As the name suggests, a heat pump moves heat from a cooler place to a warmer place, using the same principles as a common air-conditioner or refrigerator. Air-source and ground-source heat pumps can both heat and cool homes, and high performance heat pump systems with variable speed compressors can provide significant comfort benefits compared to conventional systems, as well as reducing greenhouse gas emissions.
Fortunately, moving heat as opposed to generating it directly requires substantially less energy, making these system much more efficient than conventional electric or fossil fuel systems. Fossil fuel system efficiency is below 100% since some energy goes up the flue, with typical values of 85% for oil and 90-95% for propane or natural gas. Ignoring losses in electricity generation, efficiency for electric resistance heat is 100%. A heat pump’s efficiency relative to electric resistance, called the Coefficient of Performance (COP), is typically a factor of 2.5 to 4 for air-source heat pumps (meaning 250% – 400% efficient), or 4.5 to 6 for ground-source heat pumps. This is substantially higher than fossil fuels even taking into account the losses in electricity generation. Note that efficiency in this case can be above 100% since heat pumps extract heat from the outdoor environment.
Heat pumps for home heating consist of an outdoor compressor and one or more indoor units connected by refrigerant lines. Air-source heat pumps (ASHPs), which use the outside air as the source of heat, are more common and come in two basic types, ductless or ducted (taking advantage of existing ducting). They can be installed to heat and cool just one room, a few rooms or an entire home. Often, a backup heating system is used to provide heat on the coldest winter days, when the ASHPs heating capacity falls below the home’s requirements.
Ground-source heat pumps (GSHPs), also referred to as geothermal, are the highest efficiency systems available, because they extract heat from the ground which stays at a fairly constant temperature year round. GSHPs are generally whole-home systems, requiring one or more wells near a home from which to extract the heat. This makes them a sizable investment, which is partly offset by Federal tax credits and other rebates.
For more information, please check out our video series which has both technical information and testimonials from several heat pump owners in our towns.