involves equating the water-side and air-side heat transfer
equations. i.e.:
qm cp (θ 1 – θ 2) = Km ∆ θ n
(1.69)
The mean water temperature, θ m = (θ 1 + θ 2) / 2, where θ 2
is the leaving water temperature (°C). Therefore, from
equation 1.63:
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The variation with mean water temperature depends upon
the type of heat emitter, particularly whether they rely on
natural or forced convection. The following does not apply
to floor heating systems or other heat emitters with high
thermal mass.
120
Relative heat output / %
1.8.9.6
Heating
(θ 1 + θ 2)
∆θ = ———– – θ ai 2
(1.70)
Hence, substituting into equation 1.54 and solving for the
unknown temperature θ 2 gives:
θ2 = θ 1 – Km {(θ 1 + θ 2) / 2 – θ ai}n / qm cp
(1.71)
Equation 1.56 contains θ 2 on both sides of the equation.
Once a starting value is inserted in the right hand side of
the equation, the value of θ 2 may be obtained by iteration.
Equation 1.49 will then readily yield the heat output.
Figure 1.56, which was obtained using the above method,
shows the effect on emitter output for flow rates less than
nominal. It can be seen that whatever the design value of
water temperature drop (θ1 – θ2), an appreciable reduction
in water flow rate causes little reduction in heat output.
Thus, except when full heat output is required (during the
pre-heat period), there is no need for the pumps to run at
full speed. Similarly it can be seen that increasing the flow
above the design flow does not boost the heat output
appreciably. A change in flow temperature from 75 °C to
65 °C does not make a significant difference to the shape of
the curves.
Forced convectors
The same theory of heat exchanger effectiveness is employed
as used when assessing the impact of changes in water inlet
temperature. However, as the mass flow rate of water is
changing so the effectiveness is not constant. CIBSE Guide
C, chapter 3, gives equations for determining the
effectiveness of heat exchangers such as fan coil units.
Steam systems
1.8.9.8
1-105
Underfloor heating
Control of heat emitters
The normal method of controlling room temperature is to
modulate the water flow temperature using a three-port
valve connected to a room thermostat.
Natural convectors, radiators and radiant panels
Such emitters are normally controlled using individual
thermostatic radiator valves (trv). These low-cost, standalone devices may be adjusted by room occupants to provide
local user control. They normally contain a simple element
which senses room temperature and expands or contracts
and in the process restricts or increases the valve opening
so adjusting the hot water flow rate through the emitter.
However, as is discussed above, heat output rate of radiators
and radiant panels is only poorly related to water flow rate
resulting in very poor control. It is therefore usual to also
provide weather compensated flow temperature to circuits
containing such emitters. The flow temperature
compensation allows for changes in room heat loss rate due
to increased outdoor temperature whilst the trv needs deal
only with the impact of incidental heat gains
In older buildings with higher heat losses compared to
modern buildings, the heat output rate of the heated floor
may not be sufficient to meet peak heat losses in winter. In
such cases it is common to install an additional, fast
response, heat emitter such as a fan convector or fan-coil
unit. This heat emitter should be controlled separately to
ensure that the heat output from the floor is maximised. It
may be preferable to operate the underfloor heating
continuously in cold weather.
In the case of long, linear convectors where water flow rates
are relatively high compared to individual convectors, the
trv is usually replaced with a two-port motorised valve
controlled from a room sensor.
Steam systems use dry saturated steam to convey heat from
the boiler to the point of use, where it is allowed to condense
thereby giving up its latent heat (and some sensible heat).
Steam has a high latent heat of evaporation so that relatively
large quantities of heat can be delivered with small mass
flow rates. Control of heat output is generally by variation
of the steam saturation pressure within the emitter. The
resulting condensate is returned to the feed tank or hot
well, where it becomes a valuable supply of hot feed-water
Active beams and fan coil units
This equipment is usually used in conjunction with cooling.
Control is discussed in CIBSE Guide B, chapter 2.
1.9
Steam systems
1.9.1
General
Steam
Pan
Pan
Process
vessel
Steam
Space
heating
system
Condensate
Vat
Vat
Make-up
water
Condensate
Steam
Feedtank
Boiler
Feedpump
Figure 1.57 General arrangement of a
steam heating system (simplified)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The high thermal capacitance of heated floors results in a
very slow response to any changes in water inlet flow rate or
temperature. At the same time, the temperature difference
between exposed floor surface and room temperature is
very small – typically only around 5 to 8 K and the heat
output rate very nearly directly proportional to this
temperature difference (see equation 1.61). As a
consequence, any change in room temperature will result in
a change in heat output rate from the floor that is very
nearly self-correcting. This allows room temperature to be
controlled within satisfactory limits despite the slow
response.
Heat emitters require some form of control so that room
temperature can be maintained within acceptable limits.
That is, heat output rate needs to be modulated so that it
balances the net room heat loss. At start-up, the room
temperature will be below design set point and heat output
will need to be at or close to maximum so as to achieve an
acceptable pre-heat time. At all other times, incidental heat
gains are likely to significantly reduce the need for heat,
even during cold weather. As changes in incidental heat
gains often occur very rapidly, emitter heat output control
needs to be rapid so as to minimise temporary overheating
leading to discomfort and unnecessary energy consumption.
CIBSE Guide H provides a detailed discussion of control
systems including heat emitters and heating systems.
1-106
Heating
for the boiler. The flow of steam is generated by the pressure
drop that results from condensation.
This section describes the general principles of steam
heating systems. Steam boilers are described in section 1.7:
Heat generators. A more detailed treatment of steam and
condensate systems is contained in CIBSE TM58: Design
and operation of modern steam systems and CIBSE Guide G,
chapter 9. Tables and guidance on steam and condensate
pipework sizing are provided in CIBSE Guide C, chapter 4.
For tables used to determine the key properties of steam,
see CIBSE Guide C, chapter 2.
1.9.2
System design
A typical steam circuit is shown in Figure 1.57, showing a
main pipe carrying steam from the boiler and a second pipe
returning condensate to the feed tank. Branch pipes connect
individual pieces of equipment or loads to the mains.
Condensate from the feed tank is returned to the boiler by
the feed pump, which is controlled to maintain the water
level in the boiler. Treated water is supplied to the feed tank
as required to make up for losses incurred through leaks or
venting. The water temperature in the feed tank is
maintained at 80–90 ºC (usually by steam injection) to
remove as much oxygen from the water to minimise the
amount of water treatment chemicals required for corrosion
control in the boilers.
The working pressure at which steam must be circulated
depends upon:
——
the pressure/temperature required where each piece
of plant is connected
——
the pressure drop along the distribution pipework
due to resistance to flow
——
pipe heat losses.
As steam at high pressure occupies less volume per unit of
mass than steam at low pressure, smaller distribution
pipework can be used to achieve a given mass flow rate.
This leads to lower capital cost for the pipework and
associated valves, flanges and pipe insulation. Higher
pressure also offers the advantages of drier steam at the
point of use.
Condensate
Steam
Make-up
feedwater
Load
Feedtank
Boiler
Boiler
feedpump
Boiler blowdown to heat recovery system or drain
Figure 1.58 Typical feedwater system
1.9.3
Distribution
1.9.3.1
Steam traps
Steam traps are used to drain condensate automatically
from the system while preventing the escape of steam. They
operate according to four main principles, as follows:
——
Thermostatic steam traps: operate in response to
change in temperature and open when condensate
temperature falls below a pre-set threshold; they
are available in various types suited to particular
applications.
——
Mechanical or balanced pressure steam traps: operate
by sensing the difference in density between steam
and condensate; they include ‘ball float’ and
‘inverted bucket’ types, which both operate by
simple mechanical means.
——
Thermodynamic steam traps: these are operated in
part by the formation of flash steam from
condensate; hot condensate released under pressure
closes the trap when it evaporates.
——
Fixed venturi orifice steam trap: these continuously
remove condensate from a steam line through a
small orifice machined into the trap. When
condensate is present in the steam line, the trap
allows it to pass into the condensate return system.
When no condensate is present, the trap releases a
small amount of live steam, as the steam expands it
‘chokes’ the throughput and therefore the amount
of live steam escaping the orifice is negligible.
There are no moving parts, so minimal maintenance
is required. However as the orifice can be very small
they can be prone to blocking and therefore a very
clean system is required (i.e. free of products of
corrosion).
The choice of steam traps for particular applications
involves a number of considerations, including air venting,
condensate removal (either continuous or intermittent),
capacity, thermal efficiency and reliability. The avoidance
of water hammer may also depend upon the selection and
positioning of traps, as the presence of water hammer may
cause traps to fail. Dirt is another factor to be considered in
trap selection; traps that operate intermittently with a blast
action are less susceptible to dirt than those that depend on
small orifices for their operation. Table 1.42 shows a range
of steam traps, together with typical applications.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Steam offers great flexibility in application and is long
established as a medium for heating in buildings. However,
it is not frequently chosen as a medium for heating buildings
when that is the sole requirement. This is because of more
stringent safety requirements and far more onerous
maintenance and water treatment requirements than are
required for lthw systems. It is much more likely to be
appropriate when there are other requirements for steam,
such as manufacturing processes, sterilisation or large
humidification loads. In such cases, steam may be the most
satisfactory medium both for space heating and for domestic
hot water generation. In many cases, it will be appropriate
to use steam to generate low temperature hot water in a heat
exchanger for distribution in a standard hydronic heating
system as this will minimise the extent of the steam and
condensate distribution system.
Direct steam users and
unreturned condensate
Steam systems
1-107
Table 1.42 Types and characteristics of steam traps
Type
Schematic
Notes
Disadvantages: can be damaged by water-hammer and corrosive condensate; normally
three or four differently sized valves and seats are required to cover the normal working
range.
Advantages: can be made for high pressure and superheated steam; will withstand
water-hammer; can be made of corrosion resisting materials; a check valve should be
fitted at the inlet where used with superheated steam; working parts are simple.
Inverted bucket
trap
Disadvantages: wasteful of steam if oversized; does not respond well to severe
fluctuations of pressure and discharges air slowly; a thermostatic air vent fitted in a
by-pass is recommended; should be lagged when used outdoors.
Notes: no longer manufactured but some may still be found in service; open top bucket
traps have similar advantages and disadvantages.
Thermostatic
steam trap
Advantages: compact; automatically discharges air; valve is wide open on start-up, so
cool condensate and air discharge quickly; capacity is high; unlikely to freeze if
condensate can run from trap outlet; maintenance is easy; traditional elements have
corrugated brass or phosphor bronze bellows, newer designs have a stainless steel
bellows or diaphragm-type element.
Disadvantages: older type elements liable to damage by water hammer, corrosive
condensate or superheated steam (stainless steel elements are more robust and some
designs are suitable for use with superheated steam).
Liquid expansion
steam trap
Unlagged cooling leg
Approx 3 m
Advantages: can be used with superheated steam and at higher pressures than balanced
pressure traps; valve is wide open on start-up, so cool condensate and air discharge
quickly; capacity is high; operates by continuous discharge, so quiet in operation and
unaffected by vibration, steam pulsation and waterhammer; automatically discharges
air.
Disadvantages: does not respond quickly to change in load or steam pressure; element
can be damaged by corrosive condensate. Note: because element is on discharge side of
valve orifice, trap will hold back condensate. This permits use of some sensible heat
from condensate provided that water-logging of steam space is acceptable; if this is not
the case, a cooling leg must be fitted before the trap.
Bi-metallic
steam trap
Advantages: usually small and robust; when cold valve is wide open and air is freely
discharged; capacity is greatest when condensate is coolest; some types are not damaged
by freezing; withstands water hammer and some are unaffected by corrosive
condensate; suitable for use on high pressure and superheated steam; will work over
wide range of pressures without need to change size of valve orifice, although position
of orifice may need to be adjusted; holds back condensate until cooling occurs thus
using some of the sensible heat.
Disadvantages: will not discharge condensate until it has cooled below saturation
temperature, so unsuitable for use where condensate must be cleared as soon as it forms
unless a cooling leg is provided; responds slowly to changes in steam pressure and
condensate load.
Thermodynamic
steam trap
Advantages: very compact but has large discharge capacity; will work over full range of
pressures without adjustment; can be used with superheated steam and can withstand
vibration or severe water-hammer; normally made of stainless steel and therefore can
withstand corrosive condensate and is not damaged by being frozen.
Disadvantages: normally requires a minimum pressure differential in order to function;
on starting up, if pressure at trap builds up slowly it can discharge a lot of air, but if
pressure builds up quickly the resulting high velocity air can shut the trap in the same
way as steam and it will air bind; operation of trap can be noisy; due to blast, discharge
operation sight glasses and check valves should be fitted about 1 metre from the trap.
Fixed venturi
steam trap
Advantages: no moving parts and very compact.
Disadvantages: Small orifice may be prone to blockage.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Advantages: suitable for widely fluctuating loads and pressures; easy to install and
maintain; removes condensate continuously as it forms; types with balanced pressure
air vents automatically discharge air.
Float trap
1-108
Heating
Table 1.43 Types and characteristics of air vents
Type
Schematic
Notes
Similar to balanced pressure steam trap. Valve is wide open when plant is
cold; as temperature surrounding the element approaches steam temperature
the internal liquid expands thereby generating a pressure within the element
which closes the valve seat.
Liquid expansion
air vent
Similar to liquid expansion steam trap. Changes in temperature cause the oil
air vent filled element to expand or contract causing the valve to move
towards or away from its seat.
1.9.3.2
Air vents
Steam traps are capable of venting air from steam systems
but separate air vents are fitted in certain situations,
particularly at the end of a steam main. An automatic air vent
typically consists of a thermostatically operated valve, see
Table 1.43. It is best installed at a location where the
temperature is low enough for steam to have condensed
before reaching it, but where condensation does not collect.
In practice this is typically at the top end of a 300 mm length
of pipe arranged as a ‘collecting bottle’, which is left unlagged.
Feed tanks are made from various materials, including cast
iron, carbon steel and austenitic stainless steel.
The supply of water to the steam boiler comes from the feed
tank. The water in the feed tank must be kept at a high
temperature to minimise the content of dissolved oxygen
and other gases. Feed tanks should be maintained at a
temperature of at least 80 ºC. This minimises the quantity
of oxygen scavenging chemicals required as well as
preventing thermal shock in the boiler.
1.9.3.4
1.9.3.3
Feedwater equipment
A typical feedwater system is shown in Figure 1.58. The
feed tank (often referred to as the hot well) receives
condensate returned from the system and treated water as
required to make up losses from the system. The feed pump
takes water from the feed tank and supplies it into the boiler
at the rate required to maintain the water level in the boiler.
The treatment of make-up water is vital to the longevity,
safe operation and efficiency of the system. In particular, it
seeks to avoid scaling, corrosion and caustic embrittle­ment
in boilers by removing dissolved and suspended solids and
dissolved gases thereby keeping the pH value of the water
within defined limits. BS 2486 (BSI, 1997) gives
recommendations for treatment of water in steam heating
systems.
Make-up water is provided by either base exchange water
softening, to remove scale-producing ions, may be carried
out using (in ascending order of effectiveness) baseexchange methods, de-alkalisation, or de-mineralisation.
Recently however reverse osmosis (ro) water is being used
more frequently. This is a process where softened water is
forced through a semi-permeable membrane to produce
almost pure water, leaving a concentrated solution of
impurities, which is rejected to waste. This removes 96–98%
of the dissolved solids. No chemicals are used and operating
costs are relatively low. The principal benefit of ro
treatment when used to treat water used for steam boilers is
that by removing 96–98% of the dissolved salts present in
the water, boiler blow-down, i.e. the discharge of hot water,
can be reduced by approximately 80%, providing significant
energy savings.
Heat emitters
Space heating by steam often uses a heat exchanger to
transfer heat from the steam to a secondary low hot water
circuit, which uses standard hydronic heating equipment.
Figures 1.59(a) and (b) shows two alternative types of heat
exchanger, controlled to maintain a constant secondary
flow temperature.
New installations will typically comprise a plate heat
exchanger. These are designed to sub cool the condensate
below saturation temperature corresponding to the steam
pressure to minimise the amount of flash steam when
passed to the low pressure condensate return (which is
vented to waste) resulting in optimising their efficiency.
Older installations typically comprise a shell and tube heat
exchanger with a second shell and tube exchanger to cool
the condensate below the saturation temperature. These
units are used infrequently in new installations as they have
much larger footprint for the same heat transfer as plate
types due to their lower heat transfer efficiency. They also
have additional cost associated with them for maintenance
and insurance inspections.
To prevent unsafe temperatures being reached in the lthw
system, it is important to include a self-acting control valve
in the steam supply (in addition to the temperature control
valve) to automatically close the steam supply on sensing a
high temperature in the lthw flow pipework that could
occur should the temperature control valve fail. This is
shown in Figure 1.59(a).
For further examples of steam to water and steam to air heat
exchangers and both storage and non-storage calorifiers
refer to CIBSE Guide G, chapter 9.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Balanced pressure
air vent
Steam systems
1-109
Electropneumatic
control system
Flow
high
pressure
supply
Return
Condensate
K
C
K
F
E
Figure 1.59(a) Application of
steam-to-water plate heat exchanger
with self-acting high limit
temperature control facilities
(Courtesy of Spirax Sarco)
G
A
N
H
E
P
L
R
J
S
D
M
J
T
W
V
B
Standard connections
A Steam
B Condensate
C Secondary flow
D Secondary return
E Pressure gauge
1.9.3.5
F Safety gauge
G Vent
H Thermometer
J Drain
K Lifting lugs
L Heater
M Cooler
N Stop valve
P Strainer
R Steam trap
S Sight glass
T Receiver vent
V Receiver drain
W Overflow
Steam pipework sizing
Oversized steam pipework results in excessive capital costs,
greater than necessary condensate formation, and poor
steam quality. Undersized pipework causes excessive steam
velocity and higher pressure drops, which can cause steam
starvation at the point of use as well as a greater risk of
erosion and noise.
Figure 1.59(b) Typical steam-towater shell and tube heat exchanger
with condensate cooler (Courtesy of
Ormandy Rycroft)
Pipe sizing may be carried out from consideration of the
steam velocity required to match the loads around the
circuit. In practice, limiting the velocity to between 15 and
25 m·s–1 will avoid excessive pressure drops and problems
with noise and erosion. Velocities of up to 40 m·s–1 may be
acceptable in large mains. Sizing may also be carried out
from consideration of the steam pressure required at
particular pieces of plant.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Pilot
operated
pressure
reducing
valve
1-110
Heating
1.9.4
Refer to Guide G, chapter 9 for detailed guidance.
Condensate
It is essential to fit a pressure relief or safety valve on the
downstream side of the reducing valve. The relief valve and
its discharge pipe must be sized and located to discharge
steam safely at the upstream pressure for the maximum
capacity of the reducing valve, should it fail wide open.
Effective condensate removal and return to the boiler is
essential for steam systems to operate properly. As
mentioned above, it is important to trap the steam main at
low points along its length to ensure that dry steam is
available at the point of use.
Pressure reducing sets
Steam distributed at a higher pressure than the equipment
served requires pressure reduction. The main component
in a pressure reducing set is the reducing valve, often a
spring loaded diaphragm or bellows type. Simple direct
acting reducing valves can be used where the load is small
or remains fairly steady. For larger and varying loads a
more elaborate, pilot-operated valve may be necessary.
Table 1.44 Regulations, guidance and standards relevant to steam heating systems
Regulation/Standard
Scope
Statutory Instrument 1989 No. 2169:
The Pressure Systems and Transportable Gas Containers
Regulations (HMSO, 1989)
Provides the legal framework for pressurised vessels
Pressure Systems Safety Regulations (HMSO, 2000)
Owner and user obligations for operation and maintenance of pressurised systems
HSE PM60 (HSE, 1998)
Covers bottom blow-down
HSE IND 436 (HSE, 2011)
Safe management of industrial steam and hot water boilers
BG01/2011 (joint document by the Safety Assessment
Federation and Combustion Engineering Association
produced in consultation with the Health and Safety
Executive) (SAF_CEA, 2011)
Guidance on the safe operation of boilers
BS 1113 (1999)
Covers the design and manufacture of water-tube steam generating plant
BS 2790 (1992)
Covers the design and manufacture of shell boilers of welded construction , including
aspects such as stop valves
BS EN ISO 4126 (2013) and BS EN 12953 (2003)
Covers the requirements for protection against excessive pressure including the
specification of safety valves
BS 759-1 (1984)
Covers valves, mountings and fittings for steam boilers above 1 bar gauge
BS EN 837-1 (1998)
Cover pressure gauges
BS 3463 (1995)
Covers level indicators
BS EN 13480 (2012)
Covers drainage of steam lines
BS 2486 (1997) and BS EN 12953-10 (2003)
Recommendations for water treatment for steam heating systems and requirements for
feedwater and boiler water quality
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
To prevent water or dirt entering the reducing valve it is
good practice to install a baffle-type separator and strainer
upstream of the valve. Pressure gauges are usually fitted
either side of the reducing valve to set the valve initially
and to check its operation in use.
Condensation takes place in steam mains even when they
are well insulated and provision must be made for drainage.
Steam mains should be installed with a fall of not less than
100 mm in 10 m in the direction of steam flow,using
appropriate steam traps. Where possible, branch
connections should be taken from the top of the main to
avoid the entry of condensate. Low points in branch lines,
such as those that occur in front of a control valve, will also
accumulate condensate and need provision for trapping
and drainage. Steam traps must be sized to remove
condensate at the rate needed for cold start-up. A general
rule of thumb is to size the condensate return system for
twice the mean condensing rate at the operating differential
pressure. The characteristics of steam traps and their
suitability for particular applications are described in
section 1.9.3.1.
1.9.3.6
Steam systems
1-111
1.9.4.1
Condensate pumping
A condensate pump set usually comprises an open vented
vessel mounted above one or more electric motor pumps or
pressure operated lifting pumps, the latter most often using
steam but compressed air or other gas may also be used.
Condensate from steam traps is piped to discharge into the
receiver vessel by gravity.
Electric pumps are usually switched on and off by level
controls in the receiver vessel. Special measures regarding
electric pumps need to be taken with high pressure steam
systems, where condensate temperatures can equal or
exceed 100 °C.
Pressure operated pumps work by displacing a volume of
collected condensate in the pump body. Check valves are
fitted on the condensate inlet and outlet of the pump to
ensure correct water flow. When the pump body is full of
condensate from the receiver an internal mechanism opens
the pressurising gas inlet valve. The condensate is pushed
through the outlet check valve. At the end of the discharge
stroke the mechanism closes the inlet valve and opens an
exhaust valve. The ‘used’ pressurising gas within the pump
body then vents either to atmosphere or to the space from
which the condensate is being drained. When the pressures
are equalised, more condensate can flow by gravity from the
receiver into the pump body, and the cycle repeats.
1.9.4.2
Condensate return mains
There are essentially two types of condensate return:
gravity and pumped. Traps draining a steam main or device
that is always at full steam pressure can vertically lift
condensate a limited distance before discharging into a
gravity return main laid to fall towards the boiler feed tank.
As mentioned above, traps draining heat exchange
equipment normally discharge condensate by gravity into a
vented receiver from where it is pumped into a separate
return main. Gravity condensate return lines carry both
condensate and incondensable gases, together with flash
steam from the hot condensate. The pipework should be
sufficiently large to convey all the liquid, gases and flash
steam. An adequately sized pipeline is capable of accepting
condensate discharged from traps with different upstream
pressures. However, if the pipeline is too small, excessive
velocities and pressure drops may arise, particularly where
condensate at high pressure and temperature enters the
line, giving off flash steam. Such situations often give rise
to water hammer.
Pumped condensate pipes carry only water and can be sized
for higher velocities than gravity lines. Trap discharge pipes
should not connect directly into pumped condensate
pipelines. Flash steam released from additional condensate
flowing into a flooded pipe will invariably result in water
hammer.
The pressure and temperature of the condensate can have a
large influence on the size of pipe needed. Condensate
discharging from steam traps is a mixture of hot water and
flash steam (see CIBSE Guide G, section 9.9.4). Because the
volume of the flash steam formed is so much greater than
the hot water, the size of the condensate pipe must take
flash steam into account where it is known to exist.
There are three main types of condensate lines:
——
drain lines: connecting the process to traps (no flash
steam present)
——
discharge lines: discharging condensate from traps
(flash steam present)
——
pumped lines: discharging liquid condensate from
pumps (no flash steam present).
CIBSE Guide G, chapter 9, gives a sizing chart for all types
of condensate lines together with examples of each.
1.9.4.3
Steam systems warm up
It is essential that when a boiler is brought on line, it is
done in a slow, safe and controlled manner to avoid the
following:
——
Water hammer: where large quantities of condensate
lie inside the pipe and are then pushed along the
pipe at steam velocities. This can result in damage
when the water impacts with an obstruction in the
pipe, e.g. a control valve.
——
Thermal shock: where the pipework is being heated
so rapidly that the expansion is uncontrolled,
setting up stresses in the pipework and causing
large movement on the pipe supports.
——
Priming: where a sudden reduction of steam
pressure caused by a large, suddenly applied load
may result in boiler water being pulled into the
pipework. Not only is this bad for plant operation,
the boiler can often go to ‘lock-out’ and it will take
some time to return the boiler to operating status.
The discharged water can also give rise to water
hammer in the pipework.
With small systems the main stop valve is usually used to
safely bring a small boiler on line, which should be opened
as slowly as possible.
On larger plants, however, the rate of warm-up is difficult to
control using the main stop valve. This is because the main
stop valve is designed to provide good isolation; it often has
a flat seat which ensures good seal when under pressure but
the valve is not characterised and will pass approximately
80% of its capacity in the first 10% of its movement.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Temperature control of steam process equipment and heat
exchangers is usually achieved by throttling the flow of
steam. Consequently, steam pressure falls inside the
exchanger. When the steam pressure inside the exchanger
is equal to, or lower than the pressure at the outlet side of
the steam trap, condensate will not flow. To prevent the
exchanger from flooding with condensate it is necessary to
locate the trap below the exchanger outlet to provide a
hydrostatic head to enable condensate to pass through the
trap by gravity, the outlet side of the trap normally being
kept at atmospheric pressure. A vacuum breaker is often
fitted at the steam inlet point of the heat exchanger to admit
air in the event that steam pressure inside the exchanger
falls below atmospheric pressure. If condensate is to return
to the boiler feed tank through pipework at a higher level
than the trap, as is usually the case, then the condensate
must be pumped, see below.
1-112
Heating
Induced air
Nozzle
1.9.5
Air jet
Water
outlet
header
Figure 1.60 Induced jet warm air
heating (reproduced from GPG303
by permission of Energy Efficiency
Good Practice Programme)
Water
intlet
header
Staggered
tubes
Air systems
Insulated
duct walls
1.10.1
General
Air in
Air systems use air as the heat distribution medium. The
air is heated centrally within an air handling unit and
distributed to each zone by means of ductwork. Such
systems are normally used where there is a need for a
mechanical ventilation system. Warm air heating is also
commonly incorporated in all-air air conditioning systems.
Systems that combine ventilation, heating and cooling are
considered in section 1.2. It is important to ensure that as
much care is given to the successful distribution of heated
air as is given to the distribution of ventilation air, since
airflow characteristics and circulation patterns will differ
between modes.
Water out
Air out
Fraction design mass flow rate of hot watre
Figure 1.61 General arrangement of an lthw heater battery
1·0
Water inlet
temperature / °C
0·9
0·8
70
0·7
60
0·6
50
0·5
0·4
0·3
0·2
0·1
0
0
0·5
0·2
0·3 0·4 0·5 0·6 0·7
Fraction design heat output
0·8
There are many regulations, standards and guidance
documents relevant to steam systems, including those in
Table 1.44.
1.10
Water in
Fins
Guidance and standards
0·9
1·0
Figure 1.62 Two-port control of a typical heater battery with weather
compensation
For this reason it is good practice to install a line size
motorised control valve after the main stop valve. A control
valve has a profiled plug, which means that the relationship
between an increase in flow and the movement of the plug
is much less severe. Consequently the flow rate, and hence
warm-up rate, is better controlled. A typical warm-up
arrangement could be that the control valve is closed until
the boiler is required. At this point a pulse timer opens the
control valve very slowly over a predetermined time period.
On large distribution systems however a line size control
valve is still often too coarse to provide the required slow
All of the heat output is provided in convective form so the
room air temperature is greater than the operative
temperature during the heating season. Warm air systems
generally have a much faster response time than hydronic
systems. They can, where the terminal diffusers are not
correctly selected or positioned, cause excessive temperature
stratification, with warm air tending to collect at ceiling
level. This may be particularly unwelcome in buildings
with high ceilings, although it can be overcome by either
the use of destratification systems or radial type diffusers
which can automatically adjust the direction of airflow
from horizontal to vertically downwards depending on the
temperature of the supply air. This type of terminal device
can reduce the pre-heat period.
Warm air systems may be used to provide full heating to a
space or simply supply tempered ‘make-up’ air to balance
the heat loss and air flow rate from exhaust ventilation
systems. A slight excess air flow can be used to pressurise
the heated space slightly and reduce cold draughts.
Alternatively, ventilation air can be supplied at room
temperature with a separate and independent system, e.g.
lthw radiators, offsetting the room fabric and infiltration
heat loss.
In tall industrial and warehouse buildings, specialist central
plant warm air heating systems are also used. They typically
rely on high-temperature, high-velocity primary air supply
at high level, supplemented by induction of room air at
discharge points to provide good air circulation and even
temperatures in the occupied zone. An example is shown in
Figure 1.60.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
warm-up. In these circumstances a small control valve in a
loop around an isolation valve could be used. This also has
the advantage that where parallel slide valves are used for
isolation, the pressure can be equalised either side of the
valve prior to opening. This will make them easier to open,
and reduces wear.
High velocity
distribution duct
Air systems
1-113
1.10.2
Heat sources
1.10.2.1
Hot water coils
The general arrangement of a lthw heater battery is shown
in Figure 1.61.
The water tubes are arranged in a bank to produce a number
of parallel circuits, all perpendicular to the air flow
direction, connected by means of headers. This arrangement
is known as a cross-flow heat exchanger. The water inlet is
at the same end as the air outlet. This maximises the average
temperature difference between the air and the water.
The performance of a cross flow heat exchanger is specified
in terms of its effectiveness (see CIBSE Guide C, chapter 3).
This allows the performance of the heat exchanger to be
assessed under different operating conditions to the
manufacturer’s test conditions. Using this approach, Figure
1.62 was produced showing how a typical heater battery
responds to two-port control.
A constant air mass flow rate has been assumed. The heat
output is controlled by throttling the flow rate of lthw
through the battery in order to maintain a constant air-out
temperature (of 20 °C in this example) in response to an
increase in the air-in temperature. A design lthw flow
temperature of 70 °C has been used. As can be seen, at the
design lthw flow temperature, the water flow rate needs to
be reduced to only 20% of its design value in order to
provide 50% heat output. If weather compensation is
employed to reduce the lthw flow temperature as outdoor
temperatures increase, significantly less throttling is
required which will lead to better control.
Whilst fins and extra tube rows increase the effectiveness of
the heater battery, they do significantly increase the
resistance to air flow and hence fan energy consumption.
1.10.2.2
Electric heating coil
The electric heating coil comprises a simple coiled tube
enclosing a resistance heater. Unlike lthw heating, fins are
not normally used as the surface of the heating element is
typically around 400 °C (compared to about 70 °C for
lthw) and can provide a high heat output per unit volume
of heat exchanger. Air velocity is typically 2 to 6 m/s.
Heat output rate is either by simple step-control of the
current or modulating using a thyristor. Even heating of
the air depends upon good mixing. Electric heating is
virtually 100% efficient but results in very high CO2
emissions and running costs when using grid electricity. It
is most often used as frost protection on fresh air intakes
where lthw coils would be prone to freezing.
1.10.2.3
Heat pumps
Heat pumps are used extensively in air heating as a
relatively low condensing temperature is usually acceptable
so improving cop. The condenser of the heat pump becomes
a dx (direct expansion) air heating coil, removing the need
for an intermediate heating medium. The various types of
heat pump and their performance is discussed in section
1.7.
1.10.2.4
Direct gas-fired heater
In this system, the gas is burnt directly into the airstream
within an air handling unit and the products of combustion
are distributed into the heated space and so must be used
with care. A ‘cheese grater’ burner configuration is usual,
with a perforated stainless steel V-shaped shroud around
the burner tube. Modulating control of heat output is
usually provided. Control of combustion and ventilation is
critical to ensure that sufficient dilution of the combustion
gases is achieved. In particular the ventilation requirements
of BS 6230 (2011) should be met to ensure that CO2 levels
are kept low enough to avoid adverse effects on health and
comfort. Flueless appliances may only be used in accordance
with the requirements of the Building Regulations Part J
(England).
Direct fired gas warm air heating is used due to its high
efficiency (100% net, 92% gross) and relatively low
installation cost. It is used in industrial and other spaces
where a high fresh-air ventilation rate is needed. Care
should be taken to ensure that products of combustion,
which includes water vapour, do not have adverse an effect
on items stored in the heated space.
1.10.2.5
Indirect gas and oil heaters
In these systems, the flues through which the combustion
gases flow, form a heat exchanger over which the air to be
heated flows prior to the combustion gases being released
to atmosphere. Though not as efficient as the direct gas
heater, no products of combustion enter the air stream.
The gas or oil burner is provided complete with gas/oil
train and all controls and the complete package installed
within the air handling unit. It is common practice to use
several relatively small modules rather than a single unit
for heating loads exceeding about 200 kW. On/off,
off/low/high or fully-modulating control is available.
The use of this system removes the necessity for hot water
distribution pipework from central heat generators and
removes the risk of freezing.
In the UK BS 6230 (2011) specifies the installation
requirements for direct and indirect gas-fired forced
convection air heaters for space heating that are designed
for commercial or industrial applications and to which The
Gas Appliances (Safety) Regulations (HMSO, 1995) apply.
The standard does not specifically cover lpg/air for which
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heating is commonly provided by a lthw heater battery
supplied from a hydronic system located within the air
handling unit. This comprises a heat exchanger with lthw
flowing within the tubes and the air to be heated flowing
over the tube bundle. The tubes are finned on the air side to
improve the overall output rate per unit volume so allowing
a more compact heat exchanger to be used. Control is by
means of an air temperature sensor in the duct downstream
of the coil and a two or three-port valve.
It is imperative that a thermal cut-out is provided to operate
on excessive air temperatures that could occur if a fan failed
or the air volume was reduce significantly. These are usually
manually reset. A fan run on period is also required when a
plant is shut down to ensure any residual heat in the heater
battery is dissipated.
1-114
Heating
Oil fired forced convection heaters are covered by BS EN
13842 (2004).
1.10.2.6
Heat recovery
Heat recovery involves transferring heat from exhaust air
to the supply air. There are a number of devices for doing
this including recuperators, thermal wheels, run-around
coils and heat pipes. These are described in more detail in
Guide B, chapter 2.
When designing heat recovery systems, the additional
pressure drop introduced on the air side can be considerable
and must be taken into account when assessing the overall
environmental and cost benefits. In particular, the devices
installed will create a year-round increase in air resistance
whereas the heat recovery device may be required to operate
for only a few months per year. Nevertheless, well-designed
heat recovery systems can have payback periods of only one
or two years often making them a first choice when reducing
energy consumption in existing buildings.
1.10.3
Distribution
Heating systems involving comprehensive ducting are
usually combined with ventilation systems and are therefore
also covered in Guide B, chapter 2.
Where displacement ventilation is used to provide fresh air
and cooling the general rule is: ‘Don’t heat the room by the
ventilation air’ but either provide separate heating by either
radiant heating or convectors unless the heating loads are
very small. Heating the room by the warm air may be used
to supplement the heating at the start of the working day.
