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).[1] This requires at least mechanical exhaust but 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.

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Air leaving the building when the heat pump's compressor is running is usually at around −1° in most versions.[2] Thus, the unit is extracting heat from the air that needs to be changed (at a rate of 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. Exhaust air heat pumps have minimum flow rates so that when installed in a small flat, the airflow chronically over-ventilates the flat and increases the heat loss by drawing in large amounts of unwanted outside air.[3] There are some models though that can take in additional outdoor air to negate this and this air is also feed to the compressor to avoid over ventilation.For most earlier exhaust air heat pumps there will be a low heat output to the hot water and heating of just around 1.8 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 could be substantial ( if you select the wrong unit), and when a unit with a 6 kW immersion heater operates at the full output it will cost £1 per hour to run.[4][5]