Heating systems are pretty easy to understand. In a furnace or boiler, fuel is burned and the heat energy transfered either to air or water which is then circulated throughout the home through ductwork or piping.
Air conditioners and heat pumps don’t so much cool or warm air as transfer heat from one place to another. The heart of these systems is the compressor. Its purpose is to compress and thus condense vaporized refrigerant back into its liquid state. As refrigerant vaporizes, it picks up heat from its surroundings. When it condenses back into a liquid, that heat is released.
In a standard air conditioner, this heat transfer goes in one direction. During the warmer months, a heat pump works exactly the same. Heat is picked up as inside air is circulated over coils inside the home's air handler and dispersed outside when a fan blows air over outside coils holding compressed refrigerant. In the process, humidity condenses on the inside cooling cool and is then drained away.
In a heat pump, this flow may be reversed for the colder months by means of a special valve. During most of the heating season, the heat pump gathers heat from the outside air because while cool, it is still warmer than the refrigerant. On really cold days — only about 20% of our relatively mild winters — a supplemental resistance heater kicks in.
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