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Energy Efficiency E-module - Guidance
Efficient Operation of Low Temperature Hot Water Boilers in
the Public Sector
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 2
Contents
1 Introduction 3
2 Learning Objectives and Outcomes 3
2.1 Learning Objectives 3
2.2 Learning Outcomes 3
3 Overview and Principles of LTHW Heating Systems 4
3.1 Conventional Boilers 4
3.2 Condensing Boilers 5
3.3 Wall Hung Boilers 6
3.4 Modular Boilers 7
3.5 Boiler Efficiencies 7
3.6 System Components 8
3.7 How to Survey a Boiler House 10
4 Energy Saving Opportunities 12
4.1 Boiler Replacement 12
4.2 Boiler Compliance - Ventilation for Gas Systems 13
4.3 Boiler Compliance - Gas Safety Regulations 14
4.4 Improving an Existing Boiler House 14
4.4.1 Insulation Improvements 14
4.4.2 Hydraulic Layout 14
4.4.3 Pump Upgrades 14
4.4.4 Combustion Efficiency Checks 15
4.4.5 Heating System Maintenance - Poor Practice 15
5 Building the Business Case 17
5.1 Before Considering a Boiler Upgrade Project 17
5.2 Information Gathering 17
5.3 Business Case - Case Study 17
6 Useful Links and References 19
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 3
1 Introduction
This guidance follows the format of the associated e-module, “Efficient Operation of Low
Temperature Hot Water (LTHW) Boilers in the Public Sector”. It provides further details on
the subjects covered in the module.
Please note that module users working in a healthcare environment should always refer to
the relevant Scottish Health Technical Memorandum (SHTM) prior to considering installation
of the measures suggested in the module. The advice given in the SHTM may conflict with
the advice given in this module, as it has been developed for the wider public sector. The
relevant SHTM can be found on the Health Facilities Scotland website.
2 Learning Objectives and Outcomes
2.1 Learning Objectives
The learning objectives from this module are to:
Understand the different LTHW boiler types and their applications;
Understand the main principles of how LTHW boilers and distribution systems operate;
and
Understand the different measures which can be implemented to improve boiler house
efficiency.
2.2 Learning Outcomes
The learning outcomes from this module are for the reader to be able to:
Understand the basic principles regarding how different boiler types work;
Understand where the opportunities for improving LTHW heating systems exist in
Scottish public sector sites and buildings;
Describe the main boiler technologies including their typical efficiency range, and the
advantages and disadvantages of each technology;
Carry out an audit of a boiler house and identify opportunities for making
improvements;
Identify when a boiler should be replaced and what technology could be applied;
Prioritise the opportunities for improving LTHW systems in public sector buildings; and
Understand the key aspects in relation to LTHW boiler projects when building a business
case.
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 4
3 Overview and Principles of LTHW Heating Systems
Table 3.1 provides a brief overview of some of the different LTHW boilers that are currently in operation.
Table 3.1 – An Overview of LTHW Boilers
Boiler Type Description Typical Seasonal
Efficiency
Conventional
Floor Standing, atmospheric or
pressure jet burners,
large in size. Boilers of this type are likely to be 15 years or older
45 -70% (depending
on condition)
Condensing (Floor Standing)
Smaller than conventional boilers. Have extra heat exchanger surfaces. Installed since 2005
85 - 90% (depending on heat
emitters)
Condensing (Wall Hung)
Smaller than floor standing units. Have
extra heat exchanger surfaces. Installed since 2005. Typically
used for domestic applications
85 - 90% (depending on heat
emitters)
Modular (condensing/
condensing and conventional)
Several boilers linked to give more flexible and
efficient output to service larger heat loads
Depends on boiler type
3.1 Conventional Boilers
This section introduces the basic principles of LTHW heating systems and the different boiler types available.
A LTHW boiler works by burning a fuel and then using the heat energy from this combustion
to heat water which is pumped around the heating distribution system, typically to
radiators, air handling units or fan convectors. A basic cross section of a conventional gas
boiler layout is shown in Error! Reference source not found..
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 5
Figure 3.1 - Conventional Boiler
When the boiler starts up, the burner fan first blows cold air through the combustion
chamber to purge any residual gases from the previous combustion cycle. Once this process
is complete, the burner then ignites and burns fuel in the combustion chamber. The hot
gases from this combustion process then pass over the boiler heat exchanger, heating the water before passing out of the boiler through the vent.