Underfloor heating can heat the supply air so much that the
air ascends due to buoyancy, thereby destroying the
displacement airflow pattern. If warm air is supplied at
floor level in a cold room, the warm, fresh air will rise due
to buoyancy, and be extracted when it reaches the ceiling.
Thus, the fresh air will short circuit into the outlet openings
and little of the fresh air will reach the occupied spaces.
1.10.3.1
Ductwork and diffusers
Under current Part L Building Regulations for England, all
ductwork supplying warm air must be insulated to a
minimum standard. These requirements are set down in
the Non-domestic Building Services Compliance guide
(NDBSCG) (DCLG, 2013e). Similar requirements apply in
Wales, Scotland and Northern Ireland.
Duct systems for induced jet heating are usually circular in
cross-section and installed at high level in the roof space.
Purpose designed nozzles and induction hoods are used to
provide the necessary induction and throw, normally
producing high duct velocity.
Diffusers are considered in Guide B, chapter 2. The
characteristics of various types of air terminal devices are
described, including information on typical face velocities
and noise levels. Diffusers may be radial, part radial or
linear and normally utilise the Coanda effect and/or swirl to
avoid excessive room air movement.
Particularly in high halls (where the diffusers could be
located 10 m above floor level) and there are large thermal
load fluctuations radial type diffusers are often used. These
can automatically adjust the direction of airflow from
horizontal to vertically downwards depending on the
temperature of the supply air can either ceiling or duct
mounted. This type of terminal device can reduce the preheat period and avoid stratification of warm air at ceiling
level.
1.10.4
Heating combined with air
conditioning
Buildings with central air conditioning systems normally
include provisions for heating, cooling and ventilation.
These are described in chapters 2 and 3 of Guide B. The
Table 1.45 Standards relevant to warm air heating
Standard
Scope
BS 5990 (2006)
Specification for direct gas-fired forced convection air heaters with rated heat inputs greater than 330 kW but not
exceeding 2 MW for industrial and commercial space heating. Safety and performance requirements (excluding
electrical requirements)
BS 5991 (2006)
Specification for indirect gas fired forced convection air heaters with rated heat inputs greater than 330 kW but not
exceeding 2 MW for industrial and commercial space heating. Safety and performance requirements (excluding
electrical requirements)
BS EN 525 (2009)
Non-domestic direct gas-fired forced convection air heaters for space heating not exceeding 300 kW
BS EN 621: (2009)
Non-domestic gas-fired forced convection air heaters for space heating not exceeding 300 kW without a fan to assist
transportation of combustion air and combustion products
BS EN 1020 (2009)
Non-domestic gas-fired forced convection air heaters for space heating not exceeding 300 kW incorporating a fan to
assist transportation of combustion air and combustion products
BS 6230 (2011)
Specification for the installation of gas-fired forced convection air heaters for commercial and industrial space heating
BS 5864 (2010)
Installation and maintenance of gas-fired ducted air heaters of rated heat input not exceeding 70 kW net (2nd and 3rd
family gases)
BS EN 13842 (2004)
Oil-fired convection air heaters
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
additional requirements might be necessary. The standard
applies to the installation of direct gas-fired air heaters
within the scope of BS EN 525 (2009) and to indirect gasfired forced convection air heaters within the scope of BS
EN 621 (2009), BS EN 1020 (2009).
Air systems
1-115
following is an overview of the principle systems used from
the point of view of heating:
Dual duct systems
Two separate ducts are employed to supply cooled and
heated air from central plant to zonal mixing boxes.
Thermostatic controls in each zone ensure that air from the
hot and cold ducts are mixed in appropriate proportions to
achieve the required supply air temperature to meet the
heating or cooling load whilst maintaining a constant
supply air volume. The heating, cooling and fan energy
consumption is high in such systems, as are capital costs
and space requirements, so that dual duct systems are
normally used only where it is imperative to maintain
stable room air pressures, e.g. in some laboratories and
hospitals.
1.10.4.2
Variable air volume (VAV) systems
These offer significantly improved energy efficiency
compared with constant volume systems, although both
systems represent a significant energy cost. Air is supplied
through a single duct to the terminal units from the central
plant at about 14 °C to provide cooling and ventilation air.
A heating coil, lthw or electric, is installed in each vav
terminal unit. Each terminal unit is provided with its own
controls that allow the volume flow rate of air to be
modulated down to a minimum value in response to falling
room temperature (but large enough to ensure the room air
change rate is adequate to ensure good air distribution and
mixing for heating purposes and that adequate fresh air is
provided) . Should the temperature in the room continue to
fall then the terminal will operate at constant volume and
bring on the heating coil. The heating coil will need to be
capable of providing sufficient heat to meet the room heat
loss plus the ventilation heat loss. There are a number of
issues that must be addressed with this system:
——
The supply air diffuser/grille used should be capable
of operating at maximum supply air temperature
and minimum flow rate in heating mode without
causing stratification. If need be, the minimum air
volume flow rate should be increased.
——
Heating coils must be installed on vav terminal
units in internal rooms/zones where there is no
room heat loss as the coils are also providing
ventilation heating and overcooling of the space
could occur.
——
The heating coils must be allowed to operate allyear round to avoid over-cooling of areas where the
actual heat gains are much smaller than the design
value, this can result in cooling and reheating
which is very poor use of energy.
1.10.4.3
Flue
Supply
Figure 1.63 Suspended unit heater
(reproduced from GPG303 by
permission of Energy Efficiency
Good Practice Programme)
Figure 1.64 Cabinet heater
(reproduced from GPG303 by
permission of Energy Efficiency
Good Practice Programme)
Figure 1.65 Gas-fired radiant tube heater (reproduced from GPG303 by
permission of Energy Efficiency Good Practice Programme)
Fan coil systems
A fan coil is a packaged assembly comprising coils(s),
condensate tray, circulating fan and filter. These are mostly
provided with two coils, one for heating and one for cooling
using chilled water. Heating is usually by lthw although
electric coils are available. The fan recirculates air from the
space continuously through the coil(s) either directly or via
the void in which the fan coil is located. A fresh air supply
is sometimes provided in which case the heating coil must
provide for both room heat loss and ventilation heat loss.
Figure 1.66 Gas-fired radiant plaque heater (reproduced from GPG303
by permission of Energy Efficiency Good Practice Programme)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1.10.4.1
The most common control arrangement is to maintain a
constant air volume flow rate and vary the supply air
temperature in response to a room temperature controller.
This minimises the risk of stratification in heating mode.
This may be achieved using either water-side control (in
which the heating and cooling is activated in sequence) or
by air-side control (in which control dampers modulate the
1-116
Heating
1.10.4.4
Active (ventilated) beams
An active beam is a packaged assembly comprising heating
and cooling coils served by a primary ventilation supply.
The primary air is discharged inside the unit so as to create
an induction effect within the beam that causes room air to
be drawn over the coils. The mixture of primary and
induced air is then discharged at ceiling level via an
integrated diffuser. Heating using active beams, and
measures needed to minimise the risk of stratification, is
discussed in section 1.8.6.4.
1.10.5
Controls
1.10.6
Other standards and guidance
Table 1.45 lists other standards relevant to warm air heating.
1.11
Unitary systems
1.11.1
General
Unitary systems are those where the heat source is located
within the zone being heated rather than the heat being
generated at a central point and then being distributed (e.g.
by water, steam or air) to each zone. Examples are:
——
——
gas- and oil-fired:
——
indirect gas and oil-fired unit and cabinet
heaters
——
direct gas-fired heaters
——
direct gas-fired radiant heaters
direct electric heaters:
Control strategies for warm air systems can be kept
reasonably simple.
——
air curtains
——
convectors
The best control of room temperature is obtained using
modulating control of the heater output. This can be
provided on most forms of warm air heater, but the
turndown ratio is limited on some indirect gas- or oil-fired
heaters. Modulation can be used to maintain a constant
room temperature or a constant leaving air temperature.
The latter is usually used when the warm air is providing a
tempered make-up air supply rather than full space heating.
A low-limit control is usually required to prevent the
modulating control from reducing the leaving air
temperature to such a level as to cause discomfort.
——
radiators
——
storage heaters
——
radiant heaters
De-stratification systems should be controlled to prevent
build up of unacceptable temperature gradients. For low
velocity systems the fans should be controlled to run during
the full heating period (often from the heater time control).
For high velocity systems thermostatic control is preferable
to avoid cool drafts.
CIBSE Guide H provides more detailed information on
control systems.
——
electric underfloor heating
——
stand-alone heat pumps
1.11.2
Indirect gas- and oil-fired
heaters
1.11.2.1
Suspended unit heaters
These are small independent gas-fired heaters, with outputs
up to 100 kW, typically comprising a burner and heat
exchanger inside a painted steel casing, see Figure 1.63. A
low powered axial fan blows recirculated air horizontally
across the heat exchanger and directly into the heated
space. The basic form uses an atmospheric gas burner,
usually of the ladder type, firing into a simple pressed steel
heat exchanger, which is aluminised or similarly treated to
provide corrosion protection. The degree of modulation
possible is limited by the need to avoid condensation in the
heat exchanger and flue; 60% of full output is the normal
minimum output. Flues are usually single skin stainless
steel terminating with a cowl at least 1 m above the roof. A
draught diverter is usually built into the heater itself.
Variations on this basic design include:
Figure 1.67 Electric radiant heater (reproduced from GPG303 by
permission of Energy Efficiency Good Practice Programme)
——
stainless steel heat exchangers for use in aggressive
environments or with fresh air inlet
——
room sealed units with induced draft fan and ducted
combustion air inlet
——
condensing heat exchangers
——
on/off, two stage or modulating control
——
centrifugal fans, for use with air distribution
ducting.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
air flow over the heating and cooling coils to achieve the
desired mixed flow temperature). Alternatively, the fan coil
unit may operate as a variable volume device by varying the
speed of the fan and maintaining supply air temperature. In
this scenario, as with vav units, the minimum air volume is
usually limited to approximately 6 air changes per hour.
Unitary systems
Heaters are normally mounted at heights between 2.5 m
and 3.5 m above floor level, but higher mounting is possible.
Only limited distribution ducting is possible due to the low
available fan discharge pressure.
Cabinet heaters
These are larger (up to 300–400 kW output) individual
heaters, gas- or oil-fired, and used in industrial premises
where quiet operation and close environmental control are
not essential. They are usually floor mounted, but some
versions are suitable for high level mounting.
A typical unit comprises an externally mounted forceddraught burner firing into a steel combustion chamber,
with flue gases passing through a tubular heat exchanger
before exiting through the flue, see Figure 1.64. Some low
cost designs use atmospheric burners. Stainless steel or a
protective coating may be used to increase longevity.
A centrifugal fan in the base of the heater blows air across
the heat exchanger and the heated air is discharged
horizontally through discharge louvres on the top. Alter­
natively, air may be discharged through distribution
ductwork, although the limited fan pressure available on
some heaters can mean that extensive ducting is impractical.
Inlet air is usually recirculated room air but some heaters
can have a ducted inlet for combustion air and/or ventilation
air. Flues are usually single skin stainless steel terminating
with a cowl at least 1 m above the roof.
Condensing gas-fired cabinet heaters are available which
include an additional stainless steel heat exchanger to cool
the flue gases to con­densing point.
heights between 3.5 and 20 m and are mostly used for
general area heating, rather than local spot heating. Lowlevel mounting is avoided to ensure even distribution of
heat and to minimise the effects of noise. Reflectors are
usually made of polished stainless steel or rigid aluminium,
shaped for optimum heat distribution. Tubes are usually
steel, often blackened for maximum efficiency. Stainless
steel may be used for the first section of tube from the
burner, particularly with high output burners. Minimum
ventilation requirements for unflued heaters are given in
BS 6896: 2011.
Radiant plaque
A typical radiant plaque heater is shown in Figure 1.66.
Heaters of this type offer outputs typically in the range of
5–40 kW. They operate at around 900 °C and are often used
for local spot heating. Due to the high operating
temperatures, the ceramic burners glow red/orange in use.
Like unflued radiant tube types, they must be located
where ventilation rates are high to avoid condensation and
to dilute flue gases. A cone configuration is available to
provide 360° coverage of a particular location; patio heaters
are small-scale portable versions of this type of heater.
1.11.3
Direct electric heating has a relatively high running cost
and results in high greenhouse gas and other emissions at
the power station. For this reason, direct electric heating
should only be considered where other fuel sources are not
available or heating demand is very small.
1.11.3.1
1.11.2.3
Direct gas-fired heaters
These flueless gas-fired heaters are usually of the cabinet
type. The gas is burnt directly in the main ventilation
airstream (with no heat exchanger) and the products of
combustion are therefore distributed into the heated space.
These are identical to the direct gas-fired heating system
described in section 1.10 but are stand alone and equipped
with integral fan, filters, discharge grille, all located within
a cabinet. The same safety issues of contamination of the
air within the occupied space apply.
1.11.2.4
Gas-fired radiant heaters
Gas-fired radiant heaters are typically of two types: radiant
overhead tube heaters and radiant plaque heaters. Radiant
tube heaters may be either flued or unflued. Radiant plaques
are unflued and offer very high efficiencies and are wellsuited to spot heating. The relevant British Standard is BS
6896 (2011), Specification for installation and maintenance
of gas-fired overhead radiant heaters for industrial and
commercial heating.
Radiant tube
Figure 1.65 shows a typical overhead radiant tube heater.
Radiant tube heaters are available in several configurations:
U-tube (as shown), linear and continuous (multi-burner).
Outputs from individual units are typically in the range of
10 to 40 kW and up to 180 kW can be obtained from multitube or continuous tube assemblies, operating at a
temperature of around 500 °C. They may be mounted at
Direct electric heaters
Electric air curtains
Electric warm air unit heaters are typically only used in
restricted circumstances, such as air curtains at entrance
doors, due to their relatively high running cost. Air curtains
are described in BSRIA Application Guide: Air curtains —
commercial applications (Alamdari, 1997).
1.11.3.2
Electric convector
These are individual convectors where the electrical
resistance element is placed inside a cabinet and induces
room air to flow by natural convection over the element.
Grilles are provided at low and high level. The air flow rate
is a function of the height difference between the inlet and
outlet grilles and the surface temperature of the heating
element. A simple on/off thermostatic control is normally
incorporated with the temperature sensor located just
inside the inlet grille.
1.11.3.3
Electric radiator
These are panel or column radiators filled with oil
containing an electric immersion heater. In terms of heat
output the radiator performs in an identical manner to a
conventional lthw radiator (see section 1.8) with similar
surface temperature and radiant/convective proportions.
Electric convectors and radiators would normally only be
used in small non-domestic buildings where annual heating
energy demand was very low taking advantage of the low
installation and maintenance costs.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1.11.2.2
1-117
1-118
Heating
Table 1.46 Minimum heights for radiant heat emitters (BSRIA, 1996)
Emitter type
Minimum mounting
height / m
13
3.0
22
3.6
38
4.3
Gas-fired radiant
plaque
13.5
4.2
27
7.0
Electric quartz
3
3.0
6
4.5
Gas-fired radiant
U tube
1.11.3.4
Storage heaters
Electric storage heaters utilise nighttime off-peak electricity,
storing the energy as heat which is then released during the
occupied period. Off-peak electricity is cheaper than
normal daytime rates depending on the details of the tariff.
Utilising off-peak electricity reduces the daytime demand
on the grid and the carbon intensity is generally lower in
most countries. However, national building energy codes
might not accept this lower intensity.
As these units use radiant heat transfer they heat objects
and people, not air. This will allow a lower air temperature
to achieve the same comfort level that would be achieved
with a convector and hence less energy will be used.
This type of heater can pose a significant fire hazard because
of their high temperature.
1.11.4
Electric underfloor heating
Electric under floor heating has similar design
considerations as lthw underfloor heating. This is
discussed in section 1.4.7.7 and section 1.8.6.5.
1.11.5
Stand-alone heat pumps
Stand-alone heat pumps are frequently used in the form of
air to air split-systems in which the condensing coil is
placed in the room and the compressor and evaporating
unit contained within a single cabinet located outdoors.
The two components are connected by refrigeration
pipework. In some cases the system is reversible so allowing
cooling to be provided.
The heat is generated using a simple resistance heater and
stored in a ceramic block. The heaters are relatively large
and heavy for their rated heat output. Heat output is mainly
by uncontrolled radiation and convection from the outer
casing, supplemented by the controlled opening of vents to
provide additional convection either natural or fan assisted.
This additional heat output may be operated manually or
incorporate time and room temperature control. A midafternoon top-up (at peak electrical tariff rates) is available
with some models so that physically smaller units may be
selected.
Alternatively, through-the-wall units are available. These
come as a single component, installed in an external wall
with the condenser on the room side and the evaporator on
the outside. Again, these are usually reversible. Through
the wall units are usually noisy whereas the split unit
locates the compressor outdoors.
The amount of energy drawn during the charging period is
in some cases controlled by sensing room temperature in an
attempt to minimise energy consumption (required under
some national building energy codes). Whilst the efficiency
of electrical to heat conversion is 100%, the actual efficiency
of storage radiators is reduced as heat output rate is poorly
controlled with the room temperature likely to be above the
set-point during the early part of the occupied period.
1.11.6
Although the ceramic block is insulated from the outer
casing, the outer casing temperature will rise towards that
of the block if the heat loss from the case is reduced by
placing objects on or against it (such as furniture or papers).
This will result in a serious fire risk as the block is often
heated to temperatures higher than the ignition temperature
of commonly used materials.
1.11.3.5
Electric radiant heaters
Electric radiant heaters typically use quartz-enclosed
radiant elements operating at up to 2000 °C and parabolic
reflectors. They have good directional properties and 100%
efficiency in converting from electricity to heat; however,
energy costs are high and upstream carbon dioxide
emissions are high when electricity is generated from fossil
fuels. They are mostly used for local spot heating, mounted
Heat pumps are discussed in section 1.8 and in the BSRIA
guide Heat pumps – A guidance document for designers
(Brown, 2009).
Radiant systems characteristics
In general, systems are considered to be radiant when more
than 50% of their output is radiant, which corresponds
broadly to those with emitter temperatures greater than
100 °C. This definition includes medium temperature
systems, such as high pressure hydronic systems, steam
systems and air heated tubes, which operate at temperatures
up to 200 °C. High temperature radiant systems, such as
those with electric radiant elements or gas heated plaques,
produce a higher proportion of their output in radiant form
and are particularly effective when heat output needs to be
focussed and directed to specific locations.
Radiant heating is particularly useful in buildings with
high air change rates or large volumes that do not require
uniform heating throughout, e.g. factories, and
intermittently heated buildings with high ceilings. The key
characteristics of radiant heating are as follows:
——
heat transfer occurs by radiation directly on
surfaces, including building occupants and the
internal surfaces of buildings and fittings. The
surrounding air need not be heated to the same
temperature as would be required with convective
heating
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Heat output rate
/ kW
at levels between 2 and 4 m. A typical unit is shown in
Figure 1.67.
Unitary systems
a rapid response can be achieved because the effect
of the thermal inertia of the building is bypassed by
direct radiation
——
after an initial warm-up period, radiant heating
directed downwards towards floor level is aug­
mented by re-radiation and convection from
surfaces at the level occupied by people
——
radiant asymmetry is a potential problem and may
place restrictions on design.
Radiant heating can require less energy than convective
heating because it enables comfort conditions to be achieved
at lower air temperatures. As a general rule it is likely to
have an advantage in this respect whenever ventilation heat
losses exceed fabric heat losses. Further savings may be
achieved when only some zones within a large open area
require heating and local radiant temperature can be raised
by well directed radiant heat. In such cases, large volumes
of surrounding air may be left at much lower temperatures
without a detrimental effect on operative temperature in
the working zones.
Spot and total heating
There are two basic approaches to radiant heating design:
——
——
spot heating: applies to the situation described in the
preceding paragraph, in which the intention is to
heat only a small part of a larger space. In such
cases, comfort depends mainly on direct radiant
output from the heaters and there is little effect on
the overall air temperature in the building
total heating: applies to situations in which the
whole space must be heated to a uniform
temperature.
Detailed guidance on the design of radiant heating systems
is given in BSRIA Application Guide AG3/96 (Brown,
1996). Also refer to BS EN 15316-4-8: 2011: Heating systems
in buildings. Method for calculation of system energy requirements
and system efficiencies Space heating generation systems, air
heating and overhead radiant heating systems.
For spot heating, standard heat loss calculations are not
appropriate for calculating the output required from
emitters. Relatively high levels of irradiance are required to
produce the necessary operative temperature and it is
necessary to determine the distribution of radiant energy
within the space. To achieve this, it is necessary to know the
directional characteristics of each heat emitter. For an air
temperature of 15 °C, the maximum irradiance
recommended (Brown, 1996) at floor level is 80 W·m–2,
which places limitations on the mounting height of
emitters. Total spherical irradiance at 1.8 m above floor
level is recommended not to exceed 240 W·m–2. These
figures are considered conservative for industrial heating
applications and may be exceeded with caution. However,
account should be taken of temperatures reached on
surfaces close to heaters, e.g. on the tops of shelving. When
considering the use of spot radiant heating, it is important
to consider relative humidity of the air in the building.
Contact between moist air and cold surfaces away from the
heated areas may cause problems with condensation,
particularly where flueless gas radiant heaters are used.
The energy consumption of a spot heating radiant system
can be estimated using BS EN 15316-4-8 (2011d).
When designing for total radiant heating relying on low
and medium temperature emitters, the procedure is similar
to that required for other heating systems, involving
consideration of fabric and ventilation heat loss and the
calculation of total heat output required. Designs typically
assume that air temperature will be around 3 °C below
operative temperature.
Restrictions of use
Physical restrictions on the mounting of radiant emitters
apply. High temperature emitters must not be placed where
they can come into contact with people or objects that
cannot withstand the resulting surface temperatures. Also,
the irradiance from emitters limits their proximity to
working areas. Consequently, radiant heating may be
considered unsuitable for use in buildings with low ceilings.
Table 1.46 shows typical restrictions on mounting height
for various types of radiant heat emitter as functions of heat
output rate.
Despite its obvious advantages for partially heated
buildings, ‘spot’ radiant heating does not offer good control
of temperature. It should not be considered, therefore,
where close temperature control is required.
1.11.7
Convective heating
characteristics
Purely convective heaters may lead to feelings of stuffiness
when the room air temperature exceeds room radiant
temperature. This is discussed in section 1.3.3. Convective
systems also result in stratification and so are not
recommended for rooms with high ceilings unless some
method of redirecting the warm air back to low level is
provided.
The basic equations relating to convection are presented in
outline in section 1.8.6.
For individual warm air heaters it is usual to provide a
separate thermostat or sensor to control each heater
although, exceptionally, up to four small heaters in one
space may be controlled together. Time control is usually
by simple time-switch, since the fast response of warm air
heaters makes optimum start/stop of limited benefit.
1.11.8
Controls
The sensing of temperature for the control of radiant
heating presents difficulties both in sensing operative
temperature and in finding an appropriate location for the
sensor. A black-bulb thermometer needs to be located
centrally in a zone to avoid influence by proximity to a wall.
Hemispherical black-bulb sensors are available for wall
mounting, but are often difficult to set in relation to
perceived comfort conditions.
Air temperature sensors may be used to control radiant
heating, particularly where total heating is provided.
However, they tend to underestimate operative temperature
during warm up and cause waste of energy.
Stand-alone controllers may be used or the control function
may be integrated into a building management system
(bms).
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
1-119
1-120
Heating
Table 1.47 Classification of dhw systems
System
Options available
Heat provided indirectly to dhw via heat exchanger,
normally using lthw or district heating from chp. Heat
generator is remote and may be dedicated to dhw or be
combined with those serving space heating.
Heat generator is an integral part of the system supplying
heat directly to the dhw system without the need for an
intermediate heat transfer fluid. Examples are direct gas
fired (condensing or non-condensing) and electric
immersion.
Hot water storage
No hot water storage – dhw generated instantaneously in
response to opening of a draw-off point
Hot water storage is provided. Heat generator responds to
fall in temperature of stored water registered by
thermostat control.
Open/closed vent
dhw system is permanently open (vented) to atmosphere
dhw system is sealed to the atmosphere and requires
appropriate fittings to allow for expansion of water and
incorporating pressure and excess temperature relief to
comply with Building Regulations
Localised/centralised
Localised — dhw system sited adjacent draw-off point(s)
Where individual heaters are used it is usually sufficient to
provide time control by time-switch or optimiser with on/
off temperature control using an air tem­perature sensor or
thermostat. Manufacturers generally offer these simple
controls as part of their equipment. bms or other centralised
control can be used but is often not considered necessary.
It is normal practice for each heater to have a dedicated
room thermostat to provide individual control. When small
output heaters are used it is sometimes possible to control
more than one heater from a thermostat but four is
considered to be the practical maximum. Averaging from
several sensors is not normally used except for central
systems since control zones are rarely large enough to
justify averaging.
High/low and fully modulating burner control is available.
Most individual gas and oil heaters incorporate a fan runon circuit, so that the main fan continues to run in order to
cool the heat exchanger after the burner has been switched
off (for energy efficiency and to reduce heat exchanger
stress) until a pre-set low-limit leaving air temperature is
reached.
1.12
1.12.1
Domestic hot water
systems
General
A domestic hot water (DHW) System is the term used to
describe the system that produces and supplies hot water to
sanitary fittings (e.g. sinks and showers), and equipment
including washing machines and dishwashers (terminal
points). After use, the water is discharged to drain. The
heat exchanger producing the DHW is thus subjected to a
continuously replaced flow of water with resultant issues of
scaling and corrosion.
The term applies to systems serving both domestic and
non-domestic buildings, though this section deals only
with the latter. At its simplest the system will consist of a
point-of-use hot water heater (typically electric) serving a
single outlet. Larger systems will include extensive
distribution pipework serving many outlets spread
Centralised — dhw system can be remotely located and
serves a number of distributed draw-off points
throughout the building and usually include hot water
storage vessel and in some instances the heat generator
itself.
The demand for DHW varies greatly depending on building
type, e.g. office, hotel, but also varies considerably between
buildings of the same type. Prediction of DHW consumption
and the sizing of DHW systems is therefore a difficult
engineering task and may result in oversizing with a
subsequent reduction in energy efficiency. At the same
time, as buildings generally become more energy efficient
in terms of HVAC and lighting so the energy consumption
by DHW systems is becoming increasingly significant and
the optimum choice and design of DHW system ever more
important. Solar energy and other renewable technologies,
heat pumps and heat recovery systems are becoming
common, as are measures to reduce hot (and cold) water
consumption.
In addition to the need to reduce energy and water
consumption, DHW systems present health and safety risks
to building users through scalding and bacterial infection
in addition to the potential for explosion due to inadequate
provision for expansion of the water as it is heated. As a
result, the design, installation and maintenance of these
systems are heavily regulated.
This section looks at:
——
regulations applicable to DHW
——
design temperatures and prevention of scalding and
bacterial growth
——
generic types of DHW systems and their applications
——
DHW demand and energy consumption
——
solar-thermal collectors and other means of
reducing greenhouse gases
——
the principles of sizing DHW
1.12.2
Classification of DHW systems
systems are categorised according to several key
features and are available in any combination as set out in
Table 1.47. In England, more detailed definitions and
descriptions are given in the Non-Domestic Building
Services Compliance Guide (NDBSCG) (DCLG, 2013e)
DHW
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Indirect/direct systems
Domestic hot water systems
1-121
Table 1.48 Regulations and Codes relevant to dhw in the UK
Purpose (with respect to
dhw systems)
Comments
Water Supply
(Water Fittings)
Regulations
(HMSO, 1999)
To avoid waste and
over-consumption of
water and the
contamination of water
Applies to all installations supplied with water from a licensed water supplier. Now partly superseded
by Schedule 1 Part G Approved Document 2014 of The Building Regulations 2000.
Schedule 1 Part G
Approved
Document 2014 of
The Building
Regulations,
England (DCLG,
2014a)
To ensure that adequate
dhw is provided in the
building and it is safe to
use
Part G1 requires ‘wholesome’ dhw is supplied at sufficient flow and pressure to meet needs of
sanitary fittings and that it is delivered in such a way so as to avoid waste or misuse and without
undue consumption or contamination.
Part G3 requires that all parts of the dhw system are able to deal safely with the temperatures and
pressures that could be reasonably expected under fault conditions as well as normal operation.
Methods of demonstrating compliance are set out in detail for both vented and unvented systems.
Further details are provided below.
Part G3 also requires that the system is designed to minimise risk of growth of Legionella bacteria.
Schedule 1 Part L
Approved
Documents 2014 of
the Building
Regulations,
England (DCLG,
2013)
To reduce energy
consumption and CO2
emissions
Applies to new-build and replacement. Sets minimum standards for
•
efficiency of heat generators
•
system controls
•
insulation of pipework and storage vessels
•
energy metering
•
commissioning
•
handover documentation
Details are given in Non-Domestic Compliance Guide. Sections 2 and 3 deal with non-dedicated and
section 8 with dedicated hot water generators. Solar hot water system requirements are included in
the Domestic Compliance Guide. Section 11 gives requirements for insulation of pipe work.
The Control of
Legionella bacteria
in Water Systems
Prevention of
Legionnaires’ Disease
ACoP and Guide
L8 (HSE, 2013)
This requirement arises under the Health and Safety at Work Act 1974 and the Control of Substances
Hazardous to Health (COSHH) 1999. The requirement is that the designer has a duty of care to
ensure, as far as is reasonably practicable, that it is safe when used at work and enables safe and easy
operation, cleaning and maintenance.
The Approved Code of Practice (ACoP) sets out in detail the measures that should be taken in the
design of dhw systems.
Education (School
Premises)
Regulations 1996
Prevention of scalding
This requires that in schools the dhw to baths and showers is limited to less than 43.5 °C. In practice,
Health Guidance
Note – Safe Hot
Water and Surface
Temperatures
Prevention of Scalding
Current guidance for all health care premises. Sets maximum temperatures at outlets to 43 °C. NHS
Estates Specification D 08 is specification for thermostatic mixing valves to achieve these
temperatures.
Gas safety
Applies to gas installations including direct gas-fired water heaters for commercial and catering gas
appliances and the Factory Act applies for the industrial appliances.
dhw in schools is usually limited to this at all outlets.
(NHS, 1998)
Gas Safety
(Installations and
Use) Regulations
(HMSO, 1998)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Regulation/ Code
of Practice
1-122
Heating
that accompanies the Part L Building Regulations 2010 for
England.
Regulations relevant to DHW
systems
The design, installation and operation of DHW systems are
regulated so as to:
——
reduce the risk of scalding
——
limit the growth of bacteria within the water
systems
——
reduce the risk of over-pressurisation and explosion
——
minimise water consumption
——
minimise energy consumption.
For England, these are summarised in Table 1.48. Some of
these regulations, or very similar, apply to Wales, Scotland
and Northern Ireland. Note that this section deals only
with DHW systems for non-domestic applications. There
are additional regulations for domestic applications.
1.12.3.1
Prevention of scalding
Scalding of DHW users can occur when temperatures at
DHW outlets exceed about 40 °C. The young, old and infirm
are particularly vulnerable. As a result, there are UK
regulations limiting the temperature of DHW at some
outlets in schools and health care buildings to 43 °C. The
regulations within the work place (the Health and Safety at
Work etc. Regulations 1974 (HSWA) and the Workplace
(Health and Safety and Welfare) Regulations (WHSWR))
requires the employer to provide “suitable and sufficient
washing facilities” and be responsible for the reasonable
protection of the employees in the workplace including
protection from excessive hot water temperatures. The
Water Regulations Advisory Scheme Water Regulations
Guide (WRAS, 2001) requires “terminal fittings or
communal showers in schools or public buildings, and in
other facilities used by the public, the hot water should be
supplied with water through thermostatic mixing valves so
that the water discharged at the outlets does not exceed
43 °C”.
The Chartered Institute of Plumbing and Heating (CIPHE,
2009) recommend the following maximum temperatures:
Prevention of bacterial growth
Bacteria thrive in water close to body temperature where
there are nutrients available (including products of
corrosion). DHW systems can provide ideal conditions for
bacterial growth. Of particular concern is Legionella
pneumophilia which causes a form of pneumonia known as
Legionnaires’ Disease. This is often fatal when contracted
by the old or infirm. The bacterium is present in small
quantities in potable water. To infect a person, the bacterium
must be inhaled. This can happen when a spray of water is
created, most easily in a shower but also when splashing
occurs at water outlets.
This, and other bacteria, will not reproduce in water above
50 °C and starts dying at temperatures above about 65 °C.
In the UK two principal guidance documents, CIBSE
TM13 and Health and Safety Executive Approved Code of
Practice L8 (HSE 2013) have been produced that set out a
number of recommended measures to reduce the risk of
Legionella and other bacterial growth:
——
use materials that will not encourage bacterial
growth (WRAS approved)
——
use materials that will resist corrosion
——
avoid or limit length of dead-legs
——
avoid multiple storage tanks
——
standby plant to be easily disconnected
——
provision for future increase in demand to use
modular plant
——
provide access for cleaning
——
store water at a minimum of 60 °C
——
maintain flow to outlets at minimum of 55 °C using
pumped return or trace heating
——
achieve not less than 50 °C and within 30 seconds
after fully opening the tap (WRAS Water
Regulations Guide)
——
for low use or remote DHW outlets use point of use
water heaters
——
in large storage vessels fit time-controlled destratification pumps
——
size system to meet normal daily fluctuations in
demand without drop in supply temperature
——
carry out regular monitoring and maintenance of
DHW systems.
——
38 °C for bidets
——
41 °C for showers and washbasins
As the water temperatures specified are well above those
that could cause scalding of users, the use of thermostatic
mixing valves (TMVs) is essential. TM13 and ACoP L8
recommend that:
——
44 °C for bath fill from hot tap
——
——
< 46 °C from tap for DHW at any other outlet.
the TMVs should be as close as practicable to DHW
termination outlets so as to limit the amount of
pipework operating at low temperature
——
TMVs are not used to supply spray taps where
occupants particularly susceptible to Legionella.
However, they warn that serious scalding can occur above
40°C depending on the user’s medical condition.
Part G of the Building Regulations 2000 England, Schedule
1, G3(4) requires water delivered to a bath not to exceed
48°C.
1.12.3.3
Provision for thermal expansion
During normal operation, the water in the DHW system will
attempt to expand as its temperature increases. Water has a
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1.12.3
1.12.3.2
Domestic hot water systems
Under normal operation, the DHW control thermostat will
limit the temperature of the water to the design value.
Should this thermostat fail, water temperature may exceed
safe limits. For an indirect DHW system served by a heat
generator such as a boiler or heat pump, water temperature
will be limited by the flow temperature from the heat
generator. For DHW systems served by direct electric or by
solar thermal panels, the temperature in the DHW system
under fault conditions could exceed 100 °C.
EV
IV
Hot water
outlets
NRV
T+PRV
Cold
main
H
Figure 1.68 Typical arrangement for point of use unvented electric water
heater
1.12.3.4
Minimising water and energy
consumption
DHW systems are regulated services under Part L of the
Building Regulations (England) and as such must meet
energy efficiency requirements. The Non-Domestic
Building Services Compliance Guide (NDBSCG) (DCLG,
2013e) sets standards for:
——
maximum heat loss rates from storage vessels
Part G3 of the Building Regulations (England) (DCLG,
2015) sets minimum safety requirements for all DHW
systems to prevent excessive water pressures or temperatures
occurring. These are set out below. Similar regulations exist
for Wales, Scotland and Northern Ireland.
——
minimum standards of distribution pipework
insulation
——
minimum seasonal efficiencies of heat generators
——
minimum levels of controls.
