Heat pumps use electricity to transfer heat, making them an energy-efficient alternative to furnaces and air conditioners. In 2019, the state of Maine made residential heat pumps a core piece of its climate strategy and set a goal of installing 100,000 of them by 2025. By any measure, they were successful beyond their expectations, so much so that they achieved their goal two years early. Now the state has set a new goal of installing an additional 175,000 heat pumps by 2027.

But some people still question if heat pumps are a good fit in cold-climate states. They ask, can heat pumps work effectively in the cold? Do people need to have supplemental or backup heat sources? Are homeowners satisfied with heat pumps as the primary heating appliance in their homes?

Efficiency Maine, an independent, quasi-state agency that works to implement energy efficiency programs in the state, wanted to answer those questions. So in 2021, Efficiency Maine commissioned the research firm DNV to conduct a study of heat pumps in various types of homes. They found that heat pumps perform really well in all types of homes and in all seasons — answering the question “do heat pumps work well in cold climates?” with a definitive yes.

The Homes

The study was modest, tracking a sample of 10 homes in Maine that used heat pumps as their primary heat source. The goal was to learn more about the performance of whole-home heat pumps as a primary, year-round heating solution in different types of Maine homes and to get detailed information about performance, system configuration, and homeowner satisfaction.

DNV metered participating homes from February 2021 through June 2021. This let Efficiency Maine track the performance of the heat pumps across all kinds of weather as the seasons changed.

Efficiency Maine chose a diverse set of homes of varying ages and sizes. Nine were single-family homes and one was a condominium. Some homes were new and had heat pumps installed as the primary heating source. Others were older and had installed heat pumps to replace fossil fuel heating systems. Nine of the homes were heated with ducted air-source heat pumps. One was heated with an air-to-water heat pump and radiant heating distribution system. All the homes had heat pumps installed as their primary heat source before the start of the study.

The Results

Seven out of the 10 homes did not use supplementary heating to any significant degree during the study period. In those cases, the heat pumps functioned as the main heating source without any supplemental heating, even during the winter. Most residences did have a wood stove or fireplace that provided a small amount of additional heat. But homeowners reported that the main function of their fireplaces was to provide “atmosphere,” and not as a primary heating source.

Those that used supplementary heating did so through pre-existing electric and oil heaters. In all but one residence, the heat pumps were able to provide at least 80% of the residential heating load even at outside temperatures down to -5°F. The sole exception was a residence where the heat pump was supplemented by a regularly cycling oil boiler.

At the end of the study period, all the homeowners reported that they were satisfied or very satisfied with their heat pump systems and the comfort of their homes. “My house feels warmer than before with baseboard heat,” one homeowner reported. Another reported that “we’re very happy with the comfort level the system provides.” A third described their whole-house ducted system as working “flawlessly.”

Homeowner concerns were relatively minor. One family was dissatisfied with the thermostats that came with their system. Another said that their heat pump was slow to recover from low temperatures. Several homeowners kept backup generators and heating units to have on hand in event of a power outage.

The Takeaway: Heat Pumps Work Fine in Maine Winters

Efficiency Maine’s Whole Home Heat Pump Study demonstrates that heat pumps can function as a primary heat source in different types of homes during cold temperatures. The metered heat pumps proved to be effective year-round primary heating systems for all the homes in the study, despite their varying sizes, ages and heating systems. The high level of homeowner satisfaction demonstrated that real people were happy with the performance of their heat pumps, even in the middle of a cold Maine winter.

These results corroborate other findings that modern heat pumps work quite well in cold climates, further busting the myth that heat pumps are only good in more temperate parts of the country. As Maine continues to lead the way in heat pump installation, their future looks warm and energy-efficient.