Installing Heat Pumps in Historic Buildings

Assessing heating demand

Heat pumps work best when sized to match the peak heating load. The peak heating load is determined by calculating the heat loss through all of the building fabric components, including the walls, ground, roof and windows. The uncontrolled heat loss through the fabric, known as infiltration, will also need to be considered. Knowing the proportion of the heat loss through each fabric component allows the designer to appraise the impact of any proposed fabric improvements. Rule of thumb heat loss calculations are appropriate for estimating the heating capacity of heat pumps, but more detailed heat loss calculations are required for accurate sizing.

For most installations, the infiltration will be estimated based on the form and exposure of the building. This can result in inaccuracies and for sensitive installations where the heating and/or energy performance is vital for the end user and how the building is used, it can be useful to determine the amount of infiltration via testing.

The amount of heat that is lost through each fabric component is based on a heat transfer coefficient, known as a U value. The U value can be accurately calculated in building fabric constructions where all of the materials and their thicknesses are known. In older buildings where the exact building fabric composition is unknown, estimating the U value can result in inaccuracies. For sensitive installations where the heating and/or energy performance is vital for the end user and how the building is used, it can be useful to determine the U value via testing.

Thermal imaging can be used to identify areas of the building where the most heat loss is occurring. Whilst thermal imaging can be helpful in identifying these areas of high heat loss, it will not quantify the amount of heat loss. For more information visit: Thermography in Historic Buildings.

Reducing heating demand

It is always advisable to see if any improvements could be made to the building to reduce heat losses and heat demand. Minimising heat losses will not only reduce the size of the heating plant and plant room space required, but also the costs of running the heat pump and its carbon impact. Our Energy Efficiency pages provide guidance on how to improve energy efficiency and insulate historic buildings.

Building condition strongly influences energy performance. Maintenance is vital. Draughts from cracks and poorly maintained doors and windows will contribute further to heat loss. Our Looking After Historic Buildings pages feature advice on maintenance.

Building usage

When designing any heating system, it is essential that there is a good understanding of the building usage patterns. Think about what times different areas of the buildings are used, who they are used by, and what activities take place. If the building use is likely to change in the future, then this is important to consider. This information will be helpful to the designer in ensuring the heating system meets the needs of the people using the spaces.

System configuration

Heat pumps can be sized to cover the entire peak heating load with no other heat generation source. These are known as monovalent systems. Alternatively, heat pumps can be sized to just provide the base heating load, with gas boilers (or another heat source) used to meet the peak load. These are known as bivalent or multivalent systems; they are normally implemented to reduce the size of the heat pump for space and cost reasons.

One disadvantage of bivalent systems is that they will still use a secondary heat source which will have higher operational carbon emissions than a monovalent heat pump system. The controls are often more complex in a bivalent system and it is important to check that the heat pump is operating as per the heating design strategy. Bivalent systems may be preferred where there is limited capital budget for the installation or if there is limited electrical capacity.

An example of a bivalent system is the ground-source heat pump scheme at Shrewsbury Flaxmill Maltings, where the ground-source heat pump system is designed to provide an estimated 69% of energy usage for the Main Mill and Kiln, with the remainder provided by natural gas boilers.

Heat pumps can also be used in a community scheme where they serve multiple buildings. The Bunhill Heat and Power Network in Islington is an example of a district heating scheme where hot air is extracted from the underground train tunnels to provide heat for local homes, schools and leisure centres. There are other examples of district heating schemes where heat pumps are installed in public parks or waterways.

Existing heating systems

Many historic buildings will have existing heating systems with pipework and emitters that are in good condition. It should not be assumed that these systems will need to be entirely replaced.

Reusing heating systems should always be considered, as the embodied carbon associated with replacing the equipment can be considerable. While upgrading or replacing components such as radiators and pipework can involve a significant upfront embodied carbon impact, this may be justified where it enables a heat pump system to operate more efficiently at lower flow temperatures. In such cases, the initial embodied carbon can be offset over time by reductions in operational carbon emissions and energy use.

