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Field trial proves heat pump efficiencies

UK: Real world monitoring is said to have shown that air-source heat pumps can operate with higher efficiencies than gas boilers, even in cold weather conditions. 

According to interim heat pump performance data released as part of the Electrification of Heat Demonstration Project, the performance of ASHPs has increased significantly compared to data from past field trials, and performed well even on the coldest observed days, with only a relatively small reduction in performance.

Also, high temperature ASHPs had comparable efficiencies to low temperature ASHPs, indicating that they are a viable solution to reduce home retrofit requirements.

Funded by the Department for Energy Security and Net Zero (DESNZ), the Electrification of Heat Demonstration Project is working to understand the technical and practical feasibility, and constraints of a mass rollout of heat pumps into British homes.

A total of 742 heat pumps were installed by the delivery contractors – Warmworks, E.ON, and OVO Energy – into a broad spectrum of housing types and ages. These heat pumps are being monitored throughout the trial to assess their performance.

The interim data and results of this monitoring – conducted between November 2020 and August 2022 – have been released by Energy Systems Catapult; the Net Zero Innovation Centre appointed to lead the management contractor consortium. 

Marc Brown, business leader – homes, at Energy Systems Catapult, said: “The interim findings of the Electrification of Heat Demonstration Project highlight just how far the industry has come in innovating to improve the performance and efficiency of heat pumps.

“With the release of this data, we can finally put to bed the notion that heat pumps do not work in cold weather conditions and that they are inefficient to run. We’ve observed the exact opposite. They are three times more efficient than gas boilers and work in cold weather conditions. Innovation is changing the game in the heating sector.”

The project uses seasonal performance factors (SPFs), a measure of the COP over the course of 12 months. The median SPF observed in ASHP systems during the EoH demonstration project was 2.80. This is reported to be a significant increase of around 0.3 to 0.4 since the Renewable Heat Premium Payment scheme (RHPP) heat pump trial was undertaken between 2011-2014. 

The report cites innovation in the industry and in the heat pump systems themselves as the likely leading factor in this performance improvement.

However, whilst the average performance has improved, the report says the performance variation remains high. It suggests that progress is still required to improve the consistency of heat pump design and installation to support a large-scale rollout of heat pumps in existing homes and to deliver positive environmental and consumer outcomes.

Cold weather performance

The EoH project analysed the performance of heat pumps on some of the country’s coldest days (where mean daily temperatures fell to as low as -6ºC) and found only a marginal decline in whole system performance. The median ASHP system efficiency was 2.44 on the coldest days of the year. 


The project used systems with three different refrigerant types: R410A in 111 systems, R32 in 82 systems and R290 (propane) in 98 systems. 

While the performance of R410A was found to be significantly worse than either R32 or R290, direct comparisons are difficult as the R290 was used solely in HT heat pumps. 

The difference between the R32 HT and LT ASHP SPF results is significant. However, the R32 HT ASHP sample is very small, so no conclusions should be drawn from this insight. 

High temperature ASHPs

The data released by Energy Systems Catapult demonstrates that the HT ASHPs used in the Project performed with similar efficiencies to LT ASHPs. 

High temperature ASHPs – those which can achieve flow temperatures similar to that of a gas boiler – are seen as a viable solution for meeting the heating demand of a property, reducing the need for deeper retrofit and providing a potential solution for less efficient homes.

Hybrid heat pump

Hybrid heat pump systems used in the project were commissioned to run ‘cost-optimally’, with the system choosing when to operate the boiler or heat pump based on which is the cheapest option at a given time. Generally, the heat pump would meet the base space heating requirements and the boiler would cover hot water production and meet peak space heating demand.

The performance data reveals that the heat pumps in hybrid systems typically met about 39% of the space heating demand. They also had a median heat pump efficiency of 2.37, which is lower than standalone ASHPs; giving an estimated overall heating system efficiency of between 1.26-1.42, depending on the efficiency of the boiler and the proportion of hot water demand.  

The report does note, however, that this result may be due to how the heat pump was operated, the type of heat pump installed and the fabric efficiency of the homes. 

“While we’re taking steps in the right direction, we should not grow complacent,” commented Marc Brown. “At the risk of sounding like a broken record, innovation and training are critical to the success of a mass rollout of heat pumps. We’ve done the hard work and demonstrated that heat pumps work – the UK is heat pump ready. Now we need to apply those learnings. Government and industry should commit to investing in upskilling existing installers in low carbon heating solutions and doing more to attract new talent into the sector.”

The analysis within the reports – Interim Insights from Heat Pump Performance Data and Interim Heat Pump Performance Data Analysis Report – will be refreshed, and additional analysis undertaken after the completion of the projects monitoring period in Autumn 2023.

Related stories:

Cost and disruption are key barriers to heat pump rollout20 December 2022
UK: There is no UK property type that is unsuitable for a heat pump, but innovation will be required to overcome the challenges to adoption, a new report states. Read more…

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