Exhaust air heat pump
See also Mechanical ventilation with heat recovery (MVHR)
Exhaust air heat pumps collect warm air as it leaves a building via the ventilation system and then reuse the heat that would otherwise be lost to the outside to heat fresh air coming into the building or to heat water. Exhaust air heat pumps operate on a similar basis to other heat pumps such as air source heat pumps and ground source heat pumps and are suitable for providing hot water and heating for buildings such as houses, apartments or flats. Exhaust air heat pumps are often used in conjunction with underfloor heating systems and have been used extensively in countries such as Germany and Sweden.
 Components and system
The system works by extracting air through a series of ventilation ducts from moisture-producing rooms such as bathrooms, utility rooms and kitchens. The heat energy in the air is passed over a heat exchanger which then transfers the energy to a refrigerant. The refrigerant boils and evaporates and as a gas, it is then compressed. As the compression takes place, heat is released which is then transferred to the hot water supply, or to the hot water heating system, or to incoming air which supplies non-moisture producing spaces such as bedrooms and living rooms.
As a series of ducts are required through the building to allow the flow of air to and from the exhaust air heat pump, installation tends to happen during the original construction of buildings. Retrofitting into existing buildings may not be practical or cost-effective.
 Benefits of exhaust air heat pumps
The main benefits of using exhaust air heat pumps include:
- Reduction in the cost of fuel bills.
- Reduction in the environmental impact of buildings through a reduction in CO2 emissions.
- Contribution to compliance with building regulation requirements.
- Continuous ventilation of building which can help improve indoor air quality and prevent condensation.
 Disadvantages exhaust air heat pumps
There are some disadvantages to exhaust air heat pump systems including:
- Installation costs can be high.
- Retrofitting can be disruptive and expensive.
- They work most efficiently with underfloor heating systems since they typically heat the water to lower temperatures than traditional radiator systems. Although radiators can work at lower temperatures, they will take longer to heat up rooms.
- It may not possible to rely on exhaust air heat pump systems in isolation for all heating needs and so a secondary system may be required.
- Heat pump efficiency can be significantly compromised by incorrect specification, poor installation, incorrect commissioning and incorrect operation. See Residential heat pump installations: the role of vocational education and training.
A recent government study is available which considers the efficiency of exhaust air heat pumps installed in social housing based on investigations carried out between 2010 and 2013. Ref DECC, Performance of exhaust air source heat pumps: Summary of detailed monitoring results.
 Related articles on Designing Buildings Wiki.
- Absorption heat pump.
- Air source heat pumps.
- Coefficient of Performance CoP.
- Domestic heat pumps and the electricity supply system.
- Dynamic thermal modelling of closed loop geothermal heat pump systems.
- Earth-to-air heat exchangers.
- Ground source heat pumps.
- Heat exchanger.
- Heat pump.
- Heat recovery.
- Mechanical ventilation with heat recovery.
- Phase change.
- Residential heat pump installations: the role of vocational education and training.
- Water source heat pumps.
 External references
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