On the ‘wet’ side of the system, when the boiler fires, the heating circulating pump switches
on to ensure that there is a flow of water going through the boiler heat exchanger. The
heated water is then circulated round the heating distribution system. This process is
described in more detail in the following section. Low temperature hot water boilers
generally heat water up to a maximum flow temperature of 90°C, with systems typically designed based on flow and return temperatures of 82°C and 71°C respectively.
Most boilers have an integral temperature sensor which controls the burner to meet the
target flow temperature. In smaller, simpler boilers this can be achieved by switching the
burner on and off. Larger, more complex boilers can have two stage burners which allow
them to alternate between low fire, high fire and off. Others have fully modulating burners
which can alter the amount of fuel burnt to meet variations in the heat load. Fully
modulating burners tend to have the highest overall operating efficiency, whilst single stage
on/off burners are least efficient. A typical conventional boiler can achieve seasonal
efficiencies of around 70-75%. This capacity to modify boiler output to meet heat demand is
a feature of gas and oil fired systems. As shown in the biomass e-learning module, this
ability is not shared by boilers burning solid fuels. These require different control strategies as a result.
Since 1997, conventional, non-condensing boilers that meet the minimum efficiency
requirements of the boiler efficiency regulations are usually marketed as ‘high efficiency’ boilers. These boilers tend to have higher seasonal efficiencies of around 82%.
Conventional boilers can come in a variety of shapes and sizes. They tend to be floor
standing and draw their combustion air from the space around the boiler making the
provision of sufficient ventilation essential. Burners tend to be either: atmospheric, relying
on convection; or forced draft. The latter use a fan to draw combustion air into the burner.
Forced draft burners allow for closer control of the flow of combustion air into the burner. This reduces flue gas volumes and thus the size of the flue required.
3.2 Condensing Boilers
Condensing boilers differ from conventional boilers in that they have a secondary heat
exchanger on the boiler flue through which the heating return water passes prior to entering
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 6
the main boiler heat exchanger. This increases the efficiency of the boiler as more heat is
extracted from the combustion gases prior to them being exhausted to the surrounding atmosphere.
Where the system can operate with return temperatures below 50°C, higher levels of
efficiency can be achieved. This low water temperature causes the moisture in the flue
gases to condense thus releasing the latent heat contained in the vapour. This is why these
boilers are known as ‘condensing boilers’. Figure 3.2 shows a basic cross section of a condensing gas boiler layout
Figure 3.2 - Condensing Boiler
Condensing boilers are best applied to systems where weather compensation can be
applied, ensuring that return temperatures can be minimised and that the boiler can
operate in condensing mode for as much time as possible. For more information on weather
compensation control, see the accompanying e-module on controls. However, it is worth
noting here that this can usually only be applied to systems which exclusively serve heating
circuits, with domestic hot water provided separately. LTHW boiler systems which also serve
domestic hot water tanks usually require constant temperature supplies which negate the system’s ability to compensate at all.
Condensing boilers tend to have fully modulating burners allowing them to vary output to meet a fluctuating load.
Condensing boilers can come in many forms including floor standing, wall hung, and
modular. Modular and wall hung boilers tend to have much lower water content than conventional boilers.
3.3 Wall Hung Boilers
Wall hung boilers tend to be smaller than floor standing boilers. These boilers can also be
flued horizontally through walls, provided a balanced flue is used and the flue terminal is sufficiently far away from any nearby openings such as windows or ventilation louvres.
Balanced or ‘concentric’ flues have two orifices. The inner tube transports flue gases to
atmosphere and the external tube supplies air to the boiler. When this arrangement is used
the ventilation requirements for the boiler room or cupboard are reduced as the combustion
air is provided via the flue. The same effect can also be achieved by using separate flues,
although these cannot be installed horizontally and must terminate at least 1 metre above
the roof level of the building in question.
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 7
Wall hung boilers are generally available in sizes ranging from 30 kW up to 120 kW. Larger
boilers are floor standing. Additionally, wall hung boilers tend to have a low water content
and therefore a high hydraulic resistance. This is an important consideration when replacing
old high water content boilers with wall hung boilers as additional pumping capability may be required.
