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PUBLICATION DATE: Q1 2009
REF NUMBER: HVAC SWG II 004
HVAC SPECIAL WORKING GROUP
HVAC System Optimisation
Methodology
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Table of Contents
1 Glossary
2
2 Introduction
3
3 Energy service requirements
5
3.1 Setpoints
3.2 Fresh air requirements
3.3 Air change requirements
5
6
6
4 System design
7
4.1 Effective system zoning
4.2 Single zone-requirements driving a multi-zone system
4.3 Waste-heat recovery
5 Controls
7
7
7
8
5.1 Setpoints
5.2 Deadband
5.3 Software interlock
5.4 Modulation of mixing sections
5.5 Control loop tuning
5.6 Enthalpy control
5.7 Time schedules
5.8 Occupancy-based control
8
8
8
9
9
9
10
10
6 Effective HVAC System Operation
11
6.1 Training and awareness
6.2 Monitoring & targeting (M&T)
6.3 Energy management systems (EnMS)
11
11
12
7 Effective HVAC System Maintenance
13
7.1 Simultaneous heating and cooling
7.2 Filter replacements
7.3 Insulation
7.4 Calibration of instrumentation
7.5 Duct leakage
13
13
14
14
14
8 Effective validation
15
9 Ancillary Equipment
16
9.1 Chiller
9.2 LTHW
9.3 Heat gains
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16
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1
1 Glossary
ACPH
AHU
BS
CIBSE
CHW
EED
EnMS
EPI
HVAC
LTHW
M&T
PIR
RH
ROI
SEU
VSD
Air Changes Per Hour
Air Handling Unit
British Standard
Charter Institte of Building Services Engineers
Chilled Water
Energy Efficient Design
Energy Management System
Energy Performance Indicators
Heating Ventilation and Air Conditioning
Low Temperature Hot Water
Monitoring and Targeting
Passive Infrared
Relative Humidity
Return On Investment
Significant Energy User
Variable Speed Drive
2
2 Introduction
This guide to Heating Ventilation and Air Conditioning (HVAC) methodology has been developed as
part of SEI’s HVAC Working Group Spin II activities. The auditing principles used are based on the
IS393 Energy Management system standard.
Most HVAC systems present strong opportunities for energy reduction due to the energy-intensive
nature of thermally treating air. This is compounded by the fact that the energy service requirement
that HVAC systems are designed to meet is seldom interrogated to determine if it is a perceived
rather than a real requirement. Often a lack of understanding of the real GMP requirements can result
in this scenario.
This in turn has an adverse affect on the control algorithms put in place to govern the system
effectively and the components selected within the system. All of this, coupled with poor
maintenance of the installed systems, results in HVAC being one of the most inefficient energy
consumers on most sites.
This methodology document aims to clarify how to:
•
determine real energy service requirements
•
optimise control strategies to meet them
• ensure that HVAC systems are adequately monitored and maintained
This guide illustrates the opportunities identified during the HVAC SPIN I & II working-group activities,
then discusses each group of results to ensure understanding, and finally puts forward an action plan
for improvement based on these identified areas.
The Venn diagram (fig. 1) illustrates the various ‘layers’ that contribute to HVAC system energy
consumption. The core of the diagram is the energy service; that being the end use requirement for
HVAC. During the design of a new HVAC system, the search for energy saving ‘ideas’ should begin
with the energy service requirement and work outwards. For systems already in operation this
philosophy should work in reverse, with remediating operational and maintenance issues resolved
first before working into the centre of the diagram and challenging the energy service requirement.
Figure 1: EED Venn Diagram
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4
3 Energy service requirements
The energy service requirements should be clearly defined at the concept stage of a project. They
should be based on minimum acceptable parameters. The basis for selecting these parameters
should be outlined and documented for each system. This exercise is imperative at the concept stage
to ensure that all requirements are taken into consideration to provide the optimum energy-efficient
solution in a given situation.
