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

I have to admit I am a bit lost!

My heat pump is too big for my house, I can’t make it work properly.

It’s a Daikin Altherma 3 9kW.

My heat loss at 21c/-3c is about 3.5kW an hour.

My installer calculated it at 7.3 kW an hour

On a day like today when it is 10c outside I need about 1.5kW of heat an hour.

My heat pump puts out a minimum of around 3kW an hour and doing that the COP is rubbish, a little over 3 at best.

My radiators aren’t big enough to cope at low flow temperatures and the heat pump doesn’t like less than 35c in any case.

The radiators heat my house quite happily to 22c when it is 7c outside flowing at 30c but the COP is less than 3 then.

I have some bigger radiators for upstairs but the ones I need for downstairs, which I am poised to order, will cost about £4,000.

I will improve my COP by fitting these but I am beginning to wonder what the point is.

Maybe I should cut my losses and change heat pump.

Even if I change the radiators I am going to end up with a house that is too hot or banging in chunks of heat every few hours.

None of that is what I wanted, I just want to trickle in a couple of kW an hour on a day like today.

I really don’t know what to do and I wouldn’t normally ask for advice like this on the internet, but I have nobody else to talk to.

Seeing as I have a heat pump already fitted, looking at it and having seen the installation I would imagine that a swap for another brand of heat pump in a more appropriate size would be relatively easy?

Is my Daikin worth anything?

Any thoughts are welcome!


Welcome to the oversized heat pump club! :laughing:

The first thing I would suggest is to collect data over the entire winter season rather than rushing to make big changes now.

You’re about to fit some extra radiators, so see how the system performs once those are in. You may learn something new, or find some more tweaks that make it work better.

It might help to include details of your system and the charts you have so far.


I know next to nothing about heat pumps, but from a purely engineering point of view, I wouldn’t want to make a decision after so little time in this year’s heating season. As Tim says, wait until the end of the heating season and in the meantime collect data. Even better, hope to have a run of a few days of “The beast from the east” (but keep it to yourself :laughing:) so that you see how your present system copes with extremes.


Hi Tim,

The radiators will cost £5,000 plus fitting (either I do that myself or pay someone).

I already spent £1,000 on the upstairs and have been deliberating over the right ones for downstairs. They will be another £4,000.

I am loathe to spend £4,000 on radiators that will improve my COP but not really the overall situation.

I am still going to end up with a heat pump that is too big for the house.

I know I am answering my own question and that’s a bit annoying. but it helps me to talk about it :slight_smile:

I know it’s early days but I am not stupid although I may appear it at times and I already knew the heat pump was too big when I commissioned the installation. I know there is no redemption for this heat pump it will never be what i want it to be.

Foolishly I thought it would just operate at a lower level, I never realised its low wasn’t really that low. It’s a de-rated 16kW heat pump.

My choice now is spend money on radiators or a replacement heat pump.

I know it sounds mad but a replacement seems the best option to me today. I think i am getting somewhere with this heat pump and then it’s just rubbish again. I make no changes and the performance is inconsistent day to day.

I am really struggling to bring myself to spend a large chunk of money making something a little bit less bad.


Hi Robert,

I appreciate giving things time.

But the last month has been pretty typical of my heating use, it’s a pretty warm part of the country we live in.

This is how it will be most of the time.

The house is too hot and if I turn it down the efficiency is even worse than it is turned up a lot.

It either uses a lot of electricity for the outside temperature with a really hot house (25c) or the efficiency is really poor. Either way I am using 50% more electricity than I need to if I had a correctly sized heat pump.

I need to spend £5,000+ changing every radiator in the house to make the heat pump, possibly, a bit less bad.

I don’t even know for sure if that will work.

Octopus may replace my heat pump with a smaller version but I can honesty say right now that I really don’t want a Daikn heat pump, I hate it at the moment.

From an engineering point of view, its minimum electrical input is about 900w and that’s a lot when I only need 1.5kW an hour of heat.

No waiting is going to make that better.

