As the name suggests this is a heat pump to produce domestic hot water (DHW) only, they can use air from outside, or air recovered from within the building, but the latter is something of a delicate balancing act between air flor rates, over extraction and what the heat pump needs, you may also here these termed as exhaust air heat pumps. 

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they are an ideal solution where the building is too small to warranty a full hydraulic heating system, or maybe in flats where the installation of a gas boiler is now strictly prohibited. The are generally, low power, and compact meaning the heat pump only DHW reheat times can be quite considerable from cold to hot, however we can mitigate th sda