Sunday, 3 May 2015

High temperature heat pumps go better with weather compensation.

The problem with heat pumps used to be that you couldn't use them with your existing radiators because they could not supply water at a high enough temperature - that is not true any more. You can now get high temperature heat pumps and the Renewable Heat Incentive (RHI) has been revised so you can get a subsidy for them too. But that doesn't mean it is a good idea to run them at a high temperature all the time. If you have a high temperature heat pump it is a good idea to get weather compensating controls.



The installers configured the system to run the radiators at 80C
Last week I went to a forum on renewable heating where one of the speakers was a householder talking about his experience of having an air source heat pump installed. The system he had installed was a Daikin Altherma HT. It can supply water at up to 80C - and in fact the installers left him with it configured to do so. That is a normal temperature for a traditional system with an old fashioned boiler but almost certainly more than you need - and definitely higher than optimal. The speaker did say he was rather horrified at his electricity use over the two weeks since the system was installed.

At theoretical maximum efficiency you need 73% more power at 80C than 45C
Heat pumps' performance is specified by giving its COP (co-efficient of performance) for different temperatures. They usually give a value for A7 W45 - that is heating water to 45C from air at 7C. At theoretical maximum efficiency, to get the same heat you would need 22% more power at 55C, 44% more at 65C, and 73% more at 80C. Real heat pumps aren't anywhere near the theoretical maximum but they still have a performance hit at higher temperatures. According to the spec sheet the Daikin Altherma 11 kW is 18% less efficient at 65C than at 45C. It doesn't give a value for 80C.

Weather compensating controls run the radiators cooler when the outside temperature allows.
Weather compensating controls tell your heating system to run the radiators as cool as possible depending on the weather. When the weather isn't too cold, the radiators don't have to work so hard and the water does not need to be so hot so you can run the heat pump more efficiently. When it really is freezing outside the radiator temperature will go up and the pumps are not so efficient but at least you will be warm. Weather compensating controls are also useful for condensing boiler systems because they are more efficient at lower temperatures too - so that the radiator return temperature is cool enough to give good condensation and capture more heat. However, this is more important for heat pumps. If you don't have a weather compensating controller you can twiddle the settings yourself - if you remember.

You will get less RHI subsidy for a high temperature heat pump
Also, the RHI gives you a subsidy for the renewable heat you generate. With a more efficient system, more of your heat is renewable so you get more subsidy. Suppose you need 20,000 kWh/year. The RHI heat emitter guide will give you an SPF (seasonal performance factor) of 2.5 for a high temperature heat pump. That means you need 8,000 kWh/year electricity to supply the required heat and only 12,000 kWh is renewable. If you have a system with larger radiators that can run with an SPF of 3.3, then you get 14,000 kWh/year renewable heat so 16% more subsidy. However, that probably isn't enough to pay for you to change your radiators.

When I wrote my post 'Do heat pumps deliver' back in February I was slightly out of date saying that you can't get the RHI for heat pumps configured hotter than 55C. However, the other advice I included about how to make sure your heat pump runs efficiently still stands.

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