3.4 Modular Boilers
Modular boiler systems are arrangements where several boilers are linked together to meet
the heating demand of a building. Typically, these boiler systems are made up of between 2
to 12 identical modules, although sometimes a combination of condensing and non-
condensing boilers will be used.
Modular boilers have a number of benefits. Like wall hung boilers, they have lower water
content, taking up less physical space than a conventional boiler. In addition, modular
boilers can offer an attractive solution to effectively and efficiently meeting the varying heat
demand of a large commercial building by allowing modules to be sequenced to operate at
maximum efficiency for as much of the time as possible.
For example, condensing boilers operate most efficiently at part load, whereas non-
condensing boilers are generally most efficient at peak load. Consider a building with a peak
heating demand of 500 kW but a summer base load of only 100 kW and it is evident that a
modular boiler can help improve seasonal efficiency. If a single 500 kW non-condensing
boiler was to be installed, it would only operate at optimum efficiency during the coldest
months of the year while the rest of the time it would be operating at part load with reduced
efficiency. If on the other hand a boiler with five separate 100 kW modules was installed,
the boilers could be sequenced to ensure that modules operate at peak load (and thus peak
efficiency) for a much greater time period.
For condensing boilers, where optimum efficiency is at part load, similar benefits can be
achieved by ensuring that the boilers are sequenced to operate at part load for as much
time as possible. There can also be benefits to using a combination of condensing and non-
condensing boilers to meet varying loads as efficiently as possible, especially in applications with a high demand for hot water.
As with wall hung boilers, the low water content of modular systems leads to high hydraulic resistance and additional pumping capability may be required as a result.
3.5 Boiler Efficiencies
Table 3.1 shows typically quoted values for boiler efficiencies.
Table 3.1 – reported boiler efficiencies
Reported Efficiency Efficiency Type
Boiler 1 102% Net Efficiency
Boiler 2 94% Gross Efficiency
System 87% Seasonal Efficiency
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 8
This shows that there is an important point to be considered regarding boiler efficiency and how they can be compared
It is not uncommon to see boiler literature advertising boiler efficiency of 102%, whereas
other figures may suggest a peak efficiency of 94%. This is because the first figure refers to
net efficiency, which assumes that the energy contained in the water vapour in the
combustion gases is recovered. The second value is gross efficiency and assumes that the energy is not recovered.
Another efficiency term often used in reference to boilers or heating systems is seasonal
efficiency. This is a weighted average of the efficiencies of the boiler at 15%, 30% and
100% output. The overall system seasonal efficiency will also be influenced by the type of
heat emitters used. For example, a condensing boiler serving only an underfloor heating
system and operating at relatively low temperatures will have a higher seasonal efficiency
than the same boiler serving a system including fixed temperature heat emitters with a flow
temperature of 82°C.
It is important to be aware of the various ways of reporting boiler efficiency when evaluating which boiler to install and to undertake comparisons on a like-for-like basis.
3.6 System Components
How the boiler is connected to the rest of the heating system and the key components of that system must be considered.
Error! Reference source not found. is an energy efficient schematic boiler house with multiple condensing boilers, connected in parallel.
Figure 3.3 - Boiler House Schematic
From the diagram, it can be noted that the boilers are connected and can be isolated in
such a way as to allow any one of them to be taken offline or even removed without the
system having to be shut down. Within the boiler flow pipework there will typically be
temperature and pressure gauges or sensors as part of the control and safety systems.
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 9
The system shown has a single circuit, though in practice there can be any number of
circuits depending on the heating load being serviced. All flow circuits should be prior to any
return connections, ensuring that each circuit gets boiler temperature water. In systems
where there are a mixture of variable and constant temperature circuits the flow temperature will be varied on the variable temperature circuits, using mixing valves.
Another component of a system of this type will be a dirt separator and deaerator. This will
be located at the hottest part of the system. These devices remove circulating air, micro-
bubbles and any small particles of dirt in the system. This is important as it ensures that air
and dirt are removed from the system to prevent corrosion, improve heat transfer, and prevent unnecessary deterioration in other system components such as boilers and pumps.
All modern boiler installations include a common header which acts as a barrier between
primary and secondary circuits, allowing for optimum control and hydraulic performance. The benefits of this component are explored in more detail later in this document.
