Pumping Heat Uphill
	The key to understanding how a heat pump works is that even when it is very "cold" outside there is heat energy in the air. Air would need to be -460 degrees F ("absolute zero") for there to be no heat energy. A day that is 20 degrees F outside is 480 degrees F above absolute zero. 70 degrees F is 530 degrees F above absolute zero. So a heat pump only has to move the heat uphill a relatively small distance. It uses the "refrigeration cycle" to do this.
The Heat Pump Refrigeration Cycle	A refrigerant is a fluid which vaporizes (boils) at a low temperature. The refrigerant circulates through tubes ("refrigerant lines") that travel throughout the heat pump. We'll begin our description of the refrigeration cycle on the illustration below, which describes the heat pump when it is heating the house.
	The refrigerant is a cold liquid -- colder than the outdoor air. The refrigerant flows to the outdoor coil. This coil is a "heat exchanger" with a large surface area to absorb heat from the air into the colder refrigerant. The heat added to the refrigerant causes the fluid to vaporize, so this heat exchanger is called the "evaporator coil" during the heating cycle. When materials change state (in this case from liquid to gas), large amounts of energy transfer take place. At this point, the refrigerant is a cool gas, having been warmed and vaporized by the outdoor air. It is too cool to warm the house, so that's where the compressor comes in. The compressor raises the pressure of the gas. When that happens, the gas temperature rises. One way to think about it is that the compressor concentrates the heat energy. The compressor is often thought of as the "heart" of the heat pump, since it does most of the work of forcing heat "uphill." The compressor also forces the now hotgas further into the cycle.
	The indoor coil is where the refrigerant gives up its heat to the indoor air. A fan blows air past the indoor coil to distribute heat to the house. This cools the refrigerant to the point where much of it condenses, forming a liquid. In the heating season, the indoor coil is called the "condenser coil." This change of state results in a large transfer of heat energy. The warm mixture of liquid and gas continues through the cycle to the expansion device (sometimes called a "metering device"). This device reduces the pressure, causing the refrigerant, to become cold again - cold enough so that it is once again ready to absorb heat from the cool outdoor air and repeat the cycle.
Cooling with a Heat Pump	One advantage of a heat pump is that in the summer it works just like an air conditioner to cool the house. Most heat pumps have a "reversing valve" to switch the flow of refrigerant so that it pumps heat from indoors to outdoors.
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