3.1 Setpoints
When auditing an existing system it is important to determine the parameters that drive the energy
consumption of the system and to ascertain the impact each has on the energy service requirement.
The parameters should then be challenged and the reasoning behind their requirement questioned.
The critical requirements that should be examined include:
•
Temperature
•
Humidity
•
Fresh air percentage
•
Air change per hour
•
Air quality
3.1.1 Temperature requirements
The temperature requirements of an area have a large impact on the quantity of heating or cooling
energy required by a HVAC system. The temperature of an area can be product-critical as some raw
materials or processes can be affected if temperatures are too high or too low. However, the principal
reason for specifying temperature levels is to maintain an environment that is comfortable for
personnel. Typical area temperatures are in the range of 18-24°C depending on the activity taking
place, the garments being worn by the occupants and the time of year.
It is important to determine the correct temperature requirement for a given zone and to have the
optimum control strategy in operation to achieve the desired conditions in the most energy-efficient
manner. Control considerations are discussed in detail in Section 0.
3.1.2 Humidity requirements
Humidity control should only be in operation in an area if it is a critical requirement for process or
product quality. Areas with relative humidity (RH) requirements are typically maintained at levels in
the range of 30-60% RH. However, some areas require even closer RH control, which can result in a
range of 40-50% RH. In extreme cases a fixed RH may be required.
It is important to determine the correct humidity requirement for a zone and to have the optimum
control strategy in operation to achieve – as with temperature – the desired conditions in the most
energy-efficient manner.
If humidity control is in operation to maintain comfortable conditions for occupants, the requirement
should be challenged. Critical areas with legitimate humidity requirements should be served by
separate, specifically designed units.
There should not be a mixture of energy service requirements whereby a larger volume of air is
conditioned for humidity purposes than is required. For example, a single area in a multi-zone system
requires air at 40-50% RH but all the other areas served by the unit do not have a humidity
requirement. The RH-dependent zone hence drives the energy service requirement of the HVAC unit,
resulting in RH-controlled air being supplied unnecessarily to areas with no requirement for it at
substantial HVAC-related costs. Therefore the energy service requirement must be determined by
zone and activity. A system where multiple areas are served by a common AHU with varying zone
energy service requirements will by its nature need a larger than necessary energy consumption. This
is because all of the supply air is being conditioned to the requirements of the most stringent zone on
the system.
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3.2 Fresh air requirements
A minimum quantity of fresh air is required to produce a healthy working environment. Insufficient
fresh air could lead to lethargy, headaches, dry or itchy skin and eye irritation among occupants.
Table A1.5 of the CIBSE Concise Handbook outlines the recommended fresh-air supply rate in litres
per second per person. For most area types and activities it is recommended to maintain a minimum
fresh air supply rate of 10 litres per second per person.
When modulating dampers are in operation on the mixing section of an air-handling unit (AHU), the
quantity of fresh air entering the unit will change depending on the condition of the return air and
the fresh air. The dampers will alter the mixing ratio in order to achieve the optimum condition of air
exiting the mixing section in order to minimise the heating or cooling load on the unit. This is
discussed in more detail in Section 0.
3.3 Air change requirements
Air Change Rates are the single most significant parameter that will affect the energy intensity of a
HVAC system. Great care and attention must be taken when agreeing the requirements of an area
when prescribing these rates. The volume of large pieces of equipment should be removed from
stated room volume when calculating air changes per hour (ACPH).
Some common approaches to determining air changes are that ‘Area A’ has 20 ACPH and therefore
20 ACPH is required in ‘Area B’. Design air-change rates are also often set higher than those required
by quality or GMP requirements for factor of safety purposes. For example, the design air-change rate
may be set at 20 ACPH but 12 ACPH may be sufficient to meet the standards.
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4 System design
4.1 Effective system zoning
A HVAC system can be controlled via a single-zone strategy or a multi-zone strategy. With a singlezone strategy, all areas served by the system receive the same amount of heating, cooling or air
conditioning as defined by the control logic of the unit. However, different areas can have different
end energy use requirements depending on a number of factors as outlined in Section 2 above.
