Not only do heat pumps function in freezing temperatures — they work far more efficiently than fossil-fuel heating systems in the cold.
That’s according to a team of researchers in Europe affiliated with the independent nonprofit Regulatory Assistance Project. They published a study in Joule this week that provides yet more evidence to debunk the myth that heat pumps can’t handle cold climates.
Electric heat pumps both heat and cool indoor spaces by moving warmth into or out of them as needed. And while global sales grew by 11 percent in 2022, according to the International Energy Agency, heat pumps still only account for about a tenth of the world’s building heating. To achieve the Paris Agreement’s target of net-zero emissions by 2050, heat pumps will need to replace far more fossil-fuel boilers and furnaces — including in places with frigid winters.
Extreme cold historically has been a barrier for the technology, with major utilities and fossil fuel interests asserting that heat pumps don’t work below freezing and pointing to drops in efficiency as evidence. But as the new study and examples from places including Norway and Maine have shown, modern heat pumps are reliable and outperform fossil-fuel heating in the cold.
To find out how well air-source heat pumps work as temperatures plummet, the team analyzed data from seven field studies across three continents, drawing on observations of different heat-pump models from Canada, China, Germany, Switzerland, the United Kingdom and the U.S.
These studies reported on heat-pump performance in the depths of winter — January — using a metric called the “coefficient of performance.” COP measures how much thermal energy you can get out of a heating system for every unit of energy you put in. Heating technologies that burn fossil fuels or use electric resistance convert one form of energy into another, so they hit a thermodynamic limit at 100 percent efficiency, or a COP of 1.
But heat pumps cleverly move heat around using refrigerant. They can routinely achieve COPs of 3 to 4, though higher values are possible, according to Duncan Gibb, senior advisor at the Regulatory Assistance Project and co-author of the new study. For instantaneous measurements (as opposed to those averaged out over a day), “I’ve seen some data where it gets up to 7.”
Efficiency declines when temperatures drop, however, as the gap between outdoor and desired indoor temperature widens. (This is the major reason why geothermal heat pumps, which draw heat from the earth and are much more insulated from the temperature swings in the ambient air, are more efficient.)
Even though the COP of air-source heat pumps declines as temperatures fall, the team found that heat pumps surpass fossil and electric-resistance systems in efficiency — including at temperatures that define, for many Europeans, “the coldest days of the year,” Gibb said.