Air source heat pumps

An air source heat pump (ASHP) is a system that transfers heat to outside a building, or vice versa. Under the principles of vapor compression refrigeration, an ASHP uses a refrigerant system involving a compressor and a condenser to absorb heat at one place and release it at another. They can be used as a space heater or cooler, and are sometimes called “reverse-cycle air conditioners”. In domestic heating, an ASHP absorbs heat and air-conditioning, as hot air, hot-water-filled radiators, underfloor heating and / or domestic hot water supply. The same system can often do the reverse in summer, cooling the inside of the house. When properly specified, ASHP can provide a full hot water heating solution and domestic hot water up to 80 ° C.

Air at any temperature above zero absolute zero some energy. An air-source heat pump transfers (‘pumps’) some of this energy to one place to another, for example between the outside and inside of a building. This can provide hot water and hot water. A single system can be designed to transfer heat in one direction, to heat or cool the interior of the building in winter and summer respectively. For simplicity, the description below focuses on the use of interior heating. The technology is similar to a refrigerant or air conditioning unit. Just as the pipes on the back of a refrigerator become warm as the interior, so an ASHP warms the inside of a building. The main components of an air-source heat pump are: A “standard” domestic air source heat pump can extract useful heat down to about. At colder outdoor temperatures the heat pump is less efficient; it could be switched off and supplemented by the supplemental heat (or emergency heat). There are some examples of this, while giving up some performance in cooling mode, will provide useful heat extraction to even lower outdoor temperatures.

An air source heat pump designed specifically for very cold climates can extract heat from ambient air as cold as -20 ° F or even -25 ° F (-30 ° C). Manufacturers include Mitsubishi and Fujitsu. One Mitsubishi model provides heat at -35 ° C, but the Coefficient of Performance (COP) drops to 0.9, indicating that resistance heating would be more efficient at that temperature. At -30 ° C, the COP is 1.1, according to the manufacturer’s data, the manufacturer’s marketing literature claims a minimum COP of 1.4 and performance at -30 ° C. Although the best air source heat pumps are still more effective than the best ground source heat pumps, air source heat pumps can be more economical or practical choice. A study by Natural Resources Canada found that cold climate air source heat pumps (CC-ASHPs) do work in Canadian winters, based on testing in Ottawa, Ontario in late December 2012 to early January 2013 using a ducted CC-ASHP. The record low for Ottawa is -36 ° C. The CC-ASHP provided 60% energy (though not energy cost) to natural gas, when considering only energy efficiency in the home. However, it is more likely to be used with CC-ASHP, relative to natural gas heating, in provinces or territories (Alberta, Nova Scotia, and the Northwest Territories) where coal-fired generation was the predominant method of electricity generation. (The energy savings in Saskatchewan were marginalized.) Despite the significant energy savings related to gas in the provinces (using 2012 retail prices in Ottawa, Ontario) made natural gas the less expensive energy source. (The report did not calculate the cost of operating in the province of Quebec, which did not have electricity, nor did it show the impact of using electricity.) The study found that in Ottawa was CC-ASHP cost 124% more to operate than the natural gas system. However, in areas where natural gas is not available to homeowners, 59% energy cost savings can be achieved relative to heating with fuel oil. The report noted that about 1 million residences in Canada (8%) are still heated with fuel oil. The report shows 54% energy cost savings for CC-ASHPs related to electric baseboard resistance heating. Based on these savings, the report showed a five-year payback for converting from one fuel oil to one electric CC-ASHP. The report did not consider it possible to calculate the potential for conversion of fuel oil. room temperature with the heat pump due to its defrost cycles. Based on these savings, the report showed a five-year payback for converting from one fuel oil to one electric CC-ASHP. The report did not consider it possible to calculate the potential for conversion of fuel oil. room temperature with the heat pump due to its defrost cycles. Based on these savings, the report showed a five-year payback for converting from one fuel oil to one electric CC-ASHP. The report did not consider it possible to calculate the potential for conversion of fuel oil. room temperature with the heat pump due to its defrost cycles.

Air source heat pumps can last for over 20 years with low maintenance requirements. Unreliable sources: -> There are numerous heat pumps from the 1970s and 1980s in the United States that are still in service in 2012, even in places where winters are extremely cold. Few moving parts reduce maintenance requirements. However, the outdoor heat exchanger and fan must be kept free of leaves and debris. Heat pumps have more than an equivalent electric resistance heater or fuel burning heater. Ground source heat pumps do not need fans or defrosting mechanisms.

