From Gas Boiler to Air Source Heat Pump

By Richard Mould-Ryan, January 2021


A few years ago we switched energy supplier to a provider of 100% renewable electricity (Ecotricity), and the next obvious step to reduce our direct carbon footprint was to do away with gas for central heating.  This meant some sort of heat pump, and like most people we don’t have a huge garden to dig up and lay pipes into (for a ground-source heat pump), so it would need to be an air-source heat pump (ASHP).

Before and since having ours installed, I’ve come across quite a lot of misconceptions, myths and even misinformation about heat pumps, such as:

  • They only work with underfloor heating (not true, though they do work very well together);
  • They don’t produce water hot enough for a bath or to keep your house warm (untrue);
  • You’ll have to replace all your radiators (extremely unlikely);
  • They’re very noisy (not true, though they’re not silent);
  • or perhaps the strangest – they have to be fitted to a warm south-facing wall (not true).

Some of these may have been true in the past, or have been true with poor quality installations, but a modern pump is very efficient, and will be suitable for many people, but you may have to use it differently to your gas boiler.  We’ll come back to this and these myths a bit later.

This is our experience of choosing an ASHP supplier, installation, and 3 months’ use.  When you read it, please bear in mind that I’m not in the business myself, this is just what I’ve learnt from our supplier and a reasonable amount of research.  I hope it will help you to make a decision if you’re considering installing an ASHP, but don’t take my word for it, ask any potential suppliers yourself.

Selecting a supplier and system design

We were prompted to look into an ASHP after being reminded about the Renewable Heat Initiative (RHI), which makes quarterly payments for seven years based on the amount of heat you generate, and should eventually cover a good chunk of the installation cost.  To be eligible for this, and for the new Green Homes Grant (GHG), you need to use an installer who’s registered with the Microgeneration Certification Scheme (MCS), so we found three companies using their website who had previous experience of installing ASHPs in domestic situations.

From these, we chose JEM Energy, (, 08081 646568/01252 900052, contact James Rodwell), mainly because we were impressed by James’s commitment – they’re based near Farnborough, but he came down for a pre-visit to give a general idea of feasibility and price before the detailed survey – and because they were nearly the cheapest.  He charges for the system with an additional fixed cost for each radiator which needs to be upgraded (see below), whereas another charged a price including for upgrading up to 10 radiators, more expensive if you don’t need to have so many upgraded.  That pre-visit was really useful, as it confirmed where the pump would go, an estimate of how many radiators might need to be upgraded, and some other key details at an early stage.  If you contact JEM Energy, please tell James that Richard Mould-Ryan gave you his contact details.

The first step was for James to arrange a heat loss survey by an independent surveyor.  This was included in the overall cost, but if you decide not to go ahead you have to pay, in which case it’s about £200.  The survey did a number of things:

  • Assessed whether our insulation needed to be upgraded; to qualify for the RHI or GHG you need to have cavity wall insulation and at least 100-150mm of loft insulation.
  • Calculated heat loss on a room-by-room basis, which determined the output of the heat pump needed, and how many of our radiators needed to be upgraded. We have a 9kw pump which fits neatly below our kitchen windowsill (see pic).
  • Provided us with an Energy Performance Certificate (EPC) for the house, which we’ll need if we move, and haven’t already got one.
  • Suggested whether we’d need a larger hot water tank.
Our heat pump in situ, showing the power isolation switch and the water feed pipes

We’re moving into the territory of the myths & misinformation now, and most have probably arisen for one reason: a gas boiler heats water to 60-70C, whereas a heat pump puts it out at around 50C.  (They can produce hotter water, but are less efficient at higher temperatures).  This is still hot enough to do the job, the heating system just does it slightly differently, which is where changes to the rest of the system may be needed.  Because the water in the radiators isn’t so hot, the radiators may need to be larger to provide the same total amount of heat output needed to keep your house warm, as calculated by the heat loss survey.  In our case, we upgraded 5 without much disruption, either by making them larger or replacing single panel versions with doubles, or in one case a double-panel with a triple.

For the same reason you might also need to increase the size of your hot water tank: because the water isn’t quite so hot, you may need more of it to run baths because you couldn’t use so much cold water in the mix.  We had quite a small 120L tank but never had any problems with not having enough hot water, probably because we share the bath water!  The original suggestion was for a 240L tank, which would have used all the space in our airing cupboard, which is very useful to us, and seemed a bit OTT.  So, I did some research & found that we’d be Ok with 180L which still left useful space in the airing cupboard.  The space we lost for drying towels was replaced by changing the small radiator in the bathroom for a large towel rail, and we’re very happy with it.  If you don’t have a tank cupboard, it’s also possible to install one in the loft.

