In regular air conditioning system for air conditioning the heat from inside your home is absorbed by your inside coil (evaporative coil). Then the heat collected from the inside coil is then released outside from the outdoor coil (condenser coil). To change from cooling to heating , the indoor and the outdoor coils trade function due to the reversing valve to change the flow of the refrigerant. The outdoor coil becomes the evaporator. and the indoor coil becomes the condenser. In the heating cycle, heat is absorbed by the outdoor coil which is now the evaporator, and discharged to the inside by the indoor coil which is now the condenser.
 

History of Heat Pumps

The popularity of the heat pump rose with the increasing cost of electrical energy used for heating in the residential and small commercial buildings. Heat pumps generally cost less to operate than fossil fuel furnaces and cost only about a third of what it cost to operate as electric strip heaters. The heat pump was actually developed by Lord Kelvin in 1852. The first practically applied heat pump was installed in Scotland in 1927. Between 1927 an 1950's, heat pumps were installed in hundreds of residential and small commercial applications through out Europe and the southern part of the United States. Many of these installations were merely air conditioning units converted to heat pumps by the addition of the reversing valves and the applicable controls. These early heat pumps had high failure rates because they did not have defrost controls, crankcase heaters, or suction accumulators. Heat pumps acquired bad reputation as a result of this high failure rate. Although improvements in design made heat pumps more reliable, the early high failure rates caused problems in the market for years. Further, the relatively low cost and energy during this same period almost destroyed this market.