Air Source Heat Pump. Experiences

We live in north Derbyshire and replaced our gas boiler with an Air Source Heat Pump (ASHP) early last year. We’re very happy with it as our house is warmer with the equivalent running cost as our old boiler. Now it looks like it will be quite a bit cheaper to run with the new time based plunge tariffs being trialed by octopus… Who else has an ASHP and are you happy with it?

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Interesting just had my gas boiler serviced and the engineer said you wouldn’t believe the number of ASHP’s we have uninstalled and converted back to a gas boiler. Seems they are not for everyone.


Will very much depend on how well your house is insulated I’d imagine, good for A rated but after that it’s a question mark


I’m curious how you employ a heat pump in the UK. Does it run your HVAC (Heating Ventilation Air Conditioning) or does it replace a boiler for hot water and heating?

We replaced an aged HVAC system last year and it’s saved considerable amount from our heating and cooling costs. Performs better at cooling than the traditional HVAC downstairs but is a tad slower at heating. I work from home and all day comfort suffered at times of the year before the new system went in.

I have a conventional gas boiler for Winter Heating and three wall mounted air conditioning units that provide heating during coolish periods during spring and summer. This works well and cuts cost as well as being cool during hot summer days.

The down side of ASHP as always is the the efficiency plummets once you are into regular defrost cycles when the air temperature is low.
As demand increases your heat pump has less capacity to deliver.

It’s like having a four cylinder car drop to three cylinders when you start going up a hill.

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We had an ASHP installed in January, along with rooftop PV panels. It replaced an old oil fired boiler that needed money spending on it. I was a bit nervous about it in an old house in an exposed position but it seems to work well and costs aren’t as high as I had feared. The solar also works pretty well too. Early days to assess how much it will generate, but its looking like it will go a long way towards offsetting the increased electricity bills.

I’ve had mine since May 2022. The chalet bungalow is 300m2 spread over two floors and the ASHP has used 3200kWhs in the last 12 months.

Whilst during the cold weather it is affected by defrost cycles and needs to run at 40C or more, these are relatively short and much of the time it runs with a COP or over 4. I don’t try to run it 24/7 on a lower temperature with weather compensation. Mostly it runs continuously during up to three heating periods.

I have solar and 20kW of batteries and use Octopus Flux. Now I can control charging and discharging to grid at peak time I can earn quite a bit, and increasingly I can charge the EV with excess solar (the priority is to export during peak time). I may switch to Agile though as aside from exporting at close to the standard unit rate it has been 30% cheaper for imports. The ideal would be to use both at different times of year.

Our house was EPC band C prior to getting rid of the gas boiler.

It did take a while to get the heat pump running efficiently. In my case the ultra powerful circulation pump was totally unnecessary and didn’t allow the plate heat exchanger to work properly. We had a properly designed micro bore system. Now back to a small Wilo smart pump.

I would say the days of being cosy are over, but for now we are happy wear jumpers. Agile would allow us to run 24/7 in the cold weather without using standard rate juice. With Flux I kind of like to avoid using much standard rate electric so limited to 20kWh plus heating etc during the cheap 3 hours.

Hope this helps


I have a feeling this is largely because the install engineers never do a proper sizing job. With a Gas boiler, most engineers will just oversize it because it will work, and they haven’t thought about efficiencies. With Heat Pumps, the engineers really shouldn’t be using finger in the air guesses at what you will need. There are a number of calculations required to measure the heatloss of your house, then installing a heat pump that meets that requirement during the winter months. @ChrisSU @Bokermonz I’d be really interested to know if they did that for your installation.

From my understanding, one thing that seems to change with ASHP is that you need to keep the heating on all day, and I suspect this may suit people who spend a lot of time at home.


Yes, they did the calcs, changed a few radiators, and selected a fairly high powered heat pump, so I guess they got it about right.
We don’t keep it running all day, but you do need to get used to it heating up the radiators more slowly than a boiler and allow for that when setting timers.
The controller is app based, so you can access it when not at home, and you can set multiple on/off periods at whatever temperature you want.