All open vented systems
In order to maintain a minimum water temperature at the
draw off points as required to prevent the growth of
Legionella and other bacteria, systems with extensive
distribution pipework will require either a constant
circulation of hot water (see Figure 1.69) or trace heating.
This will reduce water consumption by reducing the runoff of cool water by the user but will increase energy
consumption due to increased heat losses. Generating and
storing DHW at 60 °C or above may also reduce the efficiency
of the heat generators. For example, condensing boilers will
not operate in condensing mode when the return water
temperature is above about 55 °C; most heat pumps cannot
achieve these storage temperatures or do so at the expense
of reduced COPs. The designer must appreciate these
conflicting requirements of user safety and energy and
water efficiency when selecting the most appropriate type
of DHW system.
To prevent excessive pressure occurring, a vent pipe must
be installed which is open to atmosphere and terminates
above the water level in the cold water cistern. The vent
pipe must be adequately sized but must be no less than
19 mm diameter. The cold cistern must be capable of
operating safely when full of hot water.
In addition, to prevent excessive water temperatures, either
a thermal cut-out switch or a temperature relief valve (or a
combined temperature pressure relief valve) is required.
The thermal cut-out must disconnect the heat source from
the DHW system on over-temperature and is in addition to
the thermostat used for normal operation. Where the DHW
system is direct, the cut-out switch must require manual
re-setting. Where a temperature (or temperature-pressure)
relief valve is installed, it should discharge the hot water
safely to a drain via a tundish.
All unvented systems
Two independent safety devices are required. One must be
a thermal cut out that will disconnect the heat source in the
event of over temperature. The cut out must require manual
resetting and be in addition to the thermostat used for
normal operation. The second device should be a
temperature (or combined temperature/pressure) relief
valve that will discharge hot water safely to drain via a
tundish.
Unvented hot water storage systems of less than 500 L
storage capacity must be in the form of a proprietary unit or
package. For storage systems of more than 500 L capacity
the system must be designed by an appropriately qualified
engineer.
1.12.4
Generic DHW systems
The following sections describe the basic types of dhw
system together with common variations.
1.12.4.1
Point of use water heaters
A separate hot water heater is typically provided for each
hot water outlet or small group of adjacent outlets. It
contains its own heat generator, most commonly electric
although direct gas-fired units are available, and may
incorporate a small water storage volume.
A common example is the electric water heater installed
beneath the wash hand basins in many office buildings.
This is connected directly onto the cold water supply pipe
(unvented). This offers the advantages of low installation
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
very high bulk modulus, as do all liquids. That is, a very
large static pressure is required to compress them. If heat is
added to water in a sealed vessel with no provision for
expansion, the static pressure will rise very rapidly until
the vessel ruptures resulting in an explosion. All DHW
systems therefore need to provide adequate provision for
thermal expansion. Vented systems use a vent pipe which is
open to atmosphere. An example is shown in Figure 1.70.
Unvented systems incorporate an expansion vessel sized to
accommodate the full expansion volume of the water
without excessive increase in static pressure. Examples are
shown in Figures 1.68 and 1.69. As the expansion vessel
could be a potential site for the growth of Legionella, it
should be located on the cold water inlet side of the DHW
system.
1-123
1-124
Heating
T and PRV
HWS return
EV
Primary
HX
Cleaner’s
sink
Range
of whb’s
Cleaner’s
sink
Range
of whb’s
Cold
feed
Pump set
Drain
Figure 1.69 Central dhw system
with storage (un-vented, indirect)
Open
vent
Cold
feed
HWS return
Cold water
cistern to
drinking
water
quality
HWS return
Primary
HX
From
independent
heat source
Pump set
Drain
costs and space savings due the absence of distribution
pipework circuits. Although electricity is relatively
expensive (p/kW·h) and has a high carbon intensity
(kgCO2/kW·h) compared to other fuels, system energy
losses are very small. The point of use system also offers
little risk of the growth of bacteria. However, point of use
systems are prone to the scaling up and corrosion of the
heating element and sacrificial anodes.
The function of the small water storage volume is to reduce
the capacity of the heating element and allow the water
heater to serve more than one hot water terminal outlet. The
duty of an instantaneous water heater is given by equation
1.72 (see section 1.5). Applying this equation to a single wash
hand basin (spray) tap with a typical flow rate of 0.1 L/s and
with a temperature lift of 30 K (10–40 °C), gives a duty of
about 12 kW. However, by incorporating a storage volume of
about 10 L, a 3 kW heating element is sufficient to provide
this flow rate for up to 100 seconds with a recovery time of
7 minutes. Depending upon the frequency of use, it could be
used to serve several wash-hand basins.
Figure 1.68 shows a typical arrangement of an unvented
point of use electric water heater. This is shown
incorporating a temperature/pressure relief valve (usually
integral with the heater) and an expansion vessel. The
electric water heater also contains a non-self-resetting
thermal cut-out as well as a thermostat for normal operation
(see section 1.12.3.3, ‘Provision for thermal expansion’). A
pressure relief valve may be required on the cold fill to
ensure that the expansion vessel and t&prv can operate
correctly.
Cleaner’s
sink
Range
of whb’s
Cleaner’s
sink
Range
of whb’s
Figure 1.70 Vented central storage
dhw system
For unvented hot water heaters with a water storage
capacity of 15 L or less, the equipment manufacturer should
be consulted regarding compliance with Part G of the
Building Regulations (England).
1.12.4.2
Centralised DHW system with storage
Figure 1.69 shows a centralised system with water storage.
In this case, the system is unvented and indirectly fired
from an independent heat source (e.g. a boiler) though both
vented and directly-fired options are common. Note that all
valves and controls are omitted for simplicity. No immersion
back-up heater within the storage cylinder is shown, though
these are commonly included.
An expansion vessel and a pressure or combined
temperature and pressure relief valve are required together
with a non-self-resetting thermal cut-out is required (not
shown). All dhw systems connected directly to the cold
water main require a non-return valve. A pressure relief
valve may also be required to ensure proper operation of the
ev and t&prv.
Heat is supplied from a remote lthw generator to the heat
exchanger within the water storage vessel. The temperature
of the water in the vessel is controlled by varying the flow
of lthw through the heat exchanger using either a three or
two-port control valve. Steam or mthw may be used as an
alternative to lthw. Direct fired (gas or oil) water heaters
in either non-condensing or condensing configuration and
electric immersion or electrode type are also available in
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
HWS return
Domestic hot water systems
which the heat generator is integral with the water storage
vessel.
Distribution pipework must be adequately insulated under
building energy codes. Nevertheless, water within the
pipework would cool if there was no flow and within about
30 minutes the next user would need to run-off all the water
in the pipe before acceptably hot water reached the outlet.
This would result in wasted water and energy as well as
creating conditions at which Legionella and other bacteria
could breed. To minimise this, dhw systems should
incorporate some means of preventing the water cooling
under periods of no draw-off. This may be achieved by
providing a constant re-circulation within the (insulated)
distribution pipework or by providing trace heating.
Figure 1.69 shows the use of a pumped secondary return.
Take off pipework from the supply circuit to individual
dhw outlets is kept as short as practicable and in compliance
with WRAS Water Regulations Guidance (WRAS, 2001),
Table 18. In this way, the temperature of the water
circulating can be maintained and there is minimal waiting
time for hot water at the outlets. This is one method of
showing compliance with the regulations relating to the
prevention of legionnaire’s disease and wastage of water.
The water storage set point temperature is required to be at
60 °C and the minimum temperature of the water returning
to the storage vessel is 50 °C. The take-off pipework to
outlets form dead-legs where water will be stationary and
cool down between water being drawn off. This will provide
opportunity for bacterial growth and lead to wastage of
water as the content of the dead-leg will need to be drawn
off before hot water reaches the outlet. Both the Legionella
Code of Practice and Part G Building Regulations for
England (DCLG, 2010) set limits on the maximum length
of dead-legs related to the pipe diameter.
As an alternative to installing a pumped return, the
distribution pipework may be trace heated using self
regulating, temperature maintenance electric heating tapes
with a set point of 55 °C or higher if required. The
temperature of the cable adjusts at any point along the pipe
dependent upon the local conditions on the pipe network.
This means that the pipe is heated everywhere in proportion
to how much it cools down minimising energy use, If hot
water is flowing, the thermal output from the tape is
reduced. On extensive distribution systems, this may have
a lower installation cost and use less energy than a
traditional recirculation system.
As dhw outlets will generally require water at below
50–60 °C to minimise the risk of scalding (see above),
thermostatic mixing valves should be used as close to the
outlet as is practicable.
Scale and other matter will accumulate in the bottom of the
storage vessel. This, combined with the lower temperature
of the stratified water will encourage the growth of Legionella
and other bacteria. Provision should therefore be made for
inspection and removal. In some instances, particularly
where large volumes of hot water are stored, it may be
advisable to install a shunt pump across the storage vessel,
operating on a time clock to periodically mix the contents
and achieve a temperature throughout of at least 60 °C. In
systems serving buildings with particularly susceptible
occupants, such as care homes and hospitals, this can be
combined with pasteurisation in which the water contents
are periodically raised to 70 °C.
The open vent arrangement was extensively used in the
past. Figure 1.70 shows a typical arrangement. This is
simpler in design to the unvented system but it is not
always convenient to provide a cold feed and hot water
expansion into the cold water cistern/tank and rising
pipework. Further, the tank introduces additional
maintenance requirements and increases risk of bacterial
growth.
1.12.4.3
Central instantaneous DHW systems
Figure 1.71 shows a typical arrangement for an unvented,
direct-fired, instantaneous, central dhw system. A pumped
return is shown. The instantaneous central dhw system
avoids the needs for storage vessels and the consequential
measures required to avoid the growth of Legionella bacteria.
However, the peak demand placed on the hot water
generator (kW) is very much higher than with the storage
system and output has to be closely matched to demand in
order to maintain reasonable control on the temperature of
water supplied. It is usual therefore to provide several hot
water generators in parallel and firing in sequence using
step control. Direct fired water heaters are able to meet the
high but short duration loads.
An unvented system is shown, being the most common, but
open vented arrangements are possible. Likewise, a trace
heated supply distribution pipe could be used rather than a
pumped return.
As an alternative to using several hot water generators, a
single generator may be used, but containing a small
(integral) water storage volume. This storage volume will
reduce the required duty (kW) of the heater somewhat but
is intended principally to improve control of the
temperature of the water supplied to the distribution
pipework.
Where an indirect system is preferred, this can be achieved
through a flat-plate high performance heat exchanger as
shown in Figure 1.72(a). These are often used with a buffer
vessel to improve control as shown in Figure 1.72(b).
1.12.5
Choice of DHW system
The designer should ensure that the most appropriate type
of dhw system is selected based on considerations that
include:
——
space heating system – potential for integration
——
availability of fuels, including renewable energy,
chp and district heating
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The hot water storage vessel is usually in the form of a
vertical or horizontal cylinder with the cold feed at the base
and outlet at the top to promote stratification, usually
assisted by a baffle on the cold inlet. The heat supply to the
heat exchanger within the hot water storage vessel is
controlled so as to maintain a set point temperature in the
upper section of the vessel. In this way, a reasonable fraction
of the hot water in the cylinder can be drawn off before heat
is called for. A high dhw output (kW) can be achieved
compared to the heat input rate from the heat source
allowing a large number of dhw outlets to be served
including outlets with high flow rates such as cleaner’s
sinks.
1-125
1-126
Heating
4
1
2
5
Gas
3
Cold supply
3
Hot
outlets Figure 1.71 Central, instantaneous,
direct-fired dhw system.
Bronze circulating pump
Primary
Th
Low velocity heater
S
Cold feed
Figure 1.72(a) Central indirect
instantaneous dhw system.
T&P valve
S Th
To/from
low velocity
header
Mains or boosted
water supply
Buffer vessel
sized to suit
requirement
Secondary
pump
DOC
*Unvented system
controls
Figure 1.72(b) Central indirect
instantaneous dhw system with
buffer vessel.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1 Strainer
2 Pressure reducing valve
3 Non-return valve
4 Expansion vessel
5 Expansion valve
and tundish
Domestic hot water systems
location and distribution of dhw outlets
——
peak demand and demand pattern
——
space available
——
capital costs
——
running costs and environmental impact
——
ease of maintenance and cleaning
——
risks associated with bacterial growth.
Sport and leisure
Retail
Sport and leisure
Hotel and catering
Retail
Health
Hotel and catering
Government
Health
Education
Government
Commercial
offices
Education
Table 1.49 describes some typical applications.
1.12.6
20
40
60
80
100
120
Commercial offices0
DHW estimated annual energy consumption / kWh/m2
0
20
40
60
80
100
120
DHW estimated annual energy consumption / kWh/m2
Figure 1.73(a) Indicative energy consumption by dhw
Sport and leisure
DHW demand and energy
consumption
Figure 1.73 gives dhw energy consumption data for a range of
existing buildings in various sectors both as an annual
consumption (kW·h/m2 gross floor area) and as a percentage of
total annual energy consumption. These are based on energy
consumption data from Digest of UK Energy Statistics (DUKES)
(DECC, 2014a). dhw is responsible for 5–15% of total annual
energy consumption in existing buildings in the UK.
This data does need to be used with caution. Firstly, annual
energy consumption for dhw is rarely available, being
invariably lumped with energy for space heating. The
DUKES data is therefore based on some assumptions
regarding split between dhw and space heating. Secondly,
the building use categories embrace a wide range of building
types. For example, hotels and catering include from small
to luxury hotels which will have widely differing dhw
requirements.
The data is based on metered energy consumption in
existing building stock and so incorporates energy losses in
the dhw systems. Such losses in some systems can be very
large – possibly exceeding the useful energy delivered at the
draw-off points.
Energy losses from the system can be attributed to:
——
inefficiency of heat generator
——
standing losses from storage vessels
——
heat losses from pipe work
——
energy used in circulation pumps.
Retail
Sport and leisure
Hotel and catering
Retail
Health
Hotel and catering
Government
Health
Education
Government
Commercial
offices
Education
Commercial offices0% 2% 4% 6% 8% 10% 12% 14% 16%
Ratio of DHW to total annual metered energy consumption
0% 2% 4% 6% 8% 10% 12% 14% 16%
Ratio of DHW to total annual metered energy consumption
Figure 1.73(b) dhw as percentage of total energy consumption
The Simplified Building Energy Model (SBEM), used in the
National Calculation Methodology (www.ncm.bre.uk) for
demonstrating compliance with Part L of the Building
Regulations for England, includes details of dhw
consumption to be used for energy estimation purposes
(BRE, 2014). A selection of these are included in Table 1.50.
However, such data is based on specific zones within a
building as defined by usage and are not averages for the
whole building. In particular, areas such as swimming pools,
halls and sport halls are allocated zero dhw demand in sbem
as the allowances are included in changing/showers zones.
1.12.6.1
As such, published energy data has only limited application as
a benchmark for new-build and refurbishment projects where
more efficient systems should be designed and installed.
Opportunities to reduce energy
consumption
There are a number of methods by which energy
consumption by dhw systems, and resultant CO2 emissions,
can be reduced as described in Table 1.51. However, these
HWS
Unvented
feed
HWS
Unvented
feed
Solar
panel
Solar
panel
From
boiler
From
boiler
Cold feed
Combined
cylinder
Figure 1.74 Typical arrangement for solar assisted dhw system
Cold water feed
(unvented)
Pre-heat
cylinder
Figure 1.75 Twin cylinder solar hot water system
Top-up
cylinder
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
1-127
1-128
Heating
Table 1.49 Typical dhw applications
Applications
Point of use
•
where dhw outlets very remote from others and connection to a central system would require excessive additional
distribution pipework
•
where dhw demand is very small or infrequent
•
where occupants highly susceptible to Legionella
•
for ease of metering in multi-tenancy building
•
where inadequate space for distribution pipework
•
where groups of outlets are located conveniently close
Central
•
where low carbon heating system such as solar hot water, biomass, chp, district heating available
•
where dhw demand is relatively high
•
where central plant is preferred for ease of maintenance
Table 1.50 Allowances for dhw daily consumption (BRE, 2014)
Building
category
Zone – by use
Commercial
offices
General work areas
0.2
Reception
0.03
Food preparation
Retail
Restaurants
Education
DHW demand
/ (L/day per
m2 floor area)
Building category
Zone – by use
Leisure and
sport
Reception
0.03
Auditoria
0.15
0.33
Circulation (public)
0.06
Eating/drinking
6.0
Changing rooms with showers
Changing rooms with showers
30
Food preparation
0.33
Sales areas
0.04
Eating/drinking
6.0
Food preparation
0.33
Reception
0.03
Eating/drinking
6.0
Bedrooms (including en suite)
6.4
Offices
0.2
Hall (with seating)
0.15
Food preparation
0.33
Food preparation
0.03
Eating/drinking
6.0
Eating/drinking
6.0
Lecture theatres
0.15
Staff offices
0.2
Classrooms
1.35
A&E
0.21
Staff offices
0.19
Wards
3.0
Food preparation
0.33
Operating theatres
1.3
Eating/drinking
6.0
Staff offices
0.17
Changing rooms with showers
30
must be consistent with the need to minimise risk of growth
of Legionella bacteria.
1.12.7
Solar hot water heating
Solar hot water is becoming increasingly common in nondomestic applications. It is a recognised technology under
the UK Renewable Heat Incentive. Section 1.7 describes
the types and efficiency of solar hot water collectors.
Figure 1.74 shows a typical arrangement for connecting the
solar hot water circuit. Valves and controls have been
omitted for clarity. The solar circuit utilises a water/glycol
mixture to minimise the risk of freezing. A well designed
solar system will be able to provide approximately 50% of
the annual dhw demand. A top-up heating source is
therefore required. It is now common practice to utilise a
single hot water vessel that accommodates two heating
Hotels
Hospitals
DHW demand
/ (L/day per
m2 floor area)
120
coils. The top-up heating coil should be sized to provide
100% of the dhw demand during inclement weather and
located above the solar circuit heat exchanger. This will
ensure that the solar circuit heat exchanger is located in the
coolest part of the storage vessel and so increase the
efficiency of the solar circuit.
Where solar hot water has been added to an existing dhw
scheme, in some instances the existing cylinder has been
retained and a solar cylinder installed that acts as a pre-heat
to the existing as shown in Figure 1.75.
This however increases the risk of Legionella and other
bacterial growth as the solar storage vessel will operate at
low temperature for long periods. It is recommended
therefore that this system is not installed without first
assessing the health risk and how it may be minimised
including the use of pasteurisation.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Type of dhw system
Domestic hot water systems
1-129
Table 1.51 Reducing the energy consumption of dhw systems
Measure
Comments
Choice of dhw system
Carry out analysis for local and central
systems, with and without storage.
Include solar hot water.
Requires realistic data on both daily dhw demand and demand profile to
assess energy consumption.
Pumped return or trace
heating
Carry out analysis for both methods of
maintaining water temperature
Need to include pump energy. Assess for both energy and CO2 emissions.
Choice of fuel
chp
chp is often cost effective where there is high, year round demand for dhw
such as hospitals, hotels and leisure centres.
District heating
Ensure can comply with any restriction on maximum return water
temperature specified by district heating scheme operator
Consider going beyond minimum
standards required under Building
Regulations
Currently, minimum requirements are close to maximum efficiencies available.
Heat generator
efficiency
Some clients make life cycle analysis mandatory when selecting systems. Solar
installations benefit from the Renewable Heat Incentive (see section 1.2).
dhw systems generally require higher flow and return temperatures from/to
lthw boilers and heat pumps compared to space heating so that actual
efficiency/cop achieved may be lower than for space heating.
Insulation to storage
vessel and pipework
Consider going beyond minimum
standards required under regulations
Requires life cycle analysis.
dhw demand
Reduce demand for dhw by using
low-flow fittings
Manufacturers now produce aerated taps and showers operating at greatly
reduced flow rates. Consider use of spray washbasin taps. For healthcare and
schools, check if acceptable.
Reducing water consumption can also increase BREEAM rating.
System sizing
Avoid oversizing system
Oversizing generally reduces seasonal efficiency. If planning for future increase
in duty, consider modular or separate systems
Metering
Provide metering on demand side
Metering generally required under UK Building Regulations on fuel used by
heat generators but not on dhw used. Adequate metering provides better
opportunities for energy management.
Heat recovery
Carry out feasibility study on recovery of
heat from waste water or recovering
waste heat from cooling systems – e.g. by
fitting desuperheaters to water chillers in
air conditioned buildings or process
cooling systems.
This is more likely to be financially feasible where large quantities of dhw are
used in a central location, e.g. in shower rooms for leisure centres. Waste water
will be at less than 40 °C but may be a good source of heat for a heat pump.
1.12.8
Desuperheaters can raise incoming cold water temperature from 10 ºC to 60 °C
to supplement heat form boilers.
Sizing of DHW systems
Table 1.52 Daily dhw consumption for a range of building types
Table 1.52 shows indicative dhw demand in litres of water
per person per day. This is extracted from CIBSE Guide G.
This is based on data from existing buildings.
BS EN 15316-3-1 (2007) gives daily hot water consumption
data for various building types to be used in determining
annual energy consumption. CIBSE Guide G gives a
methodology for sizing dhw systems in terms of storage
capacity, heating power and pipe sizing.
1.12.8.1
Instantaneous systems
dhw demand
/ (L/person/day)
School (day)
15
Hospital (general)
136
Hotel (5-star)
136
Hotel (2-star)
114
Office
14
Sports pavilion
40
Table 1.53 Loading units used in EN 806-3
For instantaneous systems the useful heating power
required is given by the equation:
φdhw = m Cpw (θw2 – θw1)
Type of building
(1.72)
where m is the peak design mass flow rate of the dhw (kg/s),
Cpw is the specific heat of water (J/kg·K), θw2 is the required
flow temperature of the dhw and θw1 is the temperature of
the cold water entering the dhw heater (°C).
Loading units
When determining the peak design flow rate for systems
serving several outlets, it is unlikely that all outlets will be
Sanitary fitting
(domestic)
Loading unit
Volume flow rate
/ (L/s)
Wash hand basin tap
1
0.1
Bath
4
0.3–0.4
Shower
2
0.15–0.2
Sink
2
0.15–0.2
operating simultaneously. CIBSE Guide G recommends
that a probabilistic approach is adopted. BS 8558 (2011)
recommends the probabilistic method set out in BS EN
806-3 (2006) in the absence of data based upon experience.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Item
0
1-130
1·2
Volume flow rate / L/s
Volume flow rate / L/s
30
40
50
60
Loading units
70
80
90
100
Heating
0·6
0·4
8
7
6
5
4
3
2
1
0
10
20
30
50
60
40
Loading units
70
80
90
100
9
EN 806-3
utilises the concept of loading units (lu). Each
8
sanitary fitting is allocated a lu based on its flow rate,
7
typical
operating time and likely frequency of use. A
6
selection
of these is given in Table 1.54. The same lus are
used5for both hot and cold water. In addition, BS 806-3 lists
a number
of methodologies used in individual member
4
states.
3
2
The 1peak simultaneous flow rate in each pipe section is then
found by adding up the total number of lus for the fittings
0
served0 by that1000
pipe section
the lus into
2000 and converting
3000
4000
5000
volume flow rate. The relationship
between
volume
flow rate
Loading units
and lu is nonlinear as shown in Figure 1.76(a) and (b).
lus are also used in BS 6700 (2006) (now superseded by BS
EN 8558: 2011) and the same lus used in the Chartered
Institute of Plumbing and Heating Engineering's Plumbing
Engineering Services Design Guide 2002 (CIPHE, 2002).
The allocation of numerical values of lus to fittings is
arbitrary. Typically a wash hand basin is given a convenient
whole number and all other fittings a multiple of this based
upon relative flow rate, period and frequency of use. Thus,
as shown in Table 1.53, a bath is deemed to be equivalent in
terms of impact on peak simultaneous hot water flow rate as
four wash hand basins. As a result of this arbitrary allocation
of numerical values, the lus used in BS EN 806-3, BS 6700
(2006), CIPHE and CIBSE differ.
0
0
Storage volume (L/person)
Figure 1.77 Typical sizing graph for a storage dhw system
2000
3000
Loading units
4000
5000
Figure 1.76(b) Relationship between LU and volume flow rate used in
EN 806-3 (0–5000 LU)
The relationship between lu and volume flow rate is based
upon (binomial) probability theory. This theory is set out in
CIBSE Guide G in a discussion on the estimation of design
peak simultaneous flow rate in drainage systems serving
sanitary fittings (where discharge units are allocated to
different fittings in the same way as are lus). Starting with
the probability of a fitting being in use at any one time, this
mathematical theorem allows the probability (p) to be
calculated of more than a certain number (n) of (equivalent)
fittings out of a total (m) being in use simultaneously. That is,
p is the risk of more than n out of m fittings being used
simultaneously at any time. Typically, a risk factor of 1% is
chosen. As the number of equivalent fittings, as quantified
by the number of lu increases, so the rate of increase in
volume flow rate falls as shown in Figure 1.76.
The probability of a single fitting being in use at any one
time is numerically equal to the ratio of its period of use
divided by the time between use (known as usage ratio).
Usage ratio will vary widely with building type. For
example, a wash hand basin in a theatre, during an interval
will have a much higher usage ratio than a wash hand basin
in an office. This will have an effect on the relationship
between lu and volume flow rate. BS EN 806-3 results in
significantly lower peak simultaneous flow rates than BS
6700, but unlike BS 6700 does not specify the usage ratios
used. The designer must ensure that appropriate usage
ratios are employed together with risk factors, relative
loading units and fitting flow rates are used when using a
probabilistic method of determining design peak
simultaneous flow rates and is advised to refer to CIBSE
Guide G.
1.12.8.2
Decreasing
recovery time
1000
Storage systems
CIBSE Guide G contains a number of charts used in the
selection of both storage volume and heating power. These
are provided for a number of building types. Storage volume
and heating power is related to the recovery time – the time
taken to heat up the storage volume using the heating
power available.
An example of such a graph is shown in Figure 1.77. A
point on the line representing the desired recovery time is
chosen and the value of heating power and storage volume
selected. The line in the chart represents the upper limit of
a combination of storage volume and heating power based
upon data gathered from existing buildings where the size
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
0·8
Figure 1.76(a) Relationship between loading units (LU) and volume
flow rate used in EN 806-3 (0–100 LU)
10
Volume flow rate / L/s
20
9
0·2
Heating power (kW/person)
10
10
1·0
0
0
Connecting to heat networks
1.13
Connecting to heat
networks
The aim of this section is to provide an overview of the
issues that the designer of a heating system must address
when intending to utilise a heat network as the source of
heat. The intention is not to provide guidance on the design
of the heating networks: this is dealt with in a number of
recent technical publications including the Building
Research Establishment (BRE, 2014b) and Euroheat
(Euroheat, 2013). CIBSE, in collaboration with the
Association for Decentralised Energy (ADE), has recently
published Heat Networks: A Code of Practice for the UK
(CIBSE, 2015b). This sets out the aims and objectives of
each of the several stages involved in a heat network
development from initial feasibility through to operation
and discusses relevant regulations and legislation.
1.13.1
Introduction
A heat network is the generic term for a heating installation
supplying heat to a number of users or buildings. This
includes:
——
communal heating (for example to an apartment
building)
——
district heating (an extensive heat network usually
supplying a mix of domestic and non-domestic
users)
——
site heating (for example a hospital site or a
university campus containing multiple buildings).
Heat networks offer the following advantages over
conventional heat generators as they:
——
remove the need for heat generators in individual
buildings
——
utilise greater diversity in heat demand
——
can more easily incorporate a range of heat
generation technologies including heat from waste,
CHP, biomass and recently, large scale solar heating
(S K Solar, 2015).
This can lead to reductions in both the cost of the heat
delivered (p/kW·h) and the carbon emissions (kg·CO2/kW·h)
provided that capital and operating costs and efficiency are
carefully controlled.
In the UK there are approximately 2000 heating networks
(DECC, 2015a). The majority of these are small, communal
networks and site networks. A map of major networks in
the UK is available on-line from the ADE (ADE, 2015).
Generally, other European countries utilise heating
networks, particularly district heating, to a much greater
extent than the UK. For example, only about 1% of the
domestic sector in the UK is served by some form heating
network whilst in France the figure is over 7% and in
Denmark 61% (Euroheat, 2013).
To encourage and assist the installation of district heating
by local authorities in the England and Wales, the UK
government has set up the Heat Networks Delivery Unit
(HNDU) which provides technical assistance and funding
(DECC, 2015a). Some planning authorities, such as the
Greater London Authority, have policies that promote the
use of heat networks, (GLA, 2015).
1.13.2
Existing UK heat network
performance
Table 1.57 lists information from the recent Government
survey of UK heat networks, (DECC, 2015b) This
information is based on detailed data provided for only
seven networks and so needs to be treated with caution.
The networks ranged in size from about 2 to 50 MW.
Table 1.57 Typical characteristics of UK heat networks (DECC, 2015b)
Bulk networks*
Non-bulk
networks*
Flow water temperature (oC)
82–92
81–85
Return water temperature ( C)
55–75
46–54
Seasonal heat factor (%)
o
20–28
13–24
Distribution heat losses (as % of
annual heat energy supplied)
6.0–11
12–43
Pump energy (as % of annual
heat energy supplied)
1.0–4.0
1.7–2.0
Tariff (p/kW·h)
4.9–6.9
4.6–9.9
†
* Bulk networks are those where the network operator sells the heat to
large users (who may then distribute and resell to final users).
Non-bulk networks are those where the network operator sells the
heat directly to the final user, including individual households.
†
Seasonal heat factor is the ratio of thermal energy supplied per
annum to the amount that could be produced if the plant ran
continuously at full load for the whole year.
It is clear that in distribution heat losses can be very large
in some cases though the DECC report states that the figure
of 43% is an anomaly and that heat energy losses are
expected to be in the order of 10%.
The tariffs need to be compared to whole-life heating tariffs
of convectional heating installations, that is allowance
made for capital, installation and maintenance costs of the
heat generators. DECC estimates that these are typically 5.7
to 10.2 p/kW·h so that the tariffs currently charged by
network operators are very favourable in most cases.
The DECC report does not state the CO2 emissions for the
seven networks above. In any event the CO2 emissions will
vary greatly depending upon the fuel and heat generating
technology used. The UK Government does publish carbon
emission factors for community heating schemes in SAP
(BRE, 2012) which takes into account a range of fuels and
technology.
1.13.3
Key design points for heat
networks
Of key importance in the feasibility of heat networks is the
capital and operating costs. The DECC study described
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
of the dhw storage system was deemed adequate to meet
demand. This implies that in most cases a storage system
sized using this method is likely to be oversized rather than
undersized. Oversizing will reduce the efficiency of a dhw
system.
1-131
1-132
Heating
——
flow water temperature should be kept as low as
possible, consistent with the needs of users, so as to
minimise heat losses.
——
flow and return water temperature difference (DT)
should be kept as large as possible as this will reduce
both the mass flow rate of water required to meet
the heating load and the mean water temperature in
the pipes. This in turn will result in:
——
——
smaller pipe diameters with consequential
savings in capital cost
——
reduced pumping energy due to lower flow
rates
——
reduced heat loss due to smaller external
surface area of the pipes;
——
reduced heat loss due to lower mean water
temperature in the pipe
variable volume pumping should be used to
minimise pumping energy.
Where the heat network serves DHW, careful consideration
needs to be given to the flow temperature to avoid risk of
bacterial growth within the users DHW system, (for example
Legionella). With relatively high flow temperature, the
designer should assess the risk of overheating where
extensive lengths of the network pipes run within a building
(for example, an apartment building).
For further guidance see the CIBSE CP1 Heat networks:
Code of Practice for the UK (CIBSE, 2015b), Part A.
1.13.4
Network-consumer interface
Normal practice is to provide complete hydraulic separation
between the water in the heat network and the heating
system installed within individual buildings by means of a
heat exchanger. For convenience, packaged units are
available which include plate heat exchanger(s), valves,
controls and a heat meter. Such packages are known by a
variety of terms including heat interface units (HIU) for
individual dwellings and small buildings and thermal
exchange substations (TES) for larger applications.
The essential function of the HIU or TES is to replace the
heat generators in a building. All other components of the
building heating system remain. A typical HIU is shown in
Figure 1.79.
Figure 1.79 HIU with cover removed (reproduced courtesy of SAV
Systems Ltd)
1.13.5
Implications for design of
building heating system
1.13.5.1
Return water temperatures
The operators of the network will often apply a penalty
tariff to users who allow return water temperatures to rise
as this increases both network heat losses and pump energy
consumption. The design of the building heating network
should therefore be such as to ensure low return water
temperature at all times. As discussed in section 1.8.6,
secondary heating circuits utilising 3-port constant volume
control show a rise in return water temperature on a fall in
heating demand: two-port variable volume control leads to
a fall in return water temperatures.
Conventional DHW storage systems can lead to a high
return water temperature on the primary side due to the
need to maintain storage temperatures above 60 °C to
reduce the risk of bacterial growth (see section 1.12).
Consideration should be given to the use of instantaneous
DHW heating. This will lead to reduced primary return
water temperatures as the cold water will be entering at
10–20 °C. DHW storage systems require a very much smaller
heat input rate than instantaneous and so are often preferred
in buildings that contain their own heat generators so as to
minimise their size. However, heat networks overcome this
problem: the network contains a high thermal capacitance
and short duration, high DHW heating loads will be greatly
diversified over the whole network.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
above, found that capital costs for pipework in the network
varied between £422/m and £1472/m. On large networks,
costs of pipework will therefore be a major concern. So too
will network heat losses and pump energy as seen in
Table 1.57. In view of this:
Operation, maintenance and energy management
1.13.5.2
From heating
R
network
F
HIU/TES
Back-up
Space heating
PHE
DHW PHE
Whilst a heating network will incorporate standby plant, if
the network fails or is being maintained then the building
will lose its only heating supply. The risk of losing the
heating supply needs to be examined.
Where the DHW is of prime concern, for example in a leisure
centre, DHW storage should be considered. This can still
utilise the TES arrangement shown in Figure 1.80 with
DHW entering the storage vessel at design storage
temperature. Electric emersion heaters can provide
additional back-up.
1.14
Operation, maintenance
and energy management
CWS
General
This section is aimed at the designer of the heating system
who should ensure that systems can be installed and
operated safely and that the building owner has adequate
information to operate the systems efficiently. In addition,
the designer should, as a professional, seek to ensure that
the heating system can be operated and maintained to
minimise environmental impact and running costs whilst
achieving its performance specification.
1.14.2
‘Commissionability’ and
‘maintainability’
All designs must take account of the environment in which
the system will be installed, commissioned and operated,
considering both safety and economy.
Apart from matters affecting safety, designers must take
account of maintenance cost over the lifetime of the systems
they specify. In particular, it is important to ensure that the
client understands the maintenance requirements,
including cost and the need for skills or capabilities. CIBSE
Guide M: Maintenance engineering and management contains
guidance on issues that need to be addressed by the building
services designer.
Approved Document L2A 2013 of the Building Regulations
(England) (DCLG, 2013b) requires the provision of a
‘commissioning plan that shows that every system has been
inspected and commissioned in an appropriate sequence’.
This implies that the designer must consider which
measurements are required for commissioning and provide
the information required for making and using those
measurements. Also, the system must be designed so that
the necessary measurements and tests can be carried out,
taking account of access to the equipment and the health
and safety of those making the measurements. Approved
Space heating
circuits
Figure 1.80 Two stage heating of DHW utilising the return from the space
heating plate heat exchanger (PHE)
Document L2A states that one way of demonstrating
compliance would be to follow the guidance given in the
CIBSE Commissioning Codes or BSRIA Commissioning
Guides.
