Air source heat pumps can be effective in historic properties, according to study

Historic England has published a study of different air source heat pump installations in historic buildings and concluded that the technology is able to deliver cost-effective heating at low temperatures when allowed to run for long periods without interruption.

The publication, in its series Heat Pumps in Historic Buildings, is designed to provide guidance for those considering installations in historic buildings – assessing projects for a range of factors, from noise to defrosting to capacity. The researchers for Historic England found a good level of satisfaction from occupiers, whilst concluding that a number of commonly reported issues could be mitigated relatively easily.

The publication looked at ten ASHPs in smaller scale historic buildings, including homes, churches and offices. The case studies were mostly 18th and 19th century buildings and included a range of building construction types, including six listed buildings, the body said. All of the properties had ASHPs installed as their primary space heating. Five of the properties used monobloc air-to-water ASHPs, while five had air-to-air ASHPs.

The body noted that where there was occupant dissatisfaction with temperatures in the installations it assessed, it was because the occupants did not have the necessary knowledge to extend their system run times. It concluded that occupants would benefit from additional training on system operation. Historic England added:

Occupants who operated their ASHPs continuously had the best perception of thermal comfort and were generally happy with their heating system.

They found that the noise from external ASHP units wasn’t reported as an issue at any property, which they said was ‘unexpected’ in light of public discussion around ASHP often focusing on the outdoor unit’s noise. They noted that one set of occupants reported noise when the external unit first started up but it was found to be mounted to the house using a wall bracket with little anti-vibration dampening.

No occupants reported an issue with the cold air discharged from the external unit. ASHPs discharge a plume of cold air while running, the body noted, but said that 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, due to an advantage of very old buildings – their wall thickness. The authors said:

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 [but] the heavyweight construction of some historic buildings makes them resilient to short interruptions in heat.

The authors were also surprised to find that only one property attempted to conceal the external unit, despite the juxtaposition with a historic building. The authors said:

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 researchers said that the quality of the electrical installations ‘appeared to be high in all cases’. They concluded this was a likely consequence of the prescriptive and regulated nature of the electrical industry in the UK.

The researchers found that none of the case study properties with monobloc ASHPs had a buffer vessel installed. Since buffer vessels can improve efficiency and ASHP run times, they suggested that buffer vessel installation could be a valuable addition on a project by project basis..

They also stressed that in historic buildings, as much as in any building, it is important not to oversize the ASHP or the heat emitters. They said:

Radiators that allow an ASHP to work at 35 deg C rather than 50 deg C could be up to five times more expensive and will take up more wall space. There is a balance to be struck here, and a good system designer will help select the best solution on a case-by-case basis.

They reported significant variation between the case studies and found that a high installed capacity did not always correspond to a comfortable or efficient building:

The person who is optimising the system needs to consider the heating system specification, building insulation values and user expectations. Consequently, striking the correct balance between comfort and running costs is best done by the user.

However, here they found that users were often not sufficiently familiar with how to optimise the heating curve using the controller’s advanced features.

The cost of running the systems was another element considered by the researchers. At the time, only one user considered their ASHP heating system to be a cheap alternative to other forms of heating. But the authors concluded this was probably because in 2021/22 at the time of the survey, electricity was approximately four times more expensive than gas.

They concluded that for the installations of ASHPs in historic buildings it was essential to have a good design approach:

Monobloc and DX systems each have their strengths and weaknesses. Their differences should be considered when selecting the type of ASHP to match the building and its uses. Several of the case studies had high-quality installations but were let down by the choice of heat emitter or the detailed hydraulic system design…A slow response underfloor heating system is well suited to a building that is used several times a week. However, in a building that is occupied only occasionally, it can seem like a waste of money to heat an empty building.