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You clearly know much more about your house than I do, however my point is weather conditions, i.e. a strong wind from a particular direction can change the rate of heat loss from what it is today, this week or this month even.

My thinking - and I inherited gas fires, no wet system at all - is I’d aim to use supplementary electric convector heaters on the coldest days when I finally do get a heat pump or two (which will probably be air-to-air), and I’d try to size those to avoid exactly the situation you find yourself in.


I am just angry Robert.

My installer did a bad job.

Thinking about it rationally, £5,000 will buy me 25,000kWh of electricity on my Cosy Octopus tariff.

I have a lot of solar and 38kWh of usable battery storage.

My electricity cost is 20p per kWh allowing for losses (maybe more but the maths is easy!) so if I use 1,000kWh more of electricity a year than I could then that is 25 years worth.

I just feel a bit, or maybe even a lot, cheated and let down.

I am in a similar situation to you, as I’ve mentioned before.

It might be worth crunching some sums on this. I get a COP of around 3 with my over-sized heat pump, in fact less with the fixed flow temp setup I’ve implemented these last two days. Today I have only pumped in 14kwh of heat into the house and it’s still 21C in the house, I definitely don’t need an 11kw heat pump. COP is naff but I’m not using as much electricity either.

I measured my annual heat demand over the last couple of years, it’s around the 9000kwh per year. Crunching the difference of a SCOP of 3 vs a SCOP of 4 saves about £200-400 a year depending on tariff. If is likely to cost £4000min to replace the pump, so it’s a 10+ year payback. You’d better off getting on Intellegent Octopus Go if you can, and charging your batteries at 7.5p/kwh in a financial sense.

It’s frustrating, I am in this situation, but depends whether you want to be top of the SCOPs or just have a cost effective/carbon reducing heating system that keeps you warm for a reasonable price.

(FYI - I want to change my system and maybe I’d be able to sell the old pump on eBay for a bit)


Hi Sam,

I with you, exactly the same position.

I think my problem is that my install was five months ago and I am only one month into heating.

I knew I had a problem almost immediately.

I know I have demonstrated a bit of stupidity and ignorance on my other threads but i do understand it really.

I have nobody to talk to about it so I vent a bit, sorry.

I have spent a lot on stuff this year to make this work and it was quite a journey to get a heat pump installed. It was never about money and payback, it was a project for me to prove to myself what was possible. Unfortunately the heat pump process was plagued by planning issues from an uncooperative local authority. I have no idea why as the heat pump is barely audible but the brochures say otherwise. Another example of form filling and bureaucracy getting in the way of reality.

But, I am devastated by this.

I had it all set up in my head to be as efficient as possible and now I am stuck with something that isn’t.

Changing the house will only partly remedy that and I still won’t be happy.

And the cost of doing that is a bit pointless I am coming to realise.

It’s either change it or live with it.

I am not worried about being at the bottom or near the bottom of a performance table apart from that highlighting every time I look at it what a poor job has been done.

L cannot find it in me to let them get away with it, my disappointment and anger is immense, I can’t remember the last time I have been so disappointed in something that I had such high hopes for.

It has been over two weeks since I expressed my concerns to Octopus and I have heard nothing.

If it goes on much longer I will have to contact the MCS.


So there are some basics you can do to try and improve your system and heat pumps are a huge learning curve so you won’t get it right all the time.
1st thing to do is make sure your circuits are as open as possible, if you have TRV’s remove them. The object is slow and low slow flow rate and low temps.
2nd is to balance your system when you have an open circuit with a delta t of between 5-8c
3rd is to reduce your flow rate depending on size of pipework and tweek your delta t.
the longer you can keep the heat pump running prevents short cycling which not only wears the compressor out but costs you money every time it ramps up.
I will post pics of my heat pump app to show you what mine looks like and compare.
I would also take with a pinch of salt scop and cop ratings because most are unobtainable unless you live in the Octopus facility. if you get 3.5 and above that’s average.


Here is my data, fyi it’s a 6 kw grant aerona3 r32.
I am a gas and heat pump engineer by trade.


