Installing Heat Pumps in Historic Buildings

Assessing heating demand

Heat pumps need to be sized to match the heating demand. As with any heating system, it is important to calculate the peak heating load for the building and the extent of heat loss in order to be able to determine the amount of heat energy required to maintain comfortable conditions. There have been examples of oil and gas heating systems that use approximate values to calculate heat losses. However, you can end up with an over- or under-sized system. With heat pumps, the heat loss should be assessed on an elemental basis, calculating what is lost through the walls, ground, roof, and windows to maximise system efficiency. Knowing the proportion of the heat loss through each fabric component allows the designers to appraise the impact of any proposed fabric improvements.

To gain a thorough understanding of the building’s environmental performance, it is advisable to carry out monitoring and modelling through on-site investigations. This could include in-situ U-value measurements, hygrothermal and dynamic thermal modelling, air pressure testing, thermal imaging, and on-site weather data collection. More information can be found in our research report 'A Retrofit of a Victorian Terrace House in New Bolsover: A Whole House Thermal Performance Assessment'.

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 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. 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 demand 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.

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 energy originally used to make the equipment can be considerable. 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. The type of 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. These are more expensive than heat pumps that use conventional refrigerants, which typically supply heated water up to 55°C. The use of these types of heat pumps is usually limited to commercial domestic hot water systems, due to the limitations of the refrigeration cycle.

Underfloor heating systems

The lower flow temperatures of most heat pumps make them particularly suitable for use with underfloor heating systems. However, 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 due to the lower temperature difference between flow and return pipework than that of conventional heating systems. This means that the flow rate through the pipework will increase in order to provide the same amount of heat. As such, pipework and pumps need to be reviewed by a technically competent person.

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. Early engagement with the energy provider is essential to check if the existing supply has the capacity or if a new supply is 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.

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.

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. 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. 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, which makes them more suitable where quick warm-up times are required.

Indoor units always incorporate a fan and therefore make noise when heating or cooling, making them less suitable for residential applications.

Most air-to-air systems also allow the unit to be reversed and run in a cooling mode.

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. Some manufacturers can colour the external units so that they 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.

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

Heat emitters need to be carefully sited if the interiors are of historic interest. There is a trend to site them high up on the wall like an air conditioning unit. This does not need to be done. They can be sited lower than a conventional radiator, and the chassis can be removed and fitted into a decorative wooden or metal enclosure to match the interior.

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 can be fitted in the building.

Air source heat pump units 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.

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 or not the building is historic

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 loop 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.

Ground loops

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 large trenches or boreholes with large plant machinery. Consents will be needed if the works are in the setting of a listed building or affect a scheduled monument, or the site is in a Conservation Area. Possible harm to wildlife and habitats will also need to be considered. There may be other considerations too, such as existing underground services and cables, and access to the site.

Before starting work on-site, it is important to assess the possibility of unrecorded buried archaeology. In cases where there is known or suspected buried archaeology present, an archaeologist should be commissioned to undertake a watching brief during the groundworks.

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.

Documentation of the GSHP loop should be kept, as it could be easily damaged by later work on-site if the location is not known.

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 by an estimated 46%, from 45 tonnes to 23 tonnes per year.

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 on the second floor of the New South Engine House. This building 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.

The ground source heat pump installation was managed by Historic England’s main contractor Croft Building and Conservation Ltd with advice from E3 Consulting Engineers LLP, and JPR Mechanical and Electrical Services Ltd as the subcontractor. The specialist heat pump contractor was Geo Green Power.

Water source heat pumps

Water source heat pumps extract heat from lakes, rivers, canals, or the sea. The water temperature is often higher than the ambient air temperature in the winter and a heat pump can multiply this up to four times. Like GSHPs, the collector can be an open or closed loop system. However, water source heat pumps are only viable if the water body is close to the building to be heated. If the body of water is not large enough, there will be a net cooling effect, lowering the average temperature of the water body and reducing the efficiency of the heat pump.

Water, together with anti-freeze, is pumped around a loop located in the water body/watercourse. The heat is transferred by the heat pump into the heating system. Trenching is needed between the building and the water source for the flow and return pipework to run.

In an open loop system, water is extracted from the source and passed through a heat pump to increase the temperature. The water is then pumped back into where it came from but in another location, ensuring short-circuiting will not occur. Some type of filtration system is needed.

It is also possible to use water source heat pumps in a cascade configuration to increase the flow temperature from a low-grade heat source, such as ground source heat pumps. In this configuration, the water source heat pump can be designed to only operate seasonally when the heating demand is high or to raise the design flow temperature for part or all of the heating system.

The Chartered Institution of Building Services Engineers’ Code of Practice: CP2 Surface Water Source Heat Pumps: Code of Practice for the UK - Harnessing Energy from the Sea, Rivers, Canals and Lakes provides guidance on standards and best practice.

Britain’s largest marine source heat pump is at the National Trust’s Grade I Plas Newydd mansion on Anglesey. Sea water from the Menai Strait is pumped through pipes to and from a heat exchanger on shore, and then up 30 metres of cliff face to the mansion’s boiler house. The heat pump system replaces the house’s two oil heating boilers.

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.

Documentation should be kept of the loop installation as there is a risk future water management work could damage it.

Designers and installers

When commissioning a heat pump system, it is important the designer has the knowledge and experience necessary to work on historic buildings as well as heat pump systems. They will usually be chartered engineers and members of the Chartered Institution of Building Services Engineers (CIBSE) or the Institution of Mechanical Engineers (IMechE). It should be possible to view examples of the installer’s previous work, which will give you a good indication of whether they are the right company to match to your particular needs. They should be able to demonstrate their knowledge of commissioning and optimising heat pumps to maximise efficiency and reduce energy costs and carbon emissions.

Industry associations such as the Ground Source Heat Pump Association and the Heat Pump Association provide information on installing heat pumps and contractors. The Microgeneration Certification Scheme (MCS) has its own accreditation scheme for installers. However, these organisations have yet to identify specialist installers for historic buildings, so it is important to look at examples of designers and installers work and check their understanding of the constraints of working in a historic building. This should include how to minimise damage to historic fabric, as well as knowledge of commissioning and optimising heat pumps to maximise efficiency and reduce energy costs and carbon emissions. It will help the installers to provide an accurate quote if you can specify as much detail as possible. For example: where pipes are going to run; how they will be fixed; ground array locations; depths of ground source systems; and the location of any joints or manifolds where maintenance access may be required in the future. It is worth considering asking the installer to visit two months after the work is completed to check it, as well as during the following heating season.

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.