An air source heat pump (ASHP), the most common typeWater or Ground source heat pumps are the other type of heat pump, the water either taken from streams, or circulating as brine in the ground of heat pump, can absorb energy (heat) sourced from cold ambient air outside a building, and release the energy at a higher temperature to heat the building, either via hot air or hot water. Electricity powers the mechanical pump (compressor), the used electric energy providing typically 3 or 4 times more pumped thermal energy than simple resistive Joule heating.
Heat pumps use the same vapor-compression refrigeration process and much the same equipment as an air conditioner, but in the opposite direction, or rather source and target can be exchanged, vice versa. With ambient air accessible nearly everywhere, ASHPs are the most common type of heat pump and, usually being small, tend to be used to heat individual houses or flats rather than blocks, districts or industrial processes.
Air-to-air heat pumps provide hot or cold air directly to single rooms, but do not usually provide hot water. Air-to-water heat pumps use water pipes and radiators or underfloor heating to heat a whole house and are often also used to provide domestic hot water.
An ASHP can typically gain 4 kWh thermal energy from 1 kWh electric energy, thus its coefficient of performance or COP is 4. They are optimized for flow temperatures between , suitable for buildings with heat emitters sized for low flow temperatures. With losses in efficiency, an ASHP can even provide full central heating with a flow temperature up to .
about 10% of building heating worldwide is from ASHPs. They are the main way to phase out gas boilers and furnaces from houses, to avoid their greenhouse gas emissions.
Air-source heat pumps are used to move heat between two heat exchangers, one outside the building which is fitted with fins through which air is forced using a fan and the other which either directly heats the air inside the building or heats water which is then circulated around the building through radiators or underfloor heating which releases the heat to the building. These devices can also operate in a cooling mode where they extract heat via the internal heat exchanger and eject it into the ambient air using the external heat exchanger. Some can be used to heat water for washing which is stored in a domestic hot water tank.
Air-source heat pumps are relatively easy and inexpensive to install, so are the most widely used type. In mild weather, coefficient of performance (COP) may be between 2 and 5, while at temperatures below around an air-source heat pump may still achieve a COP of 1 to 4.
While older air-source heat pumps performed relatively poorly at low temperatures and were better suited for warm climates, newer models with variable-speed compressors remain highly efficient in freezing conditions allowing for wide adoption and cost savings in places like Minnesota and Maine in the United States.
In Europe, the boycott of Russian gas has accelerated the shift toward alternative. If 14 million were to switch to heat pumps, consumption could be reduced by 13 billion cubic metres. Heat pumps are up to five times more energy-efficient than conventional boilers, and in the UK, HVAC are now about 27 percent cheaper to operate than gas boilers.
Technology
Air at any natural temperature contains some heat. An air source heat pump transfers some of this from one place to another, for example between the outside and inside of a building.
An air-to-air system can be designed to transfer heat in either direction, to heat the interior of a building in winter and cool it in summer. Internal ducting may be used to distribute the air. An air-to-water system only pumps heat inwards, and can provide space heating and hot water. The description below focuses on use for interior heating.
The technology is similar to a refrigerator or freezer or air conditioning unit; the different effect is due to the location of the different system components. Just as the pipes on the back of a refrigerator become warm as the interior cools, so an ASHP warms the inside of a building whilst cooling the outside air.
The main components of a split-system (called split as there are two units, set up inside and outside) air source heat pump are: An outdoor evaporator heat exchanger coil, which extracts heat from ambient air Pipes with refrigerant fluid/gas connecting the units One or more indoor condenser heat exchanger coils. They transfer the heat into the indoor air, or an indoor heating system such as water-filled radiators or underfloor circuits and a domestic hot water tank.
Less commonly a packaged (single unit or monobloc) ASHP has everything outside in one box, with hot (or cold) air sent inside through a duct These are also called monobloc; they keep flammable propane outside the house. or via water pipes.
An ASHP can provide three or four times (COP 4) as much heat as an electric resistance heater for the same amount of electric energy. Burning gas or oil emits carbon dioxide and also NOx, which can be harmful to health. An air source heat pump itself does not issue carbon dioxide, nitrogen oxide or any other kind of gas, but emissions may occur where the electricity is generated. It uses a small amount of electrical energy to transfer (pump) a significantly larger amount of heat energy.
