Volumisers in Heat Pump Systems: Does Placement Matter?

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Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

Last Post by SUNandAIR 20 minutes ago

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JamesPa

(@jamespa)

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11115 kWhs

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18/03/2025 11:13 am

Posted by: @abernyte

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"we are briefly going to steal some heat from the house or DHW to defrost the HP"

... Isn't great advertising! Or is that too cynical?

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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SUNandAIR

(@sunandair)

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3431 kWhs

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Posts: 465

18/03/2025 6:54 pm

Posted by: @abernyte

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Thank you gentlemen/people (can't be too careful now) perfectly reasoned arguments that fit the observed evidence. The only fly in this ointment is why the manufacturer does not mention this in it's otherwise clear and technical explanation of defrost. Taking a "tiny bit of heat from the HE at the end of the process" is the only mention. One would think if they were setting out to make such a video then the simplest route would be to say "we are briefly going to steal some heat from the house or DHW to defrost the HP" But they don't.

Sigh....split jury, should never have studied law!

@abernyte - you really can’t be too careful.... alas there is possibly another complication in the evidence..... @davidnolan22 has a Low Loss Header fitted and he has been discussing poor recovery after defrosts (presumably the poor recovery is of room temps via the heating circuits and isn’t a failure of the heat pump)

A Low Loss header is entirely capable of masking the size of the cooldown during a defrost if the flow rates are not balanced on both sides of the LLH.

If the exit flow to his rad circuit and his 2 UFH circuits does not match the flow rate of his primary pump then it is entirely possible that the heat pump just sees a smaller cool down temperature than is actually happening. A small distortion might not be a problem But it mightn’t be worth investigating.

If the chilled water from the defrost was sent in to the secondary circuit (radiators and UFH then there could be SLIGHTLY hotter water sent straight back to the return thermistor to give a false reading.

This graph shows our heat pump during a defrost and you can see quite a high temperature drop compared to David’s graph

This graph is very clear and our system has no LLH or buffer or separate circuits. In other words there is no opportunity for distortion (or mixing)

here is the same graph where ive tried to sketch the DT as an overlay to show what the flow Andy return temps would have been like had ther not been a defrost. Hopefully you can better see the true drops in temps for both flow and return.

As was said earlier these defrosts could be repeat events every hour which could significantly upset the heating continuity of the home.

ive posted David’s graph below for comparison and I hope it can be seen the temp drop does not look that severe, when in reality it is likely to be just as large a temp drop as my defrosts.

So @davidnolan22 this was one thought I had and perhaps you’ve done it. Regarding recovery after multiple defrosts- Have you checked with thermostats what the temperatures are on all your connection ports on your Low Loss Header? This might give you an indication where the dominant circuit is or if the circuits are well balanced.

This post was modified 17 minutes ago by SUNandAIR

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