Decisions should therefore be informed by a whole life carbon perspective, balancing the conservation of existing fabric and services with long-term operational performance.

An assessment of the existing heat emitters and pipework should be undertaken. However, it may be that radiators need to be supplemented or replaced with larger versions, in order to be suitable to work with the lower water flow temperatures that some heat pumps require.

Early examples of heating systems and emitters are likely to be of historic interest and should be conserved. For more information, see our advice on recording and conserving historic building services.

Heat emitter and pipework design

The efficiency of most heat pumps increases as the water supply temperature is decreased. Therefore, to maximise efficiency and reduce running costs and carbon emissions, the heating systems are designed to operate at lower flow temperatures. Heat pumps circulate heated water at much lower temperatures than conventional gas and oil boilers, so heat emitters and pipework need to be sized to be large enough to provide adequate heat to rooms.

Heat pump systems also have a low temperature difference between flow and return pipework which means that the flow rate through the pipework will increase in order to provide the same amount of heat. The pump and the size of emitters and pipework needs to be assessed by a building services engineer.

There are heat pumps available that can provide supply water temperatures of up to 110°C, however most commercial high temperature heat pumps provide heated water up to around 75 to 80°C. These are less efficient than the majority of heat pumps, which typically supply heated water up to 55°C.

Underfloor heating systems

The lower flow temperatures of most heat pumps make them particularly suitable for use with underfloor heating systems. However, most underfloor heating systems have a slow response time, which means that they take a long time to reach the desired room temperature. Accordingly, underfloor heating systems are best suited to a building that is used frequently throughout the week.

Raising floor levels, or changing or replacing them, to accommodate underfloor heating may need listed building consent due to the impact of the alterations not only to the floor and substrate, but also to changes to skirtings, doors, and steps.

Electrical supply

Heat pumps use electricity to generate heat and require a power supply. Domestic installations will normally only need a single-phase supply, but larger installations will require a 3-phase supply. Assessment of the existing electricity supply would need to be undertaken to understand what type of electricity supply there is and its size.

Depending on the outcome of the assessment it may be that an upgrade will be required to the power supply for the heat pump so engagement with the energy provider may be required.

In some rural communities, this may be challenging where there have not been upgrades to the electricity network yet to support the electrification of heating. On some larger projects, the electricity capacity can be capped by the network and increased in stages, often on an annual basis. This can be useful where a hybrid heating system of heat pumps and boilers is installed because the total amount of heating provided by the heat pump can be increased over time as the network upgrades the electrical infrastructure.

In larger buildings, an increase in electrical capacity may require a new substation. Early engagement with the Distribution Network Operator is recommended because substations can be expensive and have a significant impact on the project programme.

Buffer vessels

Buffer vessels can be added to increase the heating system volume, maximise heat pump run times and efficiency, and buffer any discrepancy between the heat pump flow rate and the heating system flow rate. Buffer vessels also have a role in defrost cycles.   

Buffer vessels are not required in all heating systems, as in some heat pumps the management of the minimum system volume is incorporated into the design. They can also result in reduced heat pump performance depending on how they are installed.

Domestic hot water

A heat pump can also be used to heat domestic hot water. The heated water is transferred to a hot water cylinder, much the same as with a conventional indirect hot water system. Domestic hot water has to be stored above 60^oC to prevent legionella bacterial growth and associated health risks.

Most heat pumps provide hot water at 55°C, so an internal or external electric immersion is needed to increase the temperature of the water to 60^oC. The consumption patterns need to be calculated so that the heat pump can be sized to meet the domestic hot water demand.

Consents and permissions

The installation of a ground source heat pump or a water source heat pump at domestic premises is usually considered to be permitted development, not requiring planning permission. Similarly, air source heat pumps are considered permitted development. However, these must comply with conditions listed on the UK Planning Portal website.

