The efficiency of air source heat pumps increases as the flow temperature decreases. In other words, for maximum efficiency we want to produce water as cool (or less hot!) as possible.

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We refer to this temperature as the ‘design flow temperature’. Historically, gas and oil boilers have been designed to run at high temperatures (around 70c), whereas we typically design heat pump systems to run at 50c of less (<50c for radiator systems and <40c for underfloor heating).

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High temperature air source heat pumps are available these days, however, their efficiency is not as great as a low temperature heat pump. For example, the Daikin H HT model can achieve up to 70c flow temperatures, and the Hitachi Yutaki S80 6HP can achieve a flow temperature up to 80c.

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Despite their lesser efficiencies, high temperature air source heat pumps do have a use and can be used in scenarios where it is not feasible to install larger radiators, or the required output in each room is too great to be achieved at low temperatures (often in older, poorly insulated properties).