• Measured real-world performance of Air Source Heat Pumps (ASHPs) has increased significantly compared to data from past field trials.
• Heat pumps performed well even on the coldest observed days, with only a relatively small reduction in performance.
• High Temperature ASHPs had comparable efficiencies to Low Temperature ASHPs, indicating that they are a viable solution to reduce home retrofit requirements.

Real world monitoring has shown that Air Source Heat Pumps (ASHPs) can operate with high efficiencies, even in cold weather conditions, according to interim heat pump performance data released as part of the Electrification of Heat Demonstration Project.

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 – August 2022) have been released by Energy Systems Catapult; the Net Zero Innovation Centre appointed to lead the management contractor consortium. 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.

Seasonal Performance Factor (Efficiency) improvements

Seasonal Performance Factors (SPFs) indicate the in-situ efficiency of a heat pump system over the course of 12 months.

The median SPF observed in ASHP systems during the EoH Demonstration Project was 2.80 (280%)*. This is a significant increase of around 0.3 to 0.4 (30-40%) since the Renewable Heat Premium Payment scheme (RHPP) heat pump trial was undertaken between 2011-2014. Innovation in the industry and in the heat pump systems themselves is likely a leading factor in this performance improvement.

However, whilst the average performance has improved, performance variation remains high. This 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.

Good cold weather performance

The EoH Demonstration Project analysed the performance of heat pumps on some of the country’s coldest days (where mean daily temperatures fell to as low as -6^oC) and found only a marginal decline in whole system performance. The median ASHP system efficiency was 2.44 (or 244%) on the coldest days of the year. This result indicates that heat pumps continue to operate with high efficiency – providing the requisite heat to homes – in a wide range of property types even in cold weather conditions.

High Temperature Air Source Heat Pump performance

High Temperature ASHPs – those which can achieve flow temperatures similar to that of a gas boiler – are a viable solution for meeting the heating demand of a property. Use of High Temperature units reduces the need for deeper retrofit and provides a potential solution for less efficient homes.

The data released by Energy Systems Catapult demonstrates that the High Temperature ASHPs used in the Project have performed with similar efficiencies to Low Temperature ASHPs. Heat pumps are controlled to only demand higher temperatures when it’s colder outside. During the project, these weather compensation controls rarely demanded higher temperatures, allowing the units to maintain good heat pump efficiencies whilst keeping a wider variety of properties warm year-round.

Hybrid Heat Pump performance

The hybrid heat pump systems 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 (237%)*, which is lower than standalone ASHPs; giving an estimated overall heating system efficiency of between 1.26-1.42 (126-142%), depending on the efficiency of the boiler and the proportion of hot water demand**.  

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.

“While we’re taking steps in the right direction, we should not grow complacent. 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.

“This should be coupled with public and private investment in innovation to help transform the sector and deliver on our Net Zero ambitions, without losing sight of the consumer. Get this right and we can be a world leader in green heating solutions.”

*Note: SPFs and heat pump efficiencies mentioned use the SPF_H4 system boundary, accounting for electricity demand from and heat produced by the heat pump, immersion heater, back-up heater and circulation pump.

**Note: 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. It should therefore not be assumed that all heat pumps within hybrid systems will have a lower performance than standalone ASHPs.