| Heat pumps make use of the same basic technology utilized in air conditioning systems to provide for both the heating and cooling needs of a house. In cold weather, a heat pump uses the heat present in outdoor air or stored in the earth to provide heat to the inside of a house. In warm weather, the same equipment works in a reverse manner, removing heat from indoors and transferring it to the outdoors. For this reason, heat pumps are sometimes referred to as reverse-cycle air conditioners.
Heat pumps are very energy-efficient heating systems. Since they do not generate heat like conventional fuel-burning systems, but only transfer it from one location to another, they require much less energy to produce the same amount of heat as other heating systems.
While it may be hard to comprehend, there actually is enough “heat” in outdoor air or below the frozen earth surface to heat a house during the winter months. However, the amount of available heat decreases dramatically at lower temperatures. Consequently, the efficiency of a heat pump also decreases significantly as the temperature of the heat source drops below moderate levels; a factor that indicates heat pumps are not well suited for all climates. Most houses with heat pumps also need a supplemental source of heat for low-temperature conditions (typically when air temperatures fall below about 40º F or 4º C).
Heat pumps are able to transfer heat by the movement of a refrigerant through an alternating cycle of evaporation and condensation. An electric-powered compressor drives the refrigerant through the cycle – as it does in most residential air conditioning and refrigeration systems – allowing it to absorb heat at one point and give it up at another.
The primary components of a heat-pump system are basically the same as those found in air conditioning and refrigeration systems. But a heat pump uses a special four-way (reversing) valve that automatically changes the direction of the refrigerant flow – allowing the system to remove heat from outdoors in winter and from indoors in summer.
There are two basic types of heat pumps: (1) air-source and (2) geothermal heat pumps. The majority of heat pumps are air-source systems, which use outdoor air as a heat source in winter, and air as the medium to which heat is transferred to in the summer.
Geothermal systems make use of the heat available year round in the earth or water. There are two types of geothermal systems: (1) open systems, which use the water from a well as a heat source, and (2) closed systems, which use a liquid run through a continuous loop of piping buried in the ground or installed in a body of water. Geothermal heat pumps are very efficient systems but also very expensive to install.
In cold weather, a fan draws outdoor air across a coil containing an even colder refrigerant. The refrigerant absorbs the heat in the air (as heat moves from hot to cold) and is drawn as a gas into the compressor where a pressure increase creates a temperature increase. The refrigerant is then pumped to the indoor coil where a fan blows air across the coil (condenser) and the refrigerant’s heat is transferred to the cooler household air. The resulting warmed air is distributed throughout the home via a duct system. Meanwhile the refrigerant, which has condensed to a liquid, travels back to the outdoor coil where the cycle begins again.
In warm weather, the reversing value automatically changes the direction of the refrigerant flow and heat from indoors is transferred outdoors. When the outdoor air temperature dips appreciably, a supplemental heating system (often electric resistance coils) provides the needed heating boost. As in all electrically heated homes, adequate wall and ceiling insulation, as well as insulated windows or storms, is a necessity if utility bills are to be kept reasonable.
One issue to note: The temperature of the heated air provided by a heat pump (90°-100°F, 32°-38°C) is lower than that provided by a conventional warm air system (+120°F, +99°C), and may even be lower than body temperature. For this reason, a high volume/low velocity duct system with properly placed supply outlets is required to maintain heating comfort.
Heat pump manufacturers recommend homeowners consider the following operational and maintenance issues:
- Air filters should be cleaned or replaced on a regular basis (typically monthly in season).
- A heat pump works most efficiently when it maintains room temperature; avoid large temperature setbacks overnight or during the work day.
- While it may seem cold air is coming from the supply outlets during the heating season, this is a factor of heat pump design; comfort heating should still be attainable with a properly designed and used system.
- The system will periodically go into a defrost mode to remove any ice build-up on the outdoor coil in the winter.
- The outdoor unit should be positioned above the prevailing snow line with suitable clearance around the unit maintained year round.
- Expect the unit to run continuously at low outdoor temperatures, or a switch over to the supplemental heat system.
Modern heat pumps have an expected economic life span of 6- 12 years, with annual servicing and proper use.
Remember, these tips are only general guidelines. Since each situation is different, contact a professional if you have questions about a specific issue. More home safety and maintenance information is available online at housemaster.com.
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