The pressurisation unit and expansion vessel are also important components of the system.
The pressurisation unit automatically maintains pressure in sealed systems by pumping in
mains cold water when the system pressure drops below a predetermined level. As the
name suggests, the expansion vessel accommodates expansion of the fluid in the system as
it heats up. It consists of a tank with a rubber diaphragm separating the fluid from a charge
of nitrogen gas. As the system heats up, the diaphragm expands to accommodate the
increased volume. Some older systems will have a feed and expansion tank at high level which performs both functions for systems operating at atmospheric pressure.
The primary pumpset, usually comprises a twin-head pump with automatic changeover, to
give redundancy to the system. Finally on the water side, a number of balancing or
commissioning valves are present, to allow the system to be set up with adequate flow through each boiler.
Other important items are the condensate drain (often via a tundish) for condensing boilers,
and the various gas isolation valves, allowing for individual isolation of each appliance and
isolation of the gas as it enters the boiler house. This is also a requirement under the gas
safety regulations. It is also good practice to include a strainer to capture any pipeline
debris such as scale or rust before it reaches the pumps and boilers. It is important that this component is located correctly to protect these expensive plant items.
Table 3.2 contains a list of LTHW boiler system components.
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 10
Table 3.2 - Boiler system components for LTHW systems
Component Purpose Impact on Efficiency
Isolation Valves To isolate boilers, components and for gas safety
Allows removal/isolation of boilers, components etc. for maintenance or replacement but the system can still operate in most cases
Balancing and
conditioning Valves
To maintain adequate flow through boiler
Ensures the water quantity is met whilst avoiding
the entry of large quantities of potentially damaging cold water
Mixing Valves Mixing temperatures Ensures each circuit and emitter gets the correct temperature
Temperature
Gauge/Sensor
To measure the temperature within the
boiler and heat circuit
If the temperature reads below/above the range set by the manufacturer this could indicate a fault
within the system or a component
Pressure Gauge/Sensor
To measure the pressure within the boiler and
heat circuit
If the pressure reads below/above the range set by the manufacturer this could indicate a fault within the system or a component. The pressure within a system affects the quantity and the temperature of the water. Both low and high pressure can be dangerous
Dirt Separator and deaerator
Removes dirt and air from the system
Improves heat transfer, prevents corrosion and damage to pipeline and components
Common Header
Acts as a barrier
between primary and secondary circuits
Allows for optimum control and hydraulic performance
Secondary flow and return circuits
Secondary circuits providing different areas
of the building
Ensures each circuit gets boiler temperature water
Pressurisation Unit and Expansion
Vessel
Maintains pressure and contains the expansion
of system as it heats up
The pressure within a system affects the quantity and the temperature of the water. Both low and
high pressure can be dangerous
Strainer Removes pipeline debris including scale, rust etc.
Prevents damage to components
Pump Set Twin set of pumps
The pumps allow the water to flow through the
entire hot water and heating system. Two pumps allows for redundancy
Condensate Drain Removes condensate from condensing boiler
Essential for boiler operation
3.7 How to Survey a Boiler House
There are key items to look out for when surveying a boiler house. A good exercise when
surveying a boiler house is to try and sketch out a schematic (as shown in Figure 3.3) to show how the system is laid out.
Step-by-step guide to surveying a boiler house:
Identify the boiler type, capacity and age;
Identify the flow and return pipes – note arrangement of pipework and pumps;
Identify condensate drain if appropriate;
Identify expansion vessel and pressurisation unit;
Identify header and number circuits;
Identify ancillary pipework e.g. dirt separators, deaerators and location of components;
Identify water provision - separate heater, calorifiers, pump sets etc.;
Identify the flue (or flues) and where they exit the building;
Identify ventilation;
Identify control systems and how they operate; and
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 11
Check insulation.
Things to look out for:
Insulation. Are all pipes and valves well insulated? Have valve jackets been replaced
after maintenance work?
Are temperatures on heating and hot water pipes as expected?
Are multiple boilers firing together? If this is occurring, particularly in summer, it could
suggest an issue with the boiler controls.
Are there any water leaks in the system? Is the pressurisation unit coming on and off
frequently?
Are there any unusual noises suggesting plant items may have failed?