Air source heat pumps are used to provide air conditioning and cooling in climates, and can be used for air conditioning. A major advantage of some ASHPs is that the same system can be used for heating in winter and cooling in summer, but it is not true that it can not be used without air conditioning. Although the cost of installation is high, it is less than the cost of a ground source heat pump, because a ground source heat pump requires excavation to install its ground loop. The advantage of a ground source heat pump is that it has the advantage of producing heat for the environment. ASHPs are often evenly supplied with heat pumps, which can be used in the event of pump malfunctions. Since ASHPs have high capital costs, and efficiency drops as high as possible, it is not possible to achieve a possible temperature scenario, even if an ASHP could meet the full heat requirement at the coldest temperatures expected. Propane, natural gas, oil or pellet fuel furnaces can provide this supplementary heat. All-electric heat pump systems have an electric furnace or electric resistance heat, or heat strip, which is usually composed of electric heat. A fan blows over the heated coils and circulates warm air throughout the home. This serves as an adequate heating source, as temperatures go up, electricity costs rise. Electrical service outages pose the same threat as a central forced-air systems and pump-based boilers, but woodstoves and non-electric fireplace inserts can mitigate this risk. Some ASHPs can be coupled to solar panels as a primary source of energy, with a conventional grid as backup source. Thermal storage solutions incorporating resistance heating can be used in conjunction with ASHPs. Storage may be more cost-effective if electricity rates are available. Heat is stored in a high density ceramic bricks contained within a thermally-insulated enclosure. ASHPs can also be paired with passive solar heating. Thermal heated by passive solar heat can help stabilize indoor temperatures, when outdoor temperatures are colder and heat pump efficiency is lower. The outdoor section can be used where the temperature is between 0 ° C and 5 ° C (32 ° F to 41 ° F). This restricts air flow across the outdoor coil. These units employ a cycle defrost where the system switches to ‘cooling’ mode to move heat from the home to the outdoor coil to the ice. This requires the additional heater (resistance electric or gas) to activate. The defrost cycle reduces the efficiency of the heat pump significantly, but the newer (demand) systems are more intelligent and need to defrost less. As temperatures drop below freezing the trend for reducing air humidity in the air. It is difficult to retrofit conventional heating systems that use radiators / radiant panels, hot water baseboard heaters, or even smaller diameter ducting, with ASHP-sourced heat. The lower heat pump output temperature would be required to increase the temperature of the heating system.

Heating and cooling is accomplished by pumping a refrigerant through the heat pump’s indoor and outdoor coils. Like in a refrigerator, a compressor, condenser, expansion valve and evaporator are used to change states of the refrigerant between colder liquid and hotter gas states. When the liquid refrigerant is at a low temperature and low pressure coils exchanger coils, ambient heat causes the liquid to boil (change to gas or vapor): heat energy from the outside air has been absorbed and stored in the refrigerant as latent heat. The gas is then compressed using an electric pump; the compression increases the temperature of the gas. Inside the building, the gas passes through the heat exchanger coils. There, the hot refrigerant gas condenses back to a liquid and transfers the stored heat to the indoor air, water heating or hot water system. The indoor air or heating is pumped by an electric pump gold fan. The cool liquid refrigerant then re-enter the heat exchanger coils to begin a new cycle. Most heat pumps can also be used in a cooling mode where the cold is air-cooled.

The ‘Efficiency’ of air source heat pumps is measured by the Coefficient of Performance (COP). A COP of 3 means the heat pump produces 3 units of heat energy for every 1 unit of electricity it consumes. Within the range of -3 ° C to 10 ° C, the COP for many machines is fairly stable at 3-3.5. In very mild weather, the temperature may be reduced to 4. However, it is cold winter day, it takes more work to move the same amount of heat than it does to a mild day. The heat pump’s performance is limited by the Carnot cycle and will approach 1.0 as the outdoor-to-indoor temperature difference increases, which for most air-source heat pumps will -18 ° C / 0 ° F. Heat pump construction that allows a lower carbon dioxide to a temperature of less than 2 ° C down to -20 ° C, pushing the break-even figure downward to -30 ° C (-22 ° F). A ground source heat pump has comparably less than a constant change in temperature. The design of a heat pump has a considerable impact on its efficiency. Many air source heat pumps are designed primarily as air conditioning units, mainly for use in summer temperatures. Designing a heat pump specifically for the purpose of heat exchange The main changes are in the scale and type of compressor and evaporator. Seasonally adjusted heating and cooling efficiencies are given by the seasonal performance factor (HSPF) and seasonal energy efficiency ratio (SEER) respectively. In units charged with HFC refrigerants,


With HFC refrigerants are often marketed as low energy or a global technology, however, the global warming potential (GWP) and ozone depletion potential (ODP) ). Recent government mandates have seen the phase-out of R-22 refrigerant and its replacement with more environmentally sound R-410A refrigerant.

While heat pumps with backup systems other than electrical resistance heating are often encouraged by electric utilities, air source heat pumps are a concern for winter-peaking utilities if electrical resistance heating is used as the supplemental or replacement heat source when the temperature drops below the point that the heat pump can meet all of the home’s heat requirement. Even if there is a non-electric backup system, the fact that the efficiencies of ASHPs is a concern to electric utilities. The drop in efficiency of their electrical load increases steeply as temperatures drop. A study in Canada’s Yukon Territory, where diesel generators are used for peaking capacity, noted that the adoption of air-conditioning is likely to be greater than that of the United States. As wind farms are increasingly used to supply electricity to the grid, the increased winter load matches the wind turbines and calmer days.


Summer, John A. (1976). Domestic Heat Pumps. PRISM Press. .