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.
Air leaving the building when the heat pump's compressor is running is usually at around -1° in most versions, thus the unit is extracting heat from the air that needs to be changed (at around a half an air change per hour) . Air entering the house is of course generally warmer than the air processed through the unit so there is a net 'gain'. Care must be taken that these are only used in the correct type of houses. Many specifiers mis-understand that low heat loss means small. Exhaust air heat pumps have minimum flow rates that when installed in a small flat the air flow chronically over ventilates the flat increasing the heat los by drawing in large amounts of unwanted outside air. For most exhaust air heat pumps there will be a low heat output to the hot water and heating of just around 1.2 Kw from the compressor / heat pump process but if that falls short of the building's requirements additional heat will be automatically triggered in the form of immersion heaters or an external gas boiler. The immersion heater top up will be substantial and when a unit with a 6 kW immersion heater operates at full output it will cost £1 per hour to run.
A newer version from Nibe Sweden the F730 takes in additional air from outdoors to the above process - this is a more powerful type of EAHP with an inverter controlled compressor. Out put from the heat pump can vary from 1.1 to 6 kW and the discharged air from this unit can be as low as -15 °C
Between 2009 and 2013, some 15,000 brand new social homes were being built in the UK with NIBE EAHPs used as primary heating. Most owners and housing association tenants have reported crippling electric bills. High running costs are usual with exhaust air heat pumps and should be expected. This is due to the very small heat of recovery with these units. The ventilation air stream is around 31 litres per second and the heat of recovery is 750W and no more. All additional heat necessary to provide heating and hot water is from electricity, either compressor electrical input or immersion heater. At outside temperatures below 0 C this type of heat pump removes more heat from a home than it supplies. Over a year around 60% of the energy input to a property with and exhaust air heat pump will be from electricity. This will always lead to high running cost by comparison to other heating systems and also high CO2.
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.