1.14.3
1.14.1
DHW
Life cycle issues
The designer’s decisions will have consequences that
persist throughout the life of the equipment installed,
including durability, availability of consumable items and
spare parts, and maintenance requirements. Consideration
should also be given to how the heating system could be
adapted to changes of use of the building. The combined
impact may be best assessed using the concept of life cycle
costs, which are the combined capital and revenue costs of
an item of plant or equipment throughout a defined
lifetime.
The capital costs of a system include initial costs,
replacement costs and residual or scrap value at the end of
the useful life of the system. Future costs are typically
discounted to their present value. Revenue costs include
energy costs, maintenance costs and costs arising as a
consequence of system failure.
BSRIA Building Applications Guide: Whole-life costing
analysis (Churcher, 2008) presents a simple process in five
sequential steps for the practical calculation of whole-life
costs for the construction and operation of buildings. It
provides examples to show how the different stages of the
process relate to one another, to show how the results are
obtained and what they mean.
1.14.4
Construction (Design and
Management) Regulations (UK)
The HSE's Approved Code of Practice for the Construction
(Design and Management) Regulations 2007 (HSE, 2015)
place an obligation on designers in the UK to ensure that
systems they design and specify can be safely installed and
maintained. The Regulations require that a designer must
be competent and have the necessary skills and resources,
including technical facilities. The designer of an installation
or a piece of equipment that requires maintenance has a
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The HIU or TES often contain separate plate heat exchangers
(PHE) for DHW and space heating. Further, it is also
recommended (Euroheat, 2008) by some manufacturers
that a HIU/TES incorporates two-stage heating of the DHW
PHE by utilising both the primary supply to the HIU/TES
and the primary return from the space heating PHE as
shown in Figure 1.80.
1-133
1-134
Heating
1.14.5
Operation and maintenance
manuals
It is a requirement under the HSAW Act (HSE, 1974) that
no new, non-domestic building is occupied unless the
building operators are provided by an operations and
maintenance (o&m) manual. Without adequate information
on design of the building services and their intended
operation, it is unlikely that a building operator will be able
to operate the systems efficiently and safely. The importance
of comprehensive o&m manuals cannot be overemphasised.
CIBSE Guide M: Maintenance engineering and management
sets out a pro forma for an o&m manual.
In the UK, the Building Engineering Services Association
(BESA) produce a standard maintenance specification for
mechanical services in buildings, SFG 20 (BESA, 2012b).
This contains maintenance schedules for all plant and
equipment; it is continuously updated and is web-based.
1.14.6
Log books
It is a requirement under Part L of the Building Regulations
2013 (England) that a log book be provided on completion
of all new and refurbished non-domestic buildings.
The log book should contain information that will allow
the building user/operator to manage the building services
in an energy efficient manner. It should contain information
on the calculations used to demonstrate Part L compliance,
the production of the Energy Performance Certificate and
the recommendations for further reducing CO2 emissions.
The data used to calculate the building ter and ber should
be included and the log book should also contain
recommendations on how the owner or occupier might
further improve the energy performance of the building in
the future.
CIBSE TM31 provides a pro forma log book.
1.14.7
Energy management,
monitoring and targeting
CIBSE Guide F and CIBSE TM46 gives typical benchmark
consumption figures for fossil fuel consumption in
buildings (mostly serving heating and hot water). It also
describes standard monitoring and targeting methods for
analysing energy use – in particular regression and cusum.
These are described in CIBSE TM41: Degree days: theory
and application and the Carbon Trust Guide CTG075: Degree
days for energy management (Carbon Trust, 2012).
Also useful is CIBSE TM22: Energy assessment and reporting
methodology which is a systematic way of undertaking an
energy survey of an occupied building, reporting the
results, and calculating likely savings from changes in use,
technology or management. It can also be used to summarise
design information and predictions, providing a cradle to
grave benchmarking tool. The 2006 version can also be
used to assemble information for Display Energy
Certificates based on actual annual energy use, providing a
greater level of insight than the statutory method. CIBSE
provides Excel workbooks for offices, hotels, high street
agencies and mixed use buildings on disc free with the
publication.
Heating and hot water services are particularly well suited
to this type of monitoring and targeting (m&t) analysis, as
there is usually a strong correlation between energy
consumption and the weather (as described by heating
degree-days). Regular monitoring of energy use in this way
can provide strong signals about the performance of the
system, and even provide clues of where inefficiencies may
be occurring. When used in conjunction with good
maintenance regimes m&t systems can help to ensure
heating systems are run with optimum performance, can
help to prove the value of additional efficiency measures,
and can show when major failures occur.
Excess fuel consumption within a heating system may have
a number of causes ranging from overheating of the
building to degradation of the central plant performance.
In some cases, such as all air systems, failure of heating
controls may be masked by cooling systems compensating
for any resultant overheating. Many such faults cannot be
readily seen without proactive monitoring and maintenance.
CIBSE Guide M provides some good practice advice for the
maintenance and operation of heating systems.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
duty to carry out a risk assessment of the installation,
maintenance and replacement functions. Where this
assessment shows a hazard to the maintenance operative,
the designer must reconsider the proposals and try to
remove or mitigate the risk.
References
References
Action Energy (2002b) Good Practice Guide 310: Degree days for energy
management – a practical introduction (Garston: Action Energy)
ABCB (2013) Building Code of Australia (Volumes 1 & 2) (Canberra:
Australian Building Codes Board)
ADE (2015) District Heating Installation Map [Online]. [Accessed: 12
December 2015]. Available from: http://www.theade.co.uk/districtheating-installation-map_790
Alamdadi (1997) AG2/97: Air Curtains - commercial applications (Bracknell:
Building Services Research and Information Association)
ANRE (2013) Japan’s Policy on Energy conservation: EMAK 4th Workshop,
January 2013 (Tokyo, Japan: Agency for Natural Resources and Energy)
ASHRAE (2011) Handbook: Applications (Atlanta, Georgia: ASHRAE)
ASHRAE (2013a) Handbook: Fundamentals (Atlanta, Georgia: ASHRAE)
ASHRAE (2013b) ANSI/ASHRAE/IES Standard 90.1: Energy Standard for
Buildings Except Low-Rise Residential Buildings (Atlanta, Georgia:
ASHRAE)
Barnard N, Concannon P and Jaunzens D (2001) BRE Information Paper
IP6/01: Modelling the performance of thermal mass (Garston: Building
Research Establishment)
Bleicher, D (2012) BSRIA BG31/2012: Illustrated Guide to Mechanical
Building Services (Bracknell: Building Services Research and Information
Association)
BCGA (2012) Code of Practice CP33: The bulk storage of gaseous hydrogen at
users’ premises (Derby: British Compressed Gases Association)
BESA (2012a) Guide to Good Practice: Heat Metering for the RHI (Penrith:
Building Engineering Services Association)
BESA (2012b) SFG20: Standard maintenance specification for building services
(Penrith: Building Engineering Services Association)
BRE (2014) BREEAM: New Construction [Technical Manual SD5073:
2.0:2011] (Garston: BRE Press)
BRE (2012) SAP: The Government’s Standard Assessment Procedure for
Energy Rating of Dwellings (Garston: BRE Press; on behalf of DECC)
BRE (2014a) National Calculation Methodology (NCM) Modelling Guide (for
buildings other than dwellings in England) (Garston: BRE Press)
BRE (2014b) Technical guide to district heating (Garston: BRE Press)
British Gas (1979) IM/11: Flues for commercial and industrial gas fired boilers
and air heaters (London: British Gas)
Brown R (1996) BSRIA Application Guide AG 3/96: Radiant heating
(Bracknell: Building Services Research and Information Association)
Brown R (2009) BSRIA BG7: Heat Pumps: A guidance document for designers
(Bracknell: Building Services Research and Information Association)
Brown R (2011) BSRIA BG4: Underfloor Heating and Cooling (Bracknell:
Building Services Research and Information Association)
Brown R, Parsloe CJ (2011) BSRIA BG29: Pre-Commission Cleaning of
Pipework Systems (Bracknell: Building Services Research and Information
Association)
BSI (1972) BS 4856: 1972: Methods for testing and rating fan coil units, unit
heaters and unit coolers; Part 1: 1972: Thermal and volumetric performance for
heating duties; without additional ducting; Part 2: 1975: Thermal and volumetric
performance for cooling duties: without additional ducting; Part 3: 1975:
Thermal and volumetric performance for heating and cooling duties; with
additional ducting; Part 4: 1997: Determination of sound power levels of fan coil
units, unit heaters and unit coolers using reverberating rooms (London: British
Standards Institution)
BSI (1975) BS 3463: 1975: Specification for observation and gauge glasses for
pressure vessels (London: British Standards Institution)
BSI (1980) BS 5854: 1980: Code of practice for flues and flue structures in
buildings (London: British Standards Institution)
BSI (1984a) BS 759-1: 1984: Valves, gauges and other safety fittings for
application to boilers and to piping installations for and in connection with
boilers; Specification for valves, mountings and fittings (London: British
Standards Institution)
BSI (1984b) BS 6759-1: 1984: Safety valves. Specification for safety valves for
steam and hot water (London: British Standards Institution)
BSI (1986) BS 6785: 1986: Code of practice for solar heating systems for
swimming pools (London: British Standards Institution)
BSI (1989a) BS 779: 1989: Specification for cast iron boilers for central heating
and indirect hot water supply (rated output 44 kW and above) (London: British
Standards Institution) [Partially replaced by BS EN 303: 1999: Heating
boilers with forced draft boilers; Part 1: Terminology. General requirements; Part
4: Special requirements for boilers with forced draught oil burners with outputs up
to 70 kW and a maximum operating pressure of 3 bar]
BSI (1989b) BS 7074: 1989: Application, selection and installation of expansion
vessels and ancillary equipment for sealed water system; Part 1: 1989: Code of
practice for domestic heating and hot water supply; Part 2: 1989: Code of practice
for low and medium temperature hot water heating systems; Part 3: 1989: Code
of practice for chilled and condenser systems (London: British Standards
Institution)
BSI (1990) BS 855: 1990: Specification for welded steel boilers for central
heating and indirect hot water supply (rated output 44 kW to 3 MW) (London:
British Standards Institution)
BSI (1992) BS 2790: 1992: Specification for design and manufacture of shell
boilers of welded construction (London: British Standards Institution)
(1992). Partially replaced by BS EN 12953: 2012: Shell boilers
BSI (1997) BS 2486: 1997: Recommendations for treatment of water for steam
boilers and water heaters (London: British Standards Institution)
BSI (1998) BS EN 837-1: 1998: Pressure gauges. Bourdon tube pressure gauges.
Dimensions, metrology, requirements and testing (London: British Standards
Institution)
BSI (1999) BS 1113: 1999: Specification for design and manufacture of watertube steam generating plant (including superheaters, reheaters and steel tube
economizers) (London: British Standards Institution) (Partially replaced)
BSI (2000) BS ISO 15686-1: 2000: Buildings and constructed assets. Service
life planning. General principles (London: British Standards Institution)
BSI (2002a) BS EN 12257: 2002 Transportable gas cylinders – Seamless, hoopwrapped composite cylinders (London: British Standards Institution)
BSI (2002b) BS EN 13055-1: 2002: Lightweight aggregates. Lightweight
aggregates for concrete, mortar and grout (London: British Standards
Institution)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Action Energy (2002a) Energy Efficiency Best Practice Programme Good
Practice Guide GPG234: Guide to community heating and CHP —
commercial, public and domestic applications (Garston: Action Energy)
1-135
1-136
BSI (2003a) BS EN 676: 2003 +A2: 2008: Automatic forced draught burners
for gaseous fuels (London: British Standards Institution)
BSI (2003c) BS EN 12285-1: 2003: Workshop fabricated steel tanks. Horizontal
cylindrical single skin and double skin tanks for the underground storage of
flammable and non-flammable water polluting liquids (London: British
Standards Institution)
BSI (2007c) BS EN ISO 10211: 2007: Thermal bridges in building construction.
Calculation of heat flows and surface temperatures. Detailed calculations
(London: British Standards Institution)
BSI (2007d) BS EN ISO 13370: 2007: Thermal performance of buildings. Heat
transfer via the ground. Calculation methods. (London: British Standards
Institution)
BSI (2007e) BS EN ISO 6946: 2007: Building components and building
elements. Thermal resistance and thermal transmittance. Calculation method.
(London: British Standards Institution)
BSI (2004a) BS EN 10255: 2004: Non-alloy steel tubes suitable for welding and
threading. Technical delivery conditions (London: British Standards
Institution)
BSI (2007f) BS EN 15450: 2007: Heating systems in buildings. Design of heat
pump heating systems (London: British Standards Institution)
BSI (2004b) BS EN 13842: 2004: Oil fired forced convection air heaters.
Stationary and transportable for space heating (London: British Standards
Institution)
BSI (2007g) BS EN 1775: 2007: Gas supply. Gas pipework for buildings.
Maximum operating pressure less than or equal to 5 bar. Functional
recommendations (London: British Standards Institution)
BSI (2005a) BS EN ISO 11114: 2005: Transportable gas cylinders.
Compatibility of cylinder and valve materials with gas contents; Part 1 (2012):
Metallic materials; Part 2 (2013): Non-metallic materials; Part 3 (2010):
Autogenous ignition test for non-metallic materials in oxygen atmosphere; Part 4
(2005): Test methods for selecting metallic materials resistant to hydrogen
embrittlement. (London: British Standards Institution)
BSI (2007h) BS EN 15287-1: 2007 +A1:2010: Chimneys. Design, installation
and commissioning of chimneys. Chimneys for non-roomsealed heating appliances
(London: British Standards Institution)
BSI (2005c) BS EN 13341: 2005 +A1:2011: Static thermoplastic tanks for
above ground storage of domestic heating oils, kerosene and diesel fuels. Blow
moulded and rotationally moulded polyethylene tanks and rotationally moulded
tanks made of anionically polymerized polyamide 6. Requirements and test
methods (London: British Standards Institution)
BSI (2007j) BS EN 15316-3-1: 2007: Heating systems in buildings. Method for
calculation of system energy requirements and system efficiencies. Domestic hot
water systems, characterisation of needs (tapping requirements) (London:
British Standards Institution)
BSI (2008a) BS EN ISO 12241: 2008: Thermal insulation for building
equipment and industrial Installations. Calculation rules (London: British
Standards Institution)
BSI (2006a) BS EN 1057: 2006 +A1: 2010: Copper and copper alloys.
Seamless, round copper tubes for water and gas in sanitary and heating
applications (London: British Standards Institution)
BSI (2008b) BS EN 1264: Parts 1-5, various dates: Water based surface
embedded heating and cooling systems (London: British Standards
Institution)
BSI (2006b) BS 5990: 2006: Specification for direct gas-fired forced convection
air heaters with rated heat inputs up to 2 MW for industrial and commercial space
heating: safety and performance requirements (excluding electrical requirement)
(2nd family gases) (London: British Standards Institution)
BSI (2008c) BS EN 15116: 2008: Ventilation in buildings. Chilled beams.
Testing and rating of active chilled beams (London: British Standards
Institution)
BSI (2006c) BS EN 12975: 2006: Thermal solar systems and components. Solar
collectors; Part 1: 2006 + A1: 2010: General requirements; Part 2: 2006: Test
methods (London: British Standards Institution)
BSI (2006b) BS EN 12976: Thermal solar systems and components. Factory
made systems; Part 1: 2006: General requirements; Part 2: 2006: Test methods
(London: British Standards Institution)
BSI (2006e) BS EN 1993-3-2: 2006: Design of steel structures. Towers, masts
and chimneys. Chimneys. (London: British Standards Institution)
BSI (2006f) BS EN ISO 10077: 2006: Thermal performance of windows, doors
and shutters. Calculation of thermal transmittance; Part 1: 2006 General
(London: British Standards Institution)
BSI (2006g) BS EN 806-3: 2006: Specifications for installations inside buildings
conveying water for human consumption. Pipe sizing. Simplified method
(London: British Standards Institution)
BSI (2006h) BS EN 12897: 2006: Water Supply. Specification for indirectly
heated unvented storage water heaters (London: British Standards Institution)
BSI (2007a) BS EN 13831: 2007: Closed expansion vessels with built-in
diaphragm for installation in water (London: British Standards Institution)
BSI (2007b) BS EN 15287: Chimneys. Design, installation and commissioning
of chimneys; Part 1: 2007 +A1: 2010: Chimneys for non-roomsealed heating
appliances; Part 2: 2008: Chimneys for roomsealed appliances (London:
British Standards Institution)
BSI (2009a) BS EN 1319: 2009: Domestic gas-fired forced convection air
heaters for space heating, with fan-assisted burners not exceeding a net heat input
of 70 kW (London: British Standards Institution)
BSI (2009b) BS EN 12245: 2009 +A1: 2011: Transportable gas cylinders.
Fully wrapped composite cylinders (London: British Standards Institution)
BSI (2009c) BS EN 525: 2009: Non-domestic direct gas-fired forced convection
air heaters for space heating not exceeding a net heat input of 300 kW (London:
British Standards Institution)
BSI (2009d) BS EN 621: 2009: Non-domestic gas-fired forced convection air
heaters for space heating not exceeding a net heat input of 300 kW, without a fan
to assist transportation of combustion air and/or combustion products (London:
British Standards Institution)
BSI (2009e) BS EN 1020: 2009: Non-domestic forced convection gas-fired air
heaters for space heating not exceeding a net heat input of 300 kW incorporating
a fan to assist transportation of combustion air or combustion products (London:
British Standards Institution)
BSI (2009f) BS EN 1856: 2009: Parts 1 and 2: Chimneys. Requirements for
metal chimneys (London: British Standards Institution)
BSI (2009g) BS EN 1859: 2009: +A1: 2013: Chimneys. Metal chimneys. Test
methods (London: British Standards Institution)
BSI (2010a) BS 2869: 2010 + A1: 2011: Fuel oils for agricultural, domestic
and industrial engines and boilers. Specification (London: British Standards
Institution)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
BSI (2003b) BS EN 12831: 2003: Heating systems in buildings. Method for
calculation of the design heat load (London: British Standards Institution)
Heating
References
BSI (2010b) BS 5864: 2010: Installation and maintenance of gas-fired ductedair heaters of rated input not exceeding 70 kW net (London: British Standards
Institution)
BSI (2010d) BS EN 60079-15: 2010: Explosive atmospheres (London:
British Standards Institution)
BSI (2010e) BS EN ISO 21258: 2010: Stationary source emissions.
Determination of the mass concentration of dinitrogen monoxide (N2O).
Reference method. Non-dispersive infrared method (London: British Standards
Institution)
BSI (2010f) BS EN 12542: 2010: LPG equipment and accessories. Static
welded steel cylindrical tanks, serially produced for the storage of liquefied
petroleum gas (LPG) having a volume not greater than 13 m (London: British
Standards Institution)
BSI (2011a) BS 6644: 2011: Specification for installation of gas-fired hot water
boilers of rated inputs between 70 kW (net) and 1.8 MW (net) (2nd and 3rd
family gases) (London: British Standards Institution)
BSI (2011b) BS 6230: 2011: Specification for installation of gas-fired forced
convection air heaters for commercial and industrial space heating (2nd family
gases) (London: British Standards Institution)
BSI (2011c) BS 6896: 2011: Specification and installation of gas fired radiant
overhead heaters for industrial and Commercial Heating (2nd and 3rd family
gases) (London: British Standards Institution)
BSI (2011d) BS EN 15316-4-8: 2011: Heating systems in buildings. Method for
calculation of energy requirements and system efficiencies. Space heating
generation systems, air heating and overhead radiant systems (London: British
Standards Institution)
BSI (2011e) BS EN 1127-1: 2011: Explosive atmospheres. Explosion prevention
and protection (London: British Standards Institution)
BSI (2013b) BS EN 16129: 2013: Pressure regulators, automatic change-over
devices, having a maximum regulated pressure of 4 bar, with a maximum capacity
of 150 kg/h, associated safety devices and adaptors for butane, propane, and their
mixtures (London: British Standards Institution)
BSI (2013c) BS EN 14511: Parts 1 to 3: 2013: Air conditioners, liquid chilling
packages and heat pumps with electrically driven compressors for space heating
and cooling (London: British Standards Institution)
BSI (2013d) BS EN ISO 4126: Parts 1 to 7 (separate dates): Safety devices
for protection against excessive pressure (London: British Standards
Institution)
BSI (2013e) BS EN ISO 9806: Solar Energy. Solar thermal collectors. Test
methods (London: British Standards Institution)
BSI (2014a) BS EN 442-2: 2014: Specification for radiators and convectors;
Test methods and rating (London: British Standards Institution)
BSI (2014b) BS 5410-1: 2014: Code of practice for oil firing. Installations up to
45 kW output capacity for space heating and hot water supply purposes (London:
British Standards Institution)
BSI (2014c) BS EN 15502-2-2: 2014: Gas-fired central heating boilers. Specific
standard for type B1 appliances (London: British Standards Institution)
BSI (2014d) BS 4250: 2014: Specification for commercial butane and
commercial propane (London: British Standards Institution)
BSI (2014e) BS EN 17225: Parts 1-7: Solid Biofuels. Fuel specifications and
classes. (London: British Standards Institution)
BSI (2015a) BS 5918: 2015: Solar heating systems for domestic hot water. Code
of practice for design and installation (London: British Standards Institution)
BSI (2015b) BS 6891: 2015: Specification for the installation and maintenance
of low pressure gas installation pipework of up to 35 mm (R1¼) on premises
(London: British Standards Institution)
BSI (2012a) BS EN 12828: 2012 +A1: 2013: Heating systems in buildings.
Design for water-based heating systems (London: British Standards
Institution)
BSI (2015c) BS 8558: 2015: Guide to the design, installation, testing and
maintenance of services supplying water for domestic use within buildings and
their curtilages. Complementary guidance to BS EN 806 (London: British
Standards Institution)
BSI (2012b) BS EN 12952-7: 2012: Water-tube boilers and auxiliary
installations. Requirements for equipment for the boiler (London: British
Standards Institution)
BSI (2015d) BS EN ISO 9972: 2015: Thermal performance of buildings.
Determination of air permeability of buildings. Fan pressurization method
(London: British Standards Institution)
BSI (2012c) BS EN 12977: 2012: Thermal solar systems and components
(London: British Standards Institution)
BSI (2016a) BS EN 14825: 2016: Air conditioners, liquid chilling packages and
heat pumps with electrically driven compressors for space heating and cooling.
Testing and rating at part load conditions and calculation of seasonal performance
(London: British Standards Institution)
BSI (2012d) BS EN 14214: 2012 +A1: 2014: Liquid petroleum products.
Fatty acid methyl esters (FAME) for use in diesel engines and heating applications.
Requirements and test methods (London: British Standards Institution)
BSI (2012e) BS EN 12953-1: 2012 Shell boilers. General (London: British
Standards Institution)
BSI (2016b) ISO 11119: 2016: Gas cylinders. Refillable composite gas cylinders.
Design, construction and testing. Fully wrapped fibre reinforced composite gas
cylinders up to 150 l with load-sharing welded metallic liners (London: British
Standards Institution)
BSI (2012f) BS EN 15502-1: 2012 +A1: 2015: Gas-fired heating boilers.
General requirements and tests (London: British Standards Institution)
BPIE (2010) Energy performance Certificates across Europe: From design to
implementation (Brussels: Building Performance Institute Europe)
BSI (2012g) BS EN 15502-2-1: 2012 +A1: 2015: Gas-fired central heating
boilers. Specific standard for type C appliances and type B2, B3 and B5
appliances of a nominal heat input not exceeding 1 000 kW (London: British
Standards Institution)
BSRIA (1991) BSRIA Application Guide AG1/91: Commissioning of VAV
systems in buildings (Bracknell: Building Services Research and Information
Association)
BSI (2012h) BS EN 13480: 2012: Parts 1-8: Metallic industrial piping
(London: British Standards Institution)
BSRIA (1997) Application Guide AG2/97: Air curtains — commercial
applications (Bracknell: Building Services Research and Information
Association)
BSI (2013a) BS 5410-2: 2013: Code of practice for oil firing. Installations over
45 kW output capacity for space heating, hot water and steam supply services
(London: British Standards Institution)
BSRIA (1999) Application Guide AG 14/99: Variable speed pumping in
heating and cooling circuits (Bracknell: Building Services Research and
Information Association)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
BSI (2010c) BS 799-5: 2010: Oil burning equipment. Carbon steel oil storage
tanks. Specification (London: British Standards Institution)
1-137
1-138
Heating
CIBSE (2000b) Testing buildings for air leakage TM23 (London: Chartered
Institution of Building Services Engineers)
BSRIA (2013) BG 50/2013: Water treatment for closed heating and cooling
systems (Bracknell: Building Services Research and Information
Association)
CIBSE (2000c) Guide to ownership, operation and maintenance of building
services (London: Chartered Institution of Building Services Engineers)
Calor (2011) Pressure Systems Safety Regulations. Guidance notes for
commercial customers (Dublin: Calor)
Carbon Trust (2005) Good Practice Guide GPG377: Guidance on procuring
energy services to deliver community heat and power schemes (London: Carbon
Trust)
Carbon Trust (2009a) CTG012: Biomass Heating: A practical guide for
potential users (London: Carbon Trust)
Carbon Trust (2009b) Introductory Guide CTG016: Biomass Heating: An
introduction for potential users (London: Carbon Trust)
Carbon Trust (2011) Down to earth: Lessons learned from putting ground source
heat pumps into action in low carbon buildings (London: Carbon Trust)
Carbon Trust (2012a) Introducing combined heat and power: a new generation
of energy and carbon savings (London: Carbon Trust)
Carbon Trust (2012b) CTG075: Degree days for energy management: A
practical introduction (London: Carbon Trust)
Carbon Trust (2015) Biomass Decision Support Tool [Online]. [Accessed 6
April 2016]. Available from http://www.carbontrust.com/resources/tools/
biomass-decision-support-tool
CIBSE (2001a) Automatic controls Commissioning Code C (London:
Chartered Institution of Building Services Engineers)
CIBSE (2001b) Reference data Guide C (London: Chartered Institution of
Building Services Engineers)
CIBSE (2002a) Boiler plant Commissioning Code B (London: Chartered
Institution of Building Services Engineers)
CIBSE (2002b) Refrigeration systems Commissioning Code R (London:
Chartered Institution of Building Services Engineers)
CIBSE (2002c) HVAC strategies for well-insulated airtight buildings TM29
(London: Chartered Institution of Building Services Engineers)
CIBSE (2002d) Weather, Solar and Illuminance Data Guide J (London:
Chartered Institution of Building Services Engineers)
CIBSE (2003) Water distribution systems Commissioning Code W (London:
Chartered Institution of Building Services Engineers)
CIBSE (2004a) Public health engineering Guide G (London: Chartered
Institution of Building Services Engineers)
CIBSE (2004b) Environmental factors affecting office worker performance
CIBSE TM24 (London: Chartered Institution of Building Services
Engineers)
CBCA (2012) Chilled Beams and Ceilings Association: An introduction to
chilled beams and ceilings (London: Federation of Environmental Trade
Associations)
CIBSE (2006a) Energy assessment and reporting methodology TM22 (London:
(London: Chartered Institution of Building Services Engineers)
CIBSE (1989) Building control systems Guide H (London: Chartered
Institution of Building Services Engineers)
CIBSE (2006b) Building log book toolkit TM31 (London: Chartered
Institution of Building Services Engineers)
CIBSE (1989b) Condensing boilers AM3 (London: Chartered Institution of
Building Services Engineers)
CIBSE (2006c) Degree-days: theory and application TM41 (London:
Chartered Institution of Building Services Engineers)
CIBSE (1996) Air distribution systems Commissioning Code A (London:
Chartered Institution of Building Services Engineers)
CIBSE (2006d) Variable flow pipework systems KS7 (London: Chartered
Institution of Building Services Engineers)
CIBSE (1997a) Natural ventilation in non-domestic buildings AM10 (London:
Chartered Institution of Building Services Engineers)
CIBSE (2007a) Sustainability Guide L (London: Chartered Institution of
Building Services Engineers) [Withdrawn – work in hand on new edition]
CIBSE (1997b) Quality in design: engineering design calculations and the use of
margins (London: Chartered Institution of Building Services Engineers)
CIBSE (2007b) Biomass heating KS10 (London: Chartered Institution of
Building Services Engineers) [Withdrawn: superseded by AM15, 2014]
CIBSE (1998) Building energy and environmental modelling AM11 (London:
Chartered Institution of Building Services Engineers)
CIBSE (2008) Energy benchmarks TM46 (London: Chartered Institution of
Building Services Engineers)
CIBSE (1998b) Engineering design calculations and the use of design margins
Research Paper 4 (London: Chartered Institution of Building Services
Engineers)
CIBSE (2009a) Building energy metering TM39 (London: Chartered
Institution of Building Services Engineers)
CIBSE (1999a) Energy demands and targets for heated and ventilated buildings
Building Energy Code 1; Energy demands for air conditioned buildings CIBSE
Building Energy Code 2 (London: Chartered Institution of Building
Services Engineers)
CIBSE (2009b) Operational Ratings and Display Energy Certificates TM47
(London: Chartered Institution of Building Services Engineers)
CIBSE (2010) Non-domestic hot water systems AM14 (London: Chartered
Institution of Building Services Engineers)
CIBSE (1999b) Small-scale combined heat and power AM12 (London:
Chartered Institution of Building Services Engineers)
CIBSE (2012a) Energy efficiency in buildings Guide F (London: Chartered
Institution of Building Services Engineers)
CIBSE (1999c) Minimising pollution at air intakes TM21 (London: Chartered
Institution of Building Services Engineers)
CIBSE (2012b) Inspection of air conditioning systems TM44 (London:
Chartered Institution of Building Services Engineers)
CIBSE (2000a) Understanding building integrated photovoltaics TM25
(London: Chartered Institution of Building Services Engineers)
CIBSE (2013a) Minimising the risk of Legionnaire’s disease TM13 (London:
Chartered Institution of Building Services Engineers)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
BSRIA (2012) BG 31/2012: Illustrated Guide to mechanical Building Services
(Bracknell: Building Services Research and Information Association)
References
1-139
DCLG (2013a) Approved Document L1A: Conservation of fuel and power (new
dwellings) (London: NBS)
CIBSE (2013c) The limits of thermal comfort: avoiding overheating in European
buildings TM52 (London: Chartered Institution of Building Services
Engineers)
DCLG (2013b) Approved Document L1B: Conservation of fuel and power
(existing dwellings) (London: NBS)
CIBSE (2014a) Biomass heating AM15 (London: Chartered Institution of
Building Services Engineers)
CIBSE (2014b) Maintenance engineering and management Guide M (London:
Chartered Institution of Building Services Engineers)
CIBSE (2014c) Design for future climate: case studies TM55 (London:
Chartered Institution of Building Services Engineers)
CIBSE (2015a) Environmental design Guide A (London: Chartered
Institution of Building Services Engineers)
CIBSE (2015b) Heat networks: Code of Practice for the UK CP1 (London:
Chartered Institution of Building Services Engineers)
CIBSE (2015c) Design and operation of modern steam systems TM58 (London:
Chartered Institution of Building Services Engineers)
CIPHE (2002) Plumbing Engineering Services Design Guide (London:
Chartered Institution of Public Health Engineering)
DCLG (2013c) Approved Document L2A: Conservation of fuel and power (new
buildings other than dwellings) (London: NBS)
DCLG (2013d) Approved Document L2B: Conservation of fuel and power
(existing buildings other than dwellings) (London: NBS)
DCLG (2013e) Non-domestic building services compliance guide (NDBSCG)
(London: NBS)
DfE (2003) Building Bulletin 87 (BB87): Guidelines for environmental design
of schools (London: Department for Education)
DfE (2006) Building Bulletin 101 (BB101): Ventilation of school buildings
(London: Department for Education)
DECC (–) Digest of UK Energy Statistics (DUKES) (London: The
Stationery Office) [Published annually]
DECC (2010) National Renewable Energy Action Plan for the United Kingdom
[Article 4 of the Renewable Energy Directive 2009/28/EC]
CIPHE (2009) Guidance on preventing domestic hot water scalding (London:
Chartered Institution of Public Health Engineering)
DECC (2015a) Heat Networks Delivery Support [Online]. [Accessed 12
December 2015]. Available at: https://www.gov.uk/guidance/heatnetworks-delivery-support
Chrenko FA (ed) (1974) Bedford’s basic principles of ventilation and heating,
3rd edition (London: H K Lewis)
DECC (2015b) Assessment of the cost and performance and characteristics of
UK Heat Networks (London: The Stationery Office)
Churcher D (2008) BSRIA BG5: Whole life costing analysis (Bracknell:
Building Services Research and Information Association)
DECC (2016) Prices of Fuels purchased by non-domestic consumers in the UK
(London: The Stationery Office)
Clients Construction Forum (CCF) (2000) Whole Life Costing - A Clients’
Guide (BRE report funded by DETR). (London:CCF)
DEFRA (2006) Animal Welfare Act 2006 (London: The Stationery Office)
Clift M and Bourke K (1999) BRE Report BR367: Study on whole life costing
(Garston: Building Research Establishment) [Not referenced?]
DEFRA (2007) The Welfare of Farmed Animals (England) Regulations 2007
(London: TSO)
DEFRA (2012) Digest of Environmental Statistics (London: TSO)
CHPA (2011) Good Practice Guide 234: Guide to Community Heating and
Combined Heating and Power (CHP) - Commercial, Public and Domestic
Applications (London: Combined Heat and Power Association) [The Combined
Heat and Power Association is now The Association for Decentralised Energy
(ADE)
Crozier B (2000) BSRIA Application Guide AG 1/2000: Enhancing the
performance of oversized plant (Bracknell: Building Services Research and
Information Association)
CSA (1999) Technical Memorandum TM1: Standard Specification for the
commissioning of mechanical engineering installations for buildings (Horsham:
Consulting Specialists Association)
Day AR, Ratcliffe MS and Shepherd KJ (2001) Proceedings of the CIBSE
National Conference 2001: Sizing central boiler plant using an economic
optimisation model (London: CIBSE) [Not referenced?]
DCLG (2010a) Approved Document J - Combustion appliances and fuel storage
systems (London: NBS) [Republic of Ireland Building Regulations Technical
Guidance Document J covers combustion appliances in Ireland.]
DCLG (2010b) Approved Document G: Sanitation, hot water safety and water
efficiency (London: NBS)
DCLG (2010a) Building Regulations 2010: Approved Document F:
Ventilation (London: NBS)
DCLG (2010b) Building Regulations 2010: Approved Document J: Heat
producing appliances (London: NBS)
Department for Transport (DfT) (2008) Renewable Transport Fuels
Obligation (London: The Stationery Office)
Department of Finance and Personnel (DFP) (1979) Building Regulations
(Northern Ireland) Order 1979 (as amended 1990 and 2009) (Belfast: The
Stationery Office)
Department of Finance and Personnel (DFP) (2000) DFP Amendments
Booklet - AMD2: 2000 (Republic of Ireland Building Regulation Technical
Guidance Document L covers energy requirements in buildings in Ireland.)
