Study: Air-Source Heat Pumps in Cold Climates

Market: Beyond Buildings
Type: Research
Location: VT
Project Services: HVAC Systems Research & Evaluation
Sponsor: U.S. DOE Building America Program
Partner: Efficiency Vermont
Location: Cold Climate Zones 5 & 6 (New England)

Air-source heat pumps (ASHPs) have traditionally been used more in the south where cooling loads dominate and winter temperatures rarely drop much below 30°F. In the past decade, however, ASHP’s have gained popularity in colder climates. This is due to the availability of new products (largely from overseas) that have made a big impact on the U.S. market. These products often:

  • have variable-speed compressors and fans that modulate over a wide range (and allow for higher efficiencies)
  • can operate at colder temperatures, sometimes well below 0°F
  • come in a variety of ducted or ductless configurations
  • are available at a wide range of capacities – some down to ½ ton (6,000 Btu/h)

While manufacturer and laboratory tests have shown that some new ASHPs operate very efficiently and at very low temperatures, studies of their real-world performance in buildings are rare.

With funding from the U.S. Department of Energy and in partnership with Efficiency Vermont, SWA monitored the winter performance of ten ductless heat pumps in New England homes.

Overall findings:

  • Systems generally did provide their rated heat output, even at very cold temperatures
  • Efficiencies of the systems varied tremendously, and on average efficiencies were lower than expected (or desired)

A report on the study is available here.

In sorting through results, SWA identified several factors that likely contribute to poor performance:

  • Sizing – Systems that were oversized operated less efficiently.
  • Outdoor unit location – to operate properly, outdoor units need to be protected from water, ice and snow build-up.
  • Indoor unit location – ductless fan coils mounted high in a vaulted ceiling typically saw very high return air temperatures; this decreased capacity and efficiency.
  • Operation – control strategies, integration with other systems, and filter cleaning all appear to affect performance.

SWA believes that ASHPs can be an excellent choice for many (though not all) applications. Careful design, sizing, installation, and operation are crucial for getting the best performance. More information and links to other resources are available in several blog posts.