Ductwork
CIBSE Guide B3
CIBSE
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without the prior permission of the Institution.
0 September 2002 The Chartered Institution of Building
Services Engineers London
Registered charity number 2781 04
ISBN 1 903287 20 0
This document is based on the best knowledge available a t
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
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Engineers, the authors or others involved in its publication.
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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.
Foreword
A major part of a mechanical ventilation or air conditioning system is the ductwork that
transports the air from outside via the air handling equipment to the space where it is
needed, and then exhausts the air to outside. Recent innovation in ventilation systems (see
CIBSE Guide B2: Ventilation and air conditioning) is encouraging low velocity systems,
larger ducts and lower noise. There is also increased attention to health and safety, for
example relating to hygiene and fire safety issues. Ductwork design is therefore a
fundamental aspect of a successful mechanical ventilation or air conditioning system.
Ductwork should be designed so that the air is transferred in the most efficient way in
relation to energy use. This means that the pressure drop along the ductwork should be
minimised to reduce fan power. Building Regulations Approved Document L2 specifies
maximum specific fan power for new and existing buildings. The ductwork design and the
velocities for a given volume flow rate will contribute to the overall pressure drop of the air
system. Where possible bends should be avoided. Also in relation to energy efficiency,
ducts should be appropriately insulated to minimise heat loss or gain. It may be advisable
to minimise duct lengths by locating plant rooms as close as possible to the areas serviced.
There is increasing concern over air quality in buildings and its effect on occupant health,
so the air supplied to the space should not be contaminated by unclean ductwork. Build-up
of dirt in the ductwork will affect the airflow performance of the ductwork and may also be
a fire hazard. Ductwork should be regularly cleaned, and therefore access is needed. It is
important to avoid leakage to and from ducts in order to maintain good air quality and air
flow efficiency. Commissioning of ductwork is therefore also very important, especially in
large systems or sensitive environments. To enable proper commissioning and maintenance, test points are needed and again access to these is required.
Ductwork takes up space, normally both vertically and horizontally. This will have a major
impact on the building design. Vertical distribution needs to be integrated, often in
building cores. Horizontal distribution will affect floor-to-floor heights. Ducts have to be
integrated with the structure and other service elements. They may even be cast in-situ into
floor slabs. There may be a conflict in the need for larger ducts to reduce pressure drops and
smaller ducts for efficient use of space. Sometimes ducts are exposed and will affect the
aesthetic and character of the space, either intentionally or not. The architect Louis Kahn
once remarked:
“I do not like ducts, I do not like pipes. I hate them really thoroughly, but
because I hate them so thoroughly, I feel that they have to be given their
place. If I just hated them and took no care I think that they would invade
the building and completely destroy it.” (Louis Kahn - quoted in World
Architecture I, London 1964)
This Guide will provide designers with the information to ensure an efficient, effective and
safe ductwork system.
Professor Phi1 Jones
Chairman, CIBSE Guide B3 Steering Committee
Guide 63 Steering Committee
Vic Crisp (Carbon Trust) (Chairman)
Laurence Aston (AMECK)
Hywel Davies (Consultant)
Tim Dwyer (South Bank University)
Peter Grigg (BRE Environment)
Barry Hutt (Consultant)
Steve Irving (FaberMaunsell)
Alan C Watson (CIBSE) (Secretary)
Guide 63 Task Group
Professor Phillip Jones (Chairman) (Cardiff University)
Robert Kingsbury (EMCOR Drake & Scull)
Peter Koch (Coventry University)
Stephen Loyd (Building Services Research and Information Association)
Principal author
John Armstrong
Contributors
Steve Irving (Faber Maunsell)
Professor Phillip Jones (Cardiff University)
Robert Kingsbury (EMCOR Drake & Scull)
Peter Koch (Coventry University)
Stephen Loyd (Building Services Research and Information Association)
Jim Murray (Senior Hargreaves)
Editor
Ken Butcher
CIBSE Project Manager
Alan C Watson
CIBSE Publishing Manager
Jacqueline Balian
Contents
1
2
introduction
1-1
1.1
1-1
General
1.2
Symbols, definitions and abbreviations
References
1-1
Strategic design issues
2-1
2.1
2-1
Introduction
2.2
Classification of ductwork systems
Ductwork sections
2.3
Layout
2.4
Spatial requirements
2.5
2.6
Aesthetics
Approximate
sizing
2.7
Interaction with structure/building form
2.8
2.9
Zoning
2.10
Ductwork testing and air leakage limits
2.11
Fan power energy requirements
2.12
Environmental issues
2.13
Fire issues
2.14
Weight of ductwork
2.15
Testing and commissioning
2.16
Cleaning
2.17
Controlling costs
References
3
2-1
2-2
2-2
2-3
2-3
2-3
2-4
2-4
2-4
2-5
2-6
2-6
2-6
2-6
2-6
2-7
2-7
Design criteria
3-1
3.1
3-1
Introduction
3.2
Duct air velocities
3.3
Legislation
3.4
Health and safety
3.5
Airflow in ducts
3.6
Heat gains or losses
3.7
Condensation and vapour barriers
3.8
Air leakage
3.9
Air leakage testing
3.10
Access for inspection, maintenance and cleaning
Noise from ductwork and HVAC plant
3.11
3.12
Fire issues
3.13
Supports and fixings
3.14
Overseas work
References
4
1-3
3-1
3-1
3-2
3-2
3-5
3-7
3-9
3-10
3-1 1
3-1 3
3-17
3-20
3-22
3-23
System Selection
4-1
4.1
4-1
4.2
4.3
4.4
4.5
Introduction
Duct sizing criteria
Principles of design
Ductwork sizing process
Computer-based sizing methods
4-1
4-2
4-6
4-6
~
I
?
4.6
Ductwork connections
4.7
Flow regulation
4.8
Passive stack ventilation
References
5
Ductwork materials and fittings
5.1
Ductwork materials
4-7
4-1 0
4-1 0
4-1 1
Weights and thicknesses of ductwork materials
5.2
5.3
Fittings, dampers and ancillaries
5.4
Protective coverings
5.5
Connections to building openings
5.6
Sensors
References
5-1
5-1
5-2
5-2
5-2
5-3
5-3
5-3
6
Testing and commissioning
6.1
Introduction
Design provisions to facilitate commissioning
6.2
6.3
Test holes
References
6-1
6-1
6-1
6-3
6-5
7
Maintenance and cleaning
7.1
Introduction
7.2
Legislation
7.3
Maintenance
7.4
Design for cleaning
7.5
Air quality and health issues
7.6
New ductwork construction
7.7
Installation
7.8
Existing ductwork
7.9
Dust deposition
7.10 Moisture
7.11 Inspection
7.12 Cleaning methods
References
7-1
7-1
7-1
7-1
7-1
7-2
7-2
7-3
7-3
7-3
7 -3
7-3
7-4
7-4
Bibliography
B-I
Appendix A I : Recommended sizes for ductwork
A I -1
Appendix A2: Space allowances
A2-1
Appendix A3: Maximum permissible air leakage rates
A3-1
Appendix A4: Summary o f fan types and efficiencies
A4- 1
Appendix A5: Methods of fire protection
A5-1
Appendix A6: Ductwork sizing and associated calculations - worked
example
A6- 1
Index
I1
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