Air Source Heat Pumps

We recently moved into a house where, due to various issues, we were unable to get gas installed in our house and we also hated our storage heaters, like many others that I have read about on here. Last year, I installed an Air Source Heat Pump based system to provide ourselves with central heating and hot water and have written a description here in the hope that it may be of use to someone!

The system that I opted for was the Mitsubishi Heat Pump Boiler combined with a Boilermate HP that is sold as a package and are designed to work together as the Boilermate controls the HP and is fully modulating. I went for this system primarily due to ease of installation as the output from the heat pump is hot water that can be fed directly into your central heating, rather than having to deal with seperate inverters and refridgeration engineers! This system can be installed by any competent plumber. The Boilermate is an unvented cylinder that provides mains pressure hot water to the entire house and is heated by the HP (and boosted to 60degC via a 3kW immersion heater if required to meet regulations).

The first question I often get asked is 'what about the water temperature'?. Well, the temperature from the heat pump comes out at about 50-60deg C during normal use (according to the Boilermate sensors). The Boilermate HP actually has a 3kW inline electric heater to give the water a boost in temperature if required, but I don't think it is using this at the moment, and only will when the temperature drops outside. The boilermate has an external thermoneter to adjust the temperature of the water, but I have found that produces water that is too cold, so I have removed this to ensure that the water temperature stays at the maximum and then regulate the house temperature via a thermostat and TRVs. The following shows the output temperature of the heat pump verses outside temperature:


So you can see that you'd only really need to use the electric boost if the outside temperatures drop below about -10/-15degC, which is rare for where I live in the South East, if you base your radiator sizes on a water temperature of 50-60degC. In fact, looking at that graph, I think I'm losing too much heat through the pipe between the boilermate and the heat pump, but I haven't finished insulating them yet (make sure you have good stuff (~£5/2m), not the normal stuff and keep the outside pipework as short as possible). (iirc) is where I got my stuff from.

I'm not sure exactly how to convert the output temperature to a COP figure, but the HP maximum power consumption is about 2.1kW, so I'm sure someone can work it out! So I am effectively heating my entire 3 bed semi using upto 2.1kW of electric power (or 5kW in the worst case scenerio). I've posted the specs of the HP at the end of this pump for anyone who's interested.

In terms of radiator sizes, Gledhill recommend that you oversize the radiators by 20%. However, we were a bit cautious in this and actually when for a 50% increase in our living room by swapping a single skin for a double skin one, and went for a 100% increase in the main bedroom as we like to be toasty! We did this by have two single skin radiators (due to the layout of the room). Our kitchen has a plinth heater that is rated at 100% extra IIRC, but it is a fan assisted one so is the same size as a lower rated one. We are installing underfloor heating in the bathroom too with a heated towel rail.

In terms of cost, it's not cheap upfront. The boilermate and heat pump cost just over £4K to purchase. Then you have to add on the cost of installing the radiators and pipework (which I did myself) and you'll also need 10mm^2 cable running to the boilermate to provide the electric backup and 6mm^2 cable running to the heat pump outside (which of course all needs Part P certifying ;) ) & also the commissioning of the system. We needed to upgrade our consumer unit too as we had a fuse box (with wires) rather than MCDs! Personally, I got a plumber to plumb in the actual boilermate as it is quite complicated! (although remember it just needs a normal plumber, not a specialist).

I weighed the cost up against the cost of installing gas in our house, and as you have to pay the exact cost of gas installation (at c£2000 per day for the contractors, but less than £1k for a combi boiler), I think that the heat pump worked out cheaper as we'd have a very complicated gas installation due to access issues and the lay out of the land as the trench to the house would have to be dug out by hand!) and of course, there are many who don't even have the option of gas. Oil was another option, but is just as expensive to install and we didn't want a tank in our garden and oil isn't getting any cheaper! We couldn't get LPG as the tank wouldn't be in line of site to the road, so wouldn't meet regs.