The main difference with the hot water tank is that the new one is much better insulated; the old one lost so much heat that it was keeping the bathroom warm, but the new one hardly loses any at all, so the larger towel rail offsets that change.  If your tank is large enough and fairly well insulated, you probably wouldn’t have to get it changed, though it does need to have an immersion element to heat the water up to 60C+ once a week to prevent any possible problems with Legionella.

Above Left: The new very well-insulated hot water tank. Above Right: The water supply pipes – visible, but not intrusive
once painted compared to the unpainted pipes in the picture of the heat pump


The lead-in time from placing our order was about 3 months, though it could have been a bit less; with the extra demand created by the Green Homes Grant, my guess is that installers may be a bit busier now.  However that might not be a bad thing, because having no heating for 3 days would obviously be better in May or June than in February or March!

James uses separate sub-contractors for installation, and after we’d confirmed we were going ahead, they visited to confirm exact details of locations of the heat pump, electricity supply (the pump needs its own separate circuit from your consumer unit), pipes etc, and checked the radiator changes, which ended up being slightly different to the recommendations of the heat survey.

In terms of physical space, our pump is 120cm wide, 90cm high and 40cm deep, they need to be 30cm from a wall with no obstructions to the airflow from the fan, and a minimum of 1m from your boundary to comply with building regulations.  The fan creates quite a strong cold breeze, so it’s probably a good idea not to have any tender plants in front of it.

The installation went very smoothly, it was all finished in under 3 days and we were very happy with the way it went and the standard of work.  We were without electricity for an hour, hot water for just one day, and without heating for 2 ½ days, but if the weather is cold at the time, James will supply electric heaters for the duration.  The only hiccup with ours was that the pipes from the pump into the house had to run differently from originally planned.  If possible, the pipes are normally run from the pump through the hole left in the wall where the boiler flue is removed, and connect to the water pipes which fed the boiler, but because of the way our house is plumbed the pipes had to run up the outside of the house and direct into the loft (see photo).  However this wasn’t a problem in practice, there was no extra cost, and they can be painted if necessary – masonry paint stuck to them very well.

For readers with a little plumbing knowledge, we previously had a vented hot water system, but the new one is unvented and gently pressurised, so there’s a small expansion tank in the loft where the old central heating header tank used to be, and the system may need to be topped-up from time to time by turning on a small valve, but this is really simple and was covered in the clear and thorough handover by the installer.

Up and running

Inside the house, there’s a small control panel which will need to go somewhere you can reach it and fairly near where the control cables come into the house, somewhere near where your boiler was.  This is how an engineer accesses the system; you can make minor adjustments with it, but in practice we haven’t had to.  There’s also a small wall-mounted box which communicates with the Nest remote thermostat, which you can control via an app from your phone, and can go anywhere you like as it works via wifi & isn’t wired into the system.

Above Left: Inside the house: the controller unit is bottom left and the Nest wifi box is top right; the shelves are
where the boiler used to be. Top Right: A close-up of the Nest remote thermostat

The other main difference with a heat pump is that unlike with a gas boiler, there’s no timer unit; the pump is controlled just by the thermostat, and works most efficiently by using lower heat input over longer periods to maintain an established temperature.  The thermostat is set at your normal daytime comfort temperature, 20C for us, and 2C lower at night, and the pump switches on and off to maintain these temperatures as required.  During the summer, or if you’re away for longer periods, you can switch it to “eco” mode, which is a lower threshold of 15C.  The Nest thermostat also switches to Eco mode automatically if you’re away for a period of time and forget to do it yourself.  Switching it off for long periods allowing the house to become colder means the pump has to work harder and less efficiently when it has to restore the normal temperature.

During cold spells you can turn the boiler temperature up to 55C, but it’s less efficient, and we haven’t felt the need to so far.  Also during cold spells, the coolant pipes and fins frost up – not surprising as cold damp air is passing over an even colder surface, but when this happens the pump runs a defrost cycle which also produces a brief but impressive cloud of steam!

The other myth was about noise.  Our pump is rated at 60 decibels, the same as normal conversation, though obviously it’s continuous for longer periods.  It’s just outside the kitchen window, but you can’t hear it with the windows shut, and to hear what it sounds like from outside, you can watch this short video.  The iPhone microphone is very directional, so it makes the pump seem louder than it is in comparison to the other background sounds; just before the end a car goes past at the front of the house, and if you’re standing a few feet away from the pump they sound about the same volume.  It’s been working harder during the recent cold spell and at times has sounded a little louder.  Some of the noise comes from the metal panels vibrating, and putting plastic wedges in between them can reduce that quite a lot.  If you want to hear it for yourself we’re always happy for people to pop round, Covid restrictions permitting.

