My heat pump is now running at a COP of less than 1.0, help!

What’s wrong with it?

Here are my non-expert observations / guesses:

21:39 - 3 kW of electricity being drawn, yet there’s no flow and temperatures don’t budge. My guess is that this is the immersion heater in the cylinder.

21:42 - primary pump starts, and temperatures change. The flow falters a bit, but carries on. Residual heat in the pipes gets measured as “produced heat” yet heat pump is not running yet.

21:48 - electricity rises to 4 kW; this is when heat pump actually starts the compressor. Renewable heat appears shortly after.

22:14 - electricity drops back down to normal as immersion turns off again after 35 minutes. That’s 1.75 kW of electricity that can be subtracted from stats, resulting in actual COP of 3.25.

23.31 - heat pump reaches 38 C and stops.

Do you have a record of tank temperature during this period?
Are there settings on the heat pump to control when immersion is used?
Can the immersion either be disconnected or moved onto a different circuit?

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That’s heating!

It is not DHW.

It’s a problem I think and it’s done it again this morning

I just checked as back up heater and immersion have their own meters (all shown on myheatpump app), there has been no use of either.

That electricity use is all heat pump!

And the use is real as I can see it on my solar/battery inverter data.

We can see that there is something connected to that circuit drawing 3kW constantly for 35 minutes. Subtract that, and the profile of the heat pump is normal.

There should be 3kW of heat appearing somewhere, but it’s not going into space heating.

Can you chart your tank temperatures?


On my Octopus Daikin install, I disabled the BUH (Backup Heater - space heating related) and Booster Heater (bottom immersion control for cylinder) in the controller, and just to be sure also separately flicked the rocker for both of these in the outside consumer unit that they put in and dedicated to the heat pump installation.

I’d suggest turning off the power to the BUH at the consumer unit.

I have Ct clamp around cable that feeds this consumer unit as a whole.

Had no issues.

Octopus admitted they had lots of issues and callbacks on the booster heater specifically.

They had to come back and rewire my bottom immersion actually to the previous electrical connection due to the fact the Mixergy controller has its own Legionella cleansing schedule that can’t be disabled and that uses the top immersion + pump control to get cold water spraying at top of the tank. I wanted control back of the other (bottom immersion) and convinced them to do that as that is my failsafe for hot water in case either heat pump or Mixergy control unit fail.


Ah, so there are two immersion elements? I suspected that might be the case.

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@Timbones Octopus were supplying a really basic Daikin tank with single bottoms immersion when I got quoted in 2022.

They did my install re-using my 2018 Mixergy cylinder (that’s for another thread!) so I have an earlier version that had two immersions.

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Hi Tim,

There are not two immersion heaters, just one.

The heat produced by that is not measured by the heat meter for the obvious reason.

The back up heater is inside the heat pump and sits on top/next to the heat exchanger.

The heat produced by the back up heater would be measured as it provides additional heat to the water in the heat pump before it enters the house.

It is separately metered and shows no use.

Also note the BUH is inside the Daikin outdoor unit…

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You can verify this by looking in the controller settings for BUH usage consumption FYI.

Data verification is always useful if you have two sources

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I will look at the MMI when I get home.

It is doing it again right now.

My tank is at 47c and is on reheat to 50c with a hysteresis of 20c

You can look at it on, it the Ipswich heat pump.

All that I changed yesterday was the DHW from schedule to reheat only and I dropped my offset from my WD curve from +10c to zero.

Daikin controller → Information → Running Hours → BUH

Apologies for rubbish photo:

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Thanks Stephen,

I will check at the earliest opportunity.

The box is exactly the same as I have but I have three SM120 meters to the left of the three mcbs.

The BUH has it’s own supply, it cannot be getting power without going through the meter it is connected to in my installation and that would be reported.

If the BUH heater was working wouldn’t that show as heat produced?

That is its purpose isn’t it, to provide heat when normal operation isn’t enough?

Why would it possibly need the BUH now?

It was 8c outside when it happened last night and it is 8c outside now according to the Onecta app.

This has never happened before.

If it is the BUH why now?

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I may be wrong but I am convinced that my heat pump is defective.

I look at data from a 7kW Daikin installed by Octopus and it runs along quite happily at 330w and a flow temperature of 34c

Why would Daikin manufacture and supply a heat pump that is only slightly larger in capacity that uses three times the electricity to produce exactly the same result.

The type of property that needs a 9kW heat pump cannot be that much different from one that needs a 7kW heat pump.

Either it is defective or what on earth where they thinking when they designed it?

@matt-drummer All I’m saying is you have two data sources: your heat meter and the Daikin controller.

First step is to verify both match (or very closely resemble working figures)

On this note: May I suggest once a day/once a week noting down the Daikin controller input and produced heat figures? Will provide the second source of data that should resemble the heat meter, albeit possibly lower accuracy.

It of course is possible it is something else (broken), but easy to rule out BUH usage (or even booster heater), and then move onto other things.

I did the whole of last winter without BUH, so I only see it necessary if the main unit had an issue, but then could be re-enabled….that is of course assuming there wasn’t a wider issue.

I’m in the same position, and I actually didn’t report they put an 11kW rather than 9kW unit in as I figured given the derating that minimum output would be identical. And it seems to be.

I have turned this all round in my head and put a positive spin on it:

I’m now better enabled to take advantage of cheap/free electricity from the grid that is going to be more and more common. Octopus / UKPowerNetworks Power-ups are in my postcode and I also make use of Intelligent Octopus additional slots from EV charging.

@matt-drummer here is a link to my system.

Ignore data prior to today as had error in feed formula that provided heating power.

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Exactly Stephen,

What we both have, and others, is a 16kW heat pump that doesn’t actually put out 16kW.

What’s the point of the `smaller’ models?

They are not smaller and they all operate at the same lowest level by the look of it.

I won’t swear but isn’t that a really rubbish idea?

I am struggling to believe that a company as big as Daikin could come up with this.

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I have found out what it is.

I am sorry everybody, I selected the wrong feed for my graph, I chose electric energy rather than electric power.

It is the immersion heater.

It is using that to reheat the tank rather than the heat pump.

All I did was change from schedule DHW to reheat only.

The tank is now 50c so I’ll turn it off and sort it when I get home.