Air Source Heat Pumps In Historic Buildings

To gain insights into the factors influencing the performance of air source heat pumps (ASHPs), a total of ten case studies were undertaken by Historic England. These case studies encompassed various settings such as residential homes, offices, shops, and churches located in different regions including Cheshire, Cornwall, Cumbria, Gloucestershire, and London. The selected case studies predominantly featured 18th and 19th century buildings, exhibiting diverse construction styles, with six of them being listed buildings. All of these properties relied on ASHPs as their primary source of space heating. Among the ten properties, five utilised monobloc (air-to-water) ASHPs, while the remaining five employed different forms of direct expansion (air-to-air) ASHP technology.

What Are Monobloc & Direct Expansion Heat Pumps?

  1. Monobloc Heat Pumps: Monobloc ASHPs use a single outdoor unit to extract heat from the ambient air and transfer it to the indoor space or heating system. The outdoor unit contains all the necessary components, including the compressor, condenser, and expansion valve. It circulates a refrigerant through a coil that absorbs heat from the outside air and then delivers it to the indoor space or heating system through a separate water circuit. Monobloc heat pumps are typically used for heating water in a hydronic heating system, such as underfloor heating or radiators.
  2. Direct Expansion Heat Pumps: Direct expansion ASHPs consist of both an outdoor unit and an indoor unit. The outdoor unit contains the compressor and the condenser coil, while the indoor unit comprises the evaporator coil and the air handling unit. The refrigerant circulates between the two units, absorbing heat from the outdoor air at the evaporator coil and releasing it inside the building through the indoor unit. Direct expansion heat pumps are commonly used for providing heated or cooled air directly to the indoor space, often using ductwork or individual air handlers.

Case Study Findings


The noise from external ASHP units was not reported as an issue at any property. Given that the public discussion around ASHPs often focuses on the noise generated by external units, this was unexpected. The occupants at one property reported being aware when the external unit first started up but could not hear it when it was running. In this case, the ASHP was mounted to the house using a wall bracket with little anti-vibration dampening. This may have allowed the vibration from the initial compressor start-up to be transferred into the building’s structure.

Cold air

No occupants reported an issue with the cold air discharged from the external unit. ASHPs discharge a plume of cold air while running. This cold air movement can be felt up to 3m from the unit. Walking briskly through the discharge plume is unlikely to cause discomfort. However, positioning the ASHP where someone might be required to linger in front of it – when unlocking a front door, for example – would be uncomfortable and should be avoided.


Nine out of ten users did not notice the short interruption to their heating system that occurs when the ASHP is required to defrost and were, in fact, unaware of the defrost function. During certain external weather conditions, frost will form on an ASHP’s external evaporator. The frost must be periodically melted to allow the ASHP to continue to operate. This is known as a defrost cycle. Defrost times for modern ASHPs are between two and 10 minutes. While the ASHP pump is defrosting, it is normally unable to deliver heat to the building. The heavyweight construction of some historic buildings makes them resilient to short interruptions in heat.


Generally, condensation water from external units is allowed to run over the ground to a nearby drain. This water will freeze in winter and become potentially hazardous. In all the studied properties, pedestrians did not need to walk between the ASHP and drain. Therefore, ice caused by condensate water was not reported as an issue by any participant, but it was noted as a potential slip hazard at some properties. In two properties, condensate water dissipated through a gravel/slate bed that surrounded the base of the external unit, thus eliminating any potential hazard.

Visual impacts

Given that all the case studies involved historic buildings, it was assumed that most of the ASHP external units would be hidden from view. This was not the case. Only one property attempted to conceal the external unit. None of the occupants in the houses were dissatisfied with how the external unit looked, despite the fact that it was visible from their private gardens. The only property that had attempted to conceal the ASHP external unit experienced unintended consequences. To reduce the visual impact of the unit, the base had been sunk 500mm below the ground level so that the top of the unit was below the level of the garden boundary wall. However, from
the street, this slight drop in level made little difference. On the garden side, a hurdle fence had been erected to screen the unit although part of this screen had been blown down by winds. Dropping the level of the unit has caused issues with snow building-up around the ASHP in winter and leaf litter build up in autumn. The snow and leaves must be removed manually for the ASHP to operate without fault. Although no issues were reported at this property, dropping the level in this way could also leave the ASHP vulnerable to damage from flooding.

Heat Pump Running Costs

The temperature of the outlet water from an air source heat pump (ASHP) significantly influences the ratio between the electricity consumed and the amount of free heat produced. To illustrate, on a cold day with a temperature of 2°C, a modern ASHP has the capability to supply heat at around 8.0p/kWh when the water leaving the ASHP is at 35°C. This temperature of 35°C is suitable for underfloor heating systems or adequately sized radiator systems. However, if the ASHP is required to produce water at a higher temperature, such as 50°C for smaller radiators, the efficiency would decrease, resulting in heat delivery at approximately 11.8p/kWh.

By way of comparison, a modern mains gas boiler typically provides heat at a cost ranging from 7.4 to 8.1p/kWh, depending on factors like the system design and commissioning quality. It’s important to note that these cost figures are specific to the time when the surveys were conducted and may not reflect current prices.

Occupant’s Experience

The case studies found that to achieve the lowest feasible operating temperature for the system, an alternative approach is to enable prolonged heating periods. For buildings that are regularly occupied throughout the week, it is usually economically advantageous to maintain continuous heating at a low temperature. However, in the case of intermittently occupied structures like holiday homes or churches, running the heating system when the building is unoccupied may not be the most cost-effective choice. The key findings from the case studies were that:

  • The ASHPs delivered cost-effective heating at low temperatures when allowed to run for long periods without interruption.
  • Occupants who operated their ASHPs continuously had the best perception of thermal comfort and were generally happy with their
    heating system.
  • Occupants who were unsatisfied with the temperatures of their properties in the morning did not have the necessary knowledge to extend their system run times and would benefit from additional training.

Heat Pumps In Historic Buildings

Both monobloc and DX (direct expansion) systems possess distinct advantages and disadvantages. These distinctions should be taken into account when deciding on the appropriate type of air source heat pump (ASHP) that aligns with the building’s requirements and intended applications. One recurring issue was that occupants lacked awareness of how to adjust the timer settings on their ASHPs. As a result, some users resorted to using electric heaters to ensure comfortable conditions upon waking up in the morning rather than extending the runtime of their ASHPs. Full system instructions are included in your handover pack, once the heat pump has been commissioned, which will allow you to optimise its energy performance.

Based on the interviews conducted with the occupants, it was evident that the visual appearance of the air source heat pumps (ASHPs) was not a concern for them. Since all the case studies involved historic buildings, it was presumed that various measures would have been implemented to minimise the visual impact of the external units. However, only one participant had taken steps to conceal the unit behind a fence. Interestingly, when part of the fence was damaged in strong winds, it was not repaired, indicating that the visual appearance was not considered a significant issue.

In terms of noise and the release of cold air, these factors did not pose problems for the occupants in the case studies. The positioning of the external units in the residential case studies was carefully chosen, away from noise-sensitive areas and in locations where cold air plumes would not cause any inconvenience.

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