By now, you've learned that air-source heat pumps use an outdoor fan to bring air over refrigerant-filled coils. Two sets of these coils transfer this heat indoors, where it's then blown away from the coils by a second fan, and distributed through your home as cool goodness. Some air-source heat pump systems consist of a single packaged unit containing both sets of coils in one box. This box is then installed on the roof of a building with the ductwork extending through the wall. You'll see a lot of larger systems for commercial buildings installed in this way. Home heat pumps are usually split systems with an outdoor and an indoor component installed through the wall. Depending on the type of system, there may be one or more indoor components to distribute heat.
Ground-source heat pumps are a little different. They absorb heat from the ground or an underground body of water and transfer it indoors, or vice versa. The most common type of ground-source heat pump transfers heat directly from the ground by absorbing it through buried pipes filled with water or a refrigerant. These liquid-pumping pipes can be either closed-loop or open-loop systems, and they operate pretty much exactly how they sound. In a closed-loop system, the same refrigerant or water circulates through the pipes repeatedly. In an open-loop system, water is pumped out of the underground water source, like a well or a man-made lake. From there, the heat is extracted from the water, and that water returns to the well or surface lake. More water is then pumped from the well to extract more heat in a continuous open loop.
If that's not enough to blow your mind, consider theabsorption heat pump -- air-source pumps that are powered by natural gas, solar power, propane or geothermal-heated water, rather than by electricity. Absorption pumps can be used for large-scale applications, but are now available for homes on the larger side. The main difference between a standard air-source heat pump and an absorption pump is that instead of compressing a refrigerant, an absorption pump absorbs ammonia into water, and then a low-power pump pressurizes it. The heat source then boils the ammonia out of the water, and the process starts all over again.
When you go to check out an absorption heat pump, it helps to know how they're rated. Manufacturers rate them using a measurement called a coefficient of performance (COP), which sounds pretty complicated. All you need to know is to look for a COP above 1.2 for heating and above 0.7 for cooling. And don't worry, we'll discuss ratings for standard heat pumps a little later.
Air-source, ground-source and absorption heat pumps are the most common kinds of heat pumps, but they won't work in every situation. Read on to learn about special kinds of heat pumps.