Areas with similar end energy use requirements should be grouped and served from the same HVAC
system. This will ensure the optimum amount of heating, cooling or ventilation is provided to the
spaces when required.
4.2 Single zone-requirements driving a multi-zone system
The requirements of the areas being served by a unit should be as similar as possible, to prevent a
single area driving the end energy use. For example, if an area on a multi-zone system has a humidity
requirement of 40-50% RH while other areas on the system do not require humidity control, this area
should not be served by the same AHU. A larger volume of air is being conditioned for humidity
purposes than is required.
This may also result in unnecessary heating and cooling occurring as the supply air may require
cooling to remove moisture from the air and then require heating to achieve the correct supply-air
temperature. This is the most energy-intensive mode of operation for an AHU. It should be applied to
the minimum volume of supply air as is actually required, according to the real energy service
requirement.
It is important to establish the critical parameters that must be maintained in areas served by HVAC
equipment and to ascertain the impact each has on the energy service requirement. All the
parameters should be challenged and the reason for their specification questioned.
4.3 Waste-heat recovery
Waste-heat recovery devices recover thermal energy from exhaust air and transfer it to the incoming
fresh-air supply. This can result in a reduction in the energy that would normally be needed to heat or
cool air to the temperature requirements of the system. A correctly designed and installed heatrecovery device can achieve savings upwards of 10% of the running cost of the HVAC system.
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5 Controls
The following sections outline different types of control strategies that can be adopted in a HVAC
system to ensure that efficient operation of the system is maintained and the quantity of energy used
to condition zones served is minimised.
The issues outlined in the following sections should be considered when auditing an existing
installation. However, it is also important to refer to these issues when designing or implementing a
new system in a facility.
5.1 Setpoints
Different industries require diverse HVAC setpoints depending on a number of factors, including the
process undertaken in an area, product quality requirements, occupancy levels, etc. Setpoints depend
on the end energy use requirements of the areas being conditioned by HVAC systems. The setpoint
requirements used in a facility should be confirmed by quality personnel and, as previously stated,
energy service requirements should be challenged and the reasoning behind their implementation
questioned.
Control systems compare measured values (e.g. current temperature) with a setpoint (e.g. desired
temperature). The control system will apply the required actions to manipulate the measured value
up or down as required by the setpoint. Setpoints are critical to any control system.
Control systems that maintain conditions to a setpoint typically have one or more sensors reporting
back to a control device. The device's control logic uses this information to determine if heating or
cooling is required. It is important that all critical sensors responsible for the operation of an energyconsuming HVAC element are placed on an actively maintained calibration schedule to ensure that
the values reported to a control device are accurate.
HVAC systems will operate more effectively when accurate information is relayed to the control
mechanism, which results in the most efficient operation of the HVAC system.
5.2 Deadband
A deadband is an area of a signal range where no action occurs. The purpose of a deadband on a
HVAC control system is to prevent repeated activation-deactivation cycles, often referred to as
hunting. For example, in a typical workplace the heating should switch off when a temperature of
19°C has been reached and cooling should not come on until the temperature exceeds 20°C. The 1°C
gap between the setpoints prevents simultaneous heating and cooling occurring, and is referred to
as the deadband.
5.3 Software interlock
A software interlock is an important method to ensure that simultaneous heating and cooling does
not occur within a HVAC system. Without an interlock, or an appropriate control deadband, heating
and cooling elements of a system can conflict with each other in an effort to maintain an area's
temperature requirement.
A software interlock on a system will ensure that while heating occurs within a HVAC system, no
cooling elements associated with the same system will be operational – and vice versa. The only
situation where a software interlock should not be applied is when there is a humidity requirement in
an area served by an AHU. In this circumstance the cooling coil of the unit may be required to strip
moisture from the supply air by chilling the air to below its dew point. The air must then be heated to
attain the appropriate supply-air temperature to maintain the space temperature requirement.
