A heat pump is simply an air conditioner which can work in reverse to help heat the house when cooling is not needed. During the cooling season, it collects heat from the interior of the house and discharges it to the exterior, like any other air conditioner. During the heating season, the opposite is true. This is accomplished by simply reversing the flow of the refrigerant. The condenser becomes the evaporator and the evaporator becomes the condenser.
To further illustrate heat pump operation, one might consider a refrigerator as a heat pump working in the heating mode to heat a kitchen. The temperature within the refrigerator is obviously cooler than the temperature within the kitchen, yet the refrigerators cooling system is able to steal heat from the inside of the refrigerator and release the heat into the kitchen, via the coils on the back of the refrigerator.
Figure 3. Air Source Heat Pump Operation
As with conventional air conditioning, the compressor is the most critical component.
In northern climates, heat pumps are not capable of carrying the entire heating load of a house. They are only practical to operate when the outside air has enough heat that it can be collected economically. In other words, when it costs more than a dollars worth of electricity to get the heat pump to generate a dollars worth of heat, the system is no longer efficient and is shut down. At this stage, the central heating system takes over.
Depending upon the design of the heat pump and the insulation of the house, the heat pump may be forced to shut off earlier. Even if the heat pump is operating economically, it may not be able to generate enough heat to keep the house warm (the system has slipped below the balance point). If this should occur, the heat pump will shut off and the main heating source will take over. This assumes that the furnace is gas or oil. Electric furnaces and heat pumps can run simultaneously.
Figure 4. Heat Pump Balance Point
A heat pump cannot be tested during exceptionally cold periods of the year, when the heat pump is not working. During other portions of the year, if the heat pump is operating in the heating mode, it should not be tested in the cooling mode, to prevent added stress on the system. Conversely, if the system is operating in the cooling mode, it should not be tested in the heating mode.
When the heat pump is operating in the heating mode, the air coming out of the registers feels drafty to some people. It is not as warm as the air from the registers when the furnace is operating (roughly 90 degrees F. as opposed to 120 degrees F.).
The Basics of Independent Heat Pumps
In houses which are heated by some other means than a forced-air system, heat pump systems are independent of the heating. Since there is no distribution ductwork, special ducts must be added. They are similar in operation to other heat pump systems described above; however, the indoor coil does not sit in the ductwork above the furnace. Instead, it sits in independent ductwork.
On a heat pump system which shares ductwork with a heating system, the blower for the heating system is used for the heat pump. With an independent system, a separate blower must be provided.
Independent systems are normally located in attics; however, they are sometimes in the basement. Most independent systems are installed on a retrofit basis and, consequently, there are limitations to the size and location of the ductwork. Limited space dictates that many of the systems must have very small ductwork. Most of these systems employ round, flexible, insulated ductwork which has a very small diameter. To compensate for the small ducts, the velocity of the air travelling through the ducts is increased dramatically. With these systems, discharge nozzles (diffusers) mounted in ceilings are used instead of conventional heating registers. A large return air grille is typically ceiling mounted on the top floor. Other than the size and location of ductwork, and the location of the major components, these systems are identical to conventional heat pumps.
Because independent systems are normally installed on a retrofit basis, a few compromises have to be made. Attic mounted units are more difficult to service simply because of their location. Also, a system mounted in the attic tends to be noisier than a system mounted in the basement. Special care should be taken to avoid leakage of condensate into the living space below.
Independent systems are often limited in size (capacity). Therefore, they are not practical in some instances.
Since independent systems are normally installed in houses which did not have ductwork, it is not uncommon to take advantage of the ductwork to perform other functions. Therefore, electronic air cleaners, humidifiers, and plenum heaters are sometimes found with independent systems. For more information on these components, please refer to the Heating Section.
Note: Systems employing humidifiers which are attic mounted are prone to winter freezing problems.
Auxiliary Heat
Heat pumps are not capable of carrying the entire heating load during the coldest parts of a northern winter. With most systems, the heat pump has to eventually shut down and let the furnace take over because there is not enough heat in the outside air to be collected economically.
Some systems are designed to fool the heat pump. They have a burner (usually natural gas) outdoors below the evaporator coil (the coil which is collecting the heat). Just when the system is about to shut down and pass over its responsibilities to the furnace, the outdoor burner comes on and the evaporator coil thinks that there has suddenly been a heat wave. All of a sudden there is enough heat in the outside air to keep going. These systems do not require a furnace to supplement the heat pump because the heat pump never experiences the coldest part of the winter, or at least it thinks it doesn't. It's being fooled by an out-door auxiliary heater.
These systems require no furnace and no chimney.
These systems have a high initial cost. Long term durability and life expectancy are unknown, however oxidation has been noted on the burner and the coil as a result of condensation.
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