Exhaust air heat pump
An Exhaust Air Heat Pump (EAHP) extracts heat from the exhaust air of a building and transfers the heat to the supply air, hot tap water and/or hydronic heating system (underfloor heating, radiators). This requires at least mechanical exhaust, mechanical supply is optional, see mechanical ventilation. This type of heat pump requires a certain air exchange rate to maintain its output power. Since the inside air is approximately 20-22 degrees Celsius all year round, the maximum output power of the heat pump is not varying with the seasons and outdoor temperature.
Since heat pumps do not create heat but rather only move it (pump it) from colder areas to warmer areas, this type of heat pump will not work as the primary source of heat for a home. You can't heat a home using heat that is already in the home. It is a supplemental heat source that captures some (but not all) of the heat that otherwise would have been exhausted from the building and returns it to the building. It reduces the amount of heat loss caused by a ventilation system. But it cannot add heat to the house.
Between 2009 and 2013, thousands of brand new social homes were being built in the UK with NIBE EAHPs used as primary heating. Home owners and housing association tenants have been reporting crippling electric bills. 
Many families are still battling with developers today to have their EAHP systems replaced with more reliable and efficient heating, noting the success of residents in Coventry. 
- "'Efficient' heating system left families with big bills". BBC News online. 10 Sep 2012. Retrieved 21 Nov 2012.
- "Victory for neighbours on Tile Hill estate in campaign to have boilers removed". Coventry Telegraph. 31 May 2013. Retrieved 31 May 2013.