How Can Air Source Heat Pumps Help Reduce Greenhouse Gas Emissions Even in Cold Climates?

As state and local jurisdictions plan for reducing greenhouse gas (GHG) emissions, many considerations come into play regarding strategy, incentive programs, societal costs, utility costs, available energy sources, and more. As a result, the building sector will need to consider strategies for constructing high-performance homes and for decarbonizing the industry.

One piece of the puzzle involves reducing GHG emissions for residential heating. Given variable climates throughout the United States, one potential approach is installing heat pumps — a technology that’s becoming more prominent because of utility incentive programs and the Department of Energy’s Energy, Emissions and Equity (E3) Initiative to accelerate the research and adoption of heat pump technologies.

The E3 initiative also involves a Cold Climate Heat Pump Challenge, which could help further the technology and deployment of heat pumps in cold climates through air source heat pumps (ASHPs).

An ASHP works by taking energy from a cold place (like the outside air) and rejecting it to a warmer place (such as your house). To do this, the air flows through an air handler box where a heat exchange occurs. It also includes a compressor, which pumps a refrigerant fluid through a circuit, and moves heat around through compression, condensation, expansion and evaporation. The technology can typically move heat efficiently, so for each unit of energy that is put into the system, there could be up to five units of heat pushed out.

Heat pump technology is dependent on the outside air temperature; the larger the temperature difference between the outside and inside air, the bigger the “lift” and more difficult it is for the components to do their job. So in cold climates, it’s necessary to use heat exchangers that minimize the difference between the refrigerant temperature and outside air temperature. The larger the temperature lift, the higher the compression ratio, which causes the compressor component to use relatively more energy than the useful heat transferred by the technology.

New technology advances can improve performance of heat pumps in low and even sub-zero temperatures, though they tend to cost more (exact amounts depend on the local market) than traditional heat pump technology. Some cold climate techniques for heat pumps include:

• Incorporating an inverter drive into the compressor, also known as a variable-speed compressor. This allows the compressor component of the pump to adjust the flow of the refrigerant to compensate for the differences in temperature lift required throughout the day/night;
• Using vapor injection to cool the compressor to reduce the amount of energy used within the system;
• Switching the type of refrigerant used, as some refrigerants work better in colder climates than others; and
• Using oil injection technology for cooling during the compressor component.

Although the technology is still evolving and backup heating sources are typically needed to carry the heating load on extremely cold days, ASHPs designed for cold temperatures could help reduce GHGs in some areas. Builders should study proper installation to maximize performance and increase efficiency.

To stay current on the high-performance residential building sector, with tips on water efficiency, energy efficiency, indoor air quality, and other building science strategies, follow NAHB’s Sustainability and Green Building efforts on Twitter.