Our new heat pump has a low COP, help!

Hi, this is my first post. We have a 1930’s 100m2 semi bungalow near Leeds which has now been as fully retrofitted as we can manage (we are also a Superhome). The final stage (after 11 years of work) was getting a company to install an ASHP just for space heating and in time for RHI and MMSP. Although we continued to reduce out co2 hugely the COP average is 2.4 between last December and now. We have tried to get the company to install another one or two radiators but they are saying that the heat loss survey was fine and they do not seem interested in the low COP, or any performance issues, getting a bad review or helping us out . We took the complaint to NAPIT and they also said it all had been installed fine (based on looking at the heat loss survey). The company is registered with MCS and NAPIT. Although our consumption is low we know we should be getting an average COP of over 3 and we do not know who to turn to to help. Can anyone suggest an individual, organisation or company who could give advice or help us make the system more efficient?

There are a few things that impact the COP, in our experience.

  1. Output temperature. We have turned ours down to 52°C from 55, but run the heatpump for longer. ASHPs are more efficient at lower temperatures, so give a better COP
  2. Outside air temperature. We get a better COP when it’s warmer. You should find yours improves now that the weather is milder. That’s not much consolation for the winter months though.
  3. Radiator load. We upgraded 3 of our radiators to very large (attractive designer) rads which warm our three main rooms better without the pump having to work as hard.
  4. Start up and shut down cycling. Setting the pump to run much longer, keeping the house at a steadier temperature and avoiding it trying to find reheat the house each morning helped.

Although the heat pump companies promise 3-4 COP values, a report I read recently (by Herschel, the Far IR heater company) says 2.4 - 3 is more likely.


How do you know what your COP is as I understand this is difficult to measure? My ASHP (LG) does not provide this measure.

As others have said, there are many factors that influence the COP, for example, do you have a buffer tank?

What COP did your heatloss calculations suggest? Are you using more electricity than your heatloss calculations predicted? The COP may be around 3 (or more for some pumps), but drops in the winter; if you live in a particulary cold part of the country it would be lower for longer.

There is a Facebook group call UK and Ireland ASHP (or something similar) and they have a lot of experienced people who can provide more technical information. However, I suspect a COP of ~2.4 over 12 months is not so bad.

Thanks Simon
Our flow temp has been 38 for most of the time since November.
It is improving a bit with the milder weather but we hardly used it in April (90 kwh), also it doesn’t do space heating at all so it may not come on at all until october now.
We are asking for quotes from local installers to add to our radiators
We are setting the thermostat to 18 degrees pretty much all the time except winter afternoons when it goes up to 19 until we go or bed.
We are hoping that the increased radiator space will make a difference

Thanks Andrew
We divide the energy delivered by energy consumed as shown on the mitsubishi control unit.
No buffer tank, we do not have room. We have a 150L mixergy tank for hot water mainly heated by solar but not connected to ASHP.
We are using less energy than heat loss calcs. The estimated COP was 3.7!
I will checkout the Facebook group, thanks for the suggestion.

I’m not an expert, but 38°C seems low. Is that just hot water or something. I am a bit confused when you say it doesn’t do space heating.
If your radiators are only warming to 38°C then the heat transfer to the room won’t be efficient. Try increasing it 10° and see what the effect is.

Any plumber can add/upgrade your radiators, you don’t need a heatpump specialist

Simon Hudson

Thanks Simon
We have no water heating by heat pump because we use a mixergy tank with a solar diverter, and for 4 months in winter we use octopus go tariff to heat the tank overnight (12.30 to 4.30) when the grid is usually low carbon and is 5p per kWh until august when we will go up to 7.5p. (and more in October probably).
The flow temp of 38 degrees was recommended. Originally we set it at 45 degrees but COP was low. We get a more even temp in the house when we lower the flow temp. The installer recommends 45 degrees but COP decreased when we ran it at that temp. Never thought to increase it above that but we may try it next winter. We are using the summer to try to get it as efficient as possible for winter. Thanks for the info about any plumber being able to add a radiator.

Our NIBE ashp is getting COP of 3.12 for space heating for the period Dec-2021 to Apr-2022.
In terms of where to go from here I would definitely go with a couple of radiator upgrades if you can accommodate them. There are also probably so settings buried in the control parameters of the unit that can improve the COP but this can be tricky to find out and tricky to know where to start (manufacturers don’t make the advanced control settings easy to understand)

Thanks Dom
Nice to know someone else’s heat pump’s COP over a similar period. We are starting to feel confident about more and bigger rads improving things. Not sure about the deeply buried parameters though!

My feelings as a yet to be heat pump owner is that parameters should only be changed if you understand them, otherwise if you ever need an engineer to fix a problem they may find a situation they are not familiar with and either walk away or make erroneous changes.

Thanks Tim, that makes sense.

You’ll need to provide more information. The heat pump will have a lot of data about what it’s doing that might help work out what the issue is.

More radiators will provide more heating power at the same design temperatures. If you’re reaching your set room temperature with your current setup then more radiators will allow you to use lower design temperatures.

Only lower temperatures will fundamentally increase cop. So you may be dealing with incorrect install or commissioning of the heat pump.

I feel for you because we had the same problem of an installation company that was competent but completely uninterested in providing service thereafter. One problem is they are not local so the logistics of getting someone competent on site are difficult. The product failed after only 18 months so I had to find an engineer local to us (London area). After asking around local installers I found a heating engineer who quickly diagnosed the problem. We were able to source the necessary replacement component which LG reimbursed us for under guarantee.

Moral of my story: when choosing an installer make sure you are going to get competent and timely service.

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Thanks Frank
So, in order to check whether it was a problem with incorrect install then we should get another company to check the system out?

Thanks Paul,
well they answered all our questions ‘correctly’ before install, had decent reviews, are members of all the right professional bodies, and were very attentive and reassuring until we started questioning things after the install (and we had paid in full).
I am partly sharing this as a warning to people and also it in line with the research Matt mentioned on the last carbon coop zoom meeting. The research suggesting that the predicted COP from heat loss surveys does not match the real world COP. The other frustrating thing is that when we asked for help from NAPIT they supported the company, based on their ‘paperwork’ of predictions, not visiting us at all.
What would really help is if someone knew of an installer they would recommend near(ish) to us in Leeds.

Ours was a very similar story. As I said, I called a lot of other installers roughly in our area and most did not want to take on someone else’s installation. I persisted and one of them finally put me in touch with an engineer who has provided the services I needed. From here on I have someone to call for routine servicing and enhancements.

Agreed, I was simply saying that there is the scope to improve things if you know what you are doing.

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Hello Martin,
I also live near Leeds and I have a 1930s semi bungalow. I am hoping to be able to insulate and install an ASHP.
I would love to chat with you, maybe over zoom, to hear more about how you went about it, Thanks Helen

Hi Helen
Yes that would be great to chat. You would be welcome to come and have a look around as well if you like. We are on holiday in Kirkudbright until next Tuesday.
Let me know when, I am free most days. Prefer daytime.
All the best

That would be great, thanks Martin. My number is 07305 537 913, if you text me when you are back we can arrange a time
All the best