If you are considering a heat pump system rather than a traditional furnace (or boiler) and air conditioner for heating and cooling your home, you will have several types of heat pumps from which to choose. Heat pumps differ from traditional heating/cooling systems because they are designed to move heat from outdoors to indoors (or vice versa) rather the generate heat. For this reason, they are considerably more energy efficient than fuel-burning systems — but they have limitations since they may not work very well in extremely hot or cold climates.
Within this general principle of moving heat, the different types of heat pumps vary in design, with some using outdoor air as the source of heat and others using the earth itself or a body of water as the heat source.
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1. Air-Source Heat Pumps
The operating principle of an air-source heat pump is quite similar to the operation of a central air conditioner, a window air conditioner or even a refrigerator. All these devices transfer heat by the principle by which a liquid expanding into a gas cools off (in the same way that sweat evaporating from your skin provides cooling), while the same gas heats up as it is compressed back into a liquid. Loops of pipe filled with a chemical refrigerant and driven by mechanical compressors and expanders control how and where the liquid expands into a gas and is compressed back into a liquid.
The difference between a standard air conditioner or refrigerator and an air-source heat pump is that the heat pump is reversible so that the mechanism can either transfer indoor heat to the outdoors (the cooling cycle) or transfer outdoor heat to the indoors (when operating as a heater). A conventional air conditioner, on the other hand, is a simple one-direction heat pump designed to only absorb indoor heat and expel it outdoors.
Air-source heat pumps are the most common form of heat pump for residential heating/cooling systems. Originally, some air-source heat pumps, especially minisplit systems, were primarily cooling systems and were not all that effective for providing heat in regions with prolonged subfreezing temperatures. However, today's air-source heat pump systems offer a viable alternative to electric furnaces or baseboard heating systems at temperatures down to zero degrees Fahrenheit. In heating mode, air-source heat pumps can be roughly 50 percent cheaper than heating a house by electric baseboard heating or with an electric forced-air furnace. Air-source heat pumps are also somewhat more efficient than traditional air conditioner systems at removing humidity from a home.
Air-source heat pumps with a forced-air blower system typically cost $4,000 to $8,000 to install. This is considerably more than the average cost of replacing a forced-air furnace, but energy savings will quickly recoup the added investment. Compared to an electric forced-air furnace, heat pumps save 30 to 70 percent on operating costs, paying for themselves within five to 12 years.
Ductless Minisplit Heat Pumps
A very popular form of air-source heat pump for residential use is generally described as a ductless, or minisplit system. These systems work by means of two mechanical units: an outdoor compressor/condenser unit and one or more indoor air handler units containing an evaporator. The air handlers are usually rectangular boxes that sit on a wall or ceiling. Plastic pipes carry the refrigerant from the compressor to the air handler, which can be located as much as 50 feet apart. Minisplit heat pumps also incorporate a drain line to allow condensed moisture to drain outdoors, and they require an electrical cable connection to power the indoor air handler(s) and the outdoor compressor.
In cooling mode, heat is absorbed inside the house as the refrigerant expands inside coils within the air handler unit. Then, that heat is released as the refrigerant flows to the outdoor unit and is compressed. This is quite similar to the way that a standard window air conditioner works, except the compressor and evaporator components can be located quite a long distance apart. Minisplits are also much quieter than window air conditioners. A minisplit heat pump is reversible so that during cold days, the units can absorb heat from outdoor air and release it indoors.
Minisplit systems are excellent for retrofitting into a home without existing ductwork, such as a home with a steam or hot water boiler and radiators. This allows you to add air conditioning to a home where it is impractical to add ductwork for a forced-air system, and it has the added benefit of providing a heat source to relieve the load on the boiler or furnace.
These systems offer great flexibility, as a single outdoor compressor/condenser can serve as many as four indoor air handler units, each operating as a separate zone with its own thermostat and controller. The tubing that moves refrigerant between the outdoor and indoor components requires only a 3-inch hole in the wall, and since there is no ductwork to radiate thermal energy, these are very efficient systems that are up to 30 percent more efficient than forced-air central air conditioners, which lose a considerable amount of energy to ductwork running through walls or attics.
On the downside, minisplits can be more expensive to install than a central air conditioner compressor that uses forced-air ductwork, and they require an installer who is skilled in choosing the right size and positioning the indoor air handlers correctly. A system that is too large or one where the air handlers are badly placed will be expensive to operate.
