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Since the introduction of air-source heat pumps to the marketplace in the 1950s, nearly one-third of all homes built today in the United States have at least one heat pump. The convenience of a single appliance that delivers both heating and cooling is appealing especially to those building new homes or replacing older Heating, Ventilation, and Air Conditioning (HVAC) systems. Some electric utility companies also offer rebates for higher-efficiency heat pumps [a Seasonal Energy-Efficiency Rating (SEER) of 12 or higher]. Even with these advantages, however, many heat pump owners still complain about noisy operation and inadequate heating performance. The sections below describe some of the recent improvements in heat pump technology that address these complaints.
Reverse Cycle Chillers
One of the more notable innovations is called a Reverse Cycle Chiller (RCC). It offers the advantages of allowing the homeowner to choose from a wide variety of heating and cooling distribution systems from hydronic radiant floor to multiple zones for forced air. It also offers the potential for lower winter electric bills and hotter air out of the supply vents for greater comfort. An RCC is especially economical for all electric homes or in areas where natural gas is not available. Depending on other fuel rates, it may even be the least expensive heating option over all of the remaining heating fuel choices.
The system consists of a standard 12 SEER, single speed, air source heat pump (sized to the heating load rather than the usual smaller summer cooling load); a large, heavily insulated tank of water that the heat pump heats or cools, depending on the season of the year; a fan coil with ducts or a radiant floor to distribute the stored energy to the house; and some controls to regulate the water flow.
Even if you choose a standard forced air distribution system, the RCC still does away with one of the biggest complaints about air source heat pumps, which is the periodic blowing of cool air during their defrost cycle and during the initial start of the heating cycle as the distribution ducts warm up. The RCC system solves these problems because as it defrosts, it uses the stored heat in the water tank and not the room air directly. Since the temperature of the water in the storage tank is fairly hot (comparable to a gas furnace's output temperature) the cold ductwork, and you, are quickly warmed.
The RCC system also allows the heat pump to operate at peak efficiency most of the time rather than only at above freezing outdoor temperatures like an ordinary heat pump would. This provides greater comfort and economy without the need for electric resistance auxiliary heating coils. In fact, most installations don't even have electric resistance coils at all, since the RCC by is completely capable of heating the building by itself.
For example, in one Michigan installation, the RCC system supplied 115°F (46°C) water to the air handler and a radiant floor system even though the outdoor temperature was negative 15°F (-26°C). If the RCC is used as a Thermal Energy Storage (TES) system (see the section below for more on a TES) it becomes even more cost effective for areas where the electric utility offers time of use rates.
Another significant energy saving benefit is that the RCC can be equipped with a refrigeration heat reclaimer (RHR). This is similar to the common desuperheater coil found on the high-end heat pumps and air conditioners (discussed below). The main difference is that the RHR not only makes hot water during the cooling season, but also does it during the heating season by using the excess capacity of the outdoor unit during the milder winter weather to make essentially free domestic hot water. In the summer it makes free hot water by reclaiming the waste heat from the house as long as the system is also cooling the building.
The combined RCC and RHR system costs about 25% more than a standard heat pump of similar size. The simple payback on the additional cost in areas where natural gas is not available is in about 2 to 3 years.
Two Speed Compressors, Variable Speed Fans, Desuperheaters, and Scroll Compressors
Most heat pumps and air conditioners use single-speed compressors. Single-speed compressors run at full capacity, no matter what the actual needs of the house are. This wastes energy and money.
Some models of heat pumps can be equipped with a two-speed compressor. A two-speed compressor runs at the capacity that is closest to the appropriate capacity to meet the need for heating or cooling at that particular moment. This saves large amounts of electrical energy, and reduces compressor wear.
Also, unlike a gas or oil fired furnace, heat pumps (except for the RCC above) do not deliver very hot air during very cold weather. Instead, the heat pump delivers heated air at around 100°F (37.7°C). Such modest temperatures feel cool to many people who's skin temperature is around 96°F (36°C.) This is despite the fact that the heated air is more than enough to keep the house itself at a comfortable temperature.
In addition to this problem, in order to make up for the lower heating temperature, heat pumps must also push a larger volume of warm air through the ducts. Since the blower must provide this volume of warm air intermittently, it must also have a large capacity. This often makes the blower noisy, as well as wasteful, because it consumes large amounts of electricity to push the air.
Some models of heat pumps can be equipped with variable-speed or dual-speed indoor fans (blowers), outdoor fans, or both. The variable-speed controls for these fans attempt to keep the air moving at a comfortable velocity, minimizing cool drafts and maximizing electrical savings.
The two-speed compressor with a variable speed blower also operates on low capacity most of the time and only goes to its higher capacity as necessary. Low-capacity operation has many advantages. It reduces compressor on-off cycling, which is a major cause of wasted energy and compressor wear as well as temperature fluctuations in rooms. It also minimizes the initial blast of cold air often associated with more primitive models of heat pumps.
