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What modulation range do heat pumps have?

  • 10 February 2024
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There is plenty of information regarding heat pump maximum output (and COP) for a variety of ambient and flow temperatures, but where do I find the minimum output?

Most state electronic (compressor speed) control, so I assume they can modulate down. But how much?

I have downloaded umpteen data sheets for the Vaillant arotherm+ 7kw model , but have been unable to find the information.

Anyone know?

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Best answer by juliamc 12 February 2024, 19:01

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Hey @BeePee,

 

I wonder if @juliamc @hydro_sam @Jason lewis @OrphM62 have any advice?

but where do I find the minimum output?

You mean minimum outlet water temperature?

 

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Sorry I don’t know what the answer is as I’m not sure what the question is.
 

I can tell you that my Daikin has a minimum flow temperature of 25 deg C but in reality if it’s running that low it starts to cycle too often so I had my wd curve set to a minimum flow temp of 30 deg (I have radiators).

My system can modulate by 10 degrees controlled by the Madoka room sensor but apparently the efficiency is reduced by a higher modulation, so mine is set at 2 degrees.

However, I don’t think that’s what you’re asking…

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Sorry I meant minimum power output. My gas boiler has a modulation ratio of about 2 to 1 (17kW to 7kW), the modern replacement is better (down to 3 kW minimum). I hoped something like a Vaillant arotherm+ 7kW would do better.

I have tried to guess it from looking on heatpumpmonitor.org which seems to suggest a minimum of 3 kW. It also looks like that although the input power (for the 7 kW model) falls to just 500 W, that because low demand corresponds with a warm ambient and low flow temperature the COP is much higher and the output power ends up being around 3 kW.

So I think the answer is not as good as I thought they would be hence the cycling that @juliamc mentions.

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Have you looked at the discussions on the heatpumpmonitor webiste, such as https://community.openenergymonitor.org/t/how-to-reduce-cycling/25251 ? Or track down an Aerotherm owner. Someone will know !

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Thanks @juliamc for the link, some very interesting discussions over there. My very simplistic summary: if the heat pump is too large then it cycles frequently. Lots of "remedies" most of which seem to entail increasing the load (from adding radiators, increasing flow temperature, adding buffer vessels or opening windows!).

So the reason for the original question.

If a HP is sized for say -3°C, which occurs maybe 0.5% of the time, then this requires say 6 kW (21°C to -3°C and THC = 250 W/K), add a small safety margin for hot water, really cold days etc so choose a 7 kW heat pump.

The minimum output from the HP is 3 kW, so working backwards, 3 kW, 250 W/K gives 12°C difference or an ambient of 9°C. Temperatures below 9°C occur about 44% of the time, and below 15°C (accepted degree day threshold) about 77% of the time. 

  • So 0.5% of the year ambient is below -3°
  • 44% of the year below 9°C
  • 33% of the year between 9°C and 15°
  • 23% of the year above 15°C

The criteria for choosing a heat pump is based on coldest days, but for a third of the year the heat pump will actually be "too big" and will be cycling to maintain the low output required.

Whereas a HP with a 2 kW minimum, 21°C - 8°C gives 13°C ambient (77% - 66%) gives just 11% of the heating days where the HP may need to cycle.

Hence the question.

I realise that cycling (within reason) isn’t a bad thing, I just wanted to work out where the boundaries lay.

Thanks for your replies.

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Aargh - just typed an answer with a link and lost it all with an error…

If you search for Skill Builder “Don’t miss this essential step when getting a heat pump” Roger and Adam Chapman (chief heat geek) discuss this amongst other things. The suggestion from Roger was to use a fan heater on the odd occasion of unusually cold outdoor temperatures. 

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Opening a window is of course quite important unless you have mvhr. Since getting our new double glazing we have trickle vents open all the time which of course will mean a heat loss. Previously we didn’t have trickle vents so had fanlight windows open on the vent position which would have been a worse heat loss I think. It’s a balance of heating v ventilation.

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