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If that was the case, I don’t think my heat pump would ever be able to run!

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Hi Chris, it sounds like you are in a similar position to me. I have 11kW Daikin but ~5kW heat loss.10kW from previous two Octopus heat loss surveys.

My heat pump works great between +5C and -5C but struggles between +5C and +15C where it’s bouncing off the minimum electrical input it can go down to and in turn the minimum heat it can output, so it cycles rather than modulates. The overshoot masks this to a degree but in turn causes the house to overheat.

I’ve shown Octopus the following from 3 months of logging data including brief periods at or below the design temp.

  • power input sat at 850W at anything above 5C and the unit cycling, using CT clamp to log data.
  • my own HTC calcs showing <5kW heat demand at design temp based on actual use data. Manually logged data daily.
  • heatpump monitor heat demand tool showing < 5kW demand at design temp. From ESPAltherma/CT clamp
  • rule of thumb based on previous annual oil usage in worst case scenario showing <5kW heat demand.

I’ve had an Octopus engineer visit who agreed it did look like the heat pump was oversized.

Have a 3rd heat loss survey booked for this week.
I’ve been told that they would only swap the unit out for a smaller one if this survey comes back with a lower heat loss.

As I type it’s
980W
2.9C outside.
20.3C inside (target is 19C but having the target any higher and the house can reach uncomfortable temperatures)
flow target is 32C
return is at 34.5C

It’s will likely shut off either by breaching the overshoot or defrost, which ever happens soonest.

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Yep, sounds familiar! I intend to bring this up with Octopus next week after we’re done with this cold snap. I too have CT clamp data, multiple temperature sensors around the house, previous A2A system using 5 air conditioners (combined 13kw output) to heat consumed collectively about the same as the 11kw Altherma on average… something around 1.2kw, 2kw max on really cold days. If we say they are likely to be doing COP of 3, then you’ve got 6kw heat loss. Heat Punk suggests the same. I’m expecting after we’ve seen -5 this coming week, we still won’t exceed 6kw calculated heat output from the Altherma and probably no more than 2kw input at steady state.

Lots of evidence, like you, to support that heat loss survey’s aren’t always reliable.
Looks like they didn’t take into account additional internal wall insulation in one of the larger rooms, despite me pointing it out several times.

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@chrisg and @Mld

What are you hoping they will replace them with?

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One thing I’ve learned (in no small part through your experience Matt) is minimum output is just as important (if not more so) than maximum output. My gut feeling is 6kw is too close to the wire, even though we have additional heat sources here (the 5 AC units, plus a bioethanol fireplace) and since the minimum output of the 8kw unit appears to be the same as the 6 (and the 4), the 8kw will probably be the one (and I suspect easiest to get Octopus to agree to).

I’m open to thoughts and suggestions though.

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Something than can modulate lower. The 8kW is the obvious as I can’t believe any amount of resurvey will drop low enough for anything smaller.

The engineer and surveyor did briefly talk about the 6/10 kW Cosy on the phone but very quickly dismissed both.

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It’s the only option as the 9kW is the same as what you already have, just a bit worse!

You could, of course, just use your a2a when conditions are not great for the a2w?

I was in two minds about swapping my 9kW as you will have seen.

In many ways, I wish I still had the 9kW.

I would think very carefully.

The 11kW you have is a much higher quality product.

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Hi Mark,

There is no point in the 4 or 6kW versions, they are the same as the 8kW

They may offer you a 9kW depending on the survey.

We all know that is not an option.

I would be tempted by their own Cosy product, but would need to know more.

One positive is, it isn’t a Daikin!

As I said to @chrisg, think very carefully.

In many ways I would be happy to go back to my 9kW

I am not in love with what I have now!

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The huge 11kW does look somewhat comical sat outside my 101 sq meter house. It just doesn’t look right.

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Interesting. Your earlier threads on the 9kw were pretty damning. And I do get why. You have the 8kw now, right? Are you finding specific issues with it @matt-drummer ?

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Keeping the 11 has crossed my mind as better the devil you know, but it’s not just slightly oversized it’s significantly oversized.

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I was, and still am.

But just getting rid of one pile of junk doesn’t make the replacement not a pile of junk!

The 9kW to 16kW are of a higher quality, the 4kW to 8kW look and feel like a cheap toy.

None of them work properly in my mind.

I currently have an issue that shows up from time to time, being a bit of a doom merchant, I am waiting for it to get worse.

