Heat Pump outputs explained
Each heat pump manufacturer tends to give a kW output for their heat pumps. But take note! this does not usually mean the same thing as a gas or pellet boiler where the heat output is in general steady.
But the output of an air source heat pump depends on how cold the air is, and just how hot the heat pump has to make the water. If the outside air temperature outside drops, or the water temperature that the heat pump has to provide rises then the output, and efficiency, of the heat pump drops.
The output which manufacturers tend to pick is the one at 7ºC outside and at a flow temperature of 45ºC. This does allow us to compare the outputs of different heat pumps to each other and so it is useful. And yet it is vitally important to realise when sizing a heat pump because if you just assume that it is "8kW" then you are likely to be disappointed.
I suspect this is one of the main reasons why some heat pump installations fail - nothing to do with the heat pump as such, but more to do with poor specification and poor system design.
So - let's have a look at this big set of figures below - from Clivet EVO air source heat pumps:
On the lefthand side we have the outside air temp. I've put an arrow to -2/-3ºC. In Devon it does (rarely) get that cold. And I've also put an arrow by 7/6ºC - because that's a more common cold air temperature in Devon. Then I've highlighted in yellow the most challenging scenario - which is that the heat pump has to produce water at 60ºC - and a better case which is when it can run at 45ºC connected to underfloor heating or big radiators.
As you can see the outputs varies from a low of 8.66 kW (at -2/-3ºC outside, and a flow temp of 60ºC) to 12.4 kW (at 7/6ºC outside, and a flow temp of 45ºC).
So it's really important to bear all these things in mind when choosing the right heat pump - it needs to be sized correctly for the typical temperatures where you live.
And here is a line graph showing the effect that outside air temperature, and supply temperature, have on heat output.
Often heat pumps can come with an inbuilt immersion element, or you can specify an immersion for the thermal store or water cylinder.
This could then be controlled so that it only comes on for the couple of days of the year when it's so cold that the heat pump might not be able to keep up with total demand. As long as it designed correctly then will only be a short period of time when this is needed as a topup - so that should not end up costing a lot to run.
We can size the heat pump so that it covers the total heat demand, even during those most challenging days.