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Weather dependency and buffer tank

Air Source Heat Pumps (ASHPs)

Last Post by Simon S 2 years ago

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Simon S

(@simon-s)

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Topic starter 03/04/2023 4:41 pm

I have an ASHP with a buffer tank. It has been set to run a weather depent system, so normally it runs at 40 degrees C, when it's cold outside it runs at 43 degrees C. Does this mean I am wasting money heating the buffer tank by 3 degrees C when it's cold outside, and should I turn of the weather dependency>

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Derek M

(@derek-m)

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03/04/2023 4:48 pm

I would not recommend turning off weather dependent control, but from your information I suspect that it may need adjustment.

This post was modified 12 months ago by Mars

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bontwoody

(@bontwoody)

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03/04/2023 5:20 pm

Hi Simon, the reason it is a good thing to run the heat pump with weather dependency is for exactly the situation you describe. When it is cold outside the water is heated more so that you still feel toasty in your house.

A better way to think about it is that if you turned off the weather dependency then you would have to have a set water temperature of 43 degrees C all the time to account for those days which are cold, then you really would be wasting the energy in the extra 3 degrees on the warmer days.

I dont think 3 degrees sounds like a large differential, some HPs allow the weather dependency curve (line really) to be adjusted in its gradient giving more or less of a differential. If yours does you could try a bit or trial and error to find the right setting for you. For instance if your current setting is 43 degrees at -2 outside and 40 degrees at 15 outside, you might be able to adjust that to 43/-2 and 35/15 and see if that is warm enough on the warmer days.

Hope that makes sense 🙂

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Filipe

(@filipe)

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03/04/2023 5:38 pm

Posted by: @bontwoody

↑

A better way to think about it is that if you turned off the weather dependency then you would have to have a set water temperature of 43 degrees C all the time to account for those days which are cold, then you really would be wasting the energy in the extra 3 degrees on the warmer days.

I dont think 3 degrees sounds like a large differential, some HPs allow the weather dependency curve (line really) to be adjusted in its gradient giving more or less of a differential. If yours does you could try a bit or trial and error to find the right setting for you. For instance if your current setting is 43 degrees at -2 outside and 40 degrees at 15 outside, you might be able to adjust that to 43/-2 and 35/15 and see if that is warm enough on the warmer days.

Hi Simon, If you want it automated then use WC, but one can just use the control panel to adjust target water temperature up or down. If the buffer is quite large it could be more complicated. The problem with WC is that outside temperature is the only variable. Solar gain and the thermal capacity of the house tend to make a significant difference, the former quite quickly and the latter relatively slowly. Emitters also, but all the rads need to be matched across the house so that all rooms warm up at the same rate.

Phil

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JamesPa

(@jamespa)

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03/04/2023 6:53 pm

With a flow temp of 43C, properly adjusted weather compensation should save you about 15% in running costs. If this doesn't matter to you then leave it as it is, otherwise it should be adjusted correctly as suggested by bontwoody.

Filipe is of course right that ambient temp is not the only variable, but that's not really the point. Weather compensation (with balanced radiators) will get you 'most of the way there' with the lowest flow temperature for any given ambient. That makes for max efficiency.

The general recommendation for operating heat pumps efficiently is:

balance the radiators

get weather compensation adjusted so the rooms are at the right temp with the TRVs fully open

use TRVs and thermostats as temperature limiters (to shut down rooms where solar gain takes over) not temperature controllers

leave the HP on most if not all of the time , perhaps with a 3C nighttime set back if the HP controller can do that ('slow and low')

The closer you can get to this the more efficient your running will be.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

👍

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Simon S

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