Is there appropriate isolation on the boiler gas supply?
How old are the boilers? How far are they from replacement?
In most cases a gas safe register approved heating engineer should carry out any repair and maintenance work that is identified.
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 12
4 Energy Saving Opportunities
4.1 Boiler Replacement
This section covers boiler house layout and other issues to be considered when replacing
existing boilers. First, consider again the schematic of an energy efficient low temperature
hot water system, with multiple boilers as shown in Error! Reference source not found..
Figure 4.1 - Boiler House Schematic
When replacing older boilers with new, it is likely that the new units will be low water
content boilers. As a result it is recommended that a separate flow circuit is created for the
boilers to protect them from pressure fluctuations on the heating side for example when
occupants open and close thermostatic radiator valves. There are many ways to do this,
however the most common is to install a primary circuit, feeding an oversized header or
buffer vessel, which is then connected to the secondary circuit or circuits. The secondary
side would then circulate hot water to heat emitters such as radiators and fan convectors.
The return pipework for modular boiler systems should be plumbed in a “reverse return”
arrangement to prevent short circuiting of water through one boiler, i.e. ensuring the water
leaving the first boiler will return to it last. It is still common to find boilers installed in the
Scottish public sector estate feeding separate flow and return headers, even when boilers
are relatively new as they have simply been replaced in a one out/one in fashion. A
common associated issue is hot water flowing in circuits that are supposed to be isolated for example radiators being heated when only hot water for hand washing is required.
Having the boilers plumbed with a primary header has many additional benefits. These
include more stable control, better sequence control of multiple boilers, options for
connection of future heating circuits and simpler integration of renewable or alternative energy systems such as biomass or heat pumps.
If replacing conventional boilers with condensing boilers, there are key differences to be
aware of. The flue fitted to the existing boilers is unlikely to be suitable, as the combustion
products from condensing boilers would corrode a standard flue. Finding a flue route for
new boilers may also be problematic for an existing building, particularly if the boilers are located in a basement.
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 13
Another requirement is that a drainage connection is needed to remove the condensed
water from the system. There may be health and safety issues that need to be addressed,
for example finding a route for the new drainage pipework that doesn’t create a trip hazard
in the boiler room. In addition, a common issue that can be encountered in cold winters is
the potential for externally located condensate drains freezing, causing the boilers to trip.
Both issues require careful consideration of the location and design of the condensate drains.
A common problem with retrofitting modern condensing boilers to existing heating
installations is that the return water temperatures are too high to allow the boilers to
condense, and this reduces the savings that can be achieved from installing the new
equipment. Separating out the hot water systems that provide hot water to taps, commonly
referred to as domestic hot water (DHW), by installing gas fired water heaters or point of
use electric water heaters, would allow lower water temperatures to be maintained in the
system. Condensing operation can be maintained for most of the year, even if the system is
serving heat emitters that require constant temperature such as air handling units and fan
convectors, by varying the temperature of the primary circuit related with changes to
external temperature. Water temperatures should be maintained above the thermal cut out settings on fan convectors for example, typically around 50°C.
Getting the design right can mean the difference between making negligible or no energy
savings, to saving 25% or more on fuel. Strathclyde Fire & Rescue reduced the energy
consumption of their estate by over 40% since 1990 with the majority of the savings being
achieved through a rolling boiler replacement programme using the techniques described here.
4.2 Boiler Compliance - Ventilation for Gas Systems
When making improvements to a boiler house, it is important to consider compliance with
relevant standards and legislation. Two areas where older plant rooms are often non-
compliant are ventilation and gas safety.
The requirements for ventilation are set out in BS6644:20111 which is the specification for
the installation and maintenance of gas fired hot water boilers of rated inputs between 70
kW (net) and 1.8 MW (net). The quantity of ventilation required is dependent on the type
of boiler, its size, the flue type, and the method of ventilation whether natural or
mechanical. Usually, natural ventilation is preferable as it avoids the need for complicated and costly controls interlocks.
In almost all cases where boiler houses are naturally ventilated, both high and low level
ventilation will be required. This is particularly important where boilers with conventional
flues are used as the ventilation supplies air for combustion, whereas when balanced flues
are used ventilation is only required to prevent excessive temperatures. Without sufficient
ventilation, the boiler is starved of oxygen which can lead to incomplete combustion leading to production of Carbon Monoxide (CO).
