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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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swwils

(@swwils)

Estimable Member Member

Joined: 7 months ago

Posts: 90

31/01/2026 10:38 pm

@andrewj easy to forget it was £12000 cheaper! I'm sure software updates will be coming.

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 4516

01/02/2026 9:12 am

Posted by: @andrewj

↑

I’m glad I have a nerdy gene in my body to investigate and work through problems. These things definitely require more proactive aftercare from installers for the majority of people

This is such an excellent point, and very much the basis of my next video.

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Papahuhu

(@papahuhu)

Reputable Member Member

Joined: 9 months ago

Posts: 252

01/02/2026 12:46 pm

@toodles

Thinking laterally and from a non plumber.
If you need to ensure your rads are bled to get max output, then surely the converse is also true. If you have too much output from a particular volume of heating fluid then decreasing the volume by controlled introduction of air should achieve the desired outcome. Is my logic flawed?

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Toodles

(@toodles)

Famed Member Contributor

Joined: 4 years ago

Posts: 2711

01/02/2026 12:59 pm

@papahuhu I suspect that there would be a few problems even if the thermal output were reduced in this way. The circulating fluid (water plus possibly some inhibitor) would suffer from the air which would dissolve into it during circulation and this would form other gases that the inhibitor might not be able to cope with. The radiator would also likely be rather noisy and might well ‘tinkle’ too as there would be a slight waterfall effect. Overall, I don’t think I would adopt this approach. Regards, Toodles.

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

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Papahuhu

(@papahuhu)

Reputable Member Member

Joined: 9 months ago

Posts: 252

01/02/2026 1:16 pm

@toodles

I didn’t consider the redox impact, you are correct. I guess you could achieve the same result from the outside of the radiator too, my prize collection of spider webs and dust on the fins must have good insulating effects.

😂

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KevH

(@kevh)

Trusted Member Member

Joined: 1 year ago

Posts: 49

02/02/2026 10:01 pm

Posted by: @andrewj

↑

It’s a shame that Octopus:

have a heat pump that is controlled by software that limits the WC curve;

provide no access to operational data (that they have access to) except input/output power for an hour and average OAT over that hour;

Commission for, and highly encourage, running under thermostatic control. It just isn’t appropriate to heat a (most) house based on one room’s heat loss;

I agree with all of the above, although I can understand Octopus being reluctant to allow access to further operational data as those of us with enquiring minds but limited skillsets (ie me) might completely foul up the system's efficiency with that sort of data...perhaps.

That point about designing for thermostatic control really irritates me. TJ from Octopus spoke on one of the videos attached to the RH hub about this and tried to defend it and oddly nobody challenged him. It would good if he could be persuaded to revisit that in a future video (hint to the moderation team)

Your first point...could you explain what you mean by software limiting the WC. Is this alluding to the meaninglessness of the 30-36c range of the curve?

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AndrewJ

(@andrewj)

Estimable Member Member

Joined: 1 year ago

Posts: 203

Topic starter 02/02/2026 11:30 pm

@kevh yes, both ends actually. They have been pretty emphatic about not dropping the Cosy 9 below 36c and I’ve been told by one techie that it can’t modulate below 35c in any case. Having shifted the 50c point to -10, there’s little to be gained by moving it further assuming the WC curve is linear as people think (another thing Octopus don’t share.) Changing the warm end below 36c would make a bigger difference to the curve now but that is also pointless if 35c is really a hard limit. Given my radiators have been oversized, running it permanently on just overheats the house because I can’t balance the radiators output low enough. E.g. my bedrooms radiator LSV is open a bare crack and the decorator’s cap on the TRV side is not fully screwed down yet but the room is still over 22c.

That question @editor asked TJ came from me but as you say, the response didn’t really answer the question and none of the other experts said anything about the topic unfortunately.

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AgentGeorge

(@agentgeorge)

Estimable Member Member

Joined: 1 year ago

Posts: 170

02/02/2026 11:39 pm

@andrewj why is the TRV not working? With it set at 1 the room temp wont get above 18C, setting 3 is 21C

i have my spare rooms set at 2 to focus the heat in the lounge

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swwils

(@swwils)

Estimable Member Member

Joined: 7 months ago

Posts: 90

03/02/2026 12:31 am

@andrewj there are loads of reasons 35C might be a practical limit. They need to look after oil return and maintain a minimum pressure ratio. At very low compressor speeds your refrigerant velocity drops dramatically and you start to get nervous about oil circulation.

👍

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AndrewJ

(@andrewj)

Estimable Member Member

Joined: 1 year ago

Posts: 203

Topic starter 03/02/2026 12:37 am

Posted by: @agentgeorge

↑

@andrewj why is the TRV not working? With it set at 1 the room temp wont get above 18C, setting 3 is 21C

i have my spare rooms set at 2 to focus the heat in the lounge

No idea, the valve is fine, I had it set at 3 and the room is 22.8c typically. 3 should be 20c but….what can I say??

@swwils I guess I don’t care about the technical reasons, it just means the flow temp can’t be lowered enough so it is more imperative that the radiators are sized correctly.

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AgentGeorge

(@agentgeorge)

Estimable Member Member

Joined: 1 year ago

Posts: 170

03/02/2026 1:04 am

@andrewj have you tried setting it at 2 and 1 and recording what temperature the room gets to.

I did that in all my upstairs rooms with radiators and have successfully managed to control them at 18C and 19C, not much solar gain affected the readings this month

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 4717

03/02/2026 10:03 am

Posted by: @swwils

↑

@andrewj there are loads of reasons 35C might be a practical limit. They need to look after oil return and maintain a minimum pressure ratio. At very low compressor speeds your refrigerant velocity drops dramatically and you start to get nervous about oil circulation.

The other well known reason for setting min FT at 35 is that it reduces call outs from people who expect their radiators to feel warm. Just saying of course!

If 35 is a technical limit then it suggests that the system is not designed to work with UFH which quite frequently wants a FT of 30 or even less, which is bizarre in the extreme

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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