Since the system cycles less often, there is also less start-up noise. With low-capacity operation, the heat pump's heat exchanger coils operate at a temperature closer to the ambient air temperature. This reduces the differential between the outside and inside temperatures, making the unit operate more efficiently. This is especially advantageous in the winter. This feature keeps the outdoor coil from frosting, and in turn means that it needs less energy to defrost the outdoor coil.
Unlike a single-speed heat pump, a two-speed heat pump also works well for zone control systems. Zone control systems use automatic dampers to allow the heat pump to keep different rooms at different temperatures. This is a feature often found in houses larger than 4,000 square feet (372 square meters.)
Many high-efficiency heat pumps can be equipped with another energy-saving feature, known as a "desuperheater." In the heat pump's cooling mode, the unit recycles some of the waste heat from the house to generate domestic hot water. A desuperheater-equipped heat pump can heat water 2 to 3 times more efficiently than an ordinary electric water heater.
Another advance in heat pump technology is the scroll compressor. Scroll compressors differ considerably from the reciprocating compressor, which uses a piston inside a cylinder to compress the refrigerant. The scroll compressor consists of two spiral-shaped scrolls. One remains stationary, while the other orbits around it, compressing the refrigerant by forcing it into increasingly smaller areas.
A scroll compressor has a number of advantages over a piston compressor. It is simpler in design and has only one moving part. Scroll compressors are also more tolerant of compressor "flooding," and of contaminants entering the system, both of which are major causes of piston compressor failure. These advantages give scroll compressors a longer operating life. The rotary motion of the scroll compressor is also quieter. According to some reports, heat pumps with scroll compressors provide 10° to 15°F (5.6° to 8.3°C) warmer air when in the heating mode, compared to existing heat pumps with piston compressors.
Dehumidifying Heat Pipes and New Designs for Heat Exchangers
Dehumidifying heat pipes improve the ability of heat pumps and air-conditioners to remove excess moisture from the air in humid climates. They were invented for the US space program to dehumidify the cabin air inside a space vehicle, and to save electrical energy.
Dehumidifying heat pipes are a system of sealed tubes with liquid refrigerant inside. They transfer heat from one area to another without using any electrical energy. When hot, humid air passes through one end of the pipes, the pipes heat up, evaporating the refrigerant inside. This absorbs some of the air's heat. The air stream then flows through the heat pump's (or air-conditioner's) evaporator coil, where it is further dehumidified and cooled. The vaporized refrigerant moves to the other side of the heat pipes, where the cooler air passing through it condenses it, giving off the heat it had absorbed earlier, and raising the temperature of the air stream to the required level. Grooves on the interior of the pipes act as a wick to return the liquid to the opposite end of the pipes. Once there, it repeats the evaporation cycle.
Most models of heat pumps and air-conditioners can be retrofitted with dehumidifying heat pipes. They can also be built into new heat pump or air-conditioner heat exchanger coils. Tests have found that these devices increase the dehumidifying capacity of a cooling system by up to 91%, while slightly increasing electricity consumption, because the deeper coils create slightly greater air resistance for the blower fan. Heat pipes save electricity in another way as well. They allow the cooling system to be reduced in size.
There are also new types of heat exchanger coils for air conditioners and heat pumps. One is the PF (plate-fin) coil. Tests indicated that they improve the performance of a heat pump by a significant percentage. Another type of heat exchanger is the O-coil. This design permits a better airflow across the coils, and uses fewer welds, which minimizes the chances of refrigerant leaks. The O-coil also takes up less space, shrinking the unit's physical size or "footprint."
Gas-Fired Heat Pumps
This type of heat pump uses natural gas to run a small engine, which in turn drives the heat pump's compressor. To date, only the one company has the license to manufacturer this type of heat pump. Overall efficiencies are said to be in the 126% range. This is just slightly lower than the efficiency of an electric air-source heat pump. This technology also recovers much of the waste heat from the engine to either heat the house or to make domestic hot water.
The main advantage of this system is not so much energy savings as it is economics. In some parts of the nation, natural gas costs less per Btu of heat than electricity does. However, this does not include the annual "tune-up" that's necessary to keep the system operating correctly.
Ductless Heat Pumps
Ductless heat pumps (also known as mini-split systems) function the same as any other heat pump, except that they do not require distribution ducts, where some heating or cooling energy is lost. Such systems offer energy savings of as much as 50% over a ducted air distribution system. Ductless systems consist of an outside unit containing the usual heat pump components, and an indoor unit that can be located on any nearby wall. The usual heat pump (or air-conditioner) refrigerant tubing connects the two sections. These are ideal for smaller houses, remote rooms in large houses, or houses that have an open floor plan. These systems can also have several indoor units connected to one outdoor unit. This gives the occupants of the house the ability to "zone" or control heating and cooling to different parts of the house.
