Yes, I know that the following information is probably not as exciting as "American Idol", but sometimes information that is worthwhile is not always entertaining. If your heating system has let you down this winter and you're thinking of replacing it, you may want to read on. What is the most common type of heat pump, and how does it work? Chapter Five of The Virginia Energy Savers Handbook states that it is an air-to-air heat pump:
"The most common type of residential heat pump is an "air-to-air" heat pump which uses outdoor air as the heat source. Heat is extracted from the air by an outdoor unit that contains a heat exchanger and fan. The main disadvantage of air-to-air heat pumps is that they lose efficiency and output at cold (less than 35oF) outdoor air temperature. When this happens, operating cost increases and indoor comfort decreases because the air from the heat pump is not very warm. While this is a troublesome problem in colder regions of the country, it is not a severe problem in most regions of Virginia.
"Air-to-air heat pump systems are usually set up with a "two-stage" thermostat. As long as the temperature in the house remains within a few degrees of the thermostat setting, the heat pump operates normally. If the indoor temperature drops too low, the heat pumped by the compressor is supplemented by electric resistance heat and the
heat pump's efficiency drops considerably. For a typical home with a heat pump, the electric resistance heat comes on during two conditions: when the outdoor temperature
drops to about 15-25oF and when the heat pump is turned on suddenly when the house is cold.
"When outdoor air temperatures are below about 40oF, air-to-air heat pump outdoor coil temperature may be below freezing. Moisture in the outdoor air then forms frost on the outdoor coil. If too much frost builds up, the heat transfer to the coil is restricted and heat pump output and efficiency drops. To avoid this, heat pumps have a "defrost" cycle that uses energy from the house to warm the outdoor coil and melt the frost. Frost is not a problem with air conditioners since you never cool your home to 40oF.
"GHPs have a higher installation cost, but because they are more efficient and save money in the long term, they can represent a good investment. The cost effectiveness
of a GHP for a particular location depends, in part, on soil conditions and site layout since these affect the cost of the necessary excavation."
The Virginia Energy Savers Handbook 2008: Department of Mechanical Engineering of Virginia Tech Blacksburg, VA 24060
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