During the last several years it has become much more common for homeowners to choose a heat pump as the prime heating unit for their homes. Much information is available to the customer to aid him in his decision to choose a heat pump however; information is scarce as to the differences between air-source and water-source heat pumps. To grasp the concept of a heat pump in non-technical terms we can consider it to be much like a water pump in its operation.
When a water pump lifts water from one level to another it expends a certain amount of energy. The greater the lift from the source of water to the area where the water is discharged, the more energy it requires to lift it. Similarly, a heat pump transfers heat from a source (well water or outside air) at one temperature to a sink (your home) at another temperature. The greater the difference in temperature between where the heat pump has to find it’s heat and the temperature that it has to discharge it’s heat the more work the heat pump has to do and consequently the more it will cost the homeowner.
One can easily see through a picture diagram that a boy bailing water from a stream and dumping it on the river bank will use less energy than the same boy bailing water and having to climb a 30 foot tree before he can dump the water. The same situation holds true for heat energy. If we know that we must deliver air to our homes at a temperature of 100° F. to heat it, then every degree we go below that temperature to find a source from which to get our heat makes our system a little less efficient. For example heat pump “A” collecting heat from a source temperature of 47° F. and delivering it to a house at 100° F. would be more efficient than heat pump “B” collecting heat from a source temperature of 17° F. and delivering it at the same temperature of 100° F.
In other words, as the source temperature declines so will the efficiency of the heat pump. It is for this reason that a prospective buyer must look candidly at the geographic area in which he lives when choosing the “type” of heat pump. The prospective homeowner will naturally choose the type (by type we mean AIR-SOURCE or WATER- SOURCE) of heat pump which will get its heat from the source that remains at the highest temperature for most of the year.
In Canada that source happens to be the ground beneath us rather than the air. Ground water stays at a relatively constant temperature most of the year and is usually 1 or 2° F. higher than the mean average annual air temperature for any given area. It is for this reason that a water-source heat pump will yield a much higher average annual COP than does a heat pump which gathers its heat from the surrounding air.
Typical COP’s for a water source heat pump would range from 3.5 to 4.5 while that of an air-source heat pump, might yield an average COP of 1.7 to 2.2 for the year. The efficiency of a heat pump, which we talked about earlier, is called its COEFFICIENT OF PERFORMANCE or in simpler terms, what we get out for what we put in. A COP of 3.5 means that the heat pump will give 3.5 times as much heat out for a given amount of electricity as the same electricity would give were it converted directly to electric heat.