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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 27/11/2025 2:22 pm

Posted by: @jamespa

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Now that really is nuts. Hopefully tech support dont suggest to downsize the radiators!

That would be my worry. If they insist the resolution is to downsize some radiators I may instead have to go back to a thermostat to limit the temp rises as OAT drops. That would be disappointing because it brings back all the issues I had with a gas boiler and thermostat, perhaps not as bad maybe. Besides, I don't think I could face the redecorating again 😀

I'm going to take a stab that the problem isn't caused by an oversized Cosy and that a Cosy 6 would have the same limitation in the app/settings anyway. In any case, my gas usage for heating last year was 8938kW, albeit with just 100-150mm of loft insulation, and hot water 2221kW, figures I gave to the surveyor. Checking the design doc again they determined a heat requirement of:

Net Energy required to heat property: Heating 10000kWh, Hot Water 2000kWh

Existing System Consumption: Heating 10869kWh, Hot Water 2173kWh

Not that far off really. Heat Geek thought I'd get away with a 7kW Vaillant, perhaps with a supplementary heater in the flow/return for the really cold days, but given my rate of heat loss I don't think that would be required. Octopus don't do Vaillants, the choice is (was) a Cosy or Daikin.

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KevH

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27/11/2025 2:42 pm

Posted by: @andrewj

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The engineer visited this morning and checked everything over. He agrees that everywhere is too warm but he cannot drop the Cold Water Flow Temperature min value below 50c as the system doesn't allow it.

This is disappointing. Surely the engineer who called would have known this before he got in his van. I can only assume that he was expecting to find yet another uninformed end user for him to put right.

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Judith

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27/11/2025 2:46 pm

@andrewj it reads as if you really need a 6kW version or they need better turn down temperatures.

From your gas usage of 9000kW divided by rule of thumb 2900 then you really only need a 4kW max(!) pump https://protonsforbreakfast.wordpress.com/2022/04/05/what-size-heat-pump-do-i-need-a-rule-of-thumb/

Alternatively you need a software upgrade!

This post was modified 52 minutes ago by Judith

2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with it) open system operating on WC

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JamesPa

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27/11/2025 2:48 pm

Posted by: @andrewj

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I'm going to take a stab that the problem isn't caused by an oversized Cosy and that a Cosy 6 would have the same limitation in the app/settings anyway.

A lmost certainly yes to the first half but in a way could be caused by oversizing. Oversizing means bigger rads. Bigger rads means more output at any given FT.

Posted by: @andrewj

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If they insist the resolution is to downsize some radiators I may instead have to go back to a thermostat to limit the temp rises as OAT drops. That would be disappointing because it brings back all the issues I had with a gas boiler and thermostat, perhaps not as bad maybe. Besides, I don't think I could face the redecorating again 😀

Another solution would be to operate it on WC, but intermittently. 'Knocking out' an hour here or there would reduce the energy delivered at any given FT. Its stupid really, but maybe will result in greater comfort than operating on thermostat.

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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Toodles

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27/11/2025 2:51 pm

@andrewj At the time I had an 8pus (I like that term!😊) survey, they only offered Daikin; their Cosy unit was still in transition from the RED design stage. I suspect some of the deciding factors in favour of 8pus supplying just the Daikin range was for simplicity in training, probably a better price from the supplier and thirdly, the fact that using R32 refrigerant meant that there would be more flexibility in the siting of the outside unit compared to pumps using R290.

I believe the guidelines have been relaxed since but at the time we had our pump installed, R32 was the only type that could be sited near drains, windows, doors and vents. (All of which we have in the only possible siting we could use.) Regards, Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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