Heat pumps - anyone with experience?

I live in an old (circa 1705) stone cottage. I had my old combi boiler replaced with an air source heat pump in Feb this year. My first lesson was to start with the Ofgem website. My first installer contact, that I found myself, was nearly an expensive disaster. The nominated Ofgem agent will walk you through the RHI claim details and suggest an approved installer with a good track record. My installation cost about £13k. I’ll get all of that back from the government over 7 years.
The outside unit isn’t pretty, particularly outside an older property. The water tank unit is relatively huge, is not particularly attractive, and more than uses the space vacated by my old boiler.
I had to replace all of my radiators with bigger units. To satisfy the conditions of the RHI grant, I also had to get cavity wall insulation on a newer part of my house, which I probably should have done anyway.
One major benefit was that I could scrap my old electric shower and fit a simple thermostat mixer. The water pressure and flow from the new system is plenty for a good direct hot water supply.
As for cost saving, I haven’t noticed any reduction in overall energy use since Feb.
One question: Should I inform Bulb that my primary energy use has changed from gas to electric?

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Why? if your sending in monthly meter readings and getting accurate bills/statements doesn’t really make any difference which is your primary energy usage

I fitted a GSHP to my 1750s stone cottage four years ago. Like you I had to uprate all the radiators and improve the insulation. My energy use did not increase too much. My water was produced by immersion heater, now by heat pump, this reduces my electrical consumption. However heating the house with the GSHP increases it. All I did to inform my supplier was to increase my meter readings to every month, this also showed me that in summer I was using less electricity as all the hot water was from the heat pump which was more efficient than the immersion. You may find the same as you have replaced your electric shower.

Hi. I’m not sure what brand your heat pump is, but with my Mitsubishi ASHP, there are LOADS of things you can adjust. some require an engineer with a service code unless you already know the code. For the radiators, it sounds like you have done everything physical. Try increasing the flow temperature. YOu may also have a curve setting. This measures the outside temperature and uses that to set the temperature of the heating system. you can modify the curve to be more efficient for your case. The room setting sees how far the temperature is from the set temperature, and try to regulate the heat pump to keep just enough energy to maintain that temperature you have set. There are also jumpers in my FTC (flow temperature controller) to make the unit more responsive or more efficient. I have to use the flow setting, as we currently are using a separate thermostat that only tells the heat pump to turn on or off, but we will be changing this soon. I find 50 degrees flow temp to be sufficient for me. It may seem obvious, but you should also be easily able to increase the hot water temperature. I can do it in my main control or on my computer/phone (see photo). hope this helps.


My GSHP is a CTC GSi12 which appears to be slightly less flexible than yours in terms of adjustable settings (although it is a modulating system). The operating curve can be adjusted which we did after experimenting in the early days. We have also reduced the hysteresis so the device responds more frequently to changing circumstances.

Respectfully you seem to be missing the point in installing a heat pump. 50 degrees C is high for a heat pump based installation and suggests relatively low insulation levels in the property? The highest I have seen ours is around 43 degrees C on a cold day in winter.



Hi there. We are not using a curve, we are just telling the heat pump Right, turn on the central heating for 50 degrees. The reason for this is that when the installer installed our heat pump, he did not use the control panels thermostat, but a separate wireless thermostat. this is only able to turn the heat pump on or off, and cannot tell it the temp of the room, and also disables the outside temperature sensor, so it is unable to use a curve. we are planning to remove this as soon as we can, but can’t get anyone out at the moment due to coronavirus :frowning: As soon as we can, we will start using a flow curve, or a room-based system to modulate our heat pump, as it is definitely not at its most efficient yet.


I had the Mitzibushi Dan fos Air Source system installed by my Housing Association in 2015, previously I had storage heating and if I turned one of these bad boys on, my electricity bill was £70:00 per week, after installation my cost dropped, to £45,00 per week with all heaters open, I live in a 3 bedroom house and have 7 radiators, the instalation engineer set the master thermometer to 20° coming on at 08:00, and dropping to 15° from 23:00 (these times can be set to suit you) I then have a portable thermometer which sits on a caddy and you can have it in whichever room you spend the most time in, usually the living room. I then had the good sense to switch to Bulb retaining my key meter, and my winter electricity usage is in the region of £35:00 per week, whilst during the summer months it is £10:00 - £15:00 per week. Myhouse is South facing and also benifits from sunlight on a double glazed window. But a fact I must point out is that I live on the island of Lewis, in the Outer Hebridies of Scotland, and temperatures outside are considerably less than those in the South of England. I hope this is of help to you.

Hey @mackenzie86 It’s really great that you’ve noticed such a significant saving, especially in rural Scotland, where you would expect heating usage to be higher. Even better to hear that your housing association took this cost saving action for you.

Electricity only members do tend to pay more for their heating than dual fuel as well, due to the lower unit rates for gas, so it must be a relief for you to know you’ve got that weekly saving for as long as you’re at the house.