Most ASHPs are reversible and are able to either warm or cool buildings and in some cases also provide domestic hot water. The use of an air-to-water heat pump for house cooling has been criticised as being not designed for that purpose and thus not powerful enough.
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 at a low temperature and low pressure passes through the outdoor heat 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 a pressure valve into heat exchanger coils. There, the hot refrigerant gas condenses back to a liquid and transfers the stored latent heat to the indoor air, water heating or hot water system. The indoor air or heating water is pumped across the heat exchanger by an electric pump or fan.
The cool liquid refrigerant then re-enters the outdoor heat exchanger coils to begin a new cycle. Each cycle usually takes a few minutes.
Most heat pumps can also operate in a cooling mode where the cold refrigerant is moved through the indoor coils to cool the room air.
As of 2024 vapour compression is the only technology significant in the market.
Usage ASHPs are the most common type of heat pump and, usually being smaller, are generally more suitable to heat individual houses rather than blocks of flats, compact urban districts or industrial processes. In dense city centres heat networks may be better than ASHP. Air source heat pumps are used to provide interior space heating and cooling even in colder climates, and can be used efficiently for water heating in milder climates. A major advantage of some ASHPs is that the same system may be used for heating in winter and cooling in summer. Though the cost of installation is generally 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 access to the thermal storage capacity of the ground which allows it to produce more heat for less electricity in cold conditions.
Home batteries can mitigate the risk of power cuts and like ASHPs are becoming more popular. Some ASHPs can be coupled to solar panels as primary energy source, with a conventional electric grid as backup source.
Thermal storage solutions incorporating resistance heating can be used in conjunction with ASHPs. Storage may be more cost-effective if time of use electricity rates are available. Heat is stored in high density ceramic bricks contained within a thermally-insulated enclosure; storage heaters are an example. ASHPs may also be paired with passive solar heating. Thermal mass (such as concrete or rocks) heated by passive solar heat can help stabilize indoor temperatures, absorbing heat during the day and releasing heat at night, when outdoor temperatures are colder and heat pump efficiency is lower.
Replacing gas heating in existing houses Good home insulation is important. ASHPs are bigger than gas boilers and need more space outside, so the process is more complex and can be more expensive than if it was possible to just remove a gas boiler and install an ASHP in its place. If running costs are important choosing the right size is important because an ASHP which is too large will be more expensive to run.
It can be more complicated 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 temperatures means radiators (and possibly pipes) may have to be replaced with larger sizes, or a low temperature underfloor heating system installed instead.
Alternatively, a high temperature heat pump can be installed and existing heat emitters can be retained, however these heat pumps are more expensive to buy and run so may only be suitable for buildings which are hard to alter or insulate, such as some large historic houses.
ASHP are claimed to be healthier than fossil-fuelled heating such as gas heaters by maintaining a more even temperature and avoiding harmful fumes risk. By filtering the air and reducing humidity in hot humid summer climates, they are also said to reduce dust, allergens, and mold, which poses a health risk.
In cold climates
Operation of normal ASHPs is generally not recommended below −10 °C. However, ASHPs designed specifically for very cold climates (certified in the US under Energy Star) can extract useful heat from ambient air as cold as but electric resistance heating may be more efficient below −25 °C. This is made possible by the use of variable-speed compressors, powered by inverters. Although air source heat pumps are less efficient than well-installed ground source heat pumps (GSHPs) in cold conditions, air source heat pumps have lower initial costs and may be the most economical or practical choice. A hybrid system, with both a heat pump and an alternative source of heat such as a fossil fuel boiler, may be suitable if it is impractical to properly insulate a large house. Alternatively multiple heat pumps or a high temperature heat pump may be considered.
In some weather conditions condensation will form and then freeze onto the coils of the heat exchanger of the outdoor unit, reducing air flow through the coils. To clear this condensation, the unit operates a defrost cycle, switching to cooling mode for a few minutes and heating the coils until the ice melts. Air-to-water heat pumps use heat from the circulating water for this purpose, which results in a small and probably undetectable drop in water temperature; for air-to-air systems, heat is either taken from the air in the building or using an electrical heater. Some air-to-air systems simply stop the operation of the fans of both units and switch to cooling mode so that the outdoor unit returns to being the condenser so that it heats up and defrosts.