My heat pump is too big, what should I do?

Hi Ian,

I think I set it out above.

I wanted a Stelrad K3 that was 500 x 2400 but it was £800 or £900 on its own.

The cost isn’t just the radiators, it all adds up, especially if I need somebody to fit them. It’s quite a job and it may be worth paying just to get it done, it will take me a few days I would imagine.

Hi Tim,

Yes, they are what I worked out that I needed before the heat pump was installed.

I have gone slightly larger on a couple to ease fitment and minimise the cost but I have no problems with those rooms being a bit warmer, one is the lounge and it is fine.

I have more capacity now than I need I suspect and if it becomes necessary I can bring a trv into play in the lounge.

I should be able to run the lowest flow temperature possible now.

The K3s just arrived so now I have everything I need apart from the time to do it1

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almost certainly a yes to that thought, the plumbing side should be almost a case of one in one out. there will be something to change on the control side if changing brand.

what is your system on heatpumpmonitor ? can you provide some annotated commentary with days when you see different types of behaviour at the high and low (in as much as we’ve had low so far this season) ends of OAT?

My system ( is a bit oversized (but I think not by as much as yours) , I am also entering my first winter, so far I have decided to run at slightly higher LWT at the “warm weather” end of things - this for run stability, which results in better cop in the long term - and allow it to cycle on the room stat at these mild loads. today is quite a good example of that.

yes fair enough on the fitting. I DIY fitted mine but it was over the 2022 summer, with no time pressure. swapping 12 with the time pressure of it being cold is not something I’d want to be doing. if you need to re-do the tails you need to (at least partial) drain which means system offline which adds to the time pressure aspect.

I echo @HydroSam comments, I have similar battery setup to you, and a large PV. find a way to get onto a TOU tariff and load your batteries over night. I’ve just done my maths for october (not that cold yet I know) and I have spent £92 on energy (all electricity) which I can attribute as follows:
ashp: £16.60 (253kwh-> 900 kwh of heat ).
2x EVs: £37.92 (578kwh, 2000 miles ish)
everything else in the house: £38.43 (586kwh)
ALL of it has come from either 7.5p / kwh octopus TOU, or free PV.
I’m expecting to have to buy some peak rate when it gets really cold , enough for the heat pump to chew more than my 40kwh of batteries, but it massively helps the run cost regardless. obviously the batteries cost money but you already have them so I understand.

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Hi Ian,

I will have problems with pipe work, it will mean all carpets and floors up to do it properly.

But as a short term measure I will just put `dog legs’ on the flow and returns of some of the radiators to accommodate the extra width, then do it properly next summer.

I have a plan to overcome the heat pump being out of operation for a few days so that will be ok, it is just the disruption which is tempting me to pay!

I came to the conclusion that if I am going to increase radiator size I may as well take radiator limitation completely out of my equation.

It may not be the most financially viable of plans but none of what I have done this year has been entirely about money. My whole goal has been to run the house entirely from the sun with solar panels and batteries. Income in the summer pays for the heating in the winter, my own form of net zero.

The heat pump is too big so my aim is to get it running as efficiently as possible first, then deal with the excess heat. We like it warm so the excess heat is not so much of an issue but I would have preferred to have a choice in it!

My heat pump is the Octopus Daikin 9kW installation in Ipswich.

With the help of a few people here we have it running OK and I can now see the potential and I should see a COP of 5 out of it at times.

The limiting factor will always be the lowest level of electrical input this heat pump has, around 900w which is not really any good for a house that needs 2kW or less of heat most of the time. I suspect that apart from the very worst of days we will only need the minimum output of the heat pump.

We will see how it goes.

I have a formal complaint in with Octopus at the moment but even if the heat pump is changed for a smaller one the radiators will still help in my overall goal.

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Hi Ian,

I have 13.6 kW of solar panels and 38kWh of usable battery storage.

I am currently on Octopus Cosy having been on Flux in the summer, I changed tariff about a month ago.

So far Cosy is working well and I only import in the cheaper six hours.

I have a Tesla but all my charging is done at work, I never charge at home so I don’t have to pay for it, nice employer! :slight_smile:

My bill to 9 November was about £70 but I have built up more than £1,200 of credit since I started getting SEG in May/ I pay Octopus £1 a month just to keep the DD discount, I don’t expect to ever have to pay them more than that as things currently stand.

Once I get my flow temperature down and the COP up I wouldn’t ordinarily expect to use much more than 30kWh a day on the heat pump.


I have just discovered what the error is in my heat loss calculation.

My lounge has two radiators. The surveyor has included the full heat loss of my lounge against each radiator instead of halving it.

My heat loss was calculated at 6.350kW but actually went on their records as being 7.791kW due to the error.


Do you think behind the graphically pretty front end stuff there are 10,000 monkeys trying to calculate shakespeare using Excel?

It’s evidently all manual to have managed that one. Haven’t even purchased an off the shelf package to do this.