Department of Trade and Industry (DTI) (1994) An Assessment of Renewable
Energy for the UK (London: Her Majesty’s Stationery Office)
DS (2009) Danish Standard 439: Code of Practice for domestic water supply
installations (Charlottenland: Dansk Standard)
Dossat R (2001) Principles of Refrigeration (London: Prentice Hall)
Electricity Council (1980) EC 4666/11.84 DOM 8: Design of mixed storage
heater/direct systems (revised 1984, 1989) (London: Electricity Council)
EEBPP (2000) Good Practice Guide GPG303: The designer’s guide to energyefficient buildings for industry (Garston: Energy Efficiency Best Practice
Programme)
EEBPP (2003) Energy Consumption Guide ECG19 (previously known as
ECON 19): Energy use in offices (Garston: Energy Efficiency Best Practice
Programme)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
CIBSE (2013b) Ground source heat pumps TM51 (London: Chartered
Institution of Building Services Engineers)
1-140
English Heritage (2012) Energy Efficiency and Historic Buildings: Application
of Part L of the Building Regulations to historic and traditionally constructed
buildings (London: English Heritage)
Euroheat (2013) District Heating and Cooling Statistics – statistics overview
[Online]. [Accessed: 12 December 2015]. Available at: http://euroheat.org/
Statistics-69.aspx
EU (1999) Directive 1999/92/EC of the European Parliament and of the
Council of 16 December 1999 on minimum requirements for improving the safety
and health protection of workers potentially at risk from explosive atmospheres
(15th individual Directive within the meaning of Article 16(1) of Directive
89/391/EEC)
EU (2009a) Directive 2009/125/EC of the European Parliament and of the
Council of 21 October 2009 establishing a framework for the setting of ecodesign
requirements for energy-related products
EU (2009b) Directive 2009/28/EC of the European Parliament and of the
Council of 23 April 2009 on the promotion of the use of energy from renewable
sources and amending and subsequently repealing Directives 2001/77/EC and
2003/30/EC
EU (2010) Recast (revised) EPBD 2010: EPBD Directive 2010/31/EU on
the energy performance of buildings
GBPN (2007) India - Energy Conservation Building Code (ECBC) (Delhi:
Global Buildings Performance Network)
GBPN (2016) International Building Energy Codes Portal [Online]
[Accessed: 25/04/16] Available at: www.gbpn.org
GLA (2009) Powering Ahead: Delivering low carbon energy for London
(London: Greater London Authority)
GLA (2011) The London Plan: Spatial development strategy for Greater
London (London: Greater London Authority)
GLA (2014) The London Heat Network Manual (London: Greater London
Authority)
GLA, (2015) London Heat Map [Online]. [Accessed 5 April 2016]. Available
from: http://www.londonheatmap.org.uk/
for hot water boilers fired with liquid and gaseous fuels) Regulations
1994.)
HMSO (1993) Clean Air Act (1993) (c.11) (London: Her Majesty’s
Stationery Office)
HMSO (1995a) The Environment Act (London: Her Majesty’s Stationery
Office)
HMSO (1995b) The Environment Act 1995 (London: Her Majesty’s
Stationery Office)
HMSO (1995c) Gas Act 1995 (London: Her Majesty’s Stationery Office)
HMSO (1996a) Gas Safety (managemnt) Regulations 1996 (London: Her
Majesty’s Stationery Office)
HMSO (1996b) Education (School Premises) Regulations 1996 (London: Her
Majesty’s Stationery Office)
HMSO (1996c) The pipeline safety regulations 1996 (London: Her Majesty’s
Stationery Office)
HMSO (1998) Gas Safety (installation and use) Regulations 1998 (London:
Her Majesty’s Stationery Office)
HMSO (1999) Water Supply (water fittings) Regulations 1999 (London: Her
Majesty’s Stationery Office)
HMSO (2000) The pressure systems safety regulations 2000 (London: Her
Majesty’s Stationery Office)
HMSO (2002a) The notification of installations handling hazardous substances
(amendment) Regulations 2002 (London: Her Majesty’s Stationery Office)
HMSO (2002b) The dangerous substances and explosive atmospheres
Regulations 2002 (London: Her Majesty’s Stationery Office)
HMSO (2007) The sulphur content of liquid fuels (England and Wales)
Regulations 2007 (London: Her Majesty’s Stationery Office)
HMSO (2009) The control of major accident hazards (amendment) Regulations
2009 (London: Her Majesty’s Stationery Office)
HMSO (2013) The reporting of injuries, diseases and dangerous occurrences
Regulations 2013 (London: Her Majesty’s Stationery Office)
Her Majesty’s Inspectorate of Pollution (HMIP) (1993) Guidelines on
discharge stack heights for polluting emissions. Technical Guidance Note
(Dispersion) D1. (London: Her Majesty’s Stationery Office)
HMSO (2015) Construction (Design and Management) Regulations 2015
(London: Her Majesty’s Stationery Office)
HMSO (1974) Statutory Instrument 1974 no. 2160 Fuel and Electricity
(Heating) (Control) Order, 1974 (London: Her Majesty’s Stationery Offices)
HSE (1974) Health and Safety at Work etc. Act 1974 (London: Her Majesty’s
Stationery Office)
HMSO (1980) Statutory Instrument 1980 no. 1013 Fuel and Electricity
(Heating) (Control) (Amendment) Order, 1980 (London: Her Majesty’s
Stationery Office)
HSE (1992) L24: Workplace (Health, Safety and Welfare) Regulations 1992
Approved Code of Practice and Guidance (London: Health and Safety
Executive)
HMSO (1981) Chimney Heights – Third Edition of the Clean Air Act
Memorandum (London: Her Majesty’s Stationery Office)
HSE (1996a) L80: A guide to the gas safety (management) regulations 1996
(London: Health and Safety Executive)
HMSO (1989) The Pressure Systems and Transportable Gas Containers
Regulations 1989 Statutory Instrument 1989 No. 2169 (London: Her
Majesty’s Stationery Office)
HSE (1996b) L82: A guide to the Pipelines Safety Regulations 1996 (London:
Health and Safety Executive)
HMSO (1990) Environmental Protection Act 1990 (c. 43) (London: Her
Majesty’s Stationery Office)
HMSO (1993, 1994) The Boiler (Efficiency) Regulations 1993 Statutory
Instrument 1993 No. 3083 and The Boiler (Efficiency)(Amendment) Regulations
1994 Statutory Instrument 1994 No. 3083 (London: Her Majesty’s Stationery
Office) (1993 and 1994) (In the Republic of Ireland the EU Boiler Directive
is implemented by the European Communities (Efficiency requirements
HSE (1998) L56: Safety in the installation and use of gas systems and appliances
— Approved Code of Practice and Guidance (London: Health and Safety
Executive)
HSE (1998) PM60: Steam boiler blowdown systems (London: Health and
Safety Executive)
HSE (1999) Control of Accident and Major Hazard (COMAH) Regulations
1999 (London: Health and Safety Executive)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Euroheat (2008) Guidelines for district heating substations (Brussels: Euroheat
& Power)
Heating
References
HSE (2000b) L122: Safety of pressure systems: Pressure Systems Safety
Regulations 2000. Approved Code of Practice (London: Health and Safety
Executive)
HSE (2002b) Dangerous substances and explosive atmosphere regulations
(DSEAR) (London: Health and Safety Executive)
HSE (2002c) The Notification of Installations Handling Hazardous Substances
(NIHHS) Regulations (London: Health and Safety Executive)
HSE (2010) Research Report RR769: Hazards of liquid hydrogen (London:
Health and Safety Executive)
HSE (2011) INDG346: Safe management of industrial steam and hot water
boilers (London: Health and Safety Executive)
HSE (2012) Pressure systems: a brief guide to safety (London: Health and
Safety Executive)
HSE (2013a) Approved Code of Practice L56: Safety in the installation and
use of gas systems and appliances (London: Health and Safety Executive)
HSE (2013b) Approved Code of Practice L8: Legionnaire’s disease. The
control of Legionella bacteria in water systems (London: Health and Safety
Executive)
HSE (2013c) ACOP L138: Dangerous Substances and Explosive Atmospheres
(London: Health and Safety Executive)
HSE (2013d) INDG453: Reporting accidents and incidents at work (London:
Health and Safety Executive)
HSE (2014) HSG274: Legionnaires’ disease [published in 3 parts] (London:
Health and Safety Executive)
HSE (2015a) Construction (Design and Management) Regulations 2015 (CDM
2015) (London: Health and Safety Executive)
HSE (2015b) Managing health and safety in construction (London: Health
and Safety Executive)
HSL (2006) HSL/2006/59: The Hydrogen Economy - Evaluation of the
materials science and engineering issues (Buxton: Health and Safety
Laboratory)
HVCA (2000) Domestic heating design guide (London: Heating and
Ventilating Contractors Association) [HCVA is now the Building
Engineering Services Association]
IGEM (2014a) IGEM/UP/2: Installation pipework industrial and commercial
premises (London: Institution of Gas Engineers and Managers)
IGEM (2014b) UP/10: Gas installation pipework, boosters and compressors in
industrial and commercial premises (London: Institution of Gas Engineers
and Managers)
Kiwa (2013) Investigation of the interaction between hot water cylinders, buffer
tanks and heat pumps (Cheltenham: Kiwa GasTec)
Kut D (1968) Heating and hot water services in buildings (Oxford: Pergamom
Press)
LACORS (2009) Biomass and Air Quality Guidance for Local Authorities
(England and Wales) (Brighton: Local Authorities Coordinators of
Regulatory Services)
MHURD (China) 2005 GB50189-2005: Design Standards for Energy
Efficiency of Public Buildings (People’s Republic of China: Ministry of
Housing and Urban-Rural Development)
NHS (1998) Health Guidance Note: Safe Hot Water and Surface Temperatures
(London: National Health Service Estates)
National Institution of Standards and Technology (US Chamber of
Commerce) (2014) [Ref needed – for Figure 1.8, section 1.6.3.1]
National Research Council Canada (2015) National Energy Code of Canada
for Buildings 2015 (NECB) (Ontario: NRCC)
NREL (1988) Engineering principles and concepts for active solar systems
(Golden CO: National Renewable Energy Laboratory)
Ofgem (2014) Renewable heat incentive guidance: Volumes 1 and 2 (London:
Office for Gas and Electricity Markets)
OFTEC (1999) OFS T100 Polyethylene oil storage tanks for distillate fuels
(Banstead: Oil Firing Technical Association for the Petroleum Industry)
OFTEC (2000) Oil Fired Appliance Standard OFS A100: Heating boilers
with atomising burners. Outputs up to 70 kW. Maximum operating pressures of 3
bar (Banstead: Oil Firing Technical Association for the Petroleum
Industry)
OFTEC (2001) OFS T200: Steel oil storage tanks and tank bunds for use with
distillate fuels, lubrication oils and waste oils (Banstead: Oil Firing Technical
Association for the Petroleum Industry)
OFTEC (2004) Oil Fired Appliance Standard OFS A101: Oil fired cookers
with atomising or vaporising burners with or without boilers. Heat outputs up to
45 kW (Banstead: Oil Firing Technical Association for the Petroleum
Industry)
OFTEC (2010a) M/402 OFTEC Technical Book 2: Domestic and light
commercial servicing and commissioning (Banstead: Oil Firing Technical
Association for the Petroleum Industry)
OFTEC (2010b) M/403 OFTEC Technical Book 3: Domestic and
Commercial Requirements for Oil Storage and Supply Equipment (Banstead:
Oil Firing Technical Association for the Petroleum Industry)
OFTEC (2010c) M/404 OFTEC Technical Book 4: Oil fired appliance and
system installation requirements (Banstead: Oil Firing Technical Association
for the Petroleum Industry)
ISO (2004) Technical Report 15916: 2004: Basic considerations for the safety
of hydrogen systems (Geneva: International Organization for
Standardization)
OFTEC (2010d) M/407 OFTEC Technical Book 7: Commercial servicing
and commissioning (Banstead: Oil Firing Technical Association for the
Petroleum Industry)
ISO (2005) ISO 7730: 2005: Ergonomics of the thermal environment -Analytical determination and interpretation of thermal comfort using calculation
of the PMV and PPD indices and local thermal comfort criteria (Geneva:
International Organization for Standardization)
Palmer, D (2014) ‘Best Practice: Low Loss Headers’, CIBSE Journal,
February 2014
Kabele K (ed.), Hojer O, Kotrbatý M, Sommer K, Petras D (2011)
Federation of European Heating, Ventilation and Air Conditioning
Association, Guidebook 15: Energy Efficient Heating and Ventilation of Large
Halls (Brussels: REHVA)
Parsloe CJ (2001) BSRIA Application Guide AG3/89.3: Commissioning Air
Systems. Application procedures for buildings (Bracknell: Building Services
Research and Information Association)
Parsloe CJ (2010) BSRIA BG2: Commissioning water systems (Bracknell:
Building Services Research and Information Association)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
HSE (2002a) Control of Substances Hazardous to Health 2002 (COSHH)
(London: Health and Safety Executive)
1-141
1-142
Heating
TIMSA (2008) HVAC guidance for achieving compliance with Part L of the
Building Regulations (Farnham: Thermal Insulation Manufacturers and
Suppliers Association)
Parsloe CJ and Spencer A W (1996) BSRIA Application Guide AG20/95:
Commissioning of pipework systems — design considerations (Bracknell:
Building Services Research and Information Association)
UKLPG (2011) Code of Practice 22: Design, Installation and Testing of LPG
Piping Systems (Ringwood: United Kingdom LPG Association)
RCEP (2004) Biomass as a Renewable Energy Source (London: Royal
Commission on Environmental Pollution)
RICS (1998) The Surveyor’s Construction Handbook, chapter 2.2 Life Cycle
Costing (London: Royal Institution of Chartered Surveyors)
REHVA (2011) Guidebook 15: Energy Efficient Heating and Ventilation of
Large Halls (Brussels: Federation of European Heating, Ventilation and
Air Conditioning Association)
Scottish Executive (2001) Technical standards for compliance with the Building
Standards (Scotland) Regulations 1990 (as amended) (Edinburgh: Scottish
Executive)
SAF (2011) Guidance on safe operation of boilers (Joint publication by
Safety Assessment Federation and Combustion Engineering Association
S K Solar (2015) Solar thermal district heating. [Online]. [Accessed 6
April 2016]. Available at: http://www.sksolar.co.uk/solar-thermal-districtheating
Sport England (2011) Design Guidance Note: Swimming Pools (London:
Sport England)
UKLPG (2012a) Code of Practice 1: Part 2: Bulk LPG Storage at Fixed
Installations for Domestic Purposes (Ringwood: United Kingdom LPG
Association)
UKLPG (2012b) Code of Practice 1: Part 3: Bulk LPG Storage at Fixed
Installations: Examination and Inspection (Ringwood: United Kingdom
LPG Association)
UKLPG (2013a) Code of Practice 1: Part 4 - Bulk LPG Storage at Fixed
Installations: Buried/Mounded LPG Storage Vessels (Ringwood: United
Kingdom LPG Association)
UKLPG (2013b) Code of Practice 1: Part 1 2009 Edition: Bulk LPG
Storage at Fixed Installations: Design, Installation and Operation of Vessels
Located Above Ground (Ringwood: United Kingdom LPG Association)
US DoE (2013) Building Energy Codes Program (Washington: US
Department of Energy)
US Environmental Protection Agency (EPA) (2008) Catalog of CHP
Technologies (Washington: US EPA)
Wild LJ (2002) BSRIA TM 1/88.1: Commissioning HVAC systems —
Guidance on the division of responsibilities (Bracknell: Building Services
Research and Information Association)
Sport England (2012) Design Guidance Note: Sports Halls Design and
Layout (London: Sport England)
Wiltshire R (2014) Technical Guide to District Heating (Garston: BRE Press)
Strebel Ltd (undated) Installation, Operation and Maintenance Manuals:
S-CB FS 400 and 550, S-CB WH 60-180 condensing boiler range
Young L, Mays G (2001) WRAS: Water Regulations Guide (Gwent: Water
Regulations Advisory Scheme)
Teekaram A (2002) BSRIA Application Guide AG16/2002: Variable-flow
water systems: Design, installation and commissioning guidance (Bracknell:
Building Services Research and Information Association)
Zero Carbon Hub (2009) Defining a Fabric Energy Efficiency Standard for
zero carbon homes (London: Zero Carbon Hub)
Teekarum A, Palmer A, Parker J (2007) BSRIA BG2: CHP for existing
buildings: guidance on design and installation (Bracknell: Building Services
Research and Information Association)
Zero Carbon Hub (2013) Zero carbon strategies for tomorrow’s new homes
(London: Zero Carbon Hub)
Zero Carbon Hub (2014) Cost analysis: meeting the zero carbon standard
(London: Zero Carbon Hub)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Parsloe CJ (2011) BSRIA BG12: Energy Efficient Pumping Systems: A Design
Guide (Bracknell: Building Services Research and Information
Association)
Introduction
Appendix 1.A1
Hydronic system design
1-143
Thin-walled steel
publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIB
1.A1.1
Introduction
Hydronics is the use of water as the heat transfer medium
in heating and cooling systems. In building services
applications heating and cooling is commonly delivered by
means of water circulated from central plant to terminal
units via a re-circulating pipework system.
——
——
——
——
——
——
——
dirt and air removal
variable flow design considerations
flow temperature control
system pressurisation
thermal expansion
heat transfer to and from pipes
pipe and pump sizing
pipe material and jointing method selection
This guide is intended to give an overview of the main
issues that need to be considered during the design of
hydronic systems. The guide covers both design aspects
and the practical issues that need to be considered. The
scope includes:
——
commissioning considerations.
Multilayer pipe
Stainless steel is commonly used where water quality and
hygiene is a priority including food, pharmaceutical and
healthcare environments. Pipes are available in sizes up to
800 mm. Pipes are smooth bore and extremely resistant to
corrosion.
Stainless steel
Copper is commonly selected for smaller pipes, 15–25 mm
in size. In larger systems copper is often used as an
alternative to steel for final run outs to terminal units.
Although more expensive than steel, it has the advantage of
being quicker to install. It is also smoother than steel and is
less likely to corrode.
Copper to BS EN 1057 (BSI, 2006)
Alternative steel products are available which offer some
advantages over the more traditional steel pipes. Commonly
referred to as ‘thin-walled steel’, the pipes are manufactured
such that they are seamless rather than longitudinally
welded. This means that the pipe is inherently stronger and
can be manufactured with thinner walls. Because they are
thinner, they are therefore lighter and easier to handle than
normal steel pipes. The internal surfaces also tend to be
machined to a much smoother finish thereby reducing the
surface roughness. However, bare walled thin steel tubes
will corrode in water and, due to the thinness of the walls,
will fail more quickly than steel pipes with thicker walls. It
is therefore essential that an effective water treatment
regime is in place to protect the pipes as soon as they are
filled. The pipes should not be used in damp locations
where external corrosion could be a problem.
——
The guidance is applicable to medium and large scale recirculating systems for non-domestic applications up to
operating pressures of 10 bar including low, medium and
high temperature heating systems, and chilled water or
condenser cooling water systems.
Where necessary, reference is made to other publications
for more detailed guidance.
Pipes are fabricated in a variety of materials. The most
common alternatives that are applicable to heating or
cooling water applications are as follows:
1.A1.2
Multilayer pipe is an aluminium pipe that is coated
internally and externally with either cross linked
polyethylene or high density polyethylene. The aluminium
core gives the pipe the strength of a metal pipe whilst the
plastic coating makes the pipe corrosion resistant. The
aluminium layer also makes the pipe impervious to oxygen
ingress (a problem with some pure plastic pipes). Multilayer
pipe is commonly selected as an alternative to copper for
smaller pipe run-outs to terminal units. In small sizes
15–25 mm diameter, the pipe is flexible and can be bent by
hand making it quick to install.
Pipe materials and jointing
methods
chlorinated polyvinyl chloride (pvc-c)
Pipe materials
——
unplasticised polyvinyl chloride (upvc)
1.A1.2.1
Steel to BS EN 10255 (BSI, 2004)
——
A variety of pure plastic pipes are available for heating and
chilled water applications. These include:
Plastic pipes
Steel is commonly selected for pipe sizes larger than 25 mm
in diameter due to its strength and cheapness. The main
drawback of steel pipe is that it will corrode rapidly in the
presence of water and oxygen. Hence, external surfaces
should be painted with an anti-corrosion paint whilst
internal surfaces should be protected by corrosion inhibitor
chemicals.
1-144
——
Hydronic system design
medium and high density polyethylene (mpde,
hdpe)
acrylonitrile butadiene styrene (abs)
——
polybutylene (pb)
——
polypropylene (pp)
——
cross linked polyethylene (pe-x).
The pressure and temperature ratings of most pure plastic
pipes are lower than those for metal pipes and their
expansion rates are considerably higher. However, the pipes
are clean, corrosion resistant and, if flexible, may be faster
to install.
Table 1.A1.1 provides a summary of the properties of
common pipe materials.
1.A1.2.2
Factors influencing choice of pipe
material
The following factors should be taken into account when
deciding on the appropriate pipe material for a given
applications.
Pipe strength
The pipe material selected and its jointing system must be
able to withstand the maximum operating pressure in the
system without leaking. Advice on calculating the maximum
system operating pressure is given in section 1.A1.6.
Temperature
Metal pipes have temperature ratings that are well above the
normal range of heating and chilled water operating
temperatures. Plastic pipes may not have such a wide range
and need to be checked. It is sometimes that case that a plastic
pipe can withstand high temperatures for temporary periods,
but the continued operation of the system at that temperature
may reduce the overall life expectancy of the pipe.
Flexibility
Some plastic pipes are flexible thereby avoiding the need for
multiple elbow fittings. This can make the pipes quick to
install. Some pipes such as multilayer pipe are flexible but
also hold their shape and support their own weight once
bent. Some pure plastic pipes, such as polybutylene, do not
hold their shape and will sag under their own weight. These
types of pipe may need special supporting arrangements.
Oxygen diffusion
Oxygen diffusion is a problem for many pure plastic pipes.
Over a period of time, oxygen is able to diffuse through the
plastic and become dissolved in the water. This can then
cause accelerated corrosion in steel components such as
pipes and radiators. Some plastic pipes have oxygen diffusion
barriers in them that provide some degree of protection.
Multilayer pipes incorporate a layer of aluminium which
does make it impervious to oxygen ingress.
Thermal expansion
Thermal expansion must be allowed for during system
design. Expansion that is not properly catered for may lead
to misalignment and failure at joints. Section 5 of this
Life expectancy
The life expectancy of some plastic pipes can vary
significantly depending on the pressure and temperature
conditions under which it is used. Assurances should be
obtained from the manufacturer to ensure that the life
expectancy of the pipe is suitable for the application.
Plastic pipes are increasingly considered as alternative to
metal pipes. Table 1.A1.2 provides a summary of the
advantages and disadvantages of plastic pipes.
1.A1.2.3
Pipe jointing methods
The main jointing methods for pipes are described below.
Threaded
Threaded or screwed joints are commonly used for small
sized steel pipes, i.e. 50 mm diameter or less. Pipe threads
are cut by dies and the resultant threads are rough and
imperfect. A pipe jointing compound or thread sealant
must therefore be used to prevent leakage from around the
threads. The jointing compound also acts as a lubricant
when tightening the joint.
Flanged
Bolted flange joints are used for 50 mm diameter and larger
steel and plastic pipes. They are common where pipe,
piping components, or equipment must be disassembled
for maintenance purposes. Opposing flange faces are
tightened against a rubber, fibre, composite or metal gasket.
To ensure an effective seal the gasket and flange faces must
be clean and free from dirt or other obstructions. The flange
bolts must be tightened to the correct torque and in the
correct sequence following the manufacturer’s instructions.
Butt welding
Butt welded joints are commonly used for 65 mm diameter
and larger steel pipes. Butt welding is thermal welding in
which the ends of the pipe and/or fitting are welded. The
most popular method for welding pipe is the shielded
metal-arc process. Butt welding creates a weld bead, both
internally and externally. These are often left in place, but
can be removed using special tooling if necessary. In
general, welded pipe joints offer less resistance to flow than
mechanical connections such as threaded or grooved end
joints, and the overall installation costs are less.
Socket welding
Socket welded joints are formed by inserting socket
connections into the ends of each of the pipes to be joined.
These joints are almost exclusively used in joining small
bore piping. An advantage with this type of joint is that the
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
chapter explains the main options. Plastic pipes tend to
expand far more than equivalent metal pipes. Flexible
plastic pipes such as polybutylene accommodate the
expansion by bending or warping between supports. Rigid
plastic pipes such as polypropylene need to be installed
such that expansion is accommodated horizontally along
the length of the pipe, as for steel or copper pipes. For all
plastic pipes, careful attention should be given to the
manufacturer’s fixing and jointing instructions.
Copper
PVC-C
PVC-U
ABS
MDPE and
HDPE
PE-X
PB
PP
Multi-layer
Rigid/semiflexible/flexible
Rigid
Rigid
Rigid
Rigid
Rigid
Rigid
Semi-flexible
Flexible
Flexible
Rigid
Semi-rigid
Sizes available
6–150
10–600
6–159
16-160
8-315
16-315
20-1200
10-400
10-28
12-400
12 –125
(nominal diameter,
mm)
(BS EN
10255)
Lengths or coils
Lengths
Lengths
Both
Lengths
Lengths
Lengths
Both
Both
Both
Both
Both
Expansion rate
(relative to steel)
1
1.5
1.5
6
6
8
10
15
11.5
12
2.2
Expansion from
0 °C to 80 °C
0.9
1.4
1.4
5.6
5.6
7.5
9
13.6
10.4
10.8
2
260
260
200
100
60°C
70
HDPE100:
90
95
100 max
100 max
12 bar
12 bar
10 bar
20 bar
(BS EN 1057)
Pipe materials and jointing methods
Stainless steel
Table 1.A1.1: Properties of common pipe materials
Steel
(mm/m)
Maximum
operating
temperature (°C)
80
16 bar
Maximum
operating pressure
at 20 °C (bar)
16 bar
10 bar
PE80: 12 bar
PE100: 16 bar
7860
8060
8940
1540
1400
1100
PE100:
938-970
936-955
910-930
903-907
903-907
Jointing
Threaded
Compression
Compression
Solvent
Solvent
Solvent
Compression
Compression
Socket fusion
Weld
Flanges
Capillary
Flanges
Push-fit
Flanges
Push-fit
Flanges
Compression
Flanges
Socket fusion
(see also
manufacturer’s
information)
Butt fusion
Butt fusion
Flanges
Press-fit
Soldered
Threaded
Flanges
Threaded
Electrofusion
Weld
Electrofusion
Electrofusion
Grooved
Push-fit
Flanges
Butt fusion
Push-fit
Compression
Press-fit
Flanges
Electrofusion
Press Fit
Push-fit
Threaded
Press-fit
Threaded
1-145
Density kg/m³
(approximately)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
1-146
Hydronic system design
Table 1.A1.2 Advantages and disadvantages of plastic pipes
Disadvantages
Plastic pipes usually supplied in coils, therefore
individual pipe runs can be cut to length on site to
minimize joints
Plastic pipe can be subject to oxygen diffusion, where oxygen molecules can penetrate
through the tubing wall and if there are ferrous materials in the system (e.g. radiators)
general corrosion will occur. Therefore all pipes used must have an oxygen diffusion
barrier either integral or applied as a coating externally.
No painting of pipes required
Maximum temperature will vary with plastic type but limited to 80–90 °C. To achieve a life
expectancy of over 25–30 years the temperature should generally be limited to 70 °C and 6
bar gauge.
Heat free jointing, therefore no requirements for hot
working permits and lower skill level may be required
Materials cost higher than steel or copper pipe.
System is intrinsically clean after installation, and
internal surfaces will not corrode – therefore no or
minimum requirements for flushing to remove debris
Additional supports may be required for plastic pipes that sag under their own weight.
Reduced installation time
Additional allowance required for thermal expansion as coefficient of linear expansion of
plastic is much higher than copper or steel.
Lightweight, making handling and off site
prefabrication easier.
Less robust than steel, therefore probably not suitable for plant rooms and risers in some
systems.
Grooved end joints are used for jointing all sizes of steel
and ductile iron pipes. Grooves are cut into the ends of the
two pipes to be jointed. A mechanical coupling is then
fitted around the ends of the two pipes locking into the
grooves creating a secure fixing. The coupling is tightening
by fixing bolts onto a rubber gasket that fits over the ends
of the two pipes creating a watertight seal.
temperatures. The joints are therefore stronger and more
resistant to vibration or movement. Copper pipes are often
jointed using soldered joints. Socket type fittings are
provided which overlap the ends of the tubes. A space is
formed between the tube and fitting referred to as the
‘capillary space’. The solder (or filler metal) is melted into
the capillary space and adheres to the surfaces. The solder
may be applied externally or may be internally located and
supplied as part of the fitting. Heat must be applied to melt
the solder. A flux is required to encourage the solder to flow
into the capillary space. If used for potable water it is a
requirement that solders containing lead are avoided.
Compression
Solvent welded
A compression nut is tightened onto a circular ‘olive’ fitted
to the end of the pipe being jointed. This causes the olive to
compress into the compression fitting causing it to squeeze
the pipe, simultaneously gripping it and creating a water
tight seal. Compression fittings of this type are used for
connecting copper pipes and fittings. Although easy to
form, joints may not be as robust as soldered or brazed
joints. Multilayer pipes may also use compression fittings.
For these pipes an internal support sleeve is required,
incorporating rubber o-rings which create a watertight seal.
The compression of the olive then serves to achieve a strong
grip of the pipe. The use of a lubricant may be required to
avoid damage to the pipes and fittings through the use of
excessive mechanical force during assembly. The choice
and application of lubricants should be in accordance with
the manufacturer’s recommendations for the particular
material and application.
Solvent welding is used for jointing of plastic pipes. Solvent
cement is applied over the ends socket type joints which are
then pressed together by hand. Properly applied, solvent
cements can create a stronger joint than mechanical joints.
Curing times for solvent welded joint on large diameter
pipes can be up to 24 hours.
Soldered or brazed
Electrofusion
Soldering is the process of joining metals by using a low
melting point filler metal (usually an alloy of tin) to adhere
the surfaces to be soldered together. Soldering is more like
gluing with molten metal, unlike welding where the base
metals are actually melted and combined. The main
difference between soldered joints and brazed joints is that
brazing uses stronger filler metals with higher melting
Electrofusion joints are effectively welded joints for plastic
pipes in which the heat is generated by small electrical
heating circuits embedded within the fittings themselves.
A purpose made electrofusion control unit is provided by
the manufacturer to provide the correct power for the
correct time. This type of fitting enables joints to be made
in situ.
filler metal cannot enter the main pipe bore resulting in a
smooth internal surface.
Grooved end
Socket fusion welding
Socket fusion welding is used for jointing plastic pipes. The
technique employs a similar technique to that of butt welding
in heating the surfaces to be joined, in this case involving the
application of heat to the inside surface of the socket and the
outside of the pipe. A special welding tool is provided by the
pipe manufacturer for this purpose. Due to the size of the
welding tool, welds are usually made on a workbench.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Advantages
Pipe and pump sizing
1-147
Table 1.A1.3: Advantages and disadvantages of push-fit and press-fit copper/steel joints
Disadvantages
Heat free jointing, therefore no
requirements for hot working permits
Potential that electrical continuity is not maintained once the joints are made; depending on system
used earth continuity straps may be required. Checks must be made with the manufacturers.
Advantage can be made of relatively
unskilled labour
A quality assurance system is required on site to ensure all joints are made in accordance with the
manufacturers recommendations, to ensure ‘O’ rings are not damaged, pipes ends are prepared
correctly, pipes are inserted into fittings correctly, pipes are supported correctly and press-fit joints
are made correctly.
System is intrinsically clean after installation,
therefore minimum requirements for flushing
to remove flux residue
Poorly made joints can sometimes pass a pressure test, with no leakage observed during the test, but
then fail weeks or months later.
Significantly reduced installation time
Pressure and temperature of push fit systems generally limited to 90 °C and 6 bar gauge. Press fit
systems limited to 110 °C and 16 bar gauge. The systems use o-rings which will have a limited life
expectancy depending on operating temperature/pressure.
An overall cost saving can usually be
demonstrated
The system manufacturer’s proprietary tools must be used for ‘press-fit’ joints.
Special care must be taken in the design and installation of pipe supports and facilities for thermal
expansion to ensure joints are not misaligned which could cause failure of o-rings.
Table 1.A1.4: Recommended range of maximum water velocities.
Pipe diameter (mm)
Recommended maximum velocity
limits (m/s)
Copper
Steel
15-50
1.0
1.5
Over 50
1.5
3
Push-fit
Push-fit joints enable a joint to be made simply by pushing
the pipe into the fitting. Push-fit fittings are available for
both copper and plastic pipes. Generally, when a length of
tube is pushed into the joint it passes through a release
collar and then through a stainless steel grip ring. This has
a series of teeth that open out and grip the tube, securing it
so that it can only be released using some form of
disconnecting tool. Pushing the tube further into the joint
ensures that it passes through a support sleeve, which helps
to align the tube before passing through a pre-lubricated
epdm rubber o-ring. Only when the tube has passed through
the o-ring and reached the tube stop is a secure joint
created.
Press-fit
Press-fit joints are made by compressing the walls of the
fitting onto the tube being connected using a special pressfit tool provided by the joint manufacturer. Care should be
taken to ensure that due space allowance is made for use of
the tool head. Consideration should also be given to the
weight of such tools when working overhead.
Push-fit and press-fit joints are modern alternatives to the
other more traditional jointing methods for metal pipes.
Table 1.A1.3 provides a summary of the advantages and
disadvantages of these jointing methods.
Specific guidance on the installation of pipework for
different types of pipework system is provided in HVCA
publication TR20: Installation and Testing of Pipework
Systems (HVCA, 2003). This guide is structured as a set of
10 stand-alone specifications dealing with low, medium and
high temperature hot water heating, hot, cold and chilled
water service, condenser and cooling water, steam and
condensate, natural gas and oil.
1.A1.3
Pipe and pump sizing
1.A1.3.1
Pipe sizing
The following considerations should be taken into account
when selecting the appropriate pipe size for a given design
flow rate:
Pipework noise
Pipes must be sized such that the velocity of the water
running through them will not be high enough to cause
either vibration induced noise or erosion of the pipe
material. Erosion of relatively soft metals such as copper
can occur at elbows if the water velocity is excessive. Table
1.A1.4 indicates recommended maximum water velocities.
Air and dirt settlement
Small air bubbles or particles of debris carried by the
flowing fluid may settle out in the pipe at low velocities.
Ideally, full load design velocities should be maintained at
a value greater than 0.5 m/s, especially in a heat network.*
Where full load design velocities may fall below this value
additional dirt or air removal devices should be considered.
BSRIA Guide BG 29/2011 (Brown, Parsloe, 2004) provides
recommendations on the maintenance of system cleanliness.
Pump energy
Pipes must be sized such that the energy consumed by the
pump is not excessive. Smaller pipes will have a greater
resistance to flow and will therefore incur a greater pump
energy consumption compared to larger pipes. Pump
energy consumption will be roughly proportional to the
average pressure loss per metre (expressed as Pascals per
* For heat network distribution flow (hot supply) pipes
within buildings, a minimum peak velocity of 0.5 m/s­should
be achieved in all cases, see CIBSE Code of Practice CP1.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Advantages
1-148
Pipework cost
Pipes must be sized such that the cost of pipework is
maintained within acceptable limits. Pipework noise and
pump energy can both be reduced by increasing installed
pipe sizes, however, this is likely to incur an increase in the
cost of the installed system.
Practicality of installation
Pipes must be sized such that the physical sizes of pipes are
maintained within acceptable limits. Pipework noise and
pump energy can both be reduced by increasing installed
pipe sizes. However, this is likely to require an increase in
the services void areas required to accommodate the pipes.
Detailed advice on optimal pipe sizing is provided in
BSRIA publication BG 12/2011 Energy Efficient Pumping
Systems – a design guide (Parsloe, 2011).
Having decided the parameters governing pipe size
selection, actual pipe sizes can be determined using the
data provided in CIBSE Guide C, chapter 4 ‘Flow of fluids
in pipes and ducts’ (CIBSE, 2001). This guide incorporates
a spreadsheet which enables pipe sizing tables to be
generated for any pipe material, size and fluid type. Using
these tables, pipes can be selected based on their required
design flow rates that comply with the pre-determined
selection parameters (whether this is velocity limit or
pressure loss limit).