A good first step when evaluating compliance is to check that the boiler house in question
has both high and low level ventilation. High level louvres must be as high as reasonably
practicable, whilst low level louvres must be as low as reasonable practicable and within 1 metre of the floor.
If there is any doubt about the compliance of a boiler house, refer to the British Standard document.
1 A quick reference guide to this standard can be found at www.mhgheating.co.uk/wp-content/uploads/2012/04/BS-5440-BS-6644-IGEUP10-Quick-Reference-Guide-010813.pdf
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 14
4.3 Boiler Compliance - Gas Safety Regulations
All gas fired boiler equipment is required to comply with the legislation as set out in The Gas
Safety (Installation and Use) Regulations 19982. Some key points include:
There must be an emergency control valve (ECV) on the gas supply pipe immediately
after it enters the boiler house, as close as is practicable to the point of entry in order to
allow the safe isolation of the gas supply should a fire break out. Often, modern boiler
houses will have both a manual valve (which is compulsory) and an automatic valve,
which is optional. This offers the peace of mind that if the system develops a fault the
gas valve will close automatically isolating the gas supply;
The gas pipe should never pass through enclosed, unventilated voids. This is to ensure
that there can never be an undetected build-up of gas that could lead to a risk of
explosion; and There should be valves located before each user to allow individual isolation.
4.4 Improving an Existing Boiler House
There are a number of areas to consider when thinking about improving an existing boiler
house. These include:
Insulation;
Amending hydraulic layout;
Upgrading old pumps, and Boiler combustion checks
4.4.1 Insulation Improvements
For more detailed information on the theory and practice of upgrading heating system insulation, see the accompanying insulation e-module.
A good first step when evaluating an existing boiler house for potential savings
opportunities is to identify all items of pipework and valves that are not insulated and
address these.
An infrared thermal imaging survey is a quick and relatively easy method of assessing the
performance of existing insulation and heating elements within the boiler house. The
brighter the image (yellows, orange) the hotter the component and the least efficient insulation. If the image is dark (blues, purples) the insulation is working well.
4.4.2 Hydraulic Layout
It is still common in the public sector to find heating systems with separate flow and return
headers. Often, these can be converted into a common header arrangement relatively
easily. This can help with addressing common issues as noted earlier, improving control and saving energy.
4.4.3 Pump Upgrades
Pumps account for the majority of electricity consumption on LTHW heating systems and
there can be opportunities for making savings in this area. The biggest savings in the public
sector are likely to be achieved by replacing old fixed speed pumps with new, variable speed ones.
2 Approved code of practice and guidance can be found at www.hseni.gov.uk/l56_safety_in_the_installation_and_use_of_gas_systems_and_appliances.pdf
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 15
It used to be normal practice to fix the system flow rate during commissioning using
regulating valves. This works by increasing the system pressure by closing the valve until
the correct duty point is reached on the pumps system curve to provide the desired flow
rate. Any increase in system pressure means a proportional increase in power consumption.
Studies suggest that only 20% of the pump drive motors operating in this way are running
at their full rated input, mainly due to the individuals sizing them adding an additional 10% to 15% to err on the side of caution.
In addition to this, most pumps used in building services installations serve circuits with
varying pressure, for example heating circuits with thermostatic radiator valves (TRVs) that
open and close. The rate of flow through the radiator is reduced by closing the TRVs, always resulting in a duty point shift on the pump curve towards higher heads.
A pump set with an integral inverter can be used in place of regulating valves, adjusting the
pump speed instead of artificially adding system pressure to reach the desired duty point.
Variable speed pumps can therefore respond to changes in demand by varying the motor speed and saving energy when speed reduces.
4.4.4 Combustion Efficiency Checks
Having the combustion efficiency of boilers regularly checked is a good way of ensuring that
the boilers are operating efficiently, as well as giving notice of any potentially serious
issues, such as high carbon monoxide content in the flue gases. Boiler combustion efficiency
test results typically report the oxygen, carbon monoxide and carbon dioxide content in the
flue gas, along with the net and flue gas temperatures, the net efficiency and the
percentage of excess air. The net temperature relates to the temperature difference between the flue gas and the ambient air temperature in the boiler house.