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5.4 Modulation of mixing sections
Modulating dampers on the fresh-air intake, exhaust air and return-air ductwork will enable an AHU
to control the mixing ratio of air in order to achieve the optimum condition of air exiting the mixing
section. This minimises the heating or cooling load of the unit.
A unit may have these dampers in a fixed position to achieve the minimum fresh-air requirement, as
demanded by the energy service requirement. However, if the ratio of fresh air and recirculating air
allowed to enter the mixing section of the unit can be altered, this will enable the use of 'free' cooling.
Where possible, the quantity of fresh air should be altered as the first measure to satisfy the roomtemperature requirement. Only after the fresh-air percentage has been maximised and where the
temperature setpoint has not been achieved should the HVAC system enter cooling mode and the
cooling valve open and allow mechanical cooling of the air.
For example, a space requires 16°C supply air. The return air from the space is 21°C and the outside air
is 10°C. At a fixed fresh-air intake rate of 10%, the temperature of the supply air exiting the mixing
section of the unit will be 19.9°C, indicating that there is a cooling requirement within the unit.
However, if the mixing ratio is allowed to modulate to 45% fresh-air intake, this will result in a supply
air temperature of 16°C exiting the mixing section of the unit; hence the system has availed of 'free'
cooling in place of costly mechnical cooling.
Modulating dampers can control to a desired temperature setpoint exiting the mixing section of a
unit or alternatively control to an enthalpy setpoint. An enthalpy setpoint is typically used when
humidity control is a critical requirement (this is discussed further in Section 5.6).
5.5 Control loop tuning
A control loop is a mechanism that attempts to correct discrepancies between a measured variable
(e.g. the actual supply air temperature) and a setpoint (e.g. the desired supply air temperature). The
control loop will apply the required actions via an actuator that will drive the measured variable up or
down as required.
Tuning a control loop requires adjusting the control parameters to the optimum values for the
desired control response, which in turn prevents the measured variable fluctuating excessively. For
example, an incorrectly tuned system may have rapid cycling between heating and cooling to meet
energy service requirements; a correctly tuned system will provide only heating or cooling as energy
service requirements demand. This in turn ensures that simultaneous heating and cooling will not
occur.
5.6 Enthalpy control
When temperature and humidity are critical end energy service requirements, the total energy
content of the fresh and return air streams should be taken into account. This is achieved through
enthalpy control. Enthalpy is a measure of the total energy content of air. It combines the sensible
and latent heat.
Modulating dampers will ensure that the optimum mixing ratio of fresh air to re-circulated air is
obtained on exiting the mixing section of an AHU, thereby minimising the end energy use of the unit
when humidity and temperature are critical parameters that must be maintained.
A typical enthalpy control strategy would work on the following basis:
If the fresh air has a lower enthalpy and higher temperature than the return air, a modulating damper
set should provide 100% fresh air and no re-circulation.
If the fresh air has a lower enthalpy and lower temperature than the return air, then the optimum mix
of fresh air and return air should be used.
If the fresh air has a higher enthalpy than the return air, the defined minimum amount of fresh air and
maximum amount of return air should be used.
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5.7 Time schedules
Time schedules are the most basic level of automatic control. They are typically automatic on/off
switches that operate in parallel to the occupancy pattern of an area. A time schedule can also be
used to instigate a set-back pattern according to gradually increasing occupancy levels in the
morning and decreasing levels in the evening. There are a number of methods of employing time
schedules.
Time switch: Services are switched on or off in accordance with time settings.
Seven-day programmer: This is used for switching HVAC systems on, off, or to a setback mode at
different times during the week according to the occupancy levels.
Optimum time controls: These switch the HVAC systems on just in time to reach the required
temperature at the start of occupation.
5.8 Occupancy-based control
Occupancy control of a HVAC system allows for the automatic switching of a ventilation system if the
presence of occupants in an area is detected. This ensures a ventilation system is only operational
when required. The most common form of occupancy detection is passive infrared (PIR) sensors. This
type of control strategy is suitable for areas that are occupied intermittently.
