Heat pumps come in several different types, with an air-source heat pump, often called a ductless mini-split, the most common to heat a home.

Unlike a traditional furnace, which uses ducts to spread warm air throughout the home, a ductless mini-split has no duct work and it is relatively small, hence its name. As a result, mini-splits are a good alternative for existing construction. Typically, they are placed in the portion of the home the homeowner wants to heat or cool.

Air-source heat pumps can provide fairly low-cost space heating. An Energy Star-rated heat pump has a high efficiency and can provide up to four times as much heat as an electric heater using the same energy. In comparison to natural gas (or propane or oil) as a primary heat source, the lifetime cost of a mini-split heat pump will be affected by the price of electricity. To improve heat pump efficiency, the homeowner should air seal attic and basement gaps and improve the home’s insulation. If possible, spray foaming the basement sills and walls and increasing the R-value of the attic to R-49 is recommended.

Heat pump technology is similar to a refrigerator, freezer or air conditioner. Just as the pipes on the back of a refrigerator become warm as the interior cools, air-source heat pumps warm the inside of a home while cooling the outside air. Another major difference is the physical location of the different system components. Air-source heat pumps have an indoor air-handling unit and an outdoor compressor/condenser, both of which have a heat exchanger coil.

The refrigerant in the heat pump provides warm or cool air, depending on whether the space needs to be heated or cooled. A conduit connects the inside unit and outside unit, which contains the power cable, refrigerant tubing and a condensate drain.

With the advent of newer refrigerants, an air-source heat pump can extract useful heat down to about 5 below zero, perhaps lower in some cases. Because temperatures in Maine can get lower, however, a back-up heat supply is necessary. If the back-up heat source thermostat is set 10 degrees lower than the heat pump, the backup heat will turn on as the output of the heat pump is reduced.

Air-source heat pumps should be Energy Star rated, in order to obtain a high efficiency model, as well as to take advantage of any incentive offered by Efficiency Maine (www.efficiencymaine.com) and the Internal Revenue Service.

The installer must have a refrigerant handling certificate and be trained by the heat pump company. Installation usually takes one to two days and costs between $3,000 and $4,000, depending on the location and any included service. The units should be located 18 inches off the ground to keep it out of snow and have a rain/ice cap if it is subjected to snow coming off a roof. Snow inside the unit will drastically affect the efficiency.

Saving estimates are not straightforward unless comparing it to electric heat and air conditioners. Overall, compared to oil boilers and furnaces and assuming the unit will provide heat to half of the home, the fuel savings could be 20 percent to 30 percent, more if additional heat pumps are installed.

While fuel costs will decrease, the home’s electric bill will be higher. Overall, however, heating costs will go down. Also, compared to traditional air conditioners, cooling costs can be reduced by more than 60 percent.

Clough Toppan is a licensed engineer and certified building analyst/energy auditor. Involved with building science for more than 20 years, he is a member of Sustain Mid Maine Coalition’s Energy Team. For additional information, email [email protected]. Sustain Mid Maine Coalition is a grassroots organization dedicated to improving the quality of life for the people of central Maine. For more information, visit www.sustainmidmaine.org.