They have now admitted that the heat pump is too big to me on the telephone and that they are dealing with it.




Happy days.

Fingers crossed this escalates to the folks there who do know their onions and they decide to fit something appropriately sized and send a proper installer to come commission the lot.

Fingers crossed that the offer to do this doesn’t come with a gagging order asking you to delete this thread or cease contributing experience and expertise. :slight_smile:

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Nice one @matt-drummer very pleased for you. Fingers crossed it works out.

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I just had a call from Daikin.

The person couldn’t help me much and said that I need to talk to my installer.

He told me that the compressor in the 9, 11, 14 and 16kW were different sizes.

He said the performance of my pump depended on many factors and I couldn’t compare it to other installations.

But I did find one piece of information when I asked how I can cut the electrical input.

He said put it in quiet mode, it cuts the electrical input and heat output in half.

I will give this a try later.

Maybe it’s the perfect solution, a big heat pump when you need it and a smaller one when you don’t?


@matt-drummer have you had any success with “Quiet mode”?

I’ve tried changing the setting today, but not noticed any change in power consumption on my EDLA09DA3V3. I might expect that it would limit the max power consumption, but not reduce the minimum power, which corresponds to pretty quiet operation anyway.

Hi Jonathan,

The quick answer is no.

There are three quiet settings and I put it on quietest.

I can’t see any difference, the power consumption was the same.

I was hoping to reduce the minimum electricity consumption but I saw no fall and I’m not surprised really.

How could it use any less power to heat the water, if it could I suppose it would already be doing it?

Maybe it would do something with a drop in flow temperature, maybe then it can use less power, I don’t know?

I will try again now I have some radiator changes done.

The `engineer’ said that the lowest setting would reduce the power consumption and the output by 50%, I’m not sure I believe him.

I changed four of the ten radiators I need to change yesterday.

It has already made a massive difference.

I will also add one more radiator in our conservatory and my plumber is in agreement that it will be possible and easy to put a radiator in my timber office on the back of the garage. That should solve all my problems.

Here is yesterday afternoon and evening after the upstairs radiators were changed, a lot better than a COP of 3.5 or less. I also had one radiator not working downstairs after we refilled the system which I have now sorted. I need to do a bit of radiator balancing but I will really focus on that fully when all the radiator changes are complete in a couple of weeks.

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By coincidence I’ve just started a separate thread on Quiet mode. Watch that too…


My house has just been re-surveyed by Octopus and we have found the issue,

My original heat loss was 7.3kW and now it is 7.4kW

Sounds conclusive, but sadly not!

Our house is 98m2 and we have a 15m2 conservatory.

When the original surveyor came he immediately said that we would need to exclude the conservatory and remove the radiator so it was an unheated room. Fine.

When the heat loss calculation was done he said `good news’ we can include your conservatory as the radiator meets the heat loss. Great I thought, we have no trouble heating the conservatory and if we did it has electric under floor heating and/or we can close it off. But it has always been warm enough with GCH and currently installed radiator.

The original heat loss survey had the conservatory at a loss of about 800w and the rest of the house at 6.5kW.

That is why we have a 9kW heat pump.

The air changes are set by the MCS and represent a normal epc D rated house, ours is a B89 so the heat loss will be overstated a bit as the age of the house determines the air changes, not reality.

Today’s survey whilst coming up with the same over all heat loss got there in quite a different way.

The heat loss of the house is 5.1kW and the conservatory is 2.3kW.

Basically the conservatory represents about a third of our total heat loss, 45% of the actual heat loss. of the house.

Of course, the conservatory radiator cannot take 31% of our heat pumps output.

That is why our house is getting too hot and I cannot use the heat pump to its full potential.

It has been sized to heat our conservatory with no regard for the actual house and the heat pump cannot heat our conservatory as it is now (although it does as the heat loss is clearly too much, otherwise it would be freezing in there!)

So, how did this happen?

Our conservatory is all glass apart from a low brick wall on one side.

The original surveyor used a different construction, it magically acquired a tiled roof with an insulated loft space.

The Octopus designer would never spot this error, if they had seen the real construction and heat loss we would have had a smaller heat pump.

The rest of the calculation had errors/differences in it but it’s really every room, we think probably a failure to include the correct ceiling heights.

So my actual heat loss of my house is around what I predicted 4 to 4.5 kW.

I don’t know what Octopus propose to do or offer me to help.

The heat pump is too big and a smaller one will be just as capable of heating the conservatory as the one I have now is.

Overall a smaller heat pump seems the answer if it is offered.

Any views or thoughts are welcomed as always!

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Can you abandon the conservatory for winter? Is the underfloor heating resistive, rather than heat pumped?

If the pump is oversized, how do bigger emitters help? Sounds like it improves efficiency by reducing cycling, but still puts too much heat into the home.

Isn’t the heat loss figure for -2°c ambient? We’re nowhere near that in the UK yet. I’d give it two months.