Heat Pumps Take on Cold Climates

Twenty homes scattered across Canada and the northern United States are keeping warm this winter using prototypes of the latest iteration in residential heating systems: cold-climate heat pumps.

Heat pumps aren't common in homes at this latitude, because historically they haven't worked well in subzero temperatures. But heat-pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures.

To prove it, eight manufacturers are publicly testing their prototypes in the Cold-Climate Heat Pump Technology Challenge, hosted by the U.S. Department of Energy in partnership with Natural Resources Canada. The companies' task is to demonstrate a high-efficiency, residential air-source heat pump that can perform at 100 percent capacity at -15 C. Companies can choose to further test their machines down to -26 C.

Heat-pump manufacturers Bosch, Carrier, Daikin, Johnson Controls, Lennox, Midea, Rheem, and Trane Technologies have each passed the laboratory phase of the challenge, according to the DOE. They are now field-testing their prototypes in homes in 10 northern U.S. states and two Canadian provinces, where furnaces and boilers burning fossil gas, fuel oil, or propane are more commonly used.

Companies that complete the challenge won't receive cash prizes. But the DOE will help them expand into cold-climate markets by engaging with stakeholders in those regions, a DOE spokesperson told IEEE Spectrum. The challenge will conclude later this year, and prototypes will likely be ready for commercialization in 2025.

How heat pumps beat the cold

Advances in the technology came primarily through improvements in one key heat-pump component: the compressor. Heat pumps work by moving and compressing fluids. In the winter, the systems draw heat from outside the home, most commonly from the air. (There is heat in the air even in subzero temperatures.) An outdoor heat exchanger, or coil, absorbs the heat into the heat-pump system.

The outdoor air passes over a heat exchanger containing a fluid, or refrigerant, that has a very low boiling point. A common refrigerant, called R410a, boils at -48.5 C. The refrigerant boils and evaporates into a vapor, and a compressor increases its temperature and pressure. The superheated vapor then moves through an indoor coil, where fans blow air across it, moving heat into the home. In the summer, the system reverses, moving heat from inside the building to the outside, and cooling the home.

They couldn't get the lab any colder than [-30 C], so we had to cut the power to get the heat pump to turn off." -Katie Davis, Trane Technologies

The colder the temperature outside, the harder heat pumps must work to extract and move enough heat to maintain the home's temperature. At about 4 C, most air-source heat pumps currently on the market start operating at less than their full capacity, and at some point (usually around -15 C), they can no longer do the job at all. At that point, an auxiliary heat source kicks on, which is less efficient.

But advancements in compressor technology over the past five years have addressed that issue. By controlling the compressor motor's speed, and improving the timing of when vapor is injected into the compressor, engineers have made heat pumps more efficient in colder temperatures.

For example, Trane Technologies, headquartered in Dublin, played with the vapor compression cycle" so that it gets an extra injection of refrigerant, says Katie Davis, vice president of engineering and technology in Trane's residential business. It's works a little like fuel injection," she says. When the system begins to lose its capacity to heat, the system injects refrigerant to give it a boost, she says.

In the lab portion of the DOE's heat pump challenge, Trane's unit operated at 100 percent capacity at -15 C and kept running even as the lab's temperature dropped to -30 C, although no longer at full capacity. They couldn't get the lab any colder than that, so we had to cut the power to get the heat pump to turn off," Davis says.

Vapor-injection compressor technology has been around for years, but until recently, had not been optimized for heat pumps, Davis says. That, plus the introduction of smart systems that enable the indoor and outdoor units to communicate with each other and the thermostat, has enabled heat pumps to take on colder weather.

Heat pumps can reduce emissions and cut energy costs

The DOE is pushing for wider adoption of heat pumps because of their potential to reduce greenhouse gas emissions. Such systems run on electricity rather than fossil fuels, and when the electricity comes from renewable sources, the greenhouse gas savings are substantial, the DOE says. Because heat pumps transfer heat rather than generate it, they are significantly more efficient than traditional heating systems, the agency says.

A two-year study published 12 February in the journal Joule supports the DOE's claim. The study found that if every heated home in the United States switched to a heat pump, home energy use would drop by 31 to 47 percent on average, and national carbon dioxide emissions would fall by 5 to 9 percent, depending on how much electricity is provided by renewable energy. Those figures are based on heat pumps that draw heat from an air source (rather than ground or water) and includes both homes that pull heat through ductwork, and homes that are ductless.

The energy savings should lower bills for 62 to 95 percent of homeowners, depending on the efficiency and cold-climate performance of the heat pump being installed. How well a home is insulated and the type of heating system being replaced also makes a big difference in energy bills, the study found. For households that are currently heating with electric resistance heat, fuel oil, or propane, heat pumps could save thousands of dollars annually. For natural gas, the savings are less and depend on the price of natural gas in the local area.

Some homeowners are hesitant to switch to heat pumps because of what's known as temperature anxiety."

Cold-climate heat pumps will likely boost energy savings for homeowners, but will require higher up-front costs, says Eric Wilson, a senior research engineer at the National Renewable Energy Laboratory in Golden, Colo., and an author of the paper. It's generally well known that heat pumps can save money, but there's a lot of confusion around whether they're a good idea in all climates," he says. His study and the DOE's cold-climate heat pump challenge will help provide a clearer picture, he says.

The DOE is one of several government entities trying to expedite adoption of residential high-efficiency heat pumps. Nine U.S. states earlier this month pledged to accelerate heat-pump sales. Their pledge builds on an announcement in September from 25 governors, who vowed to quadruple heat-pump installation in their states by 2030. The U.S. federal government also offers tax credits and states will be rolling out rebates to offset the cost of installation.

So far, the efforts seem to be working. In the U.S., heat pumps outsold furnaces for a second year in a row in 2023, according to data released 9 February by the Air-Conditioning, Heating, and Refrigeration Institute, in Arlington, Va.

Europe is making a similar push. The European Commission has called for expedited deployment of heat pumps, and recommended that member states phase out the use of fossil-fuel heating systems in all buildings by 2035. Many European countries are subsidizing residential heat pump installation by offering grants to homeowners.

But some homeowners are hesitant to switch to heat pumps because of what's known as temperature anxiety." It's like electric-vehicle range anxiety: Homeowners are concerned about getting stuck in a cold house.

And some just like the feel of old fashioned heat. Folks who have furnaces say they really like the way that hot heat feels when it's coming out," says Davis at Trane. Heat pumps put out warm heat and it's going to do a good job heating your home, but it's not that hot heat that comes out of a furnace."

Trane's cold-climate heat pump-the one entered into the DOE's challenge-is currently heating the home of a family in Boise, Idaho, Davis says. We've had excellent feedback from our customer there, who said their energy bills went down," she says.

Heat Pump Challenge Specs

To pass the DOE's field test, heat pumps must operate at -15 C with the following specifications:

• Operate at 100 percent heating capacity without relying on backup electrical resistance heat
• Demonstrate 40 percent greater efficiency than current heat pumps on the market
• Function in homes that distribute air through ductwork
• Draw heat from the air (rather than the ground or water)