The same tables enable the pressure losses through all
straight pipe lengths to be calculated, this being the
pressure loss per metre times the pipe length. However, to
determine the overall pressure loss around a complete pipe
system, the losses due to fittings such as bends, tees
contractions and enlargements, and components such as
valves, strainers and terminal units must also be determined
and added to the straight pipe losses. Fittings losses can
represent a significant proportion of the total. In particular,
actuated control valves and differential pressure control
valves can incur significant pressure losses. CIBSE Guide
C, chapter 4 contains resistance factors for fittings and
valves from which their pressure losses can be calculated.
Valve manufacturers also publish pressure loss data for
their products.
For the purpose of sizing a pump, the maximum pressure
loss for the system, when operating at its design flow rate
value, must be calculated by summating the pipe and fitting
losses around the pipework branch or circuit with the
highest resistance. This is commonly referred to as the
‘index circuit’. The index circuit is usually (but not always)
the circuit from the pump to the most remote terminal
unit, and all other pipe branches are irrelevant in terms of
pump sizing. The reason why the circuit serving the most
remote terminal unit has the highest pressure loss is simply
because this is the circuit with the longest length of
pipework, and hence the highest pipe pressure losses. Only
where a terminal unit with a particularly high resistance is
located on a branch closer to the pump, might the index be
somewhere other than that serving the most remote unit.
1.A1.3.2
Pump selection
1.A1.3.2.1
Pump types
A pump used to force water around a closed pipework
circuit is sometimes referred to as a ‘circulator’. The vast
majority of circulating pumps consist of a single or three
phase electric motor that drives an impellor forcing water
around the pipework system.
For most building services applications the pump is a
centrifugal type, i.e. the impellor rotates in a scroll or volute
shaped casing. As the impeller rotates, water is thrown
from the blade tips centrifugally into the casing and out
through the discharge opening. At the same time more
water is drawn into the ‘eye’ of the impeller through a
central inlet opening in the side of the casing.
Centrifugal pumps may be ‘in-line’ or ‘end suction’.
For in-line pumps, the inlet pipe connection is in line with
the outlet pipe connection. In-line pumps can therefore be
connected in line with the pipe, and if small enough, the
weight of the pump can be supported by the pipework.
For end suction pumps, the water is sucked into the pump
centre horizontally and comes out from the casing at 90
degrees to the inlet. The pump and its motor are usually
mounted on a concrete base.
Pumps may also be ‘glanded’ or ‘glandless’.
The uniform feature of glanded pumps is the separation
between the pumped fluid and the electric drive motor. The
connection between the impeller and the motor is made by
either a common shaft or by coupled shaft parts. Mechanical
seals maintain water tightness between the two components.
These may be prone to damage and must be checked or
re-fitted at regular intervals.
In a glandless pump, the pump and drive motor are
encapsulated in a single casing. As a result the pumps are
quieter and more compact. Canned rotor pumps are
examples of glandless pumps. Because the motor and pump
are housed in a single casing, any heat generated by the
motor is transferred to the water. Hence, glandless pumps
are not usually considered suitable for chilled water
systems.
For pumps operating in medium or high pressure systems,
the construction and choice of materials may differ from
that in low pressure applications. Pump manufacturers can
advise on appropriate pump construction and selection for
different temperature and pressure conditions.
1.A1.3.2.2
Pump sizing
Pump manufacturers tend to express the performance of
their pumps by means of ‘pump curves’. A pump curve
indicates the relationship between the pressure differential
generated by the pump and the resulting flow rate achieved.
The aim during pump sizing is to select a pump that can
deliver sufficient flow and pressure to match the
requirements of the pipework system into which it will be
installed. Figure 1.A1.1 shows a typical pump curve (black
line).
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
metre, Pa/m) of the straight pipe lengths in the system. As
a general rule, to minimize the life cycle energy consumption
of a pipework system (i.e. the embodied energy of its pipes
plus the pump life cycle energy consumption), pipes should
be sized based on a criterion of not exceeding 200 Pa/m.
Hydronic system design
Pipe and pump sizing
1-149
Based on the design operating point, a ‘system curve’ can be
established which enables the prediction of flow rate and
pressure differential under any operating condition. The
system curve can be established by applying the approximate
square law relationship between flow rate and pressure loss.
For example, if you double the flow rate through a system,
the pressure loss quadruples and if you halve the flow, the
pressure loss reduces to one quarter of its previous value.
Hence, from a single operating point, it is possible to
estimate the system pressure loss at any flow rate. The point
at which the system curve crosses the pump curve will be
the actual operating point i.e. the selected pump, when
connected to the system will operate at this point generating
differential pressure and flow rates that can be read off from
the pump curve. A typical system curve (grey line) and
operating point are shown in Figure 1.A1.1.
Since pumps are available in a range of sizes and capacities,
it is very unlikely that the calculated design operating point
for a particular system will lie exactly on a manufacturer’s
published pump curve. The pump should therefore be
selected with a curve which lies above, but as close as
possible to, the design operating point. This will ensure
that the pump is capable of generating sufficient pressure
differential to achieve the required flow rate.
If necessary, the performance of the selected pump can be
modified so that its curve better matches the anticipated
design operating point of the system. One method of
achieving this is to ‘trim’ the pump impeller, i.e. reduce its
diameter. This can be undertaken by the manufacturer
before the pump is delivered and installed.
Pump curve at max speed
Pump curve at reduced speed
System curve
Pump efficiency curve
Net positive suction head required (NPSHr)
Design operating point
Pump efficiency point
Motor
losses
Pump
losses
Electrical
input
power
Hydraulic
pump
power
Figure 1.A1.2: Energy losses from pump and motor combinations
However, for the final setting of pump flow rate during
commissioning, the pump speed should ideally be adjusted
using an inverter drive supplied with the pump motor (this
is more energy efficient than throttling a regulating valve to
achieve the required flow rate). By either trimming the
impellor, or reducing a pump’s speed, the pump curve
effectively moves to a position that is parallel to, but below
the manufacturer’s ‘nominal’ maximum speed pump curve.
This effect is illustrated in Figure 1.A1.1.
1.A1.3.2.3
Efficiency and operating energy
As previously stated, pump sizing inevitably involves
selecting a pump with a curve that lies above, but as close as
possible to the calculated design operating point. There
may be several alternative pumps that can achieve the
required operating point, but future energy consumption
can be minimized by selecting the pump, motor and
invertor drive combination that operate with the highest
possible combined efficiency at the design operating point.
The combined efficiency of a pump, motor and invertor
combination can be defined as the hydraulic pump power
(i.e. the power available to propel water around the system),
divided by the electrical input power (i.e. the power
delivered to the electric motor and invertor drive). This
relationship is shown in Figure 1.A1.2. A low overall
efficiency is an indication that energy is wastefully
transferring to noise or heat rather than fluid motion.
Pump efficiency, motor efficiency and invertor efficiency
can be separated for any given combination. Hence, the
energy consumed by a pump per second (i.e. overall pump
input power) is given by the equation:
Pump efficiency
Pump pressure
Inverter
losses
ΔpQ
P = ———
η
(1.A1.1)
where P is the pump (electrical) input power (W); Δp
is the pump pressure (Pa), Q is the flow rate (m3/s) and η
is the efficiency.
Efficiency can be sub-divided as:
Flow rate
Figure 1.A1.1: Pump, pump efficiency and system curves
η = ηpump × ηmotor × ηdrive
(1.A1.2)
where ηpump is the pump efficiency, ηmotor is the motor
efficiency and ηdrive is the drive efficiency.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Having calculated the required system flow rate and
maximum pressure loss during the pipe sizing exercise,
(referred to as the ‘design flow rate’ and ‘design pressure
loss’ values), these can be used to plot a point relative to the
pump curve. This point is the intended ‘design operating
point’ or ‘duty’ of the pump when connected to the system.
1-150
Hydronic system design
Minimum requirements for electrical motor efficiencies
ηmotor are explained in BS EN 60034-30-1:2014 Rotating
electrical machines. Efficiency classes of line operated AC motors
(IE code) (BSI, 2014). This standard separates motor
efficiencies into four bands IE1, IE2, IE3 and IE4 where
IE1 is the least efficient, and IE4 is the most efficient. The
relevant banding will be indicated on the name plate of any
product that complies with this standard. (Note that these
bandings replace the manufacturer’s labeling scheme
which defined EFF1, EFF2 and EFF3 efficiency bands.)
Ongoing directives from the European Parliament have,
since 2015 made it a requirement that minimum efficiency
IE3 must be maintained for power ratings from 7.5 kW to
375 kW or an IE2 motor plus frequency inverter. In 2017
the threshold value will reduce to 0.75 kW.
Hence, for building services applications requiring regular
or frequent operation of the pumps, the minimum band for
motor efficiency should be IE2. Where the pumps are to be
in continuous operation, IE3 should be proposed.
Variable speed drive efficiencies, ηdrive are available from
invertor drive manufacturers. Efficiencies of 96–98% are
commonly stated.
It can be seen that the best way to minimise pump energy
consumption is to take every opportunity to reduce system
design flow rates pressure losses whilst maximising pump,
motor and invertor drive efficiencies under all anticipated
operating conditions. By enabling variable flow during
normal system operation, additional significant savings can
be achieved as described in section 1.A1.3.2.5.
1.A1.3.2.4
Net positive suction head
Net positive suction head is the term used to describe the
absolute pressure of the fluid at the inlet to the pump,
minus the vapour pressure of the fluid (i.e. it is a measure of
how far the pressure of the water is above its vaporization
pressure). Vapour pressure values for different water
temperatures can be obtained from CIBSE Guide C,
chapter 2, Properties of water and steam.
For a particular pipework system, the net positive suction
head is referred to as the net positive suction head available
(npsha) and, for a re-circulating pipework system, it can be
calculated as the absolute pressure (i.e. gauge pressure plus
1 bar) at the cold fill connection to the system, minus the
vapour pressure of the fluid, and minus any pipeline
pressure losses between the cold fill connection point and
the pump inlet.
Pump manufacturers use a similar terminology to describe
the amount of pressure required at the pump inlet to
prevent air or vapour bubbles from forming inside their
pumps. This is known as the net positive suction head
required (npshr). If allowed to form, air or vapour bubbles
can implode violently inside the pump causing significant
damage. This effect is known as cavitation. The npshr value
is a feature of each particular pump and varies with speed,
impellor diameter, inlet type and flow rate. npshr is
established by the manufacturer and is often included on
the pump performance curves.
Hence, if the npsha is greater than npshr cavitation should
not occur. However, if npsha is lower than npshr cavitation
is possible.
Options for increasing the npsha in a re-circulating system
include:
——
reducing the resistance, and hence pressure losses,
of pipes between the cold fill point and pump inlet
——
for open systems, raising the height of the feed tank
——
for closed systems, increasing the cold fill pressure
generated by the pressurisation unit.
npshr values are often plotted by pump manufacturers
relative to their operating curves, as illustrated in Figure
1.A1.1 (grey dotted line).
1.A1.3.2.5
Variable speed pumps
The ease with which modern inverters allow pumps to vary
their speed introduces significant energy saving
possibilities. This is because the design operating points
calculated for heating and cooling systems are usually based
on flow rates and pressure losses that will only be required
when systems are performing at full load, i.e. when
delivering their maximum anticipated amounts of heating
or cooling. This is only likely to coincide with the hottest
summer days or the coldest winter days. For all other times
there is scope to reduce system output and therefore pump
less water.
One method of achieving variable flow is to install multiple
pumps in parallel and controlling them such that they
switch on and off depending on system demand. However,
this method of achieving variable flow is not as energy
efficient as a single variable speed pump sized to achieve
the same duty. This is because pumps operating in parallel
do not increase the flow by a fixed amount per additional
pump, i.e. two identical pumps installed in parallel do not
achieve double the flow of the individual pumps on their
own. A single variable speed pump is therefore preferable
in most situations.
The potential energy savings achievable from variable
speed pumping can be assessed using the pump affinity
laws. For any fixed geometry pump operating against a
constant resistance at two speeds N1 and N2, the resulting
changes in flow rate, pump pressure and pump power will
be as predicted by the following equations.
Q2 = Q1 (N2/N1)
(1.A1.3)
p2 = p1 (N2/N1)
(1.A1.4)
2
P2 = P1 (N2/N1)
3
(1.A1.5)
where Q is the flow rate (m3/s) , p is the pump pressure (Pa),
P is the pump power (W) and N is the pump speed (rev/s).
If system resistance remains constant, then the affinity laws
predict that if the pump speed is halved, then:
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Pump efficiency, ηpump varies depending on the resistance of
the system to which the pump is connected. Pump efficiency
tends to be highest in the central region of a pump’s stated
operating range and reduces when the pump is connected
to a system for which the resulting operating point is near
either the top or bottom of the published pump curve. The
typical variation in pump efficiency is illustrated in Figure
1.A1.1 (black dotted line). For a particular pump, the
manufacturers’ data can be used to establish the pump
efficiency at the design operating point.
Heat transfer to and from pipes
flow rate is halved
——
pump pressure reduces to one quarter (i.e. a half
squared)
——
power consumption reduces to one eighth (i.e. a
half cubed).
This indicates the prospect of a significant energy saving if
the pump speed can be reduced during periods of low load.
For most heating and cooling systems, this represents the
majority of the time the system is in operation.
However, savings of this magnitude are only achievable if
the system is designed in such a way that pump pressure
can be allowed to reduce significantly under part load
conditions.
There are four common methods of controlling pump
speed:
——
Constant pressure control: pump speed is controlled
such that the pressure differential across the pump
is maintained at a constant value equivalent to the
pressure loss around the system at the maximum
flow rate.
——
Proportional control: pump speed is controlled such
that the pressure differential across the pump
reduces in proportion to flow rate towards a preselected value, typically equal to approximately
50 per cent of the pressure loss around the system at
the maximum flow rate.
——
Quadratic pressure control: pump speed is controlled
such that the pressure differential across the pump
reduces based on a quadratic curve relationship to
flow rate towards a pre-selected value.
——
Remote sensor control: pump speed is controlled such
that the pressure differential across the pump
reduces towards the design pressure differential
across the most remote dpcv controlled subbranches. Differential pressure sensors, wired back
to the bms, are required across the selected subbranches.
Constant, proportional and quadratic pressure control rely
on integral sensors and software supplied with the pump.
Because there is no requirement for external sensors in the
pipe work system, these options are sometimes referred to
as ‘sensorless’ solutions.
Out of these four methods, the most energy efficient
solutions are remote sensor control and quadratic pressure
control followed by proportional control. Constant pressure
control is poor in comparison with the other methods
because the full cube law reduction in pump power cannot
be achieved since pump pressure is held at a constant value.
Furthermore, pumps that are controlled in this way often
exhibit very poor efficiencies under part load conditions as
the pump attempts to maintain a constant pressure
differential against a system which, due to valve closures,
has a high resistance.
As a general rule, proportional or quadratic pressure control
is the best solution in systems where there is a fairly
uniform load pattern i.e. it can be predicted that all heating
or cooling terminal unit control valves will open or close
roughly at the same times. In more complex systems serving
multiple branches or risers with distinctly varying load
patterns, remote differential pressure sensor control is
required to ensure that sufficient pressure and hence flow is
available at system extremities under all operating
conditions.
More detailed explanation of the relative energy saving
benefits of alternative methods of pump speed control is
provided in BSRIA guide BG 12/2011 Energy Efficient
Pumping – a design guide (Parsloe, 2011).
1.A1.4
Heat transfer to and from pipes
Pipes carrying heated or chilled liquids will inevitably emit
or absorb heat to or from the surrounding air. These
emissions need to be taken into consideration during the
sizing of central heating or cooling sources, and the
selection of system operating temperatures.
National building regulations increasingly place limits on
the amount of heat that is permitted to be lost or gained
from pipework. In the UK, the Non-domestic building services
compliance guide (DCLG, 2013) provides recommended
maximum heat loss or heat gain values for Part L compliance
(in Watts per metre pipe length). These values apply to
different pipe sizes in low, medium and high temperature
heating systems and cooling water systems. To achieve
these recommended maximum values, appropriate
insulation thicknesses can be calculated according to BS
EN ISO 12241 (BSI, 2008). Typical thicknesses for
alternative operating temperatures and insulation material
is provided in the TIMSA HVAC Guide for achieving
compliance with Part L of the building regulations (TIMSA,
2008).
For heating pipes, the Non-domestic building services
compliance guide requires that pipes are insulated in all
areas outside of the heated building envelope. In addition
pipes should be insulated in all voids within the building
envelope and in spaces which will normally be heated if
those spaces might be maintained at temperatures different
from those maintained in other zones. Heat losses from
uninsulated pipes should only be permitted where the heat
can be demonstrated as always useful. It is normal that the
final connections to radiators are uninsulated.
For cooling pipes, the guide requires that pipes are normally
insulated along their full length. Heat gains to uninsulated
pipe should only be permitted where the proportion of the
cooling load relating to distribution pipework is proven to
be less than 1 per cent of the total load.
Provision may also be necessary for the control of
condensation on pipe surfaces. Advice on insulating to
prevent condensation is provided in the TIMSA HVAC
Guide for achieving compliance with Part L of the building
regulations.
For large heating or cooling systems it may be necessary to
compensate for excessive non-useful heat losses or gains.
The amount of heat lost or gained from the pipework
should be calculated and an allowance made when sizing
the central boiler or chiller plant.
In such systems there may also be a reduction in the outputs
of heating or cooling emitters due to the change in
temperature of the circulating liquid that takes place
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
——
1-151
1-152
Hydronic system design
1.A1.5
Pipework movement
Using the following equations (sourced from CIBSE Guide
C, section 3.3), the temperature change in the fluid passing
through a pipe can be calculated as:
Provision must be allowed in pipework systems for thermal
expansion or contraction. When pipes are restricted from
moving freely, large forces and moments may be imposed
on pipe supports, anchors, and connections to equipment
leading to failure. Furthermore, pipes that grow in length
between two securely fixed points may fail due to buckling
or bowing.
U ( θu – θa ) dop
∆θm = ———————
In addition to pipework thermal expansion, the movement
of pipework may also be caused by:
1330 M
(1.A1.6)
——
where ∆θm is the change in temperature per unit length
(°C), U is the overall thermal transmittance to/from
insulated pipe (W·m–2·K–1), θu is the temperature at
upstream section of pipe (°C), θa is the air temperature (°C),
dop is the outside diameter of pipe (m) and M is the mass
flow rate (kg·s–1).
Building settlement movement: whenever pipes are
routed across structural movement joints in
buildings, they may be subjected to differential
displacement.
——
Vessel settlement: pipes may be installed with rigid
connections to vessels used for storage of fluids. if
installed whilst the vessel is empty, settlement or
compression of spring mountings may occur when
the vessel is filled.
——
Plant vibration during start-up: equipment that is
installed on anti-vibration mountings may
experience excessive vibration as the motor startsup or runs-down, and passes through the resonant
frequency of the vibration isolation system.
——
Water hammer: water hammer is caused by shock
waves created due to fast closure times of automated
valves.
——
Flow induced movement and vibration: high velocity
flow of liquids in pipes can cause pipe displacements
at bends or sharp contractions.
The overall thermal transmittance to/from an insulated
pipe is given by:
1
U=
Rn +
dop
hso don
(1.A1.7)
where Rn is the thermal resistance of insulation, hso is the
outside heat transfer coefficient (or film coefficient)
(W·m–2·K–1), don is the outside diameter of insulation (m)
and where the thermal resistance of the insulation Rn is
given by:
Rn =
dop
2 kn
ln
( )
Provisions for pipework movement should be allowed in
each of these cases.
don
1.A1.5.1
dop
Relative expansion rates for pipes of different materials are
indicated in Table 1.A1.1. It can be seen from Table 1.A1.1
that plastic pipes exhibit expansion levels that are up to
13.6 mm per metre. For such materials it is recommended
that the pipe manufacturer’s specific guidance relating to
expansion is followed.
(1.A1.8)
where kn is the thermal conductivity of insulation
(W·m–1·K–1).
Using these equations the overall change in temperature
between central plant and terminal units can be determined.
Increasing circuit flow rate is wasteful of pump energy
whist increasing the emitter size adds unnecessary cost.
The best method to compensate for the change in
temperature is therefore to increase the set-point
temperature of the central plant.
Hence, if a heating system is designed based on a
temperature differential of 30 °C (i.e. 70 °C flow and 40 °C
return) but the temperature drop between the boiler and
the most remote heat emitter is estimated (from the above
equations) to be 3 °C then it would be appropriate to set the
boiler flow temperature at 73 °C to ensure that the water
reaching the emitters is at least equal to the design value of
70 °C.
Pipework expansion
For metal pipes, the expansion rates are significantly less at
around 1.4 mm per metre. However, significant forces are
generated which must be allowed for in the design of the
system and the structural planning of the building.
In general, there are two ways in which linear pipe
expansion may be controlled.
——
the use of natural flexibility
——
the use of expansion joints.
1.A1.5.1.1
Natural flexibility solution
Each change in direction requiring a bend or elbow
introduces some ‘natural’ flexibility into the pipe system.
The ability of a pipe to bend is a function of pipe material,
its nominal size, wall thickness, and the length.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
between the central plant and the emitter. For example, in
a large district heating system, due to the heat emissions
from buried pipes, the temperature of the water reaching
the heat emitters may be significantly less than the
temperature at which it left the heat source.
Pipework movement
1-153
The result of the pipe being able to bend is to reduce the
forces acting within the system and to reduce the pipe
stresses.
It can be seen that in absorbing the expansion, pipes
between anchors may move laterally. Therefore, the pipe
supports or guides between anchor points must allow for
this lateral movement.
With reference to Figure 1.A1.3, the required length of an
offset can be calculated using the following equation. For
an expansion loop, the expansion is effectively absorbed
over two offsets; hence the length of each offset can be
halved.
For steel pipes:
lo = 0.1 (d x) 0.5
(1.A1.9)
For copper pipes :
lo = 0.06 (d x) 0.5
(1.A1.10)
where lo is the length of offset (m), x is the deflection caused
by linear pipe expansion (m) and d is the nominal diameter
of pipe (m).
The stresses caused by pipework expansion can be
minimized by anticipating the amount of expansion in
each pipe, and then cutting the pipes so that their lengths,
when cold, are equal to the required length minus 50% of
the anticipated expansion length. The pipework is then
assembled cold with spacer pieces of length equal to half
the expansion, sandwiched between the connecting flanges.
When the pipework is fully installed and anchored at both
ends, the spacers are removed and the flange bolts are
tightened. When warmed through half of the total
temperature rise, the piping is at a neutral point i.e.
unstressed. At the working temperature, having fully
expanded, the piping is stressed in the opposite direction.
The effect is that instead of being stressed from 0 to F units
of force, the piping is stressed from –0.5F to +0.5F units of
force. This method of installation is known as ‘cold draw’
or ‘cold pull’.
1.A1.5.1.2
The forces experienced on anchor points tend to be greater
in pipe sections with unrestrained expansion joints than in
sections with restrained expansion joints. This is because
the pressure inside the pipework contributes to the force
exerted at the anchors. Restrained expansion joints include
restraining rods which prevent the internal pressure from
exerting a force at anchors.
Figure 1.A1.4 shows a typical arrangement of axial
expansion joints (bellows) relative to anchor points and
pipe guides.
Restrained expansion joints include a variety of expansion
joint models including lateral, hinged and gimbal expansion
joints. With all of these models, anchor forces are generally
lower than with unrestrained expansion joints, and fewer
pipe guides are required.
Lateral expansion joints are usually limited to lateral travel
only, although ‘fully articulated’ models can allow lateral
movement in any direction from their main axis. Figure
1.A1.5 shows a typical use of a lateral expansion joint used
to accommodate the movement in a short offset between
two parallel pipe runs.
Hinged (or angled) expansion joints are limited to angular
travel only, but effectively create articulating sections of
pipe when used with in groups of 2 or 3 units. Figure 1.A1.6
shows a typical use of hinged expansion joints to
accommodate the movement in an offset between two
parallel pipe runs.
Gimbal expansion joints are similar in principle to the
hinged model, but they are able to angulate in any direction
from their main axis. This makes them suitable to
accommodate thermal expansion in complex multidirectional pipe arrangements. Figure 1.A1.7 shows a
typical use of a gimbal expansion joints to accommodate
the movement in an offset between two pipes travelling in
different directions.
x
Expansion joint solution
If natural flexibility is insufficient, or the forces created by
expansion are excessive, then expansion joints must be
provided. There are several different models and it must be
decided which is the best for the pipe system being
designed. Different types of expansion joint will impose
different forces on pipes and anchor points.
Expansion joints can be divided into two main groups.
These are ‘unrestrained’ and ‘restrained’ as described
below.
Unrestrained expansion joints are essentially axial expansion
joints, this being the only model that falls into the
unrestrained group. Generally, they are designed to
accommodate movements of between 25 and 50 mm. They
0·5lo
Anchor
Anchor
x
Anchor
lo
Anchor
Figure 1.A1.3: Use of natural changes in direction to accommodate
expansion
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Figure 1.A1.3 shows how offsets, or purpose made expansion
loops can be used to accommodate pipework expansion.
Anchor points are created against which expansion forces
push. The anchor points are rigid, fixed points in the
pipework system and are usually created by welding the
pipe to a bearer which is bolted to a structural element.
are limited to axial travel only and must be suitably
anchored and guided. Pipe guides are pipe support fixings
(such as roller supports) which enable the pipe to move
axially but not laterally. These are required in order to
avoid unwanted lateral movement between anchor
locations.
1-154
Hydronic system design
Axial
movement
Primary
guides
Axial
device
Anchor
Intermediate
guides
Figure 1.A1.4 Use of axial expansion
devices
Lateral
movement
Anchor
Guide
Planar
guide
Lateralndevice
allowing movement
in one plane
Lateral
movement
Anchor
Guide
Figure 1.A1.5 Use of lateral
expansion devices
Lateral
movement
Anchor
Guide
Planar
guide
Hinged devices
allowing movement
in one plane
Lateral
movement
Anchor
Guide
Figure 1.A1.6: Use of hinged or
angular pipe expansion devices
Anchor
Planar
guide
Anchor
Guide
Figure 1.A1.7 Use of gimbal pipe
expansion devices
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Anchor
System pressurisation and expansion
Radiator
F and E pipe
Maximum
distance
150 mm
Boiler
Radiator
Close coupled open safety vent pipe and feed and expansion pipe
It can be seen that most expansion devices use a flexible
bellows which distorts in a controlled manner to
accommodate pipework movement. It is important that the
bellows is capable of withstanding the system operating
pressure in order to prevent the bellows material from
expanding and flexing outwards. Some devices may
incorporate limit rods, to prevent over-compression or
over-extension of the bellows. In the event of anchor failure,
they act to contain the system pressure thereby preventing
damage to the bellows and pipework.
There is invariably more than one way to accommodate the
movement of pipework due to expansion and it is essential
that, for whatever solution is adopted, the forces exerted on
the anchor points are calculated and notified to the
structural engineer. It is recommended that specialist
advice is sought from the expansion device manufacturer
regarding their selection and application.
1.A1.6.1
Figure 1.A1.8: Simple open feed
system
Open systems
In an open system, expansion and contraction of the fluid is
catered for by the inclusion of an open tank located above
the highest point in the system. Commonly referred to as a
‘feed and expansion tank’ the tank provides a water source
from which the system can be filled, and also accommodates
expansion of the water as it is heated. These systems must
also have an open safety vent pipe to provide an unrestricted
path from the boiler for the relief of pressure if the boiler
controls should fail. The safety vent pipe should be located
as close as possible to the boiler and with no means of
isolation between the boiler and the safety vent pipe outlet
connection. The safety vent pipe should rise to a height
above the tank sufficient to prevent any discharge occurring
under normal operation. The formula to find out the height
of the vent pipe above the water level in the tank is:
(Height (m) × 40 mm) + 150 mm
(1.A1.11)
where height is the distance from the water level in the
tank to the lowest point in the heating system.
1.A1.6
System pressurisation and
expansion
Pipework systems must be provided with a means for filling
the system and an allowance for expansion. Water expands
and contracts when heated or cooled. For chilled water
systems expansion is caused when the water heats from its
chilled condition to ambient. In heating systems expansion
is caused when the water is heated from ambient to its
design flow temperature. The resulting change in volume
must be catered for within the design of the system
otherwise excessive pressures may be generated leading to
system failure.
Table 1.A1.5: Operating temperatures ranges for heating systems
System
Temperature
/ °C
Minimum pressure
/ bar
Low temperature
<90
1
Medium
temperature
90–120
3
High temperature >120
5
Figure 1.A1.8 shows a typical open system arrangement.
Open systems are usually only found on smaller or older
systems. On larger systems they are less popular because
they provide a ready path for the ingress of dissolved
oxygen which can increase the corrosion rate of steel
components.
1.A1.6.2
Sealed systems
Larger, commercial heating and cooling pipework systems
are likely to be sealed systems where expansion and
contraction of the water is catered for by the inclusion of an
expansion vessel and associated pressurisation pumps.
Alternative methods of pressurisation are applicable to
chilled water systems and low, medium or high pressure
hot water heating systems (lthw, mthw or hthw systems),
where the operating temperatures of these systems are as
indicated in Table 1.A1.5.
It can be inferred from Table 1.A1.5 that for medium and
high temperature systems, the pressure in the system must
be maintained at all times at a value which is sufficient to
prevent the water from boiling and ‘flashing’ into steam.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
OSV
1-155
1-156
Hydronic system design
Expansion
vessel
Feed
tank
AAV
IV
IV
DOC
Safety
valve
Boiler
LSV
NRV
DOC
IV
IV
FC
NRV
IV
Figure 1.A1.9 Pressurisation by
expansion
Fill point
IV
System
pumps
Expansion
vessel
Spill
tank
AAV
IV
IV
Safety
valve
DOC
Buffer
vessel
Boiler
Pressure
sensors
LSV
NRV
IV
FC
NRV
IV
Fill point
IV
System
pumps
Buffer
vessel
AAV
or heat
exchanger
Regulated
nitrogen
supply
Spill
tank
IV
IV
DOC
Safety
valve
Expansion
vessel
Figure 1.A1.10: Pressurisation by pump
Boiler
LSV
Pressure
sensors
NRV
IV
Fill point
FC
NRV
IV
Figure 1.A1.11 Pressurisation by gas
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
IV
System
pumps
System pressurisation and expansion
1-157
pumps into the heating system and to minimize the risk of
heated water entering the expansion vessel and spill tank.
For example, if water at 120 °C is to be maintained as water,
then the corresponding vapour pressure is 198.53 kPa
absolute or 98.53 kPa gauge pressure, as indicated in CIBSE
Guide C, chapter 2. However, to ensure that boiling will
definitely not occur, a margin should be added to this
pressure. BS 7074: Application, selection and installation of
expansion vessels and ancillary equipment for sealed water
systems, Part 2 (BSI, 1989) recommends that an anti-flash
margin equivalent to 11 °C in temperature is allowed.
Hence, the minimum pressure required in the system
would be the vapour pressure of water at 131 °C which from
Guide C, chapter 2, is indicated is approximately 278 k§Pa
absolute or 178 kPa gauge pressure.
Pressurisation by gas is applicable to hthw systems and
utilizes an initial charge of gas to generate a high pressure
in the system. A pressure cylinder is connected to the
heating system filled partly by water and partly by air or an
inert gas (such as nitrogen). The initial supply of gas is
from a small air compressor or gas bottle. An initial pressure
can therefore be applied to the system at a level well above
the boiling point of the system water. The water of
expansion is discharged from the system by a spill valve to
a spill tank which is open to atmosphere and, as the system
cools and contracts, a pressure pump draws water from the
spill tank and returns it to the system. The pressure
controller regulates the admission of water from the pump
or its expulsion through the spill valve. In most units two
pumps are used to run in parallel to meet unusual demands,
and the water of expansion passes through a heat exchanger
to lower the temperature and, if possible, prevent it flashing
to steam when discharged.
There are three common methods of pressurisation for
sealed systems which are roughly applicable to chilled and
low temperature heating, medium temperature heating and
high temperature heating applications. These alternatives
are explained in the following sections.
1.A1.6.2.3
1.A1.6.2.4
1.A1.6.2.1
Pressurisation units
Pressurisation by expansion
Pressurisation by expansion is suitable for lthw systems
and chilled water systems. This involves the addition of an
unvented expansion vessel to a heating system which is
charged with gas and sealed. The function of the vessel is to
take up the increased volume of water as it is heated, and by
so doing, apply additional pressure in the system. In
practice, expansion vessels are used which incorporate a
flexible rubber diaphragm which separates the water on
one side from a factory applied charge of nitrogen on the
other. Nitrogen is used as it is less soluble than air in water
and is less likely to enter the water causing corrosion
problems. An anti-gravity pipework loop is incorporated
for heating applications to prevent heated water from the
system rising due to its natural buoyancy into the expansion
vessel. This feature is not required for chilled water systems.
For larger systems operating at low temperature, the
principles of operation remain the same. The expansion
vessel will increase in size and may even be duplicated.
1.A1.6.2.2
Pressurisation by gas
Pressurisation by pump (spill systems)
Pressurisation by pump is an alternative approach which is
suitable for mthw systems or lthw systems requiring large
expansion volumes. This system relies on the operation of
a pump to generate pressure in the system, in conjunction
with an expansion vessel to accommodate system expansion
and a spill valve that discharges water into a spill tank to
maintain constant pressure. The spill valve allows water of
expansion to escape into the spill tank once a pre-set
pressure is reached. Hence, the expansion vessel does not
have to be sized to accommodate all of the expansion water,
and the accumulation of water in the expansion vessel does
not add additional pressure to the system. While the system
remains at the design working temperature, and at constant
pressure, the spill valve remains closed and the pump is
idle. When the system temperature and pressure fall, the
pump will start and the pre-set pressure will be restored. A
buffer vessel is commonly installed between the pump and
the expansion vessel to reduce surging of water from the
Pre-packaged ‘pressurisation units’ based on each of these
aforementioned solutions are available from manufacturers.
Such units usually incorporate a ‘quick-fill’ connection
which allows temporary connection from the mains water
supply via a back-flow prevention device (usually a reduced
pressure zone valve). This connection enables the system to
be filled quickly without the need to run the pressurisation
unit pump. It should be disconnected after use. Fill water
connections should be sized such that they enable the
system to be filled within a reasonable time period.
Recommended fill connection sizes are indicated in Table
1.A1.6, corresponding to a fill pressure sufficient to generate
1.3 bar at the top of the system.
In addition to the pressurisation unit, other features are
necessary for the safe operation of heating systems including
a safety valve fitted to the boiler that is set to open if the
pressure in the boiler exceeds a set value. A high pressure
switch may also be incorporated to stop firing of the boiler
in the event of over pressure, before the excess pressure
triggers safety valve operation.
1.A1.6.2.5
Pressurisation unit sizing
For most lthw systems utilizing pressurisation by
expansion, the sizing of the expansion vessel is critical in
determining the final pressures in the system. The pressure
generated due to the expansion of water into a sealed vessel
must be calculated in order to ensure that the maximum
operating pressures of system components are not exceeded.
Table 1.A1.6: Recommended fill connection sizes
System volume
/ litres
Minimum fill pipe size
/mm
<2,000
25
2,000-10,000
40
>10,000
50
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The amount of pressure required to keep water from boiling
is known as the saturation vapour pressure and varies with
temperature. Vapour pressure values for different water
temperatures can be obtained from CIBSE Guide C,
chapter 2, Properties of water and steam (CIBSE, 2001).
1-158
Hydronic system design
——
isolation
——
flow regulation
——
differential pressure control
For example, if the boiler and cold fill point are located at the
lowest part of the system, then it is likely that the boiler’s
pressure rating will dictate the maximum working pressure.
