Heat pumps are climate-friendly in two big ways: First, they don’t burn fossil fuels. And second, they’re very efficient. They can generate more heat with less energy than traditional methods like furnaces and boilers. And all that adds up to lower greenhouse gas emissions.
But there’s a persistent belief that they don’t work when it gets really, really cold outside.
Lower emissions are great. But will a heat pump keep you warm on the coldest nights? We asked the experts.
Pulling heat from thin air
Furnaces and boilers burn fossil fuels like oil or gas to generate heat. Heat pumps, on the other hand, capture warmth from outside and use electricity to transfer it into your house.
Depending on the model, they can suck heat directly from the air or the ground. Then, they transfer it to a fluid, compress it to boost its temperature, and shoot it into your home’s heating system.
(Most models work in reverse, too. They can act as air-conditioners by moving heat out of your house.)
The process is more efficient than burning fossil fuels. For every unit of electricity used, a heat pump can produce more than 3 units of heat, whereas a standard gas boiler, for instance, produces about 0.9 units.
Even if your electricity is coming from power stations that run on fossil fuels (you can check here to find out how your electricity is made), you’re still getting more heat for less energy input. That’s why heat pumps are estimated to have less than half the lifetime greenhouse emissions of gas boilers. It’s also why projections suggest that people in every single U.S. state could reduce their greenhouse gas emissions by switching to heat pumps.
In many cases, switching to a heat pump also lowers you energy bill, but that depends on a lot of factors like where you live, your previous heating system, the efficiency of the heat pump you’re buying, and others.
When it’s colder than a penguin’s tuxedo
Even very frigid air and earth have some warm energy. But heat pumps work more efficiently if the gap between the temperature outside and the temperature you want inside isn’t too wide.
It’s fairly easy to take air from outside that’s 40 degrees Fahrenheit and use it to raise the inside temperature to 70 Fahrenheit. It’s harder pull that kind of heat from subzero air.
So the colder the air outside, the less efficient the heat pump becomes. But less efficient doesn’t mean ineffective. Far from it, in fact.
“It’s a question of perspective,” said Marek Miara, an engineer and heat pump expert at the Fraunhofer Institute for Solar Energy Systems in Freiburg im Breisgau, Germany. “It doesn’t mean that the efficiency is very bad,” Dr. Miara said.
It’s still enough to keep you cozy.
When scientists pored over more than 2,700 data points from 550 heat pumps in houses in Britain, Canada, China, Germany, Switzerland and the United States, they found that heat pumps were producing an average of 2.74 heat units for every energy unit they were using, even at temperatures as low as 14 to 41 degrees Fahrenheit, or minus 10 Celsius to 5 Celsius.
In similar tests from Finland, Minnesota and Alaska, where temperatures dropped from minus 22 degrees Fahrenheit to 14 Fahrenheit during winter, heat pumps still produced between 1.5 and 2 heat units for every energy unit.
“Even when it’s really cold, heat pumps have at least double the efficiency of the fossil fuel heating system,” said Jan Rosenow, a professor of energy and climate policy at Oxford University.
Of course, it depends on the exact type of heat pump, furnace and home and other factors. If an everyday heat pump doesn’t seem up to the task, there are special models made specifically for extremely cold climates.
Starting in 2021, the U.S. Department of Energy ran a competition to stress-test cold-weather heat pumps in households, and two years of data suggested that the heat pumps worked perfectly fine.
“Air-source heat pumps designed for cold climate work down to minus 15 degrees Fahrenheit or lower,” said Eric Wilson, a senior research engineer at the department’s National Renewable Energy Laboratory.
Cold-climate heat pumps use specialized fluids to boost heat, Dr. Rosenow noted, and are “particularly designed for that climate.”
They hit an average of 1.9 efficiency at 0 to 5 degrees Fahrenheit, and people in the Department of Energy survey were mostly satisfied with their comfort. Only in places where temperatures were consistently below 0 Fahrenheit, or about minus 18 Celsius, did people sometimes turn on some form of complementary heating.
While it is possible for very poorly insulated homes in cold climates to be heated solely with cold-climate air-source heat pumps, Mr. Wilson said, it may be more cost-effective in these specific cases to supplement with other forms of heating.
Spine-chilling stories
Tales about heat pumps not working in the cold probably come from the early days of the technology, when it was still developing and when installers were still learning how to set them up properly, Dr. Rosenow said. There might be some bad-faith actors spreading these stories, too.
Heat pumps have actually been getting more efficient each year. But, for now, the level of heat pump quality is high enough for the vast majority of homes, Dr. Miara said. And the expense of building in further improvements might not justify the gains.
“We will not see the very big jumps in efficiency in the future,” he said. “Because it’s not really necessary.”
The best proof that heat pumps are working even if it’s cold outside, Dr. Miara said, is the sheer number of cold countries widely adopting the technology. In Europe, Finland, Sweden, Norway and Estonia have the highest share of heat pumps per household, according to the European Heat Pump Association, an industry group.
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