According to national statistics, a minisplit system is roughly 30 percent more expensive to install than a traditional ducted system for new construction, though it can be more affordable for retrofit installations, where installing new ductwork can be very complicated. The national average is about $3,000 to $4,000 for a basic 12,000 BTU minisplit unit with a single indoor air handler. Plan on spending about $1,000 more for each additional indoor air handler. One national survey shows an average of $5,600 for professional installation of a minisplit system.
While this may seem expensive, over the life of the system, you may well recoup the higher cost through energy savings.
Window Heat Pumps
A relatively new entry into the heat pump world is the window-unit heat pump, which looks and operates much like a window air conditioner, with one important difference: It is reversible, so it can either cool or heat a room. The smallest units cost $500 to $700, while larger units with BTU ratings as high as 25,000 can cost close to $2,000. These units can be a good option for apartment dwellers who are looking for a unit to both cool and heat a room or for homeowners who are looking for a way to provide auxiliary cooling and heating for a new room addition or finished attic. Many leading manufacturers now make these heating/cooling window units, including Amana, LG, Comfort Aire and General Electric. They are often marketed as "heat/cool air conditioners," but the design is that of a classic air-source heat pump.
2. Ground-Source Heat Pumps
Ground-source heat pumps usually work by circulating an antifreeze/refrigerant solution that runs through an extensive pipe loop buried in the soil below the frost line. There are a variety of methods for running this tubing based on the logistics of the building site. As with air-source heat pump systems, the antifreeze absorbs and releases its heat due to an expansion/compression cycle dictated by a mechanical compressor, expansion valve and evaporator. The system collects heat at one end and releases it at the other. During the winter heating season, the system absorbs heat from the earth and releases it indoors; the process is reversed during the summer cooling season.
There are several variations in how these systems are piped. Closed-loop systems generally use an antifreeze flowing through a sealed system of tubing, but there are also open-loop systems that use groundwater or lake/pond water as the circulating fluid through the buried tubing. Such systems are possible only where local regulations and water quality allow for it.
Ground-source heat pumps cost considerably more to install than other types of heat pumps because they involve extensive excavation to run underground piping. Expect to spend $15,000 to $25,000 to install a new ground-source heat pump with underground tubing. However, they cost a lot less to operate on an ongoing basis, resulting in long-term savings. In addition, some of these systems can also heat the water for radiant flooring systems or for faucet taps in the home.
3. Water-Source Heat Pumps
A different form of geothermal pump is one that uses a body of water, such as a lake or large pond, as the source of heat. A closed and sealed system of refrigerant-filled piping is looped at least 8 feet below the surface of a suitable body of water, and this lake or pond provides the source of heat used by the system. Like ground-source systems, a compressor and evaporator change refrigerant inside the piping from gas to liquid to transfer the heat — from the water and into the house for the heating season and from the house into the water during the cooling season.
Water-source systems can be cheaper to install since there is no need for extensive excavation to bury the piping in the earth, but they are subject to many regulations since keeping the refrigerant-filled piping sealed and isolated from the water is essential.
4. Hybrid Heat Pumps
There are residential heating/cooling systems that include both a heat pump and traditional forced-air HVAC components. In addition to a forced-air furnace, the system features an outdoor heat pump unit (usually an air-source type) that handles the heating and cooling duties during low-demand periods. During very cold or very hot periods, the traditional HVAC system is activated.
A good option in extreme climates is a hybrid system in situations where a heat-pump-only system may not be able to handle the duties at all times. They are also a good option when the traditional furnace must be electric rather than natural gas. Heating with an electric furnace alone can be an expensive proposition.
5. Absorption Heat Pumps
There is another less common heat pump design that doesn't fit into the air-source category, the two geothermal types or the hybrid category. Known as an absorption heat pump, this design uses an ammonia-water absorption cycle to provide heating and cooling. In the same way as a standard heat pump, the ammonia refrigerant is condensed in one coil to release its heat, and then its pressure is released through an expansion mechanism to absorb heat. When the system absorbs heat from inside your home, it provides cooling; when it releases heat to the interior of your home, it provides heating.
The difference in design is that in the absorption heat pump, the evaporated ammonia is not directly pressurized by a compressor but is instead absorbed into water before being compressed by a relatively low-power pump. To release the dissolved ammonia from the water at the other end of the cycle, the system requires a heat source, such as a gas burner or electric heat source.
These systems are relatively rare for all but the largest residential homes. They make sense only for homes larger than 4,000 square feet.