Hi @joelewis2710 :wave:

I know your comment is from Jan 2020 but we would love to know a bit more about your experience of retrofitting air source heat pumps.

We have been talking quite a lot about heat pumps on the community forum lately from the sizing of heat pumps to looking at what other countries are doing.

We are not experts in heat pump installations so would like to know more about them.

How would you decide the size of the heat pump you install? and do most installations go ahead smoothly?

Thanks in advance,

Good Evening All,

It is sometime since I posted here but, at the request of Cara, I will add to my original posting above.

I am currently a customer of another supplier so haven’t been folllowing whatever might have been posted on this forum wrt heat pumps but I do have one big chip on my shoulder which I need to air at the outset.

The statement ‘I can’t afford a heat pump’ or some such similar nonsense. There, that should gain some peoples’ attention.

I will accept that heat pumps are, like EV’s, expensive’ish but there is a section of our society who see no problem in replacing cars every three years and taking two of more oversea’s holidays so clearly have a disposable income but they CHOOSE to spend their money contributing to the world’s problems rather than spending it in a way that might help in solving them.

Kate and I have chosen the latter path. As I have posted on other forums we replaced our old septic tank with one of these new fangled sewage treatment plant. OK a part of this will make selling this property in future much easier but it means we are not polluting our water course as we were. It also makes for a much cleaner environment for the water voles we know exist here. The install cost between £5 - 6000 all in…

We no longer have a cat in order to give the local wildlife more of a chance, however I digress…

It is now 4 1/2yrs since the heat pump and solar hot water (Evacuated Tube) system was installed.

There were problems with the hot water system which weren’t resolved by the installer - I can only say this was because they didn’t know what they were doing. They tried to fix it through the summer of 2017 and again the summer of 2018 and, quite simply, failed to do so.

Nothing was done the following summer - we had a kind of Mexican stand-off as I was refusing to pay the installers for the cost of installing a pressure relief valve (which should have a been a part of the original install).

In the end it wasn’t until the introduction of isolation at the start of the pandemic that the issue was addressed by yours truly assisted by my long suffering wife, Kate.

It had been suggested by both Navitron (I pointed the installers at this organisation for advice) and myself that the expansion vessel pressure needed to be checked.

When I checked it in February 2020 I found it be at 3bar i.e. what it would have been set at by the manufacturer. You don’t have to be a degree qualified plumbing person to work out that a system with a working pressure of 1.2bar isn’t going to see an expansion vessel set at 3bar as an expansion vessel. This explains all the problems we had been experiencing.

I bled the pressure in the expansion vessel down to 0.9bar and then armed with a garden sprayer and a washing machine hose we pressurised the hot water system to 1.2bar.

The system has been operating without any interference since i.e. 16 months.

The moral of this story is make sure the installers really know what they’re doing.

This goes along with the same installers telling me I didn’t ‘need’ to change out any of the radiators. Granted most of the radiators were already ‘oversized’ given the work we have carried out on improving the insulation values of the house in the previous 16yrs but nowhere near the ‘oversizing’ required to allow a heat pump to operate most efficiently.

The standard radiator heat output is quoted at a ∆ of 50°C. The reality is that in the real world of heat pumps such a radiator will only give out 20% of this figure and that is a fact you cannot get away from.

This means most people will need to make sure an accurate house heat loss assessment is made (backed up with real world figures from their own experience of living in the property).

Only one radiator was the ‘correct’ size, another single panel had to be upsized, the others had to changed from double to triple panel.

Prior to having the heat pump installed we had a weather compensated, modulating LPG boiler. I took the previous 4yrs LPG consumption and assumed the boiler was some 90% efficient overall to produce a total heat requirement.

Where possible you really need to ensure that the heat pump isn’t oversized (don’t listen to any installer who tells you otherwise). For far too long installers have installed oversized gas boilers, short cycling with these isn’t too much of an issue but short cycling with a heat pump is going to reduce the compressor longevity.

You’ll also need to get used to the systems running 24/ 7. This will vary depending on the age/ insulation value of your property. You don’t get instant heat.

The Achilles’ heel of a heat pump is the provision of hot water. A household of four is going to seriously struggle with a heat pump alone - the likes of a Mixergy hot water system is going to be essential.

Anybody wanting realistic advice is welcome to message me directly.

For your further information I have a 12kW Solar PV system, a PW2, a zappi, 2off eddi’s and a Renault Zoe 50.



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Interesting you had LPG/oil heating before. Can’t link for some reason but read that this was the only group who really benefit from moving to electric ‘heat pumping’.

Sounds like a hell of a lot of work with radiators. Have one or two Qs will message you.


The radiator replacement work wasn’t such a big job as all replacement radiators (except the one in the Hall) were the same width as the ones that came out. Thus it was simply a case of undoing a couple of nuts and taking one out and putting the new ones in.

