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Historic England (HE) has issued new Technical Guidance on ‘Installing Heat Pumps in Historic Buildings’, looking at 3 types of heat pumps in historic buildings: air, ground and water source.
Historic England writes:
… 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 60oC 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 60oC. 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)
Ground source heat pumps (GSHP)…
