Air-source heat pumps can operate at high efficiency in cold weather conditions, according to real-world monitoring data from the UK-based Electrification of Heat Demonstration Project.

“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,” said Marc Brown, interim business leader of Energy Systems Catapult. “We’ve observed the exact opposite. They are three times more efficient than gas boilers and work in cold weather conditions.”

The project is funded by the UK Department for Energy Security and Net Zero, which commissioned Energy Systems Catapult, a non-profit net zero innovation center, to report on the data. A total of 742 air-source heat pumps were installed in detached homes, semi-detached homes, and terraced houses, as well as flats. The age of the properties spans from before 1919 to 2001 and later.

The heat pumps were installed by three delivery contractors – Warmworks, E.ON, and OVO Energy. Their performance was monitored from November 2020 to August 2022, with seasonal performance factors (SPF) indicating their in-situ efficiency over the course of 12 months. Results show a median SPF of 2.80 across heat pump types, operational patterns, and home types.

“This is a significant increase of around 0.3 to 0.4 (30% to 40%) since the Renewable Heat Premium Payment scheme (RHPP) heat pump trial was undertaken between 2011-14,” Energy Systems Catapult said in a statement, referring to an incentive scheme put in place by the UK authorities. “Innovation in the industry and in the heat pump systems themselves is likely a leading factor in this performance improvement.”

Figures also show considerable performance variation across heat pump models, with refrigerant and flow temperature being key culprits. The median SPF for heat pumps using R32 refrigerant was 2.94, followed by propane (R290) at 2.89, and R410a at 2.66. Their global warming potentials (GWPs) are 675, 3, and 2,088, respectively.

As for operating flow temperature, data shows that heat pumps that can reach temperatures above 65 C have median SPFs between 2.89 and 2.92, while low operating temperature heat pumps have SPFs between 2.74 and 2.94. However, the report says that “operating above 65 C is not a common occurrence, with 81 of the 94 heat pumps operative above 65 C flow temperature less than 1% of the time.”

The project also analyzed heat pump performance during the coldest days of the year in the UK, when mean external temperatures varied from -5.8 C to 2 C. Their coefficient of performance (COP) was calculated for each of the cold days, with a mean of 2.44.

“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,” Energy Systems Catapult said.

Brown concluded that heat pumps have been shown to work.

“The UK is heat pump ready. Now we need to apply those learnings,” Brown concluded. “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.”