Octopus Cosy Heat Pump Owners & Discussion Thread

AndrewJ · 2025-10-23T11:37:06Z

I'd like to discuss how you find it, how you run it and so on.

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Air Source Heat Pumps (ASHPs)

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AndrewJ

(@andrewj)

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Topic starter 23/11/2025 8:50 am

Posted by: @swwils

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@andrewj Certainly mine cycles alot below 36C, it seems "happier" with the weather compensated minimum flow temperature at 36ºC or above. I think the really old manual also mentions this.

If you go back to generic basics, you must have correct pressure differential for as-designed compression, you need to avoid suction flooding, keep the inverter in its stable torque region. So if you go "too low" the pressure ratio becomes too small, and the compressor is basically pumping nothing.

Looking at some of my data, the pump really wants to maintain a set ΔT, so as you drop the minimum LWT really low the return water might only be 1–3 °C lower and destroy your COP.

At mild outdoor temps, your house may need <2 kW. So for my 9, the system must cycle and minimum flow temperature being too low makes it worse, because the compressor surges on start, return water spikes, unit trips off, repeat.

This is why dropping the minimum WC temp only increases cycling and reduces efficiency.

@swwils Thanks. I'm currently running with the Warm Weather flow temp set at 32c which, at a linear plot, drops the flow temp to 36.1 at an outside temp of 7c and 35c at 8.5c. Checking earlier in the week I saw a deltaT of 3.8 but that was with an OAT of 2.8c. I've been running with this value since the afternoon of 20th Nov: 20th Nov and 21st Nov where quite cold days and the COPs were 3.52 and 3.29; 22nd was warmer at 5.5c to 7.9c with a COP of 4.3. Today it looks like it will be a bit warmer still with the current temp of 7.9c and a COP so far of 4.34. At these temps and WC setting, the internal temp of the house is between 22c and 23c so comfortable and really what I'm aiming for.

I think what you are saying is that it is likely that as the temp heads up towards 12c, it is likely to cause problems and I'd be better with that set at 36c and adjusting the Cold Weather value? Certainly with the settings at 50c and 33c the house massively overheats and the only limit I have is by setting the thermostat but that causes comfort issues and also seems unnecessarily inefficient to heat the water just to let it drop off as the Cosy turns off. Hopefully I can get them to drop the minimum Cold Weather flow temp min value below 50c.

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