We also have woodburners, which may now get a little less use, but they gave me the confidence to go for the heat pump knowing that we had we had additional heat if it couldn’t cope.


We had an ASHP in our last apartment and it was certainly lower to run for hot water than the gas boiler in the previous place.

However, that was a hot climate.

We later moved to an extreme climate (very hot and humid for three to four months, but under snow for five to six months) and I had asked about getting ASHP in here too. But there are several weeks of the year below their -13C operating floor and extreme snowfall in these parts means you have to constantly dig the units out from being covered. In this case, we were advised to go with gas again and invest heavily on insulation.

My understanding is that they are efficient and cost effective in warm climates. At least as good if not better than gas in temperate climates. No worse than gas in cold climates. But not economical or easy to run in extremely cold climates.

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Is there a reason why you don’t run it 24/7 during winter? If the ambient air temp outside is 5C or below you are already into defrost cycles while it’s trying to warm the house up. During a defrost cycle it removes heat from your house to complete the defrost process.

My GCH has timer facilities but I don’t use them, it gets switched on when Winter temperatures arrive and switched off when consistent Spring temperatures arrive.
It’s cheaper to leave it on, flow temps are around 35-45C max, the Worcester boiler is set at default 65C but only gets close when you first switch the system on.

I think this is a myth.

For an ASHP the efficiency is dependent on the air temperature and most certainly plummets when the air temperature gets near zero and defrost is necessary.

Heat is always lost in proportion to the difference in temperature between inside and out. The insulation determines the amount. This in turn determines the temperature drop when no heating is provided. Our preference is for the house to be cooler when sleeping and this is when the heat pump efficiency is lower because of lower air temperature.

The final factor is that heat pumps are less efficient when operating at low output. They are sized for the very cold weather but in the UK that is only a few weeks a year on average. My 12kW unit is noticeable less efficient below 6kW and it also starts to cycle. The efficiency is very high when heating the radiators up because the temperature of the water leaving the unit is lower but the efficiency drops as it reaches target temperature. The idea of weather compensation is have the lowest water temperature, but I don’t think the units are smart enough yet. I’m content I know better how to heat our house efficiently and have the temperatures that let us be cooler when sleeping.


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We had a heat pump installed January 23 so we’ve had a year to experience in moving from a gas boiler, work through settings and understand how it works.
We have a 3 bed bungalow in southern England that has good insulation in the loft and walls, and double glazed windows. We also have solar panels but not no battery saving kit.

Our installer did specific surveys before designing the installation including heat loss surveys and with hindsight I’m glad they did - I’m convinced that element of the project was critical. The inclusion of external temperature sensors and a satellite tank were things I wouldn’t have thought of initially but I can see the advantages now.

We now run the pump at low temperatures all the time, but with a set back overnight as we found the house was hotter than we liked when sleeping, and find that the house stays at a nice daytime temperature (20 degrees C is fine for us) without problem, with plenty of hot water. We use weather compensation and find that as the temperature drops the house stays pretty much at 20 deg C without problems, although finding that point on the weather compensation curve took a while - again the house warmed up more than we wanted initially. The pump switches off heat when outside temperature reaches a set point.

We had to get used to much cooler radiators than experienced with the periods of high heat from a timed gas system, and also the fact that changing things is a slower process that just using a thermostat to switch the gas heating up or down, but it works well for us. Now we refer it to the temperature highs and lows of timed gas heat.

Over the last 18 months we’ve had temperatures of mins 8 degrees C but it doesn’t often get much colder than that in these parts. Cost wise it hasn’t been exorbitant - over that period it is a bit less than the cost of gas heating but it’s difficult to be exact on that as we were on LPG and a contract so I don’t know the current price per litre of LPG here. I do know I’m pleased to be off the eternal cycle of bartering for an LPG price, being locked in for periods and still wondering in bad weather if the tanker is coming to top us up (yes, even with auto ordering).

