A furnace pump is part of a category of machines called heat pumps. A heat pump is a device that uses mechanical work to move heat from one location to another. Most heat pumps move heat from an area of relatively low temperature to an area of high temperature. A furnace pump is a type of heat pump that operates in this way for the heating or cooling of a house.
An easy way to think of a furnace pump is as an air-conditioner that can switch from heating to cooling. Furnace pumps use a closed system that circulates refrigerant gas, similarly to how an air-conditioner works. The difference is that the flow of refrigerant in the system can go either of two ways.
In cool weather, the default setting on the furnace pump is heating. Cool or cold air still contains some heat, and this heat is moved from the outside to the inside of a house. The part of the system that is exposed to the outside air consists of coils like those on the back of a refrigerator. These coils can sometimes collect ice during cool weather, since they are located at the point where heat is drawn out of the air. Whatever ice is accumulated on the coils is melted periodically by the furnace pump.
When the weather is warm, the furnace pump operates in the opposite direction, taking heat from the inside of the home and depositing it outside. Most furnace pumps are electrical, but use very little energy compared to an air-conditioner or furnace. This is because furnace pumps simply move heat around rather than generating it. The more efficient furnace pumps can move up to four times the amount of energy they consume.
Energy efficiency is the main advantage of furnace pumps over other methods of heating and cooling. The principal disadvantage is that in climates with very warm summers, very cold winters, or both, a furnace pump cannot move a sufficient volume of heat to maintain a comfortable indoor temperature.
A variety of refrigerants can be used by a furnace pump, depending on when it was manufactured. Until the 1990s, the common refrigerants in furnace pumps and elsewhere were chlorofluorocarbons (CFCs). The manufacture of some CFCs was discontinued in 1995, due to new scientific evidence that they can cause atmospheric ozone depletion. One widely-used type of replacement refrigerant is the family of hydrofluorocarbons (HFCs), which are safer but less efficient. Other substitutes include liquid ammonia, propane, and pressurized carbon dioxide, especially since 2001.