In this case the maximum working pressure would be the
boiler operating pressure minus the safety valve lift margin
(typically 0.5 bar) minus a margin of 0.7 bar.
——
flow control
——
a combination of the above.
However, if the boiler and cold fill point are located at the
highest part of the system, then it may be that terminal
units located the lowest part of the system will dictate the
cold fill pressure. In this case, the maximum acceptable
cold fill pressure might be calculated based on the
recommended operating pressure of the terminal units,
minus the system static pressure (i.e. the pressure due to
the height of the system).
Having determined the maximum fill pressure to the
system, an expansion vessel can be sized to accommodate
the volume of system expansion without exceeding the
maximum fill pressure.
Example sizing calculations for expansion vessels in
heating systems are provided in CIBSE AM14: Nondomestic hot water heating systems (CIBSE, 2010), and in
BSEN 12828: 2003 Heating systems in buildings – design for
water based heating systems (BSI, 2003). It should be noted
that BSEN 12828: 2003 replaces BS7074 Part 2, and differs
in that it introduces a requirement to size the expansion
vessel with some water reserved in the vessel at the cold fill
state. Example calculations for expansion vessels in cooling
water systems are provided in BS 7074-3: Application,
selection and installation of expansion vessels and ancillary
equipment for sealed water systems Part 3, Code of Practice for
Chilled and Condenser Systems (BSI, 1989).
1.A1.6.2.6
Pressurisation unit connection point
The pressurisation unit connection point is the only point
in the system where an external pressure is applied and is
termed the ‘neutral point’ because the pressure at this point
remains constant whilst pressures elsewhere in the system
will vary due to height, pump pressures and system pressure
losses. The fill point and consequent neutral point should
normally be connected at the inlet side of boilers or chillers
so that these items always operate at a pressure that is less
than the fill pressure. Furthermore, pumps should normally
be located on the flow side from boilers or chillers so that
pump pressure is not added to fill pressure thereby
increasing the pressure inside the boilers or chillers. By
locating the neutral point at the pump inlet ensures that the
pump pressure is additive meaning that that the entire
system is always above atmospheric pressure and the
required net positive suction head (npshr) of the pump can
be maintained under all operating conditions.
1.A1.7
Valve types
All hydronic systems are dependent for their successful
operation on the inclusion of properly designed and
selected valves. Pipeline valves may perform a variety of
functions including:
The main valve types and associated pipeline components
are described in the following sections.
Double regulating valves
A double regulating valve is a regulating valve that can
perform the double function of flow isolation and
regulation. This double function is achieved by
incorporating a locking mechanism in the handle of the
regulating valve. This allows the valve to be regulated until
the required flow rate is achieved and then locked in place.
If the valve is subsequently closed for isolation purposes,
on re-opening, the valve handle will only open as far as its
locked position. Some double regulating valves have
pressure tappings across the opening making it possible to
measure flow. These are commonly referred to as ‘variable
orifice double regulating valves’.
Fixed orifice flow measurement devices
A fixed orifice flow measurement device uses the pressure
differential across an orifice plate as an indicator of flow
rate. An orifice plate is a plate with a circular opening at its
centre of a diameter that is less than the internal bore of the
adjoining pipe. Pressure tappings are fitted upstream and
downstream of the orifice plate and are used to measure the
pressure differential signal across the orifice.
Fixed orifice double regulating valves
Fixed orifice double regulating valves are so called because
they comprise a fixed orifice flow measurement device,
close coupled to a double regulating valve enabling flow
rate to be measured and regulated from a single location.
This combination is commonly referred to as a
‘commissioning set’. The two components can be cast into
a single body or screwed together. For larger sizes, they may
be linked by a short section of pipe (spool piece). The flow
measurement device must be located upstream of the
double regulating valve to avoid any flow disturbance at the
inlet to the orifice plate.
Control valves
Control valves are installed on terminal unit branches as a
means of automatically controlling the flow of water
through the terminal units, and hence, the amount of
heating or cooling they deliver. In constant flow systems,
the control valves are typically 3-port or 4-port valves, both
of which reduce the flows through terminal units by
diverting them through by-passes; overall flow rate remains
constant. In variable flow systems, the control valve is
typically a 2-port valve which simply throttles the flow.
Control valves may be operated by temperature sensitive
actuators located in the occupied space, as it the case of
thermostatic radiator valves, or by motorized actuators
linked at a remote sensor and building management system.
Advice on the selection of control valves is provided in
CIBSE Guide H: Building control systems (CIBSE, 1989) and
KS7: Variable flow pipework systems (CIBSE, 2006).
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
The maximum allowable pressure at the pressurisation unit
cold fill point will depend on the system component with
the lowest recommended operating pressure, and its
location in the system.
Design considerations
Differential pressure control valves (dpcvs)
dpcvs are self-acting valves that act in response to changes
in pressure differential across the control valves (or circuits
containing control valves) that they protect. This pressure
differential is transmitted to either side of a flexible
diaphragm inside the valve via small capillary tubes. As the
diaphragm flexes in response to the changing pressure
differential, it causes the valve plug to move thereby varying
the opening through the valve. The effect is to maintain a
constant pressure differential between the inlet to the valve
and the upstream point to which the capillary tube attaches.
This pressure setting can be varied, but once set the action
of the valve will hold the pressure differential constant
regardless of changes in the resistance of the circuit and
regardless of changes in the available pump pressure.
Pressure independent control valves (picvs)
picvs combine the 2-port valve and differential pressure
control valve into a single body. Therefore the valve is selfprotected against excess pressures. Because the integral
dpcv holds the pressure differential constant across the
integral 2-port control valve, the result is that whenever the
control valve is fully open, the flow rate through the valve
always returns, approximately, to its set value (since a
constant pressure differential across a fixed resistance
results in a constant flow rate).
The opening through the 2-port control valve can be varied
manually, and can therefore be used to regulate the flow
rate through the valve to the required design value. A flow
setting dial on the valve spindle can be used for this
purpose. Once set, the valve should perform the function of
a constant flow regulator (or ‘flow limiting valve’) whenever
the 2-port control valve is fully open. Only when the control
valve begins to close might the flow rate change from its set
value.
Constant flow regulators
A constant flow regulator is any self-acting device that
operates to hold the flow rate through the branch in which
it is installed constant regardless of pressure and flow rate
changes in surrounding branches. When used in variable
flow applications, these devices are often referred to as ‘flow
limiting valves’ since they limit the maximum flow but
allow the flow to drop to zero as control vales throttle.
The simplest type of constant flow regulator comprises a
spring loaded stainless steel cartridge inside a brass casing.
An interchangeable orifice plate forms the entry port of the
cartridge which enables a variety of flow values to be
specified. The outlets from the device are via specially
profiled holes. The pressure exerted on the orifice plate
causes the cartridge to compress against the spring thereby
restricting the outlet holes. The result is that flow rate is
held constant regardless of changes in pressure.
Constant flow regulators are seldom used for terminal
branch control in variable flow systems, since their action
might interfere with modulating flow control. However,
they can be used in system by-passes where a constant flow
is required under all operating conditions.
1.A1.7.1
Valve authority
For automatic valves that have a control function, it is
important that the valves are selected with the appropriate
authority.
The authority of a valve is an indication of how accurately
the valve will be able to modulate flow as it opens and
closes. Valves designed for isolation purposes (e.g. ball or
gate valves) often have very poor authority because as they
close, there may be no significant change in flow until the
valve reaches the last part of its travel when the flow
suddenly drops to zero.
To achieve good authority, the pressure loss across the
control valve relative to the pressure loss around the circuit
in which it is located, needs to be as large as possible. This
will ensure that small changes in the resistance of the valve
will have a large influence on flow rate.
Acceptable authority in constant flow systems using 3- or
4-port valves to control the flows through terminal units is
0.5, i.e. the pressure loss across the control valve is
approximately equal to the pressure loss across the terminal
unit that it serves, and is therefore 50% of the total loss
through the terminal unit and valve combined.
The minimum acceptable authority in variable flow system
using 2-port control valves to control flows through
terminal units is 0.2, i.e. the pressure loss across the control
valve should be not less than 20% of the total pressure loss
across the entire terminal unit sub-branch in which the
valve is located. Although 0.2 is a minimum to suit
terminals with low flow rates, where possible, valves should
ideally be selected with authorities in the range 0.25 to 0.5.
The best achievable authority is 1, i.e. the only pressure loss
in the circuit through which flow is to be controlled is the
2-port valve itself. In theory, this is the authority achievable
by a picv since the pressure differential across the 2-port
control valve is controlled at a constant value by the integral
dpcv.
Further guidance on control valve authorities and valve
selection can be found in CIBSE Guide H and in CIBSE
KS7.
1.A1.8
Design considerations
1.A1.8.1
Flow temperature control
For heating systems in particular, control of temperatures
in occupied spaces can sometimes be improved by varying
the temperature of the circulating liquid.
For example, in a heating system serving radiators, if the
water is supplied at a constant temperature of say 80 °C,
then when the set-point room temperatures are achieved
and thermostatic radiator valves begin to close, the radiators
will continue to emit heat as the radiators cool. This can
result in significant over-heating.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
dpcvs are installed in systems to prevent two port control
valves from having to close against excessive pressures.
Without this type of protection, some 2-port control valves
may generate noise or lose their authority resulting in poor
modulating control of heating or cooling outputs.
1-159
1-160
Hydronic system design
P
TP
IV
IV
TP
TP
FC
FC
IV
TP
FC
FC
IV
IV
TP
STR
Constant
temperature
IV
MV
Variable
temperature
IV
IV
IV
IV
Primary header
Figure 1.A1.12: Constant and variable temperature circuits
Hence, for this type of situation, better control of room
temperatures is achieved if the temperature of the
circulating water is varied depending on the external design
conditions. One common solution for heating systems is
the use of ‘weather compensated control’. This type of
controller measures outdoor temperatures and varies the
temperature of the water supplied to radiators accordingly.
Such a system might only provide water at 80 °C on the
coldest days in winter (coinciding with the design
condition) and at all other times the water supplied will be
at a reduced temperature, as determined by the controller.
This type of system is known as a ‘variable temperature’
system as opposed to a ‘constant temperature’ system.
Variable temperature control can be achieved by some
boilers which are able to modulate their supply water
temperature. However, where there is a need to maintain
water from the boiler at a constant high temperature
(perhaps to serve hot water heaters) then a mixing circuit
will be required in enable the supply temperature to be
varied.
Figure 1.A1.12 shows a comparison between a constant
temperature circuit and a variable temperature circuit.
It can be seen that a constant temperature circuit takes
water from the heat source and supplies it direct to the
terminal units at the same temperature. Variable
temperature circuits incorporate a three port valve to enable
mixing of the return water with water from the heat source
thereby varying the flow temperature to terminals.
For the reasons explained, heating or cooling emitters with
a slow thermal response tend to operate more effectively in
variable temperature systems. However, emitters with a
faster response operate more effectively in constant
temperature circuits. This includes forced convection units
i.e. coils for which air is blown by a fan across a heating or
cooling coil. For these types of emitter, the volume of water
inside the heat emitter is relatively small and the operation
of the fan ensures that excess heating or cooling energy is
quickly dissipated.
Variable temperature circuits can be used as a means of
achieving low return temperatures in order to maximize
the energy efficiency of low carbon heat sources such as
condensing boilers, chp or solar heating. This solution
works best in situations where the means of varying the
flow temperature is achieved by the control of the heat
source itself e.g. variable temperature boilers. The use of a
variable temperature secondary circuit may be of little
benefit if the low temperature returning water is being
mixed with high temperature flow water in a primary
circuit before returning to the heat source. Furthermore,
the adoption of a variable temperature circuit will inevitably
limit the potential for the pumps to reduce their speed
under part load conditions, thereby missing out on
potentially significant pump energy savings.
1.A1.8.2
Variable flow systems
In a variable flow system, the flow rate varies depending on
the demand for heating or cooling from the system.
Terminal units are typically fitted with two port control
valves that throttle the flow when the zone served by the
terminal unit reaches its set-point temperature value. As
two port control valves throttle the flow in the system, the
pump is made to reduce its speed thereby saving energy.
The main advantages of variable flow systems are as follows:
——
Pump energy savings: due to the cube law relationship
between pump speed and power (as predicted by
the pump affinity laws) there is the potential for
significant pump energy savings relative to constant
flow systems.
——
Larger temperature differentials: the efficiencies of
low carbon emission heating and cooling sources
are often improved when the temperature
differentials between flow and return are maintained
as high as possible. Since variable flow systems
throttle the flow when the heating or cooling load is
satisfied, their temperature differentials tend to
increase under part load conditions, whereas in a
constant flow system the temperature differential
would decrease. Hence, variable flow systems are
essential when it is important to maintain a high
temperature differential.
Advice on the design of variable flow systems is provided in
BSRIA guide BG 12/2011: Energy Efficient Pumping Systems
– a design guide (Parsloe, 2011). The following sections
describe the main issues relating to variable flow system
design.
1.A1.8.2.1
System by-passes
By-passes are required in order to ensure a constant path
for flow and to allow water treatment chemicals to circulate
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
P
TP
STR
P
TP
DRV
P
TP
Design considerations
1-161
In the case of glandless pumps where the system water is
cooling the motor, the minimum flow rate through the
pump should not be allowed to fall below 10% of the
nominal flow rate of the pump, unless otherwise indicated
by the pump manufacturer.
system layouts. The following options are considered to
achieve the best results.
——
picv
——
dpcv valve modules: the use of dpcvs as part of valve
modules located locally to the terminal units they
serve is effective because the action of the dpcvs to
maintain pressure constant across each circuit will
ensure that flows through individual terminal units
remain constant regardless of closures in adjacent
circuits. By-passes can be created by fitting terminal
unit branches at system extremities with 3- or
4-port diverting control valves.
——
Branch dpcvs: dpcvs can be located on pipework
branches serving groups of terminal units although
their locations should be close enough to the
terminals they serve to prevent excessive pressures
and hence flow rates through the terminal units. As
a general rule, dpcvs serving heating or cooling
coils with 2-port valves should be located such that
they maintain a pressure differential of no more
than 1.5 times the design pressure loss across the
highest resistance terminal unit branch. Similarly,
dpcvs located on branches serving groups of
radiators should be limited to control at a pressure
differential of no more than 10 kPa. These limits
will ensure that the flow rate across any individual
terminal unit will never exceed 160 per cent of its
design value under part load conditions.
In the case of glanded pumps, the minimum flow rate
should be set at a value which does not cause an excessive
heat gain to the circulating water. The maximum possible
increase in temperature can be calculated from the equation:
∆T = P / (cp × qm)
(1.A1.12)
where ∆T is the temperature increase of the water as it
passes through the pump (°C), P is the pump power (W), cp
is the specific heat capacity of water (J/kg·K) and qm is the
mass flow rate of water (kg/s).
The pump power at zero (or near zero) flow can be
determined from the pump manufacturer’s published data.
Having decided the flow capacity of system by-passes, their
locations and design can be decided. In general, it is
beneficial to locate by-passes at system extremities to ensure
that water treatment chemicals reach all parts of the system.
By-passes can be fitted with pressure relief valves that open
when they see a rising pressure differential. However, this
solution will not work in systems where pump pressure
reduces under part load conditions, since by-passes will
experience a reducing rather than increasing pressure
differential.
By-passes can also be fitted with constant flow regulators to
maintain a fixed minimum flow. For these systems the
amount of water by-passing the system is constant and
must be added to the pump flow duty when sizing the
pump.
To avoid wasting pump energy by circulating water through
constant flow by-passes, a more energy efficient solution is
to install 3- or 4-port diverting control valves in terminal
branches at system extremities (sized as if they were 2-port
throttling control valves). This approach will ensure that
the by-passed flow rate is included within the overall system
design flow rate value.
1.A1.8.2.2
System layout
To maximize pump energy savings and operating
temperature differentials, system design should aim to
minimize pressure variations (and hence flow rate
variations) across terminal unit branches. In general, this
can best be achieved by locating some form of differential
pressure control device as close as possible to each terminal
unit.
BSRIA guide BG 12/2011: Energy Efficient Pumping Systems
indicates comparative energy performances of alternative
control: the use of picvs to modulate flows
through terminal units is effective because the
integral dpcvs within each picv act to maintain flow
rates through individual terminal units constant
regardless of closures in adjacent circuits. By-passes
can be created by fitting terminal unit branches at
system extremities with 3 or 4-port diverting
control valves and constant flow regulators.
1.A1.8.2.3
Hot water provision
Traditional hot water storage cylinders are not always
compatible with heating systems that operate with large
temperature differentials. Large design temperature
differentials result in prolonged heat-up periods, and
inevitably, the return temperature of the heating water
must exceed 60 ºC if temperature is used to control
legionella bacteria.
To maximize the temperature differential across hot water
heating circuits, the heat exchange rate should be
maximized. This will enable water to be heated quickly
whilst minimising storage where bacteria can multiply.
One option is the use of plate heat exchangers for the
instantaneous heating of hot water within so called ‘heat
interface units’.
This type of unit heats the incoming mains cold water by
means of a plate heat exchanger. Heating water circulates
on one side of the plate heat exchanger transferring heat to
mains cold water passing through on the other side. The
heated water then feeds straight to the hot water taps. To
control the heating capacity of the unit, self-acting control
valves regulate flow rates through each side of the plate heat
exchanger.
The heat transfer across the heat exchanger is such that
heating water entering at temperatures up to 80 °C can be
cooled to around 20–30 °C as it heats the incoming cold
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
to system extremities. To maintain the system temperature
differential at as large a value as possible, the amount of
water that is allowed to by-pass heating or cooling emitters
should be kept to a minimum. The minimum acceptable
by-pass flow is usually dictated by the pump. The amount
of water that a pump can deliver before it runs the risk of
overheating can be calculated for a given situation.
1-162
Hydronic system design
Because the heating demands for hot water are for shorter
periods (relative to the periods required to heat cylinders),
there is more scope to allow for diversity of usage in the
sizing of heating system pumps and pipe sizes. This can be
further assisted by the inclusion of a buffer tank in the
heating system to deal with large simultaneous demands
for hot water.
1.A1.8.3
Air and dirt removal
Excessive amounts of either air or dirt left in the system can
lead to difficulties in obtaining repeatable flow
measurements during the commissioning process.
Furthermore, problems associated with corrosion or
bacteria proliferation are more likely during the ongoing
operation of the system.
These problems can be avoided by proper venting and degassing of the system to remove air, and flushing and
chemical cleaning of the system to remove solids.
1.A1.8.3.1
Air removal
Air vents should be located at system high points to enable
the removal of trapped pockets of air during the initial fill
of the system. This will often need to be a manual process
whereby the installer opens each of the vents in turn to
release trapped air.
This type of manual venting may prove difficult in large or
complex systems or in high rise buildings due to the sheer
number of vent locations that must be visited. Furthermore,
in large or tall systems, even if manual venting is carried
out properly, additional air bubbles may be generated as
dissolved gas within the system water comes out of solution
due either to increased water temperature through boilers,
or the gradual reduction in static pressure as the water
travels up vertical risers.
For systems that may be difficult to vent by manual means,
some form of de-aeration facility is advisable. A purpose
designed de-aeration unit can be installed in order to
remove air from the system before commissioning, with the
option to leave it in place permanently.
De-aeration units can work by either temperature or
pressure.
Temperature based de-aeration
These units are installed in-line in the hottest part of the
system (e.g. the outlets from boilers or the inlets to chillers)
and are able to catch air ‘microbubbles’ released due to the
relatively high temperature of the circulating liquid at
those points. Captured bubbles are collected by a mesh or
packing material in a low velocity chamber (under laminar
flow conditions) and rise naturally to a vent where they are
automatically released. These types of unit are limited by
the static pressure in the system and are not suited to
medium to high rise installations. Units should only be
installed at low static pressure points as advised by the
manufacturer.
Pressure based de-aeration
These units, sometimes referred to as ‘vacuum degassers’,
can be installed at any point in the system, and are more
effective than temperature based units in that they are able
to remove dissolved gases from the water. This is achieved
by generating a temporary vacuum around the liquid.
Water is extracted from the system into a cylinder, degassed
and then re-introduced to the system. The degassed water
is then circulated around the system where it is able to
dissolve any additional pockets of trapped air which can
then also be removed. Although the volume of the cylinder
is small relative to the size of the system, repetition of this
process means that over a period of time, all of the water in
the system is de-gassed.
1.A1.8.3.2
System cleaning
System flushing and chemical cleaning is an essential
precursor to the commissioning of most large scale
pipework systems. Furthermore, once the system is in
operation, there will be an ongoing requirement to dose the
system with chemicals to prevent corrosion or biofouling.
Existing and newly installed pipework will inevitably
contain various types of debris and contaminants. These
can be classified under the following headings.
Installation debris
Extraneous materials that commonly find their way into
systems during installation include millscale, welding slag,
metal swarf, cutting oil, soldering flux, jointing compounds
and grease. Furthermore, in larger pipes there is the
potential for larger objects such as tin cans or plastic bags to
inadvertently enter the system.
Corrosion products
Corrosion in steel pipework may result in increased levels
of suspended solids due to the formation of insoluble iron
compounds. Furthermore the settlement of solids in low
velocity areas of the system may give rise to localized
‘under-deposit’ corrosion or provide a hiding place for
bacteria. Microbiological induced corrosion is usually
caused by sulphate reducing bacteria. These bacteria
metabolise naturally occurring sulphate in the water to
produce sulphuric acid under clumps of bacteria resulting
in localised pitting corrosion.
Biological fouling
All natural sources of water (including tap water) contain
many different types of bacteria, some of which may
multiply and lead to problems within a pipework system if
they encounter suitable conditions for growth. Systems left
filled and untreated or which are filled and subsequently
drained, can quickly develop a biofilm layer on pipe
surfaces. The biofilm (a mixture of live and dead bacteria
and their excretions) helps the bacteria to resist the action
of biocides and seeds bacteria back into the system water. It
can also create the starting environment for the
microbiological induced corrosion previously mentioned.
Pseudomonas bacteria, in particular, have been linked with
particularly severe cases of biological fouling.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
water. Hence, whenever there is a draw-off of hot water,
return water temperatures are maintained at low values.
When there is no draw off of hot water, the self-acting
control valve inside the unit throttles the heating flow to a
minimum thereby causing the pump to reduce its speed
and hence power consumption.
Design considerations
typically have mesh sizes of 0.8 mm upwards and are
unlikely to collect this material.
Options to trap and remove fine corrosion debris include
filtration or separation solutions such as:
——
Side stream filtration whereby a proportion of the
main flow is side streamed through a filter unit
removing material before being circulated back
into the system. Because the unit is installed in
parallel with the main flow, usually with its own
pump, it can achieve a high level of filtration
without incurring an additional pressure loss across
the system pumps.
——
In-line dirt separators that combine low velocity
conditions with a specially designed packing that
encourages the settlement of fine dirt particles.
Collected particles can then be released via a
manually operated drain cock. Such units have
relatively low resistance meaning that they can be
installed in the main flow without adding
significantly to system pressure losses.
——
Magnetic separators that exert a magnetic attraction
over particles of iron oxide i.e. magnetite. Magnetite
is a black oxide which appears as small insoluble
particles which, if left, can settle out in low velocity
areas such as the bases of radiators forming a thick
sludge. Magnetite is more likely to form in closed
heating or cooling systems than rust which requires
higher oxygen levels. As inferred by its name,
magnetite is magnetic and can therefore be
separated from the flow by exposure to a strong
magnetic unit.
To enable an effective clean, in accordance with the BSRIA
guidance, it is essential that appropriate facilities and
features are incorporated in the installed pipework system.
These should be identified and planned at the design stage.
The main features recommended in the BSRIA guide are as
follows.
Flushing by-passes across central plant
By-passes across central plant items should be provided to
permit main pipework to be flushed and chemically cleaned
without having to circulate the dirty water and chemicals
through central plant, i.e. boilers or chillers.
Flushing drains on central plant connections
Suitably sized flushing drains should be provided on flow
and return connections to central plant items – to enable
these items to be flushed through.
Flushing by-passes across terminal units
By-passes across terminal unit connections should be
provided to permit pipework to be flushed and chemically
cleaned without having to circulate the dirty water and
chemicals through terminal units, i.e. fan coil units, chilled
beams or air handling units.
Flushing drains on terminal unit connections
1.A1.8.3.4
Line size drains should be provided on flow and return
connections to central plant items and terminal units to
enable these items to be flushed through.
Commissioning is defined as the advancement of an
installation from the state of static completion to full
working order to specified requirements. For hydronic
systems, it includes the setting to work of system pumps
and the regulation of flow rates. The regulation of flow rates
is particularly important; if flow rates are not properly
distributed throughout the system, the required amounts
of heating or cooling may not be delivered and the building
will not meet the comfort requirements of the occupants.
Alternatively, excess energy consumption may be incurred
due to excessive flow rates or inadequate control of flows.
Strainers
Strainers should be provided in pipework connections to
central plant items to provide ongoing protection. In
addition, strainers should be provided in front of all pumps
so that, if used for flushing purposes, there is no risk that
debris particles could enter the pumps and cause damage.
Strainers should also be provided on main branches (e.g.
off riser connections) to trap debris that might enter the
terminal unit connections.
1.A1.8.3.3
Maintenance of system cleanliness
It is essential that cleaned systems are properly maintained
by regular dosing with water treatment chemicals.
Corrosion inhibitors and biocide chemicals in particular
must be maintained at appropriate levels to prevent
deterioration of system water quality.
However, it is inevitable that over a period of time, and
despite proper treatment with corrosion inhibitors, there
will be some ongoing corrosion of steel components that
will lead to an increase in the amount of solid material in
the system. This material may in the form of fine particles
that are carried in the fluid stream, possibly settling out at
low velocity regions. Standard in-line system strainers
Commissioning
All re-circulating pipework systems should be
commissioned in accordance with the requirements of
CIBSE Code W: Water distribution systems (CIBSE, 2003) and
BSRIA Guide BG2/2010: Commissioning water systems
(Parsloe, 2011).
The emphasis of both of these documents is on building
heating and cooling systems although the recommendations
may also be applied to other types of water distribution
systems. The guidance is equally applicable to new-build
and retrofit applications and is independent of the scale of
the system.
Code W sets out the general requirements for balancing
and commissioning water distribution systems to meet the
requirements of the designer. BSRIA Guide BG2/2010:
Commissioning water systems provides a more detailed
description of the practical aspects of commissioning
procedures in a step-by-step format.
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Dynamic flushing and chemical cleaning procedures are
described in BSRIA Application Guide BG29/2011: Precommission cleaning of pipework systems (Brown, Parsloe,
2011). All newly installed pipework systems should be
flushed and cleaned in accordance with this guidance by a
suitably experienced pre-commission cleaning specialist.
1-163
1-164
Hydronic system design
References
The main commissioning activity for most hydronic
systems will be the regulation of flow rates to achieve the
designer’s specified flow rate values. Depending on the
layout of the system, and the valve types selected, flow
regulation will involve an exercise of either proportional
balancing or flow setting as described in the following
sections.
BSI (2003) BS EN 12828: 2003: Heating systems in buildings. Design for
water-based heating systems (London: BSI)
Proportional balancing
Proportional balancing is the process of bringing the fluid
flow rates throughout a distribution system into balance
with one another, in their correct proportions and within
tolerances specified by the designer.
The procedure is applicable to sub-branches fitted with
manually operated regulating valves such as fixed orifice
double regulating valves. In such systems, the adjustment
of each regulating valve will cause a change in flow rates
through all other branches, and therefore, the balancing
process must follow a prescribed procedure. The balancing
procedure must always start at system extremities and work
its way back towards the pump. Furthermore, for each
group of sub-branches to be balanced, the end (i.e. most
remote) sub-branch must be made the least favoured at the
outset by throttling its regulating valve if necessary.
For proportional balancing to be successful, regulating flow
measurement devices must be fitted in each branch and
sub-branch throughout the system.
Flow setting
Flow setting is a more appropriate term for achieving the
correct balance of flow rates in systems fitted with selfacting valves such as constant flow regulators, differential
pressure control valves and pressure independent control
valves. For each of these valve types, flow rates can be set
adjusting the valve whilst verifying flow rate at a separate
flow measurement device. Because the valves are self-acting
they will respond to any changes in pump or system
pressure and automatically adjust themselves so that the set
flow rate is maintained. Hence, there is no necessity for a
prescribed balancing procedure such as that for manually
operated valves.
Because the valves are self-acting, there is no need for
multiple valves to be installed on all system branches and
sub-branches. Suitably sized valves located at terminal unit
sub-branches (or branches feeding to groups of terminals)
are sufficient.
A detailed description of proportional balancing and flow
setting is provided in CIBSE Code W Water distribution
systems and BSRIA Guide BG2/2010: Commissioning water
systems.
BSI (1989) BS 7074: Application, selection and installation of expansion vessels
and ancillary equipment for sealed water system; Part 1: 1989: Code of practice
for domestic heating and hot water supply; Part 2: 1989: Code of practice for low
and medium temperature hot water heating systems; Part 3: 1989: Code of
practice for chilled and condenser systems (London: BSI)
BSI (2004) BS EN 10255:2004: Non-alloy steel tubes suitable for welding and
threading. Technical delivery conditions (London: BSI)
BSI (2006) BS EN 1057:2006+A1:2010: Copper and copper alloys. Seamless,
round copper tubes for water and gas in sanitary and heating applications
(London: BSI)
BSI (2008) BS EN ISO 12241:2008: Thermal insulation for building equipment
and industrial Installations. Calculation rules (London: BSI)
BSI (2014) BS EN 60034-30-1:2014: Rotating electrical machines. Efficiency
classes of line operated AC motors (IE code) (IE-code) (London: BSI)
Brown R (ed.) (2013) BSRIA BG50: Water Treatment for Closed Heating and
Cooling Systems (Bracknell: Building Services Research and Information
Association)
Brown R, Parsloe C J (2011) BSRIA BG29: Pre-Commission Cleaning of
Pipework Systems (Bracknell: Building Services Research and Information
Association)
CIBSE (1989) CIBSE Guide H: Building control systems (London: Chartered
Institution of Building Services Engineers)
CIBSE (2001) CIBSE Guide C: Reference data (London: Chartered
Institution of Building Services Engineers)
CIBSE (2003) CIBSE Commissioning Code W: Water distribution systems
(London: Chartered Institution of Building Services Engineers)
CIBSE (2006) CIBSE Knowledge Series KS7: Variable flow pipework systems
(London: Chartered Institution of Building Services Engineers)
CIBSE (2010) CIBSE Applications Manual AM14: Non-domestic hot water
systems (London: Chartered Institution of Building Services Engineers)
Department for Communities and Local Government (2013) Non-domestic
building services compliance guide (London: NBS)
Heating and Ventilating Contractors Association (2003): TR20: Installation
and testing of pipework systems (London: HVCA) [HCVA is now the
Building Engineering Services Association]
Parsloe C J (2010) BSRIA BG2: Commissioning water systems (Bracknell:
Building Services Research and Information Association)
Parsloe C J (2011) BSRIA BG12: Energy Efficient Pumping Systems: A
Design Guide (Bracknell: Building Services Research and Information
Association)
Thermal Insulation Manufacturers and Suppliers Association (TIMSA)
(2008) HVAC guidance for achieving compliance with Part L of the Building
Regulations (Farnham: TIMSA)
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
It is a requirement of Code W that water distribution
systems should be inherently commissionable, i.e. designed,
installed and prepared to specified requirements in such a
manner as to enable commissioning to be carried out. This
is most likely to be achieved if the commissioning
requirement is in the brief from the outset, and specialist
commissioning input sought early in the design process.