It is also possible to optimise the energy consumption and indoor air quality according to the number
of people in a zone at any given time. Typically, for this to function accurately, levels of CO2 are
measured in the occupied zone and used as the control input. If there are more people in a space, this
will create a higher level of CO2 in that area. The speed of the ventilation fan can be increased to
maintain the desired level of CO2. As the occupancy density decreases, so too will the level of CO2.
This in turn will reduce the speed of the ventilation to maintain the desired level of CO2.
This type of control is suitable when the occupancy density of an area varies signifiicantly throughout
the occupied hours.
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6 Effective HVAC System Operation
6.1 Training and awareness
Staff awareness of energy and energy consumption can achieve significant savings. This involves
making people aware of energy issues and energy-saving practices in their day-to-day activities.
For example, examining a work area after the occupants have left in the evening will reveal which
pieces of equipment are left on and why. These findings should be communicated to the staff, and
the amount of energy that can be saved by switching equipment off and the potential environmental
benefits can be outlined to them.
In relation to HVAC, specific training and increased awareness should be maintained in relation to the
level of energy consumption that results from changing system setpoints, opening windows while
the heating system is in operation and leaving equipment running when it is not in use.
6.2 Monitoring & targeting (M&T)
M&T is another useful technique for managing energy consumption. It usually involves predicting
levels of energy perfocmance against which future energy performance can be measured, thus
allowing the impact of energy-saving actions to be evaluated.
A simple approach to M&T is regularly reviewing energy invoices for a site, graphing data and
identifying anomalies such as high out-of-production energy usage. Corrective actions should then
be taken and their impact evaluated when the next set of invoices arrives. However, this will not give
a breakdown of how much energy the HVAC system is using.
A more advanced method is to individually record energy usage in the HVAC system by means of
energy meters (electrical & heating/cooling) which can be installed to record the electricity
consumption of the facility. Data would be fed to a central controller which could be accessed
through a PC interface to view and analyse energy data. The figures would then be compared against
historical data to determine the impact of implemented energy–saving measures.
In addition to this, the data – both historical and current – can be adapted for internal publication in
order to raise awareness of energy use among all occupants of the buildings in question and in the
development of energy performance indicators (EPIs).
An EPI is a point of reference for making comparisons on energy consumption. In general, an EPI may
be based on consumption, cost, or environmental measures. EPIs can be used to develop relative
measures of energy performance, track changes over time, and identify best practice in energy
management.
The implementation of effective EPIs enables the close monitoring of the performance of a HVAC
system with a view to identifying periods of under-performance at the earliest possible juncture at
the least cost.
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6.3 Energy management systems (EnMS)
IS393 is an Irish Energy Management Standard which establishes a systematic approach for
continuously improving energy performance. This type of management structure offers many
benefits, including:
•
Demonstrating a commitment to energy efficiency
•
Strong energy management techniques are embedded into normal operations
•
A culture of continuous improvement is fostered
• Processes are standardised so that improvements are sustained over time
An EnMS helps an organisation in identifying equipment that consumes a significant quantity of its
total energy usage.
It then ensures that the persons responsible for operating these key pieces of equipment are
identified and trained in their energy-efficient operation. It also ensures that the specific energy
service requirements of these significant energy users (SEUs) are interrogated for energy-efficiency
improvements, and documented.
Finally, it ensures that controls in the form of energy performance indicators (EPIs – see next section)
are put in place to ensure that these SEUs operate efficiently and do not consume energy wastefully.
An EnMS does all this in a structured, documented and repeatable manner in order to ensure
continuous improvement in energy performance.
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7 Effective HVAC System Maintenance
Checking that HVAC systems are operating as intended will help to prevent them from using energy
inefficiently. HVAC components should be kept free of dirt and other obstructions so they can
operate efficiently. The overall system should be serviced periodically.
Maintenance plans/schedules should be provided for each element of the HVAC system. Routine
maintenance will identify potential problems at an early stage, lower the risk of breakdown and
minimise unplanned plant downtime that leads to production losses.
