How Does an Air-to-Air Heat Pump Work?
	As the term "pump" implies, a heat pump moves heat from one place to another. In the winter, it moves heat from outside to inside to heat your home. In the summer, it moves heat in the other direction to cool your home. This reverses the natural flow of heat from warm places to cooler places. Like a water pump that uses energy to pump water uphill, a heat pump uses electrical energy to pump heat "uphill" from cool locations to warmer locations. Heat pumps use the refrigeration cycle to accomplish this. (If you want more details, read "Pumping Heat Uphill.")
Why Not Convert Electricity Directly To Heat?	That's what most electric heating systems do. The advantage of pumping heat is that it takes less electrical energy than it does to convert electrical energy into heat (as in electric furnaces, baseboards and radiant heaters). In fact-in mild winter temperatures you can get three times as much heat out of each watt of electricity as you get from an electric furnace. However, as we'll see later, heat pumps don't achieve their highest efficiencies much of the time. Energy cost savings differ for each installation. Properly sized and installed heat pumps can reduce heating costs by 30 to 50 percent compared to electric furnaces. The quality of the installation will greatly affect energy savings and longevity.
Types of Heat Pumps	Heat pumps are named for their source of heat. Most heat pumps are air-source heat pumps. They get heat from the outdoor air. There also are water-source heat pumps which get heat from water--usually well water. (Geothermal) Ground-source heat pumps get their heat from ground below the frost line. Air-to-Air heat pumps are the 2nd most efficient method of heating & cooling your home.  Geothermal heat pumps are the most cost-effective, but the upfront cost is higher. These pages discuss air-to-air heat pumps since they are by far the most common. Unless we mention ground- or water-source, assume we are discussing air-source heat pumps.
Main Parts of Heat Pumps	Most heat pumps have two main parts--the outdoor unit and the indoor unit. The outdoor unit includes the outdoor heat exchanger, the compressor and a fan. This is where the heat from the air outside is picked up during the heating season, and where the heat from inside the house is rejected during the cooling season. The indoor unit contains the indoor heat exchanger and the fan that distributes heated or cooled air to the duct system of the house. Some systems have a second indoor cabinet that contains the compressor. The two (or three) cabinets are connected by refrigerant lines that allow some flexibility in locating the indoor and outdoor units. Some heat pumps are contained in only one cabinet that mounts through a wall. These are not common in homes since they allow little design and layout flexibility. These systems may attach to ducts to deliver heating and cooling. Or they may look like a room air conditioner. But they can also provide heat.
	Another configuration that's becoming more common is to include some water heating capability in heat pumps. The equipment that, heats water is called a "desuperheater." Desuperheaters heats water with excess heat that would be wasted during the heating season. During the air conditioning season, a desuperheater can meet much of your hot water needs at much lower cost than a conventional electric water heater. The heat that's removed from the house during cooling is transferred to your water.
Next: Pumping Heat Uphill