Cost and savings

The cost of a heat pump system will depend on a number of things, principally the size of pump needed; the number of radiators which need to be upgraded; and whether you need to replace your hot water tank.  Our 9kw pump, five radiators plus new tank cost £11,400 including VAT, which is a lot of money, but there are things to offset this.

Most fairly modern boilers have a lifespan of about 15 years, and become less efficient in their later years, so if yours is over 10 years old, it’s likely to need to be replaced in the next 5 – 10 years.  Depending on your system and if you want to make any other changes this would cost £1,500 – £3,000, which you won’t have to spend if you opt for a heat pump system.

The Green Homes Grant will cover 2/3 of the cost of a system to a maximum of £5000, or the full cost up to £10,000 if someone in your household receives certain benefits.  The Renewable Heat Incentive may be a bit more generous, and is calculated based on the energy your system will generate, so a larger system should attract a higher rate of payment (this was worked out as part of the heat loss survey).  However the payments occur after installation, so the up-front cost is higher.  We’ll be receiving £888/year for 7 years, so £6,200 in total, and allowing £1,500 for boiler replacement, the net cost of our system will be around £3,700.

So will you save anything on your energy bills to offset this further?  The honest answer is that for us it’s too early to say.  We’ve had the pump for just over three months, and in that time we’ve spent £48 more on electricity than we saved on gas.  However we’ve kept gas for cooking, at least for now, but if we did without gas completely, we’d save another £23 on standing charges, so the net cost would be £25 for 3 months, which have included a couple of cold spells.  During the summer, when there won’t be any need for space heating, I’d expect to save on heating hot water, so we might break even over the year, though servicing costs appear to be a bit higher than for a boiler.

Whether this would apply to you may depend on your tariffs, how well insulated your house is, how you use your boiler now, and particularly if you already have solar panels.  If you use the thermostat to control your boiler rather than the timer, just turning the temperature down at night, then you might save more than if you use the timer to switch it on and off several times a day, but it’s impossible to say with any certainty.  Our heat loss survey provided an estimate of the cost of running the heat pump for a year compared to using gas, but the figures were very different to our actual gas costs and to the first quarter’s use of the heat pump, so they may not be reliable.  A heat pump has a life expectancy of 25 years, so in our case, if we saved £150/year, it would pay for itself over that time, but unless the cost of electricity comes down relative to gas I think that’s unlikely.

If you already have solar panels, then the situation is obviously much better, because every unit you generate which is used by the heat pump saves about 15p.  If you don’t already have solar panels, the savings might make it worth installing them; it’s something we’re considering but haven’t yet looked into in detail.


Based on our experience, the main physical reasons not to install an air source heat pump seem to be if you don’t have a suitable outside space for it, or if it’s physically impractical to upgrade or add the number of radiators needed.  For us at least, a new hot water tank and five radiator upgrades were feasible, the noise of the pump isn’t an issue, and the heating performance has been fine, even during the current cold spell.

It seems more likely that cost would be the reason for not making the change.  Depending on the changes needed to your system, after taking into account the cost of replacing your gas boiler, there will probably still be a net up-front cost of up to £4,000 – £5,000 if you apply for the GHG.  This could be £9,000 – £10,000 if you opt for the RHI, though your net bills will be greatly reduced for the next 7 years if you take this option.  You won’t necessarily save enough on your energy bills for the system to pay for itself over its lifetime, unless you pair it with solar panels.  However if you’ve read this far, cutting your carbon emissions may be more important to you than cutting your energy bills.  Moving away from gas should reduce our direct carbon emissions as a family by 1.8 tons a year; I don’t have a current estimate of our carbon footprint, but this is 5% – 10% of the UK average for four people, and will be more if you’ve already taken other steps to reduce your carbon footprint – not to be sniffed at.

In my view, if you can afford it, do it – to reach net zero by 2050 the UK needs to replace over 20 million gas boilers, and the more who make the change early, the easier it’ll be to dispel some of the myths and show that they work.  If you do decide to ask JEM Energy for a quotation, please tell James that I gave you his contact details, and if you have any other questions you can-mail me on rmr1964 [at] or contact me via the contact form here.

One comment

  1. Morning,
    Came here via the link from YouTube.
    This is a great, informative and objective description of ASHP- thank you for sharing your experience.
    Although we have opted for the solar heating route via the GHG (and are still waiting) ASHP is something we are seriously considering for the future.
    At the moment we are still using oil, but i have installed UFH in our living room and it is amazing ! We have turned the CH off to the rest of the house now the snow has gone and the UFH is set at 21°C continuously. The input temp to the manifold is set at 40°C and the boiler hardly fires up.
    Keep the updates coming with your experience.

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