7.1 Simultaneous heating and cooling
Heating and cooling control valves should be checked as part of the annual maintenance to ensure
they are not passing and to prevent simultaneous heating and cooling. Often this problem can go
undetected, but it results in increased energy consumption.
For example, if a cooling coil control valve is passing, when the HVAC system is in heating mode, an
increased thermal input will be required in the boiler to compensate for the undemanded cooling. In
this situation (see fig. 2Error! Reference source not found.) the cooling coil control valve is 0% open
but there is a 3°C drop in air temperature across the coil. This means that, instead of having to raise
the temperature of the supply air by 3°C to its setpoint of 18°C, the heating coil must now raise the
supply-air temperature by 6°C.
Figure 2: An example of a passing cooling coil
7.2 Filter replacements
Filters should be replaced when the recommended maximum differential pressure is exceeded. As a
filter becomes blocked, the resultant increased pressure drop causes the AHU fan to draw more
power to maintain the same airflow rate. Therefore, replacing a filter as it approaches its maximum
allowable differential pressure will minimise the added load on the AHU fan.
Replacing a filter on a timed basis will lead to increased capital costs as a filter could be replaced
while it is fully functional. It is also possible that a filter could still be in place when it is far in excess of
its maximum differential pressure, placing undue strain on the supply fan.
7.3 Insulation
Heat gains and heat losses can be large when pipes or ductwork are left uninsulated. This is
particularly evident when the pipework/ductwork passes through an area that is at a very different
temperature to its surroundings. These heat gains and losses increase the amount of energy needed
to maintain the medium, within the pipework/ductwork, at the required temperature.
BS 5422:2001 – Method for specifying thermal insulating materials for pipes, tanks, vessels, ductwork
and equipment operating within the temperature range -40°C to +700°C is a standard which covers
insulation levels for four applications: frost protection, condensation protection, personnel protection
and energy savings. The standard details an ‘environmental thickness’ which is the level of insulation
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that would attain the minimum level of heat loss or gain within pipework/ductwork for a given
application.
Insulation on valves and flanges may be omitted from installations for ease of access but flange boxes
or other easily removable insulation components should be installed.
Table 1: Applications and associated insulation requirements according to BS5422:2001
STATED APPLICATION
Table from
BS5422:2001
Refrigeration pipework
Table 6
Chilled-water pipework
Table 9
Non-domestic heating/hot water
Pipework
Table 12 & 13
Domestic heating/hot water
Pipework
Table 14
Process pipework
Table 15
Ductwork carrying chilled air
Table 10
Ductwork carrying warm air
Table 11
7.4 Calibration of instrumentation
As mentioned in Section 0, control systems that maintain conditions to a setpoint typically have one
or more sensors reporting back to a control device. This information is then used by the device's
control logic to determine if heating or cooling is required.
It is important that all critical sensors responsible for the operation of an energy-consuming HVAC
element are placed on an actively maintained calibration schedule to ensure that the values being
reported to a control device are accurate.
HVAC systems will operate more effectively when accurate information is relayed to the control
mechanism, resulting in the correct, efficient operation of the HVAC system.
7.5 Duct leakage
Ductwork air leakage should be a concern to both design engineers and facility managers because of
its potential impact on system performance and energy wastage. For a system to compensate for
ductwork leakage, it will require an increase in airflows and thus increased fan power consumption.
A number of recommendations should be considered when designing ductwork that will improve
the energy efficiency of the system. These include:
•
Use the minimum number of fittings
•
Seal the ductwork to minimise air leakage
•
Use round duct where possible as it has the lowest amount of friction loss for a given perimeter,
requires less support and is less expensive to seal
• When using rectangular ductwork, maintain the aspect ratio as close to 1:1 as possible
Identifying and repairing ductwork leaks is an important step to reduce energy costs. A number of
sealant types can be used to ensure adequate ductwork sealing, such as liquid sealants, mastics, tapes
or heat-applied materials. Ducts can also be purchased that have various types of flanges that
incorporate self-sealing, factory-applied gaskets.