Heat Pumps with Gas Back-up Heat (Dual Fuel)
Even though combustion furnaces that use more than one type of fuel are not new, they are less common than dual fuel with heat pumps. This is due to a lack of awareness of these appliances, and the small number of manufacturers producing them. In theory, any heat pump can be equipped with a gas (LP or natural gas) burner that supplements the heat pump when the outside temperature falls below about 35°F (1.6°C). Gas back-up heat helps solve the problem of the heat pump delivering relatively cool air during cold weather. It also lowers the electrical consumption of the equipment, because the electric resistance coils (the major source of high electric bills in cold weather) are not used.
Since there are few heat pump manufacturers that incorporate both types of heat supply in one box, these configurations are often two smaller, side-by-side, standard systems sharing the same ductwork. The combustion fuel half of the system could be propane, natural gas, or oil, or even coal and wood.
In comparison with a combustion fuel-fired furnace or standard heat pump alone, this type of system is also economical. Actual energy savings depend on the relative costs of the combustion fuel relative to electricity.
Geothermal Heat Pumps
Geothermal heat pumps (sometimes referred to as earth-coupled, ground-source, or water-source heat pumps) have been in use since the late 1940s. Geothermal heat pumps use the constant temperature of the earth as the exchange medium instead of the outside air temperature. This allows the system to reach fairly high efficiencies (300%-600%) on the coldest of winter nights, compared to 175%-250% for air-source heat pumps on cool days.
As with any heat pump, geothermal and water-source heat pumps are able to heat, cool, and, if so equipped, supply the house with hot water. Some models of geothermal systems are available with two-speed compressors and variable fans for more comfort and energy savings. They are quieter, last longer, need little maintenance and do not depend on the temperature of the outside air relative air-source heat pumps.
At least one manufacturer has combined an air-source heat pump with a geothermal heat pump. This is called a "Dual-Source" heat pump. These appliances combine the best of both systems. Dual-source heat pumps have higher efficiency ratings than air-source units, but are not as efficient as geothermal units. The main advantage of dual-source systems is that they cost much less to install than a single geothermal unit, and work almost as well.
Even though the installation price of a geothermal system can be several times that of an air-source system of the same heating and cooling capacity, the additional costs are returned to you in energy savings in 5 to 10 years. System life is estimated at 25 years for the inside components and 50+ years for the ground loop. There are approximately 40,000 geothermal and water source heat pumps installed in the United States each year.
Bibliography
The following articles have additional information about heat pump innovations.
"Dual-Compressor, Multicapacity Heat Pumps," Energy Design Update, (14:6) pp. 9-12, June 1994.
"Ductless A/C Systems Gain Recognition and Uses," Air Conditioning, Heating & Refrigeration News, pp. 13-15, April 18, 1994.
"Energy-Saving Products for Energy-Efficient Homes," J. Nisson, Journal of Light Construction, (13:4) pp. 31-34, January 1995.
"Focus AHR Expo," Air Conditioning, Heating & Refrigeration News, entire issue, February 19, 2001.
"Gas-Fired Heat Pump Gets Real-Life Test," J. Frantz, Air Conditioning, Heating & Refrigeration News, pp. 20-21, May 30, 1994.
"Getting the Most Out of Heat Pumps," Air Conditioning, Heating & Refrigeration News, p. 9, May 30, 1994.
"Heat from the Earth: Geothermal Heat Pumps," The Family Handyman, (46:9) pp. 9-12, October 1996.
"Heat Pipe Defeats Mold, Mildew; Saves Energy in Houston," G. Schultz, Air Conditioning, Heating & Refrigeration News, pp. 11-13, March 21, 1994.
"Heat Pumps Reflect Customer Needs," C. Hayes, Air Conditioning, Heating & Refrigeration News, p. 22, June 6, 1994.
"High-Velocity Forced-Air Heating and Cooling Systems," Energy Design Update, (15:4) pp. 9-12, April 1995.
" 'Infinity' Variable-Capacity Units Boast SEERS Above 16," Air Conditioning, Heating & Refrigeration News, pp. 79-80, January 21, 1991.
"Ingenious New Dual-Source Heat Pump," Energy Design Update, (14:10) pp. 11-13, October 1995.
"New Air Cooled Reverse Cycle Chiller Provides Cost Effective Heating at -15°F", Air Conditioning, Heating & Refrigeration News, pp. 22, September 8, 1997.
"New Heat Pump System Will Tap Btus in Municipal Water," Ed., Energy Design Update, (21:3) pp. 6-7, March 2001.
"Nyle Corp. Unveils New Cold Climate Heat Pump." Ed., Energy Design Update, (21:10) pp. 9-10, October 2001.
This fact sheet was reviewed for accuracy in March 2003.
NOTICE
This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.
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