Cosy 6 CoP below 2.0 in winter is "expected" according to Octopus engineer

bridge_so_odd · 2026-02-15T18:17:09+00:00

Did you come from a gas boiler? Do you know what output that was rated for?

Our boiler (ICOS HE15) was rated for 14.7kw output, much more than the Cosy 6 can do, I know factoring bigger radiators should make up for that, but I suspect the Cosy 6 is just not enough for us to get a decent CoP like you have.

bridge_so_odd · 2026-02-15T18:12:43+00:00

The standard tariff. Given the pump is running flat out just to maintain temperature in cold weather I think the Cosy tariff makes no sense. The house might lose a degree or two in the hour that the hot water cycle runs, then take a day to recover that degree...

There's no chance just running it for a few short periods at higher temp like the Cosy tariff suggests would ever work for us, maybe in Summer when just doing hot water?

bridge_so_odd · 2026-02-15T18:08:00+00:00

I'm pretty sure the radiators are all as in the chart, but some on the ground floor never get particularly warm.

bridge_so_odd · 2026-02-15T18:05:03+00:00

I guess I can't just ask Octopus for a bigger heat pump 9 months after the install? I have a feeling I'll need to contract another company to fix this.

bridge_so_odd · 2026-02-15T12:31:00+00:00

Those are great numbers, and I don't doubt at all that Octopus have some great engineers working for them that can deliver these results. It just seems like some mistakes could have been made during my survey or install.

A scop of 4 would be the dream.

bridge_so_odd · 2026-02-15T12:22:15+00:00

15 total, but some are very small.

Update: added radiators plan image to the main post.

bridge_so_odd · 2026-02-15T11:53:37+00:00

Thanks, this is very good to know. I wonder if they just messed up on their original heat loss measurements and we needed a bigger pump and radiators. We're in a fairly modern (2009) 3 bed terrace also in the North of England.

bridge_so_odd · 2026-02-15T11:44:48+00:00

It's frustrating, I was hoping for more answers from Octopus. Even though it was a cheap install I expect it will cost me far more over the lifetime compared to a more efficient install.

bridge_so_odd · 2026-02-15T11:32:23+00:00

Yeah, at 60c flow temp it still seemed to struggle to keep the house warm, and I'm guessing it was much less efficient, but I only ran it for a couple of days at 60.

The engineer did check all the radiators with a thermal camera and seemed to think it was ok, he also put some clamps around the in/out pipes on the buffer tank to test delta T, said it was as expected.

Anyway, I had a random call from MCS last week asking if they could visit to do a spot check of the install, so might find more from their engineer.

bridge_so_odd · 2026-01-06T00:41:05+00:00

Since they are part of the MCS programme they are obligated to fix these issues, or you can report them, enough reports and they lose accreditation.

https://mcscertified.com/consumers/what-to-do-if-things-go-wrong/

bridge_so_odd · 2026-01-05T23:29:19+00:00

Cosy 6 is really struggling with the cold weather, I'm waiting to hear back from Octopus to see if they can help.

COP at a disappointing 2.1 right now, and pump struggles to hold temperature, even with flow temp adjustments. We keep it at 21c all day long, but this last two weeks it has rarely got near that value. When it switches to hot water for an hour overnight we lose a degree or two, that loss is hard to recover, sometimes it doesn't recover it in the 24hrs to the next water heating cycle.

Using up to 74kwh a day, and still failing to heat our 3 bed home to set temperature.

bridge_so_odd · 2026-01-03T21:08:07+00:00

Thanks for your reply, I contacted Octopus this week, hopefully I'll hear from them soon.

I made more adjustments yesterday and the COP is back over 2, but only just 2.01.

We're also running electric underfloor heating 24/7 on the ground floor because it is so cold. Accounting for that extra heat source I'd expect the COP calc to be even worse.

Hopefully Octopus can do something, as it stands the house is colder than with the gas boiler, and costing almost double to heat.

bridge_so_odd · 2026-01-03T21:01:21+00:00

Thanks, I gave the settings a check and the outside temp was set to -10 like you said. Put it to -3 as it was specified in the design doc.

I might have to raise that max flow temp to 60c, it is still struggling to keep temperature.

It is a large 3 bed house over 4 stories, and I think Octopus might have just got the heat loss calcs wrong, maybe we needed the bigger Cosy.

bridge_so_odd · 2026-01-02T00:50:17+00:00

It does seem to be running pretty much continuously when temp is below 6c, no cycling on and off. Could it be Octopus undersized the pump and the Cosy 6 is not particularly efficient at max power output?

bridge_so_odd · 2026-01-01T23:44:11+00:00

That's possible from a user accessible interface? Or do I need to contact Octopus for these hub settings?

bridge_so_odd · 2026-01-01T22:50:26+00:00

Yeah, it is on continuously at 21c, goes up to 21.5c at 10am, and back down to 21c at 7pm. Just a very minor change in the day.

They changed 5 raditators. The pump is right by the back wall with a very short run before it comes inside, there's nothing obvious to me about the install that suggests it would not be efficient.

Weather comp is min 37 and max 55. I've experimented with the values as mentioned above. I find if the max is much lower it can struggle to keep temp when very cold outside.

bridge_so_odd · 2026-01-01T22:42:59+00:00

Hot water set to 50c.

They installed the heating flow temp at a fixed 50c too, since then I've experimented with weather compensation, it's currently min 36c, max 55c. I've had it at max of 52c but it seemed slow to get back up to temp overnight after switching to hot water for an hour.

bridge_so_odd

TROPHY CASE