What modulation range do heat pumps have?

You mean minimum outlet water temperature?

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.

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.

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.