The portion of the central air conditioning or heat pump system that moves heated or cooled air throughout a home's ductwork. In some systems a furnace handles this function.

An outdoor temperature, usually between 30° F and 45° F, at which a heat pump's output exactly equals the heating needs of the home. Below the balance point, supplementary electric resistance heat is needed to maintain indoor comfort.

The pump that moves the refrigerant from the indoor evaporator coil to the outdoor condensing unit and back to the evaporator. The compressor functions as the heart of the system, because it circulates the refrigerant in a loop.

The portion of a central air conditioning system that is located outside the home and functions as a heat transfer point for collecting heat from and dispelling heat to the outside air.

A network of refrigerant-filled tubes where heat leaves the hot refrigerant vapor. The vapor then condenses into a liquid capable of absorbing more heat.

The portion of a heat pump or central air conditioning system that is located in the home and functions as the heat transfer point for warming or cooling indoor air.

An air conditioner that contains a valve that allows it to alternate between heating and cooling.

HVAC is the acronym for Heating, Ventilation and Air Conditioning

The difference between the two is the "R" which stands for Refrigeration.

Roof Top Unit.

BTU is an acronym for British Thermal Unit. Although its name seems to suggest otherwise, it is no longer commonly used in the United Kingdom. BTU is a unit of measurement for energy. One BTU is equal to the amount of energy used to raise the temperature of one pound of water one degree Fahrenheit.

Annual Fuel Utilization Efficiency. A measure of a furnace's heating efficiency. The higher the AFUE%, the more efficient the product. The government's established minimum AFUE rating for non-weatherized gas furnaces is 80%.

Heating Seasonal Performance Factor. A measure of a heat pump's heating efficiency. The higher the HSPF, the more efficient the product. There is no government established minimum HSPF rating for heat pumps.

Seasonal Energy Efficiency Ratio. A measure of an air conditioner's cooling efficiency. The higher the SEER, the more efficient the product. The government's established minimum SEER rating for air conditioners manufactured after January 2006 is 13.