Every time I turn it on I am a little bit scared, honestly.

It’s unstable, they all seem to be (4/6/8)

I have a really good SCOP, but I could easily make it much worse if I tried to run it as designed.

In all likely hood, no better than the 9kW in terms of efficiency if I handed over control to the Madoka.

I don’t find it nice to live with.

And I am bitter and twisted, resentful of the MCS and Ipswich Borough Council who hobbled me into going with Octopus and a Daikin.

I have no issue with Octopus by the way, they have been great and I won’t have a bad word said against them, they have done their best despite the mistakes.

I just wish I had waited.

@KnightPhoenix and me were the first to deal with this as far as I know, and if I had known, I would never have let anybody install a Daikin heat pump at my house.

1 Like
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So it might be.

But that doesn’t guarantee that the other offerings will make you any happier.

I asked the question, just so neither of you were under any illusion that being offered a 9kW Daikin would improve matters, it won’t.

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My main gripe with the 11kW is the sheer amount of heat it can dump into the house. For the majority of the time is very binary, the house is too warm one minute and then a tad chilly shortly after when it cycles on/off, except below 5C where it’s just right. This has caused us too have the room setpoint lower than we would like as being a little chilly is preferable to being in a sauna.

It’s not a big house, so you are never far away from a radiator. For example when in bed I am only a foot away from a behemoth of a rad that just cant be on when in bed, you really feel the swings in temp.

I would also hope the lower modulation would play nicer with our solar/battery.

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I know @Mld, I have been there, our house is about the same size as yours.

And I had effectively the same heat pump as you.

But I am still not happy.

Just for different reasons.

Obviously, just make sure you are not offered the 9kW!

Who dismissed the Cosy, you or them?

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The Octopus engineer was on the phone to the surveyor during her visit, they dismissed the Cosy as the 6 was likely too small and the 10 might not address the underlying issue. Note I was only hearing one side of the conversation but it was clear they were thinking out loud.

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The Cosy 10 might work?

Remember your 11kw Daikin is really a 16kW heat pump.

For me, the 8kW Daikin really only goes down to about 2,500W of output.

The Cosy 10 may do the same, maybe even lower?

The Cosy 10 is R290, the Daikins are R32

The big plus, possibly, the Cosy 10 won’t have Daikin controls?

It may work like other heat pumps and actually be better than any Daikin.

The Cosy 10 is, possibly, a big drop from your 16kW Daikin.

I wouldn’t rule it out if you have the chance to find out more.

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I will certainly talk to them about it and do some research but right now there is very little info out there and I don’t think the Octopus people in the field know much right now either.

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Here’s my electrical input every 15 mins since beginning of November (x axis) histogram. Below 100W filtered out.

The 11kW isn’t breaking a sweat.

Screenshot_20250107-184126
Screenshot_20250107-1841262495×1161 211 KB

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It’s really interesting to read through these posts as we have a 9kW Daikin which seems to spend the majority of its time running at minimum output levels, happily maintaining house temperatures with 36-40ºC flow.

We had Octopus carry out a second heat loss survey last year which recalculated the figure as 9kW (was apparently 7.3kW when first assessed… and nothing’s changed physically since then except adding insulation!) yet, here we are, on a day with 3ºC outside and somehow we’re maintaining a steady temperature with the heat pump ticking along at minimum output, burning 900w or so and producing about 3500W heat - which logically must be all that the house is loosing if it’s maintaining temperatures. Seems to be a very long way away from 9kW!

We’re feeding stats into emoncms with today being a useful demonstration - https://emoncms.org/app/view?name=Altherma39kW&readkey=6d594a2b06e28cb25d7906e4f14930c5&mode=power&start=1736246235&end=1736267385&flow=1

Not really sure where to go next with this tbh. The system works well but as we all know the 900w minimum input is frustrating when an 8kW would be more than capable of producing much the same heat yet presumably uses less energy. Appreciate they’re not perfect and if we were doing this all again I’d have a look at other manufacturers than Daikin.

I’m considering getting a 3rd party to complete another heat loss survey as a comparison point and if we get the predicted colder weather over the next few days it will be interesting to have more real-world performance data to feed into the conversation.

Those of you who have engaged further with Octopus on HP sizing - has presenting performance evidence helped in your discussions with them, or are they only concerned with what their heat loss surveys calculate… regardless of how the property actually behaves or how much real-world heat is needed?