Consents are likely to be required for installing any type of heat pump in listed buildings or buildings in conservation areas, scheduled monuments, or installations that affect designated wildlife sites. Installation works need to take into account bats, birds, water voles, great crested newts and other protected species. Licences may be required. The section on water source heat pump considerations outlines the range of ecological advice you might need when installing heat pumps.

All heat pump installations have to comply with Building Regulations, and guidance is set out in the Approved Documents.

Ground and water source closed-loop systems generally do not require a permit from the Environment Agency. Where they are installed adjacent to or in a watercourse, consent may be required. Owners are liable for any adverse effects that may be caused by their system (such as a leak that could cause pollutants to enter the groundwater).

For open-loop systems, groundwater investigation consent from the Environment Agency is needed before drilling or test pumping any abstraction boreholes. If work proceeds, you will require an abstraction licence and a permit to discharge. Similarly, an abstraction license and a discharge permit are required for open-loop surface water systems. For further advice, see open-loop heat pump systems: permits, consents and licences.

Air source heat pumps (ASHP)

In an air source heat pump, the evaporator extracts heat from the outside air to warm the liquid refrigerant and turn it into a gas, which is then compressed to further raise the temperature. This allows heat to be extracted from the air, even on the coldest of days. The condenser is the part of the heat pump that delivers the heat into the building. All ASHPs house the evaporator in the outdoor unit.

There are two types of ASHPs: monobloc (air-to-water) and direct expansion (air-to-air). The difference between the two types is the location of the condenser. In a monobloc heat pump, the condenser is located inside the outdoor unit; in a direct expansion, the condenser is located remotely from the outdoor unit.

Monobloc heat pumps

A monobloc heat pump (air to water) can be connected directly to a wet heating system of radiators and underfloor heating. The pipes connecting the outdoor condenser unit and the building are filled with heating system water.

Direct expansion heat pumps

Direct expansion systems (air-to-air) transfer heat into the building using refrigerant rather than water. The remote condenser can interface with a traditional wet heating system via a heat exchanger or directly with the individual room heat emitters or indoor units(these emitters or indoor units are often called air conditioning units, particularly when they are used for cooling). Systems that use indoor units to deliver warmth are referred to as air-to-air heat pumps.

Direct expansion heat pumps can provide higher air delivery temperatures without sacrificing heat pump efficiency and the indoor units have fans which circulate the heated air. This makes them more suitable where quick warm-up times are required.

Indoor units always incorporate a fan and therefore make some noise when heating or cooling.

Most air-to-air systems also allow the unit to be reversed and run in a cooling mode. In this mode, the heat exchanger in the indoor units becomes the evaporator and the heat exchanger in the outdoor unit becomes the condenser.

ASHP installation considerations

With ASHP installations, there are fewer groundwork issues compared to other types of heat pumps. However, the visual impact of the outdoor condenser unit and the need for a clear unobstructed air path will need to be considered. There is a growing range of designs to choose from and the colour of outdoor units can vary between manufacturers. In some cases, these options may help the outdoor unit blend into their surroundings. It is also possible to screen the unit or put them in plant rooms, providing that manufacturer guidelines are followed so as not to interfere with the air intake and discharge. There is the option to install digitally printed vinyl wraps to the outdoor units, however you would need to check that these would not void the manufacturer’s warranty. 

The external heat pump will produce condensate which will need to be piped to either a soakaway or external drain.  

It is worth checking if the installation area is at risk of flooding now and in the future, so as to ensure that the outdoor unit will not be damaged by flood water. 

Heat emitters need to be carefully sited if the interiors are of historic interest. The default installation for indoor units in air-to-air systems is to site them high up on the wall. However, there are more types of indoor units available. They can be sited at a low level like a conventional radiator, and there are chassis versions which can designed to be installed within ceiling voids or inside a decorative wooden or metal enclosure to match the interior. 