The inlet/ outlets are in the same location on a triple as a double panel so there are no pipework modifications and, strangely, the mounting brackets were near identical.

Since that initial install I have replaced the bathroom radiator from an 700 x 700mm triple to a 900 x 900mm triple to reduce the maximum temperature the whole system runs at. The bathroom in these old Aberdeen stone farmhouses is often placed in the old ‘larder’ so is downstairs and it is one of the three spaces in the house we have yet to ‘renovate’ so does lose more heat than we would like.

The old radiator was used in the Living Room in place of the old double panel 800 x 700mm and necessitated buying a shiny 100mm chromed extension piece from Screwfix. This didn’t need to be done but as we had the radiator ‘going spare’ and it would further reduce the ‘peak load’ on the heat pump it seemed a no brainer.

The bathroom is due to be renovated this year and should have already started but the new equipment has yet to be supplied. We will be installing UFH but it is a small space and the amount of insulation we can install is limited by the requirement to get the bath in place so there is a heated towel radiator as back up.

We have kind of made a start as the SO6 sized 2002 installed Velux window has been upgraded to the latest replacement panel and fitted with remote opening and a motorised blind. This will make the window more thermally efficient overall and give us the option to tie the window opening to the ventilation.

The bathroom has no external walls. It does have an extract fan we installed which runs up in to the roof space then down and out through the lobby to the outside.

We will be installing a Boulder Developments DHV-04 MHVR unit so will need to put another hole in the lobby wall and run a second pipe so we have an inlet and an exhaust.



Hi everyone :wave:

I’m considering an air source heat pump. How do you find the maintenance and costs post-installation?

Hey @emilybrown :wave:

Welcome to Community! Thanks for posting.

We’ve written quite a lot about air source pumps here which I recommend taking a look at.

We also made a table to compare the different kinds here which might help break down the comparisons of costs and maintenance as well!

If you have any further questions about it do let us know- what made you pick an air source one out of curiosity? :thinking:

Thanks Holly- I’ll take a look!

I don’t have much outdoor space so don’t think ground source would be possible. I guess the main thing I’m worried about is how visible the unit will be from the outside of the property and if there might be noise that could annoy neighbours. Judging by this thread, there’s mixed opinions but have also read the noise is just similar to an air conditioning unit, so thinking it should be ok!

My problem with compressor heating (AKA heat pumps) is finding the space to put the hot water tank. We live in a 4 bed/2 bathroom bungalow with the loft converted, and simply can’t lose the lump of space in any room to fit the tank. We’re currently running an outside oil combi boiler (live on an island, so gas isn’t an option anyway) and non of the government’s “green” options seem to be able to provide like for like.

Hey @Phill001 ,

Welcome to community :wave: Thanks for getting in touch.

As you discuss that you already your outside oil combi boiler, I wonder if you would have space for an air source heat pump?

The installation process requires garden space that is suitable for digging. If you go for horizontal installation, you’ll need an area around 100 metres long and 1-2 metres deep. If you go for vertical installation, you’ll need an area around 100 metres deep and 20cm wide. Vertical installations require specialist equipment to drill a bore hole, and double the cost, more or less. Reasonably difficult to retrofit so more popular in new builds.

You can read more about the ground vs air source heat pumps here.

If you are off the gas grid, heat pumps are a far better alternative to oil or LPG and work out cheaper to operate than continuing to buy fuel.

Let me know if you have any further questions on this.

Daisy :bulb:

Thanks for coming back. I have no problem with space for a heat pump (or two) outside.
But I do not have space for a hot-water tank of the size required by the heat pump system inside.
So I’ll need some sort of “approved” system that supplies heat and hot water as per a combi boiler, and I don’t think there is one apart from wet electric - which would greatly increase my fuel bill!

I see @Phill001 ! That is a shame.

I don’t know much about Wet electric but i’ve just had a read through of a few websites.

Have you looked into quotes for this? I’d be very interested to see how it compares.

There look to be a number of benefits including:

  • Energy efficiency
  • No noise
  • Maximise benefits of off peak times
    and i’m sure many more.

Look forward to hearing back about your experience with this,
Daisy :bulb:

I design heat pump systems professionally. I read and hear of all sorts of questions and answers from people who don’t really know anything but what they have heard. The best advice on heat pumps to begin with it’s on the micro generation scheme web site. They are the ones who set the standard for the industry.

Heat pumps are still developing so keep up to date. There are new models being announced that don’t need back up heaters, are super quiet and amazing efficiency. Don’t go for the cheapest it will cost more on the long run. Use the weather compensated control not room stats.

Above all start with a full heat loss calculation to BS12831, not the EPC, that’s rubbish. The heat pump should be as close to the heat loss as possible, don’t go adding extra just in case. Make sure the emitters match the heat pump output, you can add a bit extra here.

Most important use an experienced MCS certified installer who is a member of a trade association like CIPHE.

Happy heating, WJ