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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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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 3172

24/10/2025 6:36 pm

Posted by: @andrewj

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I don't know how else to ask this and I sometimes think I must be speaking Martian as even on the Facebook Octopus group I can't get an answer: is your Cosy running continuously (i.e. ALL THE TIME, low and slow) or does it go on/off based on the temperature read by the Primary Pod letting the temp drop and thing bringing it back up again?

This will differ from house to house depending on how it's set up. Because the octopus user manual is heavily dumbed down it's impossible to tell what the actual algorithm is. This is probably why you won't get a clear answer!

Try the following:

Increase the target temperature by 3C above what you really want on the room temp control

The house will probably overheat

Then gradually, a degree at a time, and not more than once ever 12 hrs, turn down the WC curve at the low outdoor temp end.

Eventually you should get to a point where the house is either at the right temp or a bit too cool.

If the latter turn the WC up a degree.

This will have the effect of optimising your WC settings unless there is some sort of auto adapt of the WC, in which case it becomes more complex.

Before you do anything note the settings so you can reset if necessary.

The objective is just to match the heat loss of your house with the heat supplied by the radiators by getting to a WC curve that just matches these two, by getting the relationship between flow temp (which determines radiator energy output) and OAT(which determines house loss and therefore energy needed to keep it at constant temperature) correct. If it's matched the thermostat (pod) won't do anything, because it doesn't need to and it so happens doing this is both the most efficient and most comfortable operating pont in most circumstances.

Forget anything you learned about controlling boilers, it was mostly wrong for boilers and wholly wrong for heat pumps.

Lots more detail can be found / provided if you want it, it's unfortunate we didn't adopt weather compensation in the UK for boilers, unlike some more enlightened european countries. As a direct result you have almost certainly been paying 10% more for your heating up until now than you needed to, and been less comfortable, and (as if that wasn't enough) have a steep learning curve to adapt to heat pumps.

Keep asking questions and read the ABC guide to heat pumps in 'articles' on this site.

This post was modified 3 hours ago 6 times by JamesPa

This post was modified 2 hours ago 2 times by JamesPa

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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swwils

(@swwils)

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Joined: 3 weeks ago

Posts: 1

24/10/2025 8:32 pm

I have the cosy 9 and full MID monitoring. I also have technical insights to details about them.

I got octopus to deliberately chuck in a cosy 9 knowing it would be oversized. Zero radiator changes, as quick as possible job.

Ask away.

Someone signed the PCB!

This post was modified 1 hour ago by swwils

This post was modified 1 hour ago by Mars

👍

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Mars

(@editor)

Illustrious Member Admin

Joined: 5 years ago

Posts: 3623

24/10/2025 8:47 pm

@swwils that PCB is signed by Dr. Jason Cassells, former managing director of Renewable Energy Devices (RED) and unknown heat pump manufacturing legend. RED made high-end heat pumps and they were acquired by Octopus. We had the honour of interviewing Jason years ago before he sold to Octopus.

I have to say watching the intro is uber cringe! 🤣

This post was modified 1 hour ago 2 times by Mars

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JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 3172

24/10/2025 8:53 pm

Posted by: @swwils

↑

I got octopus to deliberately chuck in a cosy 9 knowing it would be oversized. Zero radiator changes, as quick as possible job.

That's interesting. I suppose youbl are experimenting with/testing how to get the best out of an oversized heat pump.

If so are you in a position to share any insights and associated explanations. There has been some discussion here (and in the other place) about this and a bit of theirising, but as yet no conclusions that have been soundly backed up by experiment.

In a similar vein there are discussions here how to deal with a situation where a buffer tank has been fitted unnecessarily (other than the obvious which is often negated by circumstances).

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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AndrewJ

(@andrewj)

Estimable Member Member

Joined: 10 months ago

Posts: 73

Topic starter 24/10/2025 8:56 pm

@swwils Sounds like you could be useful on this Cosy thread 😀 Do you have any insights in to the control choice that Octopus took with these models? I’d be very interested in how you have yours set up to run - have you had it installed for long? @jamespa has given an approach which might alter the way the Cosy runs - I haven’t tried yet as I’m still interested in how others have theirs set up.

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