With air-to-water systems, it may also be necessary to increase the number or size of heat emitters to cope with the lower water flow temperatures. In historic buildings, the location of radiators will need to be carefully planned. Higher output radiators are available, but you will need to consider if their increased size is appropriate and if larger emitters will physically fit. 

Air source heat pumps always incorporate a fan and compressor. Both generate noise when heating or cooling. The manufacturer’s noise data and the positioning of units will need to be carefully considered. In locations sensitive to noise, it is often possible to mitigate this with acoustic measures although this is an additional installation cost. An acoustician may be required to advice on acoustic measures, particularly where planning conditions require specific acoustic criteria to be satisfied.

ASHP case studies

There are many examples of ASHPs performing well in historic buildings in England. However, there are also instances where new ASHPs have not met expectations. When poorly performing systems are reported openly without exploring the underlying issues, people may conclude that heat pumps are not a suitable replacement for fossil fuel heating systems in older buildings.

In 2021, we commissioned environmental building services engineers specialising in net zero technology to review the performance of ASHPs in ten small-scale historic properties, including residential homes, offices, shops and two small churches to understand variations in the success of installations.

The key findings were:

• ASHPs worked well in a range of different historic building types and uses
• The choice of heat emitters and the type of ASHP system needs to be matched carefully with the building and its occupancy
• Building occupants need briefing on how to make best use of their ASHPs and reduce running costs
• The visual and noise impact of ASHP outdoor fan units and the cold air plume discharges were not an issue in any of the case studies. However, it is good practice to carefully consider the positioning of units to minimise impacts, whether the building is historic or not.

A further investigation was carried out looking at the viability of air source heat pumps in larger historic buildings. 

• Air source heat pumps work well in historic buildings and are a readily available means of decarbonising heating systems.  
• Issues with the heating system’s performance were usually caused by the design of the heating distribution or delivery, rather than by the air source heat pumps themselves. For example, convectively heating draughty areas with a high air change rate can be problematic, regardless of whether the heat source is a boiler or a heat pump.  
• The visual and noise impacts of the air source heat pump units were not an issue for any of the users, as also was the case in the previous air source heat pump study.  
• Carbon dioxide refrigerant systems present unique opportunities for reusing existing pipework, reducing embodied carbon emissions, and reducing unintended emissions caused by refrigerant leaks.  
• Building users need to have a good understanding of how to use system controls to maximise the efficacy of the system.

Ground source heat pumps (GSHP)

Below 1.5 metres, the ground temperature is a constant 8 to 12^oC all year round. This low-grade heat can be taken from the ground using a buried pipe known as the ground loop, around which a mixture of water and anti-freeze is pumped. The heat is then transferred from the liquid within the collector by the heat pump and upgraded to a higher temperature for heating.

Ground loops can be either closed or open, and installed vertically as a borehole or horizontally in a trench. The type selected will be dependent on the ground area available, as well as access and geological conditions. The British Geological Survey offers further information on site geology.

Boreholes

Boreholes are normally drilled where there is not a large land area available for a horizontal collector loop. The loops are usually installed into 100 and 150mm diameter boreholes, 15 to 120 metres deep. Water with anti-freeze (or brine) is pumped around these loops. 

Boreholes must be correctly spaced. If they are too close together, the net cooling effect on the ground will reduce the efficiency of the system. The cost and efficiency of the system will also be highly dependent upon location. A thermal conductivity/response test can be carried out and this would give the installer confidence about the amount of heat that can be extracted from a borehole. This can sometimes result in fewer boreholes being installed which can significantly reduce the installation cost.

Horizontal ground collectors

An alternative to using a borehole is to employ a horizontal or 'trench' loop. These are commonly laid in trenches 1.2 to 2.5m deep. Horizontal loops typically require a larger land area than borehole loops. Another way of laying horizontal loops is to use a ‘slinky’ coil of pipe, so that the pipes overlap and less area is required. Horizontal loops can be considerably cheaper than vertical boreholes. They are also usually slightly less efficient than boreholes due to the fluctuating ground temperature near the surface.

