There is, however, a down side. As the ambient air temperature goes down, it is more difficult to extract heat, and the unit must work harder. Efficiencies decline as ambient air temperatures go down. Historically, below 20 degrees, air-source heat pumps drop in efficiency to 100 percent (no better that electric radiant heat). They do not drop below 100 percent efficiency. Because of this reduced efficiency, the cost of producing the same amount of heat is a lot more than the cost of producing it with an ambient air temperature of 50 degrees. Some people even experience cold homes when low temperatures are extreme because the heat pump simply can't produce the needed heat fast enough. This is why air-to-air heat pumps are popular in the South, but see limited use in harsh Northern climates.
Many years ago someone realized that the heat pump worked great when the ambient air temperature was 50 degrees or more. It was very efficient. Was there a way to have 50-degree temperatures all the time, in any climate? The temperature of the earth is about 50-degrees even in cold climates, once you get below the frost line (about 5 feet underground). What if we could use that temperature to run a heat pump? How could we harness the heat in that 50-degree soil? If a hole was drilled or a trench was dug, a series of pipes could be installed. Properly constructed, we could circulate water through a maze of pipes and regardless of the water temperature going in, the return flow would be 50-degrees because of the ground temperature. That 50-degree water could then be used to extract heat using the heat pump. Heat from the home could also be transferred to this ground loop if cooling were needed. This process is known as a ground-source or geo-thermal heat pump.
The ground source heat pump never loses its full efficiency because the ground water loop temperature always stays the same. Some manufactures claim newer models are achieving efficiencies of up to 500 percent. That equates to 17,065 BTUs for just one kWh of electricity. This heating option, when compared to others, offers a tremendous savings.
The ground-source heat pump is not for everyone. Your home must be properly constructed and insulated to ensure that this option will work for you. Ground-source heat pumps are also very expensive to install, often costing $20,000 or more. Any serious consideration of a ground-source heat pump should weigh your investment against your expected payback in operational savings. When looking for heating alternatives, comparing electric heating options to traditional fossil fuel systems is a good idea.
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