We had a terrible instalaltion in 2019. I raised it with NAPIT and/or MCS who were useless. The installers fobbed me off.

We have an 8.5kW unit. The other quotes we received suggested a 10.5kW and 13.5kW unit. I can confirm 8.5kW is ample for our house.

Our problem was new slightly undersized primary pipework from the connecting to a maze of very old very undersized pipework to undersized radiators.

I have spend the last few years on a learning curve. ASHP are something you get then start learning about and optimising to get the most from them.

We are currently running at 32C and our deltaT is 2.2C. I know if we increased the flow temp the deltaT would increase.

My suggestions:

  • remove the TRVs and anything that slows the flow
  • get really accurate calibrated thermoments and put them in each room. Get them to log data and make sure you can view all the temperatures from a single place
  • lower that flow temperatures as much as you can
  • increase the flow rate. This will help with your efficiency


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George and Mike both advise increasing the flow rate, but on my Daikin the flow rate is set by the system. I read it’s related to the deltaT but again that’s set via the emitter type on the Daikin. Any thoughts ?

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Just a thought which may be wide of the mark but here is what has been working fine for me since 2015.

You do not mention if you have a buffer tank (volumiser) in the circuit. The way mine is done the heat pump (husky) feeds a big buffer tank (around 300 litres) which then feeds the radiators and underfloor. Weather compensation and a cunning algorithm controls the temperature of the buffer tank which is currently at 26 degrees and giving me 19c in the house.

The system was put in in stages with the underfloor being put in 5 months after the first stage due to building works, so it initially ran with just two radiators in the circuit. The big buffer kept everything running fine. No short cycling, the heat pump ran 3 times for a total of about 2.5 hours today.

power draw with the heat pump running flat out is 2.4 kw which gives me about 8.5Kw into the house which was fine even last February when it was -9c outside. I have a lot of insulation though, EPC is a high B.



just trying to help on the radiators. £4000 seems a lot. post your proposed list?

Hi Ian,

Here are the radiators

500 x 2200 K3
600x 1800 K3
600 x 1800 K3
700x 1400 K3
700 x 1600 K3
600 x 600 K2
600 x 1000 K2
600 x 1000 K2
600 x 1200 K2
600 x 1200 K2
400 x 1800 K2 vertical
600 x 1600 K2

Plus trvs that will be fully open, Danfoss RLV lockshield valves.

Plus copper pipe, fittings etc.

Plus any help I need to fit them or paying somebody to fit them if I can’t face it myself.

Easily £5,000

What radiators do you have already? Are they being replaced, or added to?

Replaced completely Tim

I probably could have reused a couple but I already bought some for upstairs before I decided on which K3s for downstairs.

The K2s were not expensive, the 600 x 1200 K2s came from Wickes at £77 each and the 600 x 1000 K2s were £70.

It is the K3s that cost but they were not that bad, I bought Ultraheat from Just Radiators.

£2,500 plus valves for the five K3s

Maybe it is not £5,000 overall, but it will be £4,000+ by the time I have paid for fitting if I can’t face it myself.

It really depends on how far I want to go in making it look nice right now, basically do I move the pipes or make up extensions and leave the pipes in the floor and then sort it properly next summer.

I was just going to replace upstairs to start with but I don’t want trvs to come into play as they just mess everything up so I need radiators whose output is in proportion to the room. If I had just changed upstairs that would have got too hot as the downstairs would need heat when the upstairs didn’t.

I’m also mindful that it is our home and it needs to look nice, the house isn’t just an outlet for the heat pump! My ocd requires symmetry and balance unfortunately.

do you have your pricing per component, exact make / model ? I don’t know what research you’ve done , maybe this is cost optimised already and I’m asking you questions in an area that you’ve already gone into in depth. But: 12 rads for £5000 is £420/rad on average, that seems awfully high to me. Some of the k3’s will be pricy but the standard k2’s should be well under 100 each if you stick with commodity plumbers merchant stuff?

Are these sized to match the heat loss of each room?