Closed loop is the most common type of ground source heat pump. A sealed loop of pipe is filled with a mixture of water and anti-freeze. The liquid increases in temperature as it passes around the loop through the warmer ground. This heat is then transferred from the loop by the heat pump.

Open loop systems are not often used. Groundwater is pumped from boreholes drilled into underground water sinks (aquifers) or water is extracted from lakes, rivers and the sea. Heat is transferred from the water via a heat pump, with the water re-injected into the ground through another borehole. The re-injected water should not mix with the extracted water, or the system will be less efficient. This mixing is known as ‘short-circuiting’.

GSHP installation considerations

Installation of GSHPs involves digging trenches or boreholes, not all sites are suitable. Works to listed buildings, scheduled monuments, burial grounds and sites of special scientific interest would require consent or licence. On scheduled monuments contact the Historic England local office. In the curtilage of a domestic dwelling or block of flats GSHPs are permitted development, but more extensive works may need planning consent. Before designing the work, contact your local government archaeological officer for advice about what may be under the ground. On known or suspected archaeological sites please hire a professional archaeologist. Possible harm to wildlife and habitats should be considered. There are also other considerations such as buried services and cables, site access, tree roots and drainage.

Pipes will need to run between the boreholes or loops and the heat pump, and also through the building's external wall. Care will be needed not to damage historic fabric. Generally, GSHP pipework enters the building below ground level. The design of the system will also need to ensure that the manifold chamber provides sufficient access for maintenance and adequate drainage. Additionally, there should be sufficient space for the indoor plant room and access to and maintenance of all the items of the GSHP plant.

Accurate as installed record drawings of the GSHP collector and distribution pipework should be provided, to mitigate the risk of damaging the pipework in the future.

Case study: Shrewsbury Flaxmill Maltings ground source heat pump

The new ground source heat pump at Shrewsbury Flaxmill Maltings demonstrates that centuries-old buildings can also adapt to use sustainable energy sources and play their part in efforts to tackle climate change. The low carbon energy source is an important part of Historic England’s regeneration project and our organisation’s commitment to climate change mitigation.

The Shrewsbury Flaxmill Maltings site has eight listed buildings. The Grade I listed Main Mill, built in 1797, was the world's first iron-framed building and paved the way for modern skyscrapers.

The restoration of the Main Mill and the Grade II listed Kiln was supported by the National Lottery Heritage Fund.

The ground source heat pump system has been designed to provide an estimated 69% of energy usage for the Main Mill and Kiln, with the remainder provided by a natural gas boiler. The floor area is 3,999m².

The heat pump reduces carbon emissions associated with space heating when compared with a heating system that only uses gas boilers.

The heat pump extracts heat from the ground via 10 vertical boreholes at a depth of 187 metres. These are underneath the line of the former Shrewsbury and Newport Canal towpath, a new green corridor with a pedestrian and cycle route.

The main heat pump plant is located in the New South Engine House. This used to house the steam engine which powered the flax-spinning machinery, a fitting home for the building’s new energy source.

The heat pump installation comprises two Stiebel Eltron 59kW pumps. They supply water up to 57°C to a 1000 litre buffer vessel, a thermal store for the heat sourced from below the ground. From the buffer vessel, hot water is distributed through the building heating pipework, radiators on the upper floors and the ground floor underfloor heating. The radiators are sized to cope with the reduced water flow temperatures from the heat pump.

If the demand from the building exceeds the output from the heat pumps, gas fired boilers provide a top-up heat source. For example, in the coldest weather conditions. The operating temperature of the overall heating system will automatically be reduced in mild weather in order to maximise the use of the heat pumps, and to optimise their efficiency.

Water source heat pumps (WSHP)

Water source heat pump considerations

The water source needs to be within a reasonable distance of the site, in order to minimise the energy required to pump the water to and from the heat pump to the water source.

