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  • Alastair McLean

Can an Air source heat pump provide enough hot water ?

YES - A heat pump is normaly in heating mode, this means it is harvesting the energy from the outside air and transferring it to the heating circuit which heats you're radiators and under floor heating. The heating mode is very different to hot water mode because in heating mode the heating circuit going around the radiators only needs to heat the rooms to about 20 deg centigrade, so the heat pump only does just enough work to achieve this to make it as efficient as possible. In some situations it can have an efficiency of over 500% which is amazing. It uses sensors inside and outside your home and a specially designed for your home heat curve profile that keeps it working within the most efficient band. The circulation temperature for heating varies from 35deg to about 45 deg C this is clearly not hot enough to heat the water in the hot water cylinder.

When the heat pump sees the is a demand to heat the hot water cylinder ie. when your programmer is set to heat it or when you set it manually on your touch screen or smartphone, the heat pump closes off the all heating valves and opens the flow to the hot water cylinder. The heat pump output flow temperature is increased to 55 deg C, for the heat pump to achieve this temperature it has to work much harder harvesting heat from the outside air and it also switches the weather compensation off to provide a steady flow at 55 deg C.

The Hot water cylinder is a specialty designed to work with a heat pump, it can transfer the heat from the heat pump much more efficiently and quickly than a standard cylinder enabling the cylinder to heat up very quickly so you always have hot water for baths and showers.

55 deg C is more than hot enough for a bath or shower.

There is also the option the use an immersion heater to raise the temperature to 61 deg C another 6 degrees once a week for a few minutes to ensure Legionnaires does not grow. There will be an assessment of this in the heat pump design.


Heat pump cylinder showing larger transfer coil
Heat pump cylinder

Hot water on manual override heating the cylinder
Evohome app hot water control


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