It is recommended to have boiler combustion efficiency checked at least once a year.
4.4.5 Heating System Maintenance - Poor Practice
Some examples of poor practice are shown in Figures 4.2. The first image shows an older
type boiler in poor condition. There are various signs of rust and fouling across the unit, as
well as signs of sulphur around the boiler. The second image shows signs of poor
maintenance, there is corrosion on the pipe and the pipes and valves are also uninsulated.
Figure 4.2 – Examples of poor practice with heating systems
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 16
Providing effective maintenance of LTHW boilers is one way to obtain optimum efficiency.
Two types of maintenance exist, preventative and reactive with the former preventing
problems and the latter being carried out after a fault has been identified. Experience shows
that in the long term, preventative maintenance can be far more cost effective than waiting for problems to occur and then responding to them.
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 17
5 Building the Business Case
5.1 Before Considering a Boiler Upgrade Project
When thinking about replacing boilers, it is important to consider all other low cost
measures which could be implemented to save fuel consumption prior to proceeding, so as
not to distort the business case. For example, it may be prudent to ask the following
questions of the project before proceeding:
1. Have the heating and boiler controls been optimised? It is important to make sure that
energy is not being wasted through poor control as this can usually be rectified for
relatively low cost.
2. Is insulation up to standard? Insulating pipework and valves is a low cost way to save
energy and money, and this should be taken account of before considering a new boiler
plant.
3. Is the best fuel being used? If the site uses oil, is there a natural gas connection
available? Could gas burners be retrofitted to the existing boilers?
4. Is the building appropriately zoned? Could improved zoning help with better control? If
so, this could be done before replacing boiler plant, or it could be included as part of the
same project.
Once all of the above have been considered, an effective business case can be made.
5.2 Information Gathering
To build an effective business case, one of the most important things is to gather accurate
information. In particular, collect the following data:
The building or boiler total fuel consumption;
The boiler combustion efficiency;
The building peak heating demand; and
The age of the existing boilers.
In addition, consider any other site specific actions which may influence the viability of the
project, for example:
Is the boiler house in the basement?
Could flue routes be complicated?
Is redundancy required?
How will hot water be provided?
5.3 Business Case - Case Study
Consider the following system:
A system comprising two 200 kW boilers was installed in 1985;
Total annual gas consumption is 712,600 kWh;
Existing boiler efficiency is 75%;
Gas price is 3.2 p/kWh;
Current annual running cost is £22,800; and
Building peak heat demand is 180 kW.
For the current system:
Heating demand = gas consumption x efficiency, which is 712,600 x 0.75 giving a total
demand of 534,450 kWh.
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 18
If a boiler with a seasonal efficiency of 91% is installed:
o New gas consumption = 534,450/0.91, equating to 587,308 kWh.
o Annual saving = 712,600 – 587,308 = 125,292 kWh.
o Cost saving = 125,292 x 0.032 = £4,009.
Assume that three 90 kW boilers will be installed, giving a scenario where one boiler could
fail but the other two could still meet the building peak heating demand.
A full boiler house upgrade, with new boilers, pipework and all ancillaries installed would cost in the region of £90,000, giving a simple payback of 22 years.
This serves to demonstrate how difficult it is to make the economics of a new boiler stack
up on energy cost saving alone, unless the existing boiler has reached the end of its
economic life and needs replacing anyway. CIBSE estimate that the life of LTHW boilers is
20-25 years. Therefore, in our example the boiler has exceeded the normal economic working lifetime and it is likely that it should be replaced.
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 19
6 Useful Links and References
Title Source Field Link
Low temperature hot water boilers The Carbon Trust LTHW Boilers www.carbontrust.com/media/7411/ctv051
_low_temperature_hot_water_boilers.pdf
BS6644:2011 Specification for the
installation and maintenance of gas-fired hot
water boilers of rated inputs between 70 kW
(net) and 1.8 MW (net) (2nd and 3rd family
gases)
British Standards Gas fired hot Water
Boilers
Guide B1 – Heating CIBSE Heating
Options Appraisal Toolkit Resource
Efficient Scotland Options Appraisal
www.resourceefficientscotland.com/resourc
e/options-appraisal-toolkit
Efficient Operation of Low Temperature Hot Water Boilers in the Public Sector | 20
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