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8 Effective validation
Careful consideration should be given to the prescribed requirements that commissioning and
validation engineers must achieve. All air-change rates and temperature setpoints should be
challenged to ensure the optimum conditions are achieved with minimum end energy use
requirements.
When defining air-change rates that must be achieved during validation, it is important to be clear
about what is required in a given area – for example, if greater than or equal to 12 ACPH is specified in
an area. At 12 ACPH, the area meets the requirement, but it is also true that the requirement will be
met at 25 ACPH. This obviously results in higher energy consumption due to increased fan power
required to supply the larger volume of tempered air, as well as the cost to condition the air within
requirements.
Therefore, it is important that maximum and minimum limits be documented on commissioning and
validation criteria to ensure energy consumption is minimised.
Other important issues and considerations that should be considered when designing, installing or
commissioning a system are outlined in the following sections.
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9 Ancillary Equipment
9.1 Chiller
Challenging the chilled water (CHW) temperature setpoint can achieve significant energy savings. It is
important to understand the drivers for the CHW temperature setpoint, investigate if the HVAC
systems can operate at a higher temperature and eliminate any parasitic loads where applicable.
9.1.1 CHW setpoint: CHW systems typically operate at 5°C or lower but the operating setpoint of all
systems should be challenged. It should be determined whether end users of the CHW need the
temperature to be as low as 5°C; and, if so, whether these users can be isolated and placed on a
separate system.
For example, a single HVAC system with a load of 30kW requires chilled water at 2°C but another 10
systems with a combined load of 400kW can operate at 7°C but are supplied from the same CHW
source. In this situation, the smaller system should be isolated and supplied separately. This allows
most of the CHW to be supplied at 7°C which achieves significant energy savings.
9.1.2 Parasitic loads: These can account for a significant proportion of the site cooling load and can
include heat gains from CHW circulating pumps and heat gains from uninsulated pipework passing
through heated areas. In winter, when demand for CHW is typically very low or non-existent, a
circulating pump and circulatory loads could account for most of the load on the chiller. This energy
wastage can be counteracted by switching off the chiller and CHW circulation pumps in winter or
enabling variable-speed-drive (VSD) control of the pump in response to load demand.
9.2 LTHW
Maintaining an energy-efficient Low Temperature Hot Water (LTHW) system can be achieved through
similar methods as outlined in the previous section – with one notable difference. To achieve a more
efficient CHW system, you would strive to increase the temperature of the water to the maximum
allowable level as determined by the end users. In the case of a LTHW system, you would strive to
decrease the temperature of the water to the minimum allowable level as determined by the end
users.
It is important to understand what the drivers are for the LTHW temperature setpoint and investigate
if the HVAC systems can operate at lower temperatures and eliminate any parasitic loads where
applicable.
9.3 Heat gains
Reducing the cooling load on a HVAC system by minimising the heat gain in an area is an efficient
method to achieve energy savings. Turning off all electrical equipment when it is not in use will result
in energy savings. This will include reductions in the facility electrical consumption and in the cooling
load placed on the HVAC system. Turning off lights in unoccupied areas will realise similar benefits.
Computer-server rooms are particularly vulnerable to large heat gains. They should be conditioned
separately from the main system to ensure that the optimum energy-efficient HVAC system is in
operation for the application. A server room should only be cooled to the maximum temperature at
which the equipment contained in it can operate effectively.
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03
PUBLICATION DATE: Q1 2009
REF NUMBER: SWGII 002
HVAC SPECIAL WORKING GROUP
Documentation available on
HVAC System Optimisation
Sustainable Energy Ireland
Glasnevin, Dublin 9, Ireland
Glas Naíon, Baile Átha Cliath 9, Éireann
T. +353 1 8082100
F. +353 1 8372848
info@sei.ie
www.sei.ie
Sustainable Energy Ireland is funded by the Irish Government
under the National Development Plan with programmes
part financed by the European Union.
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