Index
Index
absorption chillers 1-25
acid corrosion 1-84
acid dew-point 1-84
acid smuts 1-81
active beams 1-30
control of 1-105
heat transfer rate 1-103
ventilated 1-116
air change rates 1-14 to 1-15, 1-34
air conditioning 1-114 to 1-116
air infiltration
design rates 1-14 to 1-15
heat losses 1-22, 1-32, 1-34, 1-35, 1-39
highly insulated buildings 1-22
see also airtightness
air permeability 1-14 to 1-15, 1-17, 1-34
Air Quality Management Areas (AQMA) 1-69
air source heat pumps (ASHP) 1-27, 1-72
air systems see warm air systems
air vents 1-108, 1-108, 1-162
airflow velocity 1-11, 1-12
see also variable air volume (VAV)
airtightness 1-7, 1-14, 1-16, 1-22
see also air infiltration
ammonia refrigerant 1-71
animal houses 1-6, 1-13, 1-14
Approved Documents 1-15
AQMA (Air Quality Management Areas) 1-69
ash extraction and disposal 1-56
atmospheric burners 1-63
atomising burners 1-63
Australia, regulations 1-8
back-end corrosion 1-85
bacterial growth prevention 1-122, 1-125,
1-162 to 1-163
balanced draught systems 1-83
balanced/room sealed flues 1-83
biogas 1-23, 1-42, 1-75, 1-76
biological fouling 1-162 to 1-163
biomass 1-23 to 1-24
carbon emission factors 1-25
combined heat and power (CHP) 1-75
combustion 1-68
fuel specification 1-58
handling and storage 1-58 to 1-59
biomass boilers 1-17, 1-64 to 1-65
combined with conventional heat sources
1-97, 1-100 to 1-101
pollution control 1-69
regulations 1-69
bio-methane see biogas
bio-oils 1-23, 1-75, 1-76
‘blue flame’ technology 1-64
boiler horse power 1-66
boilers 1-60 to 1-67
combining different heat sources 1-97
corrosion prevention 1-85, 1-92
hydronic systems 1-26
operating temperatures 1-85
seasonal efficiency 1-17
selection of 1-60 to 1-61
shut-down 1-84
types 1-61
see also steam boilers
brazed joints 1-146
BREEAM 1-19, 1-20, 1-74
brick flues/chimneys 1-84
buffer vessels 1-95, 1-99, 1-100
building design considerations 1-7
effect on radiator performance 1-101
fabric energy efficiency standard (FEES)
1-18, 1-19
fabric thermal standards 1-17, 1-19
heating to protect fabric 1-6
thermal inertia 1-13
building emissions rate (BER) 1-16, 1-17
building log books 1-16, 1-18, 1-134
Building Regulations
Approved Documents 1-15, 1-15
future requirements 1-10
Part F 1-5
Part G 1-122, 1-123, 1-124, 1-125
Part J 1-5, 1-68 to 1-69
Part L 1-5, 1-15 to 1-18, 1-16, 1-21, 1-25,
1-34, 1-37, 1-77, 1-114, 1-123, 1-127,
1-134, 1-151
butane 1-41 to 1-42
gas pressure 1-48, 1-48
physical properties 1-41
refrigerant 1-71
butt welded joints 1-144
by-passes 1-161
boiler corrosion prevention 1-85
compensated flow temperature control 1-89
flushing 1-89, 1-163
cabinet heaters 1-115, 1-117
calorific values 1-41, 1-41, 1-42, 1-42
Canada, regulations 1-8
carbon dioxide see CO2
carbon emission factors 1-25, 1-25 to 1-26
community heating 1-131
gaseous fuels 1-41
grid electricity 1-25, 1-44
carbon intensity factors 1-9
carbon monoxide (CO) 1-67
cast iron sectional boilers 1-61
CE marking 1-62
ceiling height
for active beams 1-103
heating choice 1-22, 1-29
centralised systems 1-21, 1-32, 1-124 to 1-125,
1-126
centrifugal pumps 1-148
chain grate stokers 1-64
chimneys 1-78 to 1-84
construction 1-83 to 1-84
heat loss 1-81 to 1-82
insulation 1-78, 1-81, 1-83 to 1-84
linings 1-83, 1-84
minimum height 1-79
see also flues
China, regulations 1-8
CHP see combined heat and power (CHP)
CIBSE publications 1-4
classification of heating systems 1-21
Clean Air Act 1993 1-5, 1-69, 1-79, 1-83
clean rooms 1-14
cleanliness 1-147, 1-163
Climate Change Levy 1-10, 1-77
climate change scenarios 1-10
CO (carbon monoxide) 1-67
CO2 emissions 1-1
benchmarks 1-19, 1-20
combined heat and power (CHP) 1-77
energy sources 1-23
fuel choice 1-40
heat pumps 1-27, 1-74
Part L2A requirements 1-16
rate for regulated services 1-16
CO2 refrigerant 1-71
CO2 transcritical cycle 1-71
coal 1-43, 1-55 to 1-56
coefficient of performance (CoP) 1-17, 1-27,
1-70, 1-71
coil type steam generators 1-65 to 1-66
coking stokers 1-64
combined heat and power (CHP) 1-26 to 1-27,
1-74 to 1-78
applications to buildings 1-77
CO2 emissions 1-77
combined with conventional heat sources
1-97, 1-98, 1-99 to 1-100
domestic hot water 1-32
efficiency 1-75, 1-76 to 1-77
operating temperatures 1-75, 1-87
Quality Assurance scheme (CHPQA) 1-77
with thermal storage 1-99 to 1-100
types 1-74 to 1-75, 1-75
combustion of fuels 1-67 to 1-69
combustion products 1-67 to 1-68
combustion side corrosion 1-85
commissioning 1-133, 1-163 to 1-164
communal heating 1-24, 1-131 to 1-133
see also heat networks
compensated flow temperature control 1-89,
1-105, 1-160
compressed natural gas (CNG) 1-41
compression ignition (CI) engine CHP 1-76
compression joints 1-146
concrete chimneys 1-84
condensate drainage
corrosion prevention 1-84 to 1-85
steam systems 1-106, 1-107, 1-110 to 1-111
condensate pumping 1-111
condensate return mains 1-111
condensation control, heating for 1-6, 1-13
condensing boilers 1-61, 1-67
combined with different heat sources 1-97
corrosion prevention 1-85
efficiency/load characteristic 1-90
high and low temperature return 1-95
operating temperatures 1-87
condensing gas-fired cabinet heaters 1-117
constant flow regulators 1-159, 1-161
constant temperature systems 1-160
Construction (Design and Management)
Regulations 1-5, 1-133 to 1-134
control methods/systems
compensated flow temperature control
1-89, 1-105, 1-160
flow water temperatures 1-89, 1-159 to
1-160
heat emitters 1-88, 1-105, 1-115 to 1-116,
1-119
heat generators 1-62, 1-89
hydronic systems 1-88 to 1-89, 1-91 to 1-92,
1-95
multiple heat generators 1-90 to 1-91
two- and three-port control 1-88, 1-92,
1-112
Part L2 requirements 1-16, 1-18
pumps 1-89, 1-151
steam systems 1-111
underfloor heating 1-105
unitary heating systems 1-119 to 1-120
warm air systems 1-116
Control of Substances Hazardous to Health
1-5
control valves 1-89, 1-158 to 1-159
hydronic systems 1-88, 1-89
steam systems 1-108, 1-112
convective systems
characteristics 1-119
electric 1-117
and heat loss from room 1-28 to 1-29
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
Note: page numbers in italics refer to
figures; page numbers in bold refer to
tables
1-165
1-166
dampers, flues 1-82
de-aeration 1-162
decentralised systems 1-21, 1-32
design criteria 1-10 to 1-15, 1-20 to 1-32
design margins 1-38
design outdoor temperatures 1-6, 1-13
design process 1-2
design room temperature see operative
temperature
design strategy 1-1 to 1-10
Design Summer Years (DSYs) 1-10
design uncertainties 1-38 to 1-39, 1-39
de-stratification systems 1-116
differential pressure control valves (DPCVs)
1-89, 1-159, 1-161
diffusers 1-114
direct electric heaters 1-117 to 1-118
direct gas-fired heaters 1-113, 1-117
dirt separators 1-163
distribution losses see heat distribution losses
district heating (DH) 1-59 to 1-60, 1-97, 1-131
to 1-133
see also heat networks
domestic hot water systems 1-6, 1-120 to 1-131
centralised 1-21, 1-32, 1-124 to 1-125
choice of system 1-31 to 1-32, 1-125, 1-127,
1-128
classification 1-120, 1-120
decentralised 1-21
distribution pipework 1-125
energy consumption 1-123, 1-127 to 1-128,
1-128
flow water temperatures 1-87
heat generators 1-17
heating load 1-37
instantaneous 1-129
maximum temperature 1-122
point-of-use 1-31, 1-32, 1-123 to 1-124
pumped secondary return 1-125
regulations and codes 1-121, 1-122 to 1-123
sizing 1-129 to 1-130
solar thermal 1-27, 1-78, 1-127, 1-128
storage systems 1-32, 1-124 to 1-125, 1-130
to 1-131
storage temperatures 1-87
thermal expansion provision 1-122 to 1-123
unvented 1-123, 1-124
water consumption 1-123, 1-129
double regulating valves 1-158
draughts 1-12, 1-31
dual duct systems 1-115
dual fuel burners 1-62
ductwork 1-37 to 1-38, 1-114
Eco-Design of Energy Related Products
Directive 1-8
economic considerations see cost
considerations
economisers 1-66
electric air curtains 1-117
electric heat pumps 1-40
electric heating systems 1-27
electric convectors 1-117
electric heating coils 1-113
electric radiant heaters 1-118
electric radiators 1-117
storage heaters 1-118
underfloor heating 1-30, 1-118
water heaters 1-123 to 1-124
electricity 1-43 to 1-44
carbon emission factor 1-25
cost considerations 1-40
environmental considerations 1-40, 1-44
see also grid electricity
electrofusion joints 1-146
energy consumption
domestic hot water systems 1-123, 1-127 to
1-128, 1-128
pumps 1-92 to 1-93
energy efficiency
benchmarks 1-19, 1-20
boilers 1-17
combined heat and power (CHP) 1-75
domestic hot water systems 1-123, 1-127
to 1-128
financial incentives 1-9 to 1-10
heat emitters 1-28 to 1-29
minimum acceptable 1-17
Part L2 requirements 1-16, 1-18
pumps 1-93, 1-149 to 1-150, 1-150 to 1-151
refurbished buildings 1-18, 1-18
regulations 1-1, 1-5, 1-8 to 1-9, 1-16
energy management 1-134
energy metering 1-16, 1-18, 1-129
Energy Performance Certificates (EPCs) 1-1,
1-16, 1-18
Energy Performance of Buildings Directive
1-1, 1-8, 1-18
energy sources 1-39 to 1-59
availability 1-40
choice 1-23 to 1-25
cost considerations 1-40
fuel handling and storage 1-45 to 1-59
gaseous fuels 1-41 to 1-42
global warming potential (GWP) 1-40
liquid fuels 1-42 to 1-43
see also fuels
Enhanced Capital Allowance scheme 1-10,
1-77
Environment Act 1995 1-69
environmental considerations
choice of energy sources 1-23, 1-23, 1-40
electricity 1-40, 1-44
fuel combustion 1-68
heat pumps 1-73 to 1-74
see also CO2 emissions; pollution control
environmental performance assessment 1-15,
1-19 to 1-20
environmental performance targets 1-9, 1-15
Environmental Protection Act 1990 1-69
European Union regulations 1-8
exempted buildings 1-21
existing buildings 1-18, 1-19
external design conditions 1-6, 1-12 to 1-13
fabric energy efficiency standard (FEES) 1-18,
1-19
fabric heat loss 1-33 to 1-34
fabric thermal standards 1-17, 1-19
fan coil units 1-30, 1-103, 1-105, 1-115 to 1-116
fan convectors 1-30
fan dilution systems 1-82, 1-83
fatty acid methyl esters (FAME) 1-43
feed-in tariffs 1-10
feedwater equipment 1-106, 1-108
FEES (fabric energy efficiency standard) 1-18,
1-19
filtration systems 1-163
financial incentives 1-9 to 1-10
fire protection 1-46, 1-47, 1-53, 1-59
fire-tube boilers 1-65
fixed orifice double regulating valves 1-158
fixed orifice flow measurement devices 1-158
flanged joints 1-144
floor surface temperature
beneath boiler 1-61
underfloor heating 1-30, 1-103
flow regulation 1-87 to 1-88
flow setting 1-159, 1-164
flow water temperatures
active beams 1-103
combining of different types of heat
generator 1-97
comparison for heat generators 1-25, 1-60
control 1-89, 1-159 to 1-160
domestic hot water systems 1-87
heat networks 1-131
LTHW hydronic systems 1-86, 1-87
outdoor temperature compensation 1-88,
1-89, 1-105, 1-160
flue draught 1-80, 1-82
flue gases 1-67 to 1-68
see also greenhouse gas emissions; pollution
control
flues 1-78, 1-79
acid smuts 1-80
corrosion 1-80 to 1-81
draught production 1-82 to 1-83
draught stabilisers 1-82
efflux velocities 1-79 to 1-80
flow resistance 1-80, 1-82
heat loss 1-81 to 1-82
sizing 1-80
see also chimneys
fluorinated hydrocarbons (HFC) 1-71, 1-74
flushing 1-163
forced convection 1-102, 1-104
forced draught burners 1-62, 1-82
fouling 1-162 to 1-163
frost protection 1-6, 1-13, 1-27, 1-72
fuel cells 1-27, 1-76
fuel oil see petroleum fuel oils
fuels
calorific values 1-41, 1-41, 1-42, 1-42
choice 1-23 to 1-25
combustion 1-67 to 1-69
comparison 1-25
electricity generation 1-44
gaseous fuels 1-41 to 1-42
handling and storage 1-23, 1-45 to 1-59
liquid fuels 1-42 to 1-43
see also carbon emission factors; energy
sources
gas boosters 1-45, 1-62
gas coolers 1-71
gas distribution pipework 1-47 to 1-48
gas engine vapour compression heat pumps
1-71
gas fired tubular heaters 1-29
Gas Safe Register 1-49
Gas Safety (Installation and Use) Regulations
(GSIUR) 1-47, 1-49, 1-49
gas spark ignition (GSI) engine CHP 1-75
gas turbine CHP 1-76
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
heat transfer rate 1-101 to 1-102
infrequently occupied buildings 1-22
occupant comfort 1-28 to 1-29
copper pipes 1-143, 1-145, 1-147
corrosion mechanism 1-84
corrosion prevention 1-84 to 1-85
condensing boilers 1-61
flues 1-80 to 1-81
heat generators 1-92
corrosion products 1-162
cost considerations 1-1, 1-9 to 1-10
choice of energy sources 1-23
financial incentives 1-9 to 1-10
heat emitters compared 1-28
heat generators compared 1-25
heat networks 1-131, 1-132
integration of renewable/low carbon heat
generators 1-96
pipework 1-148
Index
Index
headers (hydronic systems) 1-93 to 1-95
health and safety
domestic hot water 1-6, 1-122
fuel handling and storage 1-46, 1-55 to
1-56, 1-59
surface temperatures 1-29
Health and Safety at Work Act 1-5
heat distribution losses 1-37 to 1-38
heat distribution media 1-31, 1-31
heat emitters
architectural considerations 1-28
choice of 1-28 to 1-31
convective versus radiant 1-28 to 1-29,
1-35 to 1-36
high thermal mass buildings 1-22
highly insulated buildings 1-22
rooms with high ceilings 1-22, 1-29
comparison 1-28
control methods/systems 1-88, 1-105, 1-115
to 1-116, 1-119
control valves 1-89, 1-105
heat output rate 1-28, 1-101 to 1-105
variation with flow rate 1-104
variation with system water temperature
1-104
location 1-31
noise 1-29
and room surface temperatures 1-12
sizing 1-22, 1-87
space considerations 1-28
speed of response 1-28, 1-29
steam systems 1-108
surface temperatures 1-29
see also secondary circuits
heat exchangers 1-25
heat gains
internal 1-7, 1-35
pipework 1-151 to 1-152
heat generators 1-17, 1-26 to 1-27, 1-59 to 1-67
choice of number and duties 1-39
combining different heat sources 1-97
comparison 1-25, 1-59 to 1-60, 1-60
control methods/systems 1-62, 1-89, 1-90
to 1-91
corrosion prevention 1-92
heat output rate 1-89
thermal stress 1-92
see also primary circuits
heat interface units (HIU) 1-132, 1-133, 1-161
heat losses
air infiltration 1-14 to 1-15, 1-34, 1-35
calculation 1-32 to 1-36
distribution losses 1-37 to 1-38
fabric 1-33 to 1-34
heat networks 1-131, 1-131
mechanical ventilation 1-37
pipework 1-37 to 1-38, 1-151 to 1-152
radiant versus convective heating 1-28 to
1-29
ventilation 1-34
heat metering 1-65, 1-92
heat networks 1-131 to 1-133
heat pumps 1-17, 1-27, 1-69 to 1-74
air to air arrangement 1-73
buffer vessels 1-99
carbon emissions 1-27
categories 1-73
coefficient of performance (CoP) 1-17,
1-27, 1-70, 1-71
combined with conventional heat sources
1-97, 1-97 to 1-99
cooling 1-27, 1-72
emissions 1-74
environmental considerations 1-73 to 1-74
heat sources 1-72
operating temperatures 1-70, 1-71, 1-87,
1-99
refrigerants 1-71, 1-74
reversible 1-70
sizing 1-98
stand-alone 1-118
supplementary heat sources 1-98 to 1-99
‘through the wall’ arrangement 1-73
types 1-70 to 1-71
for warm air systems 1-113
waste heat 1-25
heat recovery systems
domestic hot water 1-129
economisers 1-66
typical effectiveness ranges 1-37
ventilation heat load calculation 1-37
for warm air systems 1-114
heat storage see thermal storage
heat transfer 1-101 to 1-102
heated floors see underfloor heating
heating load
calculation 1-32 to 1-36, 1-37, 1-38
demand measurement 1-91 to 1-92
distribution losses 1-37 to 1-38
effect of load diversity 1-38
peak generator 1-38
pre-heat 1-36
heating oils see petroleum fuel oils
HFC (fluorinated hydrocarbons) 1-71, 1-74
high temperature hot water (HTHW) 1-31,
1-86, 1-95 to 1-96
historic buildings 1-18, 1-22
HIU (heat interface units) 1-132, 1-133, 1-161
horticultural buildings 1-14
hot water coils 1-112, 1-113
hot water storage vessels 1-125, 1-161
HTHW (high temperature hot water) 1-31,
1-86, 1-95 to 1-96
humidity levels 1-12
hydrogen fuel 1-42
combined heat and power (CHP) 1-76
regulations 1-51
transportation and storage 1-48 to 1-49
hydronic systems 1-85 to 1-105
air and dirt removal 1-162 to 1-163
buffer vessels 1-95, 1-99, 1-100
by-passes 1-161
categorisation by temperature 1-86
commissioning 1-163 to 1-164
domestic hot water 1-161 to 1-162
flow rates 1-87 to 1-88
flow temperature control 1-159 to 1-160
heat output rate of heat emitters 1-101 to
1-105
heat transfer to and from pipes 1-151 to
1-152
integration of renewable/low carbon heat
generators 1-96 to 1-101
interface between primary and secondary
circuits 1-93 to 1-95
layout 1-161
LTHW systems 1-87 to 1-88
MTHW and HTHW systems 1-95 to 1-96
operating temperatures 1-21, 1-21, 1-86 to
1-87, 1-87
pipe and pump sizing 1-147 to 1-151
pipe materials and jointing methods 1-143
to 1-147
pipework movement 1-152 to 1-155
pressurisation 1-87, 1-95, 1-155 to 1-158
primary circuit 1-89 to 1-93
secondary circuits 1-89
valve types 1-158 to 1-159
variable flow systems 1-87 to 1-88, 1-160
India, regulations 1-8
indirect fired warm air heating 1-113 to 1-114
indirect gas- and oil-fired heaters 1-116 to
1-117
indoor design temperature see room air
temperature
induced draught systems 1-82 to 1-83
induced jet warm air heating 1-112, 1-114
industrial buildings 1-6, 1-12, 1-14
instantaneous hot water 1-31, 1-32, 1-123 to
1-124
centralised 1-125, 1-126
heating load 1-37
Institution of Gas Engineers and Managers
(IGEM) standards 1-49, 1-50
insulation
ductwork 1-37 to 1-38, 1-114
heat emitters for highly insulated buildings
1-22
heat emitters for poorly insulated buildings
1-22
pipework 1-37 to 1-38, 1-151
Integrated Pollution Prevention and Control
system 1-69
intermittent operation 1-7, 1-36
intermittently occupied buildings 1-21 to
1-22, 1-29
internal design conditions 1-10 to 1-11
internal heat gains 1-7, 1-35
Japan, regulations 1-8
laboratories 1-14
LACORS (Local Authority Coordinators of
Regulatory Services) 1-69
landfill gas 1-42
LEED 1-20
Legionella 1-32, 1-71, 1-122, 1-125
legislation
fuel combustion 1-68
gas industry 1-49
see also regulations
life cycle costs see whole-life cost
liquefied natural gas (LNG) 1-41
liquefied petroleum gases (LPG) 1-41 to 1-42
carbon emission factor 1-25
gas distribution pipework 1-47 to 1-48
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
gas utilisation efficiency (GUE) 1-71
gaseous fuels 1-41, 1-41 to 1-42
combustion 1-68
regulations 1-49, 1-49
see also liquefied petroleum gases (LPG);
natural gas
gas-fired absorption heat pumps 1-71, 1-72,
1-74
gas-fired boilers 1-61, 1-62 to 1-63
gas-fired heat pumps 1-27, 1-71
gas-fired radiant heaters 1-117, 1-118
gas-fired warm air heaters 1-113 to 1-114
gauge glasses 1-67
global warming potential (GWP) 1-40
gaseous fuels 1-41
refrigerants 1-74
gravity oil supply 1-54 to 1-55
greenhouse gas emissions 1-40
see also CO2 emissions; pollution control
grid electricity 1-25, 1-44
grooved end joints 1-146
gross calorific value (GCV) 1-17
ground source heat pumps (GSHP) 1-27, 1-72
GSI (gas spark ignition) engine CHP 1-75
GSIUR (Gas Safety (Installation and Use)
Regulations) 1-47, 1-49, 1-49
GUE (gas utilisation efficiency) 1-71
1-167
1-168
magnetic separators 1-163
maintenance 1-133 to 1-134
biomass boilers 1-65
cleanliness 1-163
and fuel choice 1-41
heat emitters compared 1-28
make-up water
hydronic systems 1-85
steam systems 1-108
mechanical ventilation, heat losses 1-37
medium temperature hot water (MTHW)
1-31, 1-86, 1-95 to 1-96
metering see energy metering; heat metering
methane 1-41, 1-41, 1-42
Micro Generation Certification Scheme 1-9
microbial growths see bacterial growth
prevention
Microgeneration Certification Scheme 1-27
modular boilers 1-62
modulating burners 1-64
modulating control 1-89, 1-116
MTHW (medium temperature hot water)
1-31, 1-86, 1-95 to 1-96
multilayer pipes 1-143, 1-145
multiple boilers 1-17
multiple heat generators 1-39, 1-62, 1-88
controls 1-90 to 1-91
hydronic systems 1-89 to 1-91
identical 1-90 to 1-91
mixed 1-91
multiple heating circuits 1-89
National Calculation Methodology (NCM)
1-25, 1-77, 1-127
natural convectors 1-30
control of 1-105
heat transfer rate 1-101 to 1-102, 1-104
natural draught burners 1-63
natural draught flues 1-82
natural gas 1-41
calorific value 1-41
carbon emission factor 1-25
combined heat and power (CHP) 1-75, 1-76
gas distribution pipework 1-47 to 1-48
gas pressure 1-45, 1-48, 1-48
main supply 1-45
physical properties 1-41
natural ventilation, heat loss 1-33
NCM (National Calculation Methodology)
1-25, 1-77, 1-127
‘nearly zero energy’ fabric performance 1-18
net calorific value (NCV) 1-17
noise
heat emitters 1-29
pipework 1-147
non-steady state response 1-36
NOx emissions 1-19, 1-24, 1-26, 1-44, 1-68
benchmarks 1-19
biomass boilers 1-64
combined heat and power (CHP) 1-75, 1-76
energy sources 1-23
heat pumps 1-74
nuclear power 1-44
occupancy 1-7, 1-12
occupant comfort 1-3, 1-11, 1-28 to 1-29
oil pressure jet burners 1-64
oil storage tanks 1-51 to 1-53
oil-fired boilers 1-63 to 1-64
oil-fired condensing boilers 1-61
oil-fired warm air heaters 1-113 to 1-114
open hydronic systems 1-155
operating pressures (OP) 1-21
gaseous fuels 1-41, 1-45, 1-48, 1-48, 1-50
hydronic systems 1-21, 1-87, 1-145
operating temperatures 1-60
heat networks 1-131, 1-132
hydronic systems 1-21, 1-21, 1-145, 1-155
steam systems 1-21
see also flow water temperatures; return
water temperatures
operation and maintenance manuals 1-134
operations and maintenance 1-133 to 1-134
operative temperature 1-11, 1-12, 1-14, 1-35
outdoor temperature compensation 1-88, 1-89,
1-105, 1-160
outdoor temperatures 1-6, 1-13
oversizing 1-38
particulates 1-19, 1-23, 1-64
part-load operation
boiler efficiency 1-26, 1-90, 1-91
oil-fired burners 1-63 to 1-64
Passivhaus standard 1-19, 1-19
peak heating demand 1-6, 1-38
combining different heat sources 1-97
domestic hot water 1-125
effect of load diversity 1-38, 1-129
integration of renewable/low carbon heat
generators 1-96
percentage persons dissatisfied (PPD) 1-11
perimeter heating 1-103
petroleum fuel oils 1-42, 1-42 to 1-43
calorific values 1-42
carbon emission factor 1-25
handling and storage 1-51 to 1-55, 1-57 to
1-58
physical properties 1-42
pipework 1-54 to 1-55
pollution avoidance 1-40
regulations 1-57 to 1-58
storage and outflow temperatures 1-53
pipework
air and dirt settlement 1-147
cost considerations 1-148
domestic hot water systems 1-125
expansion and movement provision 1-152
to 1-155
gas distribution 1-47
heat distribution losses 1-37 to 1-38
heat transfer to and from 1-151 to 1-152
hydronic systems 1-143 to 1-147
insulation 1-151
insulation standards 1-37 to 1-38
jointing methods 1-144, 1-145, 1-146
liquid fuels 1-53, 1-54 to 1-55
materials 1-143 to 1-144, 1-145
MTHW and HTHW systems 1-95 to 1-96
noise 1-147
pressure loss 1-148
sizing 1-87, 1-109, 1-147 to 1-148
steam systems 1-109
strainers 1-163
trace heating 1-125
plant size ratio 1-22, 1-36
plaque heaters 1-29
plastic pipes 1-143 to 1-144, 1-145, 1-146
plate heat exchangers
domestic hot water systems 1-31, 1-161 to
1-162
heating networks 1-133
steam systems 1-108, 1-109
point-of-use water heaters 1-31, 1-32, 1-123 to
1-124
pollution control 1-9, 1-68 to 1-69
biomass boilers 1-69
chimney heights 1-79
see also CO2 emissions; NOx emissions;
particulates; SOx emissions
PPD (percentage persons dissatisfied) 1-11
predicted mean vote (PMV) 1-11, 1-11
pre-heat time 1-7, 1-12, 1-21, 1-33, 1-36
calculation 1-36
heating load 1-36
highly insulated buildings 1-22
and plant size ratio 1-36
pre-heating (heat recovery) 1-25, 1-66
pre-mix burners 1-63, 1-82
press-fit joints 1-147, 1-147
pressure see operating pressures (OP)
pressure independent control valves (PICVs)
1-89, 1-159, 1-161
pressure jet burners 1-62, 1-63, 1-64, 1-68, 1-82
pressure losses
control valves 1-159
flue ducts 1-80
gas distribution pipework 1-48
hydronic pipework 1-148 to 1-149
pressure reducing sets 1-109
Pressure Systems Safety Regulations 2000
1-26, 1-46, 1-95
pressurisation 1-87, 1-95, 1-155 to 1-158
primary circuits 1-89 to 1-93
propane 1-41 to 1-42
gas pressure 1-48, 1-48
handling and storage 1-45 to 1-48
physical properties 1-41
refrigerant 1-71
proportional balancing 1-164
pulse combustion 1-63
pump energy
heat networks 1-131, 1-132
hydronic systems 1-92 to 1-93, 1-147 to
1-148
pumped ring main oil supply 1-55, 1-56
pumps
condensate 1-111
energy consumption 1-92 to 1-93
energy efficiency 1-149 to 1-150, 1-150 to
1-151
net positive suction head 1-150
output control 1-89
primary circuits 1-92
selection 1-148 to 1-151
sizing 1-87, 1-148 to 1-149
speed control 1-151
types 1-148
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
gas pressure 1-41
handling and storage 1-45 to 1-48
physical properties 1-41
regulations 1-51
liquid fuels 1-42 to 1-43, 1-68
see also bio-oils; petroleum fuel oils
listed buildings 1-18, 1-22
LNG (liquefied natural gas) 1-41
load diversity 1-38, 1-129
log books 1-16, 1-18, 1-134
low and zero carbon technology
benchmarks 1-19
electricity generation 1-44
‘nearly zero energy’ fabric performance
1-18
low carbon energy sources 1-9, 1-91
integration of heat generators 1-96 to 1-101
Part L2A requirements 1-16
low carbon steel sectional boilers 1-61
low loss headers 1-93 to 1-95
low temperature hot water (LTHW) 1-86, 1-87
to 1-88
low water content boilers 1-62, 1-65 to 1-66,
1-92
LPG see liquefied petroleum gases (LPG)
LTHW (low temperature hot water) 1-86, 1-87
to 1-88
Index
Index
variable speed 1-150 to 1-151
push-fit joints 1-147, 1-147
room noise levels 1-29
room thermal response 1-36
rotary burners 1-63
safety valves 1-66 to 1-67
SAP (Standard Assessment Procedure) 1-9,
1-25
saturation vapour pressure 1-157
SBEM (Simplified Building Energy Model)
1-127
scalding prevention 1-122
scale prevention 1-125
sealed hydronic systems 1-155, 1-157
seasonal coefficient of performance (SCoP)
1-17, 1-27, 1-70
seasonal efficiency 1-25, 1-39
seasonal performance factor (SPF) 1-70, 1-72
secondary circuits 1-89
SEI (system efficiency index) 1-70 to 1-71
separation distances
LPG storage vessels 1-47
oil storage tanks 1-53, 1-54
shell boilers 1-65
Simple Model 1-32
Simplified Building Energy Model (SBEM)
1-127
site heating 1-131 to 1-133
see also heat networks
site-related issues 1-6 to 1-7, 1-40 to 1-41
sizing
domestic hot water systems 1-129 to 1-130
heat emitters 1-22
methodology 1-32
pipework 1-147 to 1-148
pumps 1-148 to 1-149
Smoke Control Areas 1-69
socket fusion welded joints 1-146
socket welded joints 1-144, 1-146
solar energy 1-45
solar gains 1-16
solar irradiation 1-44, 1-45
solar water heating 1-27, 1-78
combined with conventional heat sources
1-97, 1-128
combined with heat pumps 1-99
domestic hot water 1-127, 1-128
flat plate versus evacuated tube collector
efficiency 1-78
soldered joints 1-146
solid fuel boilers 1-64 to 1-65
solid fuel burners 1-64
solid fuels 1-43 to 1-44
combustion 1-68
see also wood fuel
solvent welded joints 1-146
SOx emissions 1-19, 1-44, 1-68
biomass boilers 1-64
energy sources 1-23
flue corrosion 1-80, 1-84
heat pumps 1-74
spa facilities 1-6
SPF (seasonal performance factor) 1-70, 1-72
sports facilities 1-14
stainless steel chimneys 1-83
stainless steel pipes 1-143, 1-145
stand-alone heat pumps 1-118
Standard Assessment Procedure (SAP) 1-9,
1-25
steady-state heat loss 1-32 to 1-36
steam boilers 1-26, 1-65 to 1-67
steam pipework 1-109
steam pressure 1-21, 1-109
steam systems 1-105 to 1-112
condensate removal and return 1-110 to
1-111
distribution system 1-106 to 1-110
operating temperatures 1-21
regulations, guidance and standards 1-110
system design 1-106
warm up 1-111 to 1-112
working pressure 1-106
steam traps 1-106, 1-107, 1-110 to 1-111
steel chimneys 1-83 to 1-84
steel pipes 1-143, 1-145, 1-147
steel shell and fire-tube boilers 1-61 to 1-62
step control 1-91 to 1-92, 1-95
storage heaters 1-118
strainers 1-163
straw fuel 1-43
Sulphur Content of Liquid Fuels (England and
Wales) Regulations 1-68
Sulphur Content of Liquid Fuels (SCLF)
Directive 1-68, 1-79
surface temperatures
beneath boiler 1-61
and choice of heat emitter 1-12
health and safety 1-29, 1-87
of heat emitters 1-29
and occupant comfort 1-29
underfloor heating 1-30, 1-103
surface water, heat pumps heat source 1-72
suspended unit heaters 1-115, 1-116 to 1-117
swimming pools 1-6, 1-14
system efficiency index (SEI) 1-70 to 1-71
system options 1-20 to 1-32
target emissions rate (TER) 1-16, 1-17
temperature control
hydronic systems 1-81, 1-89, 1-159 to 1-160
steam systems 1-111
see also flow water temperatures
temperature sensors 1-119, 1-120
TER (target emissions rate) 1-16, 1-17
TES (thermal exchange substations) 1-132,
1-133
Test Reference Years (TRYs) 1-10
thermal admittance 1-13
thermal comfort 1-3, 1-6
benchmarks 1-20
design criteria 1-11
vertical temperature gradient 1-12
thermal exchange substations (TES) 1-132,
1-133
thermal expansion provision 1-144, 1-152 to
1-155
thermal inertia of a building 1-13
thermal mass 1-22, 1-36
thermal performance of building 1-7
future requirements 1-10
minimum standard 1-18, 1-19
Part L2 requirements 1-16, 1-18
thermal response 1-36
thermal shock 1-111
thermal storage
with biomass boilers 1-64, 1-100
with CHP 1-99
domestic hot water 1-32
heating load 1-37
renewable/low carbon heat generators 1-96
thermal stress 1-92
thermostatic mixing valves (TMVs) 1-122
thermostatic radiator valves (TRVs) 1-105
threaded joints 1-144
three-port control 1-88, 1-92
three-stage burners 1-64
TMVs (thermostatic mixing valves) 1-122
top-up boilers 1-96
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
radial type diffusers 1-114
radiant heaters
control of 1-105
electric 1-118
and heat loss from room 1-28 to 1-29
infrequently occupied buildings 1-21 to
1-22
minimum heights 1-118, 1-119
occupant comfort 1-28 to 1-29
spot and total heating 1-119
radiant heating systems
characteristics 1-118 to 1-119
controls 1-119
convective component 1-102
heat transfer rate 1-102
radiant panels 1-29 to 1-30
radiant plaque heaters 1-117, 1-118
radiant temperatures 1-12, 1-102
asymmetry 1-12, 1-29, 1-31
radiant tube heaters 1-115, 1-117, 1-118
radiators 1-29
electric 1-117
heat transfer rate 1-104
hydronic systems 1-102 to 1-103
Rankine cycle turbine CHP 1-76
refrigerants
greenhouse warming potential 1-74
heat pumps 1-71, 1-74
regulations 1-19
refurbished buildings 1-18, 1-18
regulated services 1-16
regulations 1-1, 1-5, 1-7 to 1-9
domestic hot water systems 1-121, 1-122
to 1-123
energy efficiency 1-8 to 1-9
exempted buildings 1-18
fuel combustion 1-68 to 1-69
gas industry 1-49, 1-49
petroleum fuel oils 1-57 to 1-58
refrigerants 1-19
room temperatures 1-7 to 1-8
steam systems 1-110
relative humidity 1-12
renewable electricity 1-24, 1-44
renewable energy
financial incentives 1-9 to 1-10
integration of heat generators 1-96 to 1-101
Renewable Energy Directive 1-9, 1-43, 1-44
Renewable Heat Incentive (RHI) 1-9, 1-23,
1-27, 1-65, 1-70, 1-96
return water temperatures
active beams 1-103
combined heat and power (CHP) 1-75
combining of different types of heat
generator 1-97
condensing boilers 1-61, 1-123
corrosion prevention 1-85
domestic hot water 1-161
heat networks 1-60, 1-131, 1-132 to 1-133
heat pumps 1-99
LTHW hydronic systems 1-86, 1-87
as a proxy for heat demand 1-92
see also operating temperatures
reverse flame boilers 1-61
risk assessments
frost protection 1-13
health and safety 1-134
rivers, heat pumps heat source 1-72
room air temperature 1-6, 1-12
acceptable short term fall 1-39
regulations 1-7 to 1-8
see also operative temperature
1-169
1-170
UCOME (used cooking oil methyl esters) 1-43
underfeed stokers 1-64
underfloor heating 1-30
control of 1-105
electric 1-118
heat emission characteristics 1-30
heat transfer rate 1-103 to 1-104
supplementary heating 1-105
thermal comfort 1-29, 1-30
unison control 1-90
unitary systems 1-116 to 1-120
upgrading systems 1-18
USA, regulations 1-8
used cooking oil methyl esters (UCOME) 1-43
U-values 1-17, 1-19, 1-39
valve authority 1-159
valve types 1-158 to 1-159
see also control valves
vapour compression heat pumps 1-70 to 1-71,
1-74
variable air volume (VAV) 1-115
variable flow systems 1-87 to 1-88, 1-160
variable speed pumps 1-150 to 1-151
variable temperature systems 1-160
ventilated chimneys 1-84
ventilation
heat losses 1-22, 1-32 to 1-33, 1-34, 1-37
highly insulated buildings 1-22
ventilation exhaust
air source heat pumps 1-72
heat recovery for warm air systems 1-114
ventilation systems, frost protection 1-13
vertical temperature gradient 1-12
wall-mounted natural convectors 1-30
warehouses 1-12, 1-14
warm air systems 1-21, 1-112 to 1-116
combined with air conditioning 1-114 to
1-116
controls 1-116
distribution 1-114
ductwork 1-114
heat sources 1-113 to 1-114
standards 1-114
waste fuel 1-25, 1-43
waste heat 1-24, 1-24 to 1-25
see also heat recovery systems
water consumption 1-123, 1-129
water flow rates 1-88 to 1-89, 1-90, 1-94 to 1-95
effect on heat output of heat emitters 1-104
maximum 1-147
water hammer 1-111, 1-152
water saturation pressure 1-86
water side corrosion 1-85
water source heat pumps 1-27, 1-72
water treatment
hydronic systems 1-85
steam systems 1-108
water tube boilers 1-65
weather compensated control 1-160
weather data 1-6, 1-10
welded steel boilers 1-61
whole-life cost 1-133
wind speed effect on heating load 1-6
wood fuel 1-43
carbon emission factors 1-25
combustion 1-68
combustion products 1-69
wood chips 1-23, 1-58, 1-59, 1-64
wood pellets 1-23, 1-58 to 1-59, 1-64
zero energy see ‘nearly zero energy’ fabric
performance
zoned systems 1-7, 1-89
This publication is supplied by CIBSE for the sole use of the person making the download. The content remains the copyright property of CIBSE.
trace heating of pipework 1-125
TRVs (thermostatic radiator valves) 1-105
TRYs (Test Reference Years) 1-10
turn-down 1-89, 1-91
two-pipe flow systems 1-88
two-port control 1-88, 1-92, 1-112
two-stage burners 1-64
Index