It is advisable to identify annual temperatures and flow rates to determine if the water body is suitable. A detailed water source heat map is available to check potential heat capacity.

A full investigation of the water source then needs to be carried out to understand: how the water flows; its depth; width turn-over rate; the quality of the water; whether it is in full sunlight or shaded; and the role of the water body in the water catchment system. Where deep water is to be used, the water stratification needs to be considered. This is because, at certain times of the year, water at the bottom can be a higher temperature than water at the top. Existing services such as underground cables and drains will need to be considered.

The Environment Agency and other water protection bodies including navigation and harbour authorities, and neighbours up and downstream should be consulted as the water source heat pump installation may affect wider water systems.

The investigation will need to include a wildlife and habitat survey by a professional ecologist that includes an analysis of the temperature profile of plants and animals, nesting sites, and other uses. Local wildlife groups and fishing clubs may have useful records. The ecologist can also advise on any necessary wildlife licences and protection measures needed including the timing of installation work.

Installation work involves heavy plant machinery and mitigation against damage to water edges and setting of the water body. The work should not be carried out during the bird nesting season (February to August). Precautions must be taken not to harm other protected species, such as bats and water voles and their habitats.

As with GSHPs, there are likely to be other landscape and archaeology considerations too, especially in historic designed landscapes where the aesthetics of the water body will be important. Mitigation screening, such as reed beds, may not be desirable, although there may also be opportunities to create new wildlife habitats. Understanding the historic significance of a site will help determine design considerations and whether a heat pump is acceptable. You may need to commission advice from a historic designed landscape specialist.

Installation of a water source heat pump could also be an opportunity to undertake other maintenance work of the water body and its structures.

Pipes will need to run between the loop in the water body and the heat pump and the building. Generally, water source systems pipework enters the building below ground level. Consideration needs to be given to the design of manifold chambers and plantrooms, and access to plant for maintenance.

Accurate as installed record drawings of the WSHP collector and distribution pipework should be provided, to mitigate the risk of damaging the pipework in the future.

WSHP case studies

To learn more about the viability of WSHPs in historic buildings, we commissioned building services engineers specialising in net zero technology to review the performance of WSHPs in five historic properties. 

The key findings were: 

• Closed-loop water source heat pumps are a viable and readily available option for discreetly decarbonising heating systems in historic buildings. 
• When used for conservation heating, water source heat pumps can operate efficiently with existing radiators and pipework. 
• Many historic stately homes are located close to a body of water, because this would have been a priority when they were built. Water source heat pumps are, therefore, particularly suited to these buildings. When a body of water is available nearby, a water source heat pump system can be a more discreet solution than an air source heat pump system and a more straightforward installation than a ground source heat pump system. 
• Issues with a heating system’s performance are often due to its configuration, not the water source heat pumps itself. The open-loop system at Site 2 was the only one that experienced significant difficulties extracting heat from the source. 
• Closed-loop water source heat pumps have similar noise levels to other heating system components, such as large circulation pumps. They are installed inside a housing in a plant room, and noise reduction measures are, therefore, more readily available than they are for outdoor air source heat pumps. Acoustics should be considered, but may not require any more attention than those of a fossil fuel system. 
• Building users need to have a good understanding of how to use system controls to maximise the efficacy of the water source heat pump system. 
• The availability of skilled maintenance and installation contractors was an issue at a number of the sites. 
• Safe and effective maintenance options and a good understanding of the water source are essential when designing an open-loop water source heat pump system.

Designers and installers

Learn more

This webinar provides an overview of how heat pumps work, and how heat pumps will be key to the decarbonisation of heat and hot water generation in our buildings. The webinar covers the different types of heat pump technologies, design and installation considerations.

You can also watch the talk by Andrew McQuatt (Engineering Standards Project Leader, Max Fordham) on the viability of air source heat pumps for historic buildings from our Future of Heating conference in June 2022. This presentation highlights the considerations to take into account before installing air source heat pumps and explores ten case studies, including private residences and churches.