Heat pumps, as shown in Figure 1, have been around for a number of years, but many homeowners do not understand how a heat pump works.
Figure 1 - Heat pump
If you are going to make an informed decision on whether a heat pump is right for you, it is important to have a basic understanding of its principle of operation.
Principle operation of a heat pump.
A heat pump is a heat exchanger that uses a small amount of external energy to transfer the heat or cold of one item to another item. Hence, it can be used to heat or it can be used to cool. Most residential heat pumps are designed to do both functions. In the winter the heat pump reduces the energy costs to heat your home and in the summer it reduces the costs to cool your home.
It may be transferring the air temperature to a liquid or the temperature of a liquid to the air. It can also be designed to transfer the air temperature of one area to the air temperature of another area or the liquid temperature of one area to the liquid temperature of another area.
Every home that has any heating or cooling has a heat exchanger. A wood stove is a heat exchanger, as it transfers the heat from burning wood to the body of the wood stove, which in turn transfers the heat to the surrounding air. It is not a heat pump because it does not use any additional energy to aid in the transfer of the heat. The same applies to a hot water radiator, as the metal portion of the radiator becomes hot as the heat from the water inside it is transferred to the radiator itself. The air around the radiator becomes heated as the radiator transfers the heat to the air.
A vehicle’s cooling system is the one of the simplest forms of a heat pump and in fact is two heat pumps.
The first heat pump is in the engine block. Water is in circulated through the engine block and absorbs the heat of the engine block. A pump takes the water and circulates it through the engine block to a radiator, which is made up of a group of tubes. Air passes over the tubes, drawn across the tubes by a fan powered by the engine. The air that passes around the tubes absorbs the heat from the tubes (the second heat pump), cooling the water that is in the tubes, before that water is returned to the engine, to absorb more heat. Figure 2 is a diagram of the heat pumps working in your vehicle.
Figure 2 - Vehicle heat exchanger (cooling system)
The same principle is used in air conditioners.
In a portable air conditioner, air is passed through a set of cooling coils that are filled with a compressed gas, making them quite cool. As the air passes through the cooling coils it gives off its heat to the coils and hence reduces its temperature, providing cool air to the home.
A central air conditioner uses two heat exchangers. The first is located in the plenum of the forced air furnace. The hot air from the home is forced passed a set of cooling coils, the hot air transfers its heat to the cooling coils and the air that leaves the coils is cooler than the air that entered them. The warmed gas is then sent to a second heat exchanger, on the outside of the home where it transfers the heat from the compressed gas to the outside air.
A home heat exchanger, used as part of a total HVAC system, uses the temperature of the outside air to augment the heat that is produced by the furnace and uses the difference in the outside air temperature to augment the cooling part of the system.
Heat exchangers are now being applied to residential hot water tanks. The incoming water is circulated through a heat exchanger inside the home. As it circulates through the heat exchanger it absorbs the ambient heat from the room. This has the effect of pre-heating the water before it enters the hot water tank.
Additional information on hot water heat pumps.
Heat pumps work best in moderate climates and were very advantageous when the output of forced air furnaces was at best 70% efficient. However, new forced air furnaces work at 98 and 99% efficiency making it difficult to financially justify the added expense of the heat pump.