| By :
Nick Messe
Serving as both furnace and air conditioner, heat pumps provide a unique heating system. Simply transferring heat, rather than burning fuel to create it, they can also work very efficiently. At present, there are only two types of air pumps in general use for the home, the air-source or air-to-air heat pump and the ground-source or geothermal heat pump. They all operate on the same laws of heat transfer. The natural flow of heat is from high to low, or down hill. With a small energy requirement, a heat pump reverses the process by pulling heat out of a heat source and pumping it into your home. In its most basic form, the air-source heat pump system consists of two fans, refrigerator coils, and a compressor. Most home systems are split systems with outdoor and indoor components joined through the wall. An air-source heat pump, in heating mode, evaporates the refrigerant in the coil in the outdoor unit. During evaporation and resultant cooling, the refrigerant absorbs heat from the outside air. Compression of the warmed refrigerant takes place before it passes into the indoor coil where the refrigerant condenses, releasing heat to the inside of the house. In essence, the pressure changes caused by the compression and expansion of the refrigerant allow the refrigerant to evaporate at a low temperature outside and condense with higher heat content, or temperature, indoors. In its simplest configuration, the heat taken from the outdoor air then circulates indoors via air ducts. Such a pump is commonly referred to as an air-to-air heat pump. However, with modifications, air-source systems are compatible with other types of indoor heating systems. The reverse valve is a crucial part of a heat pump because, with the change of a thermostat setting, it reverses the flow of the refrigerant and the system provides cool air to the air ducts. The components of a ground-source, or geothermal, heat pump are the same as for the air-source pump except the outdoor refrigerator coil is replaced by underground pipes filled with refrigerant. Therefore, in heating mode, the ground-source pump pulls heat from the ground for circulation indoors. As with the air-source pump, ground-source heat pumps have a reverse valve that turns the system into air-conditioning mode. The major differences between the air-source pump and ground-source systems are the cost and source temperature. Ground-source systems cost 2-3 times the cost of an air-to-air system unless included in new construction. The air-to-air system is limited to moderate climates. It is amazing that just a few feet below ground, temperatures are nearly constant anywhere in the United States. So ground-source heat pumps can be effectively used almost anywhere. In either case, heat pumps can reduce utility bills by between 30 and 40 percent and government incentives will often defray part of the cost of installation. Higher SEER and HSFP ratings indicate a more efficient unit. Consult with a contractor you trust before choosing a system. The wrong kind of heat pump may actually increase energy costs.
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