So for our circumstances I’d say it’s definitely a workable solution, affordable (as long as electricity prices are not loaded massively with green costs in future - but then I don’t know how LPG prices will go either) and comfortable. Not sure if I would have done it if we’d been on mains gas though, possibly not if we were still working and didn’t need to heat the house during the day.

Main tips - you need a very good installer that does heat loss/design calcs, make sure they are also going to do maintenance (probably more expensive than a gas boiler) as there seems to be a shortage of good ASHP service contractors, and be prepared for a different type of system you need to understand to get the best results.

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The weather hasn’t been particularly cold since the ASHP was commissioned in late January, so it’s hard to tell what’s going on with defrost cycles.
We don’t have the house very warm by most people’s standards. The main thermostat is usually set to 18°C and we may light a woodburner to keep one room warm in the evening. The ‘off’ periods have the controller set to 10°C rather than actually turning anything off. Early days yet, but I’m going to try increasing this temperature to see what effect that has, at least during the day. It might be next winter before we start to get a feel for it.
How water is always on, and there seems to be plenty of it.

Thinking about it, I’m not sure if it’s even possible to tell when it’s doing a defrost cycle!


It’s very simple really.
To start with you need a very well insulated home.
Also an air tight home, or as good as you can get it.
Then underfloor heating in a scread, and not overlay.
If you haven’t got all the above, then you are at a loss before you even start.
The reason is that heat pumps only really work very efficiently up to 30 degrees. You are not going to get radiators that are hot. They feel more like you switched the heating off an hour ago.
That’s why underfloor heating works best, as to start off with it’s designed to work around the 30 degrees, plus once warm, you have the thermal mass in the scread to keep warm for days even when off. This helps when it’s very cold as your heatpump will lose output when the air temperature is cold.
So basically, everything needs to be right for it to work effectively and efficiently.
I know quite a few doing heatpumps for octopus energy, and i can’t see how they are getting away with installing most off them. Shocking.

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I constructed a Home Assistant WiFi temperature sensor (actually 5 on one interface). I monitor the flow in and out of the CH plate heat exchanger.

This is normal running.

This is when cold.

To defrost the unit reverses the flow direction. It uses the heat in the circuit to defrost the external heat exchanger.

I had to fit an electric monitor to know the electric used. Quite content with 3200kWh for the last 12 months. It would have been 14000kWh of gas going by similar years. Now we heat the whole house as well rather than just the rooms we need.

People need convincing that normal homes (EPC C) are suitable. Below C more insulation is important, although the EPC methodology still needs improving. There are too many rumours circulating, but there are installers who still have much to learn.


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Watching “Urban Plumbers” on YouTube gives you a feeling on how a good installation is really important here. This plumber is excellent, and is followed by many plumbers as the gold standard. @dunc would you agree?

It does occur to me that while gas prices are less than a quarter of electricity, I suspect it makes payback slow. Of course there are environmental reasons to do this, and not to mention the gains when combined with your own solar/wind power.

I took readings and worked out the cost.

Each rad has a TRV and the hall has the main algorithm type stat. The bedrooms are set much lower than the living room.

Yes, this is correct with oversized units. It may be better to have smaller ones and some supplementary heating for weather extremes.

Yeah, although it isn’t ‘off’ the system has to gain 8 degrees and to do this it will have to work hard. Try a setback to 14°C.

If it happens while you can seethe outdoor unit you will see ice melting or steam coming off the coil. It uses heat from the house to do this.

The problem is that the output needs to match the outside temperature. If 18C inside and 12c outside, then 6c outside doubles the heat loss and -6c outside quadruples it. A tandem arrangement of two smaller units would be better.

I had all my radiators upgraded. TVRs should be set to maximum because the turning on and off messes up the heat pump which is designed to aim for a steady delivery of heat. It will cycle (cut out and restart after a delay) if suddenly the delta T goes outside its limits.

What works for gas doesn’t work with heat pumps. I have lots of daily readings also. 3200kWh/years for 300 sq meters is pretty good.


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