The main drawback is the noise generated by the HP itself. It is 49dB according to the specs, but I would say that it is about the same as a modern washing machine on spin cycle. This may or may not be a problem depending on where you live. The HP needs to be as close to your house as possible to prevent heat losses through your pipes, so you can't hide it at the back of your garden! However, it is hard to hear through a double glazed window, so shouldn't be too much of a problem. However, your neighbours may not be so sympathetic. There are no regs AFAIK on siting a HP as the main problem is the noise and that is already covered by the normal environmental noise regs. If you mounted it on your boundary next to your neighbours windows, then they may well complain. In Ireland, they are suggesting that there should be a 5m distance between any HP and a boundary, so we used that as a guide for England, but our garden is only about 7m wide, so doesn't actually quite meet that but we've done our best to site it in the least annoying place. We haven't had any complaints yet, but only time will tell. As mentioned, if it is a good few meters away, and your neighbours have double glazing, it shouldn't be a problem, although I'm not offering any guarantees! Also note that as it powers your central heating, it won't be running much during summer (bar hot water), so shouldn't be much of a problem when in the garden during the summer. Also, the HP is large. About 90cm by 90cm by 30cm, so you'd need a space to put something that size and is, tbh, rather unsightly! Although this is less than the size of an oil tank I suppose. It also needs a 30cm (iirc) space behind and 50cm infront for the flow of air, so can be mounted on bracket on the wall if you buy big brackets (it weighs about 90kg).

In terms of running costs, we only installed it about a month ago, and the temerature has been between 0-10degC over the past month, bar a few sub zero days, and it seems to work out about £3 per day if it's on for most of the day, and about £2 per day when we are at work (on in morning/evening only). That is based on 8p/kwH and is the price for all our electric including cooking etc. We have cavity wall insulation, old double glazing (doesn't meet regs) and only 100mm of loft insulation so our house is fairly well insulated, but by no means well insulated.

The key point I think is that in South east of the UK where I live, it rarely drops below freezing temperature anyway, so should be operating at a reasonable efficiency for most of the time. Obviously if we have an artic winter, then running costs will increase, but during summer, it will provide our hot water for virtually free (well, not quite as cheap as solar, but cheaper than gas anyway!).

Finally, as the HP is running at greater than 100% efficiency most of time, my CO2 emissions should be less than all you gas boys, giving me a boost when we get a HIP done ;)

Any questions, then please ask and I'll do my best to answer!

Info on the boilermate HP is here (under alternative energies);
and info on the HP is here:
This is the spec sheet of the HP from the manual.


  • Cardew
    Cardew Posts: 29,034
    Name Dropper First Anniversary First Post Rampant Recycler
    Most interesting - thanks.

    A couple of quick questions.

    I have read that the minimum temperature for Domestic Hot water was supposed to be 65C - prevention of Legionnaires' disease? Could that be the reason for the 'booster' immersion heater?

    I appreciate that it is early days yet, but do you think that moving to an Economy 7 tariff would be cheaper?

    Never thought about the noise problem.
  • Cardew wrote: »
    Most interesting - thanks.

    A couple of quick questions.

    I have read that the minimum temperature for Domestic Hot water was supposed to be 65C - prevention of Legionnaires' disease? Could that be the reason for the 'booster' immersion heater?
    Yes, the cylinder has a 3kw immersion heater to bring the temperature upto 60/65degC (not sure which, but it is preset by Gledhill to meet whatever regulation it is).
    I appreciate that it is early days yet, but do you think that moving to an Economy 7 tariff would be cheaper?
    I'm still a bit undecided on this, I don't think that it will as the cylinder is purely for hot water, it isn't a 'buffer' for the heating as the heating is effectively driven direct from the heat pump so most of the electric will be used whilst the heating is actually on, i.e. during peak times. I guess it would be advantageous for the cylinder, but I don't think that that will out weigh the extra cost of the heating.
  • adr0ck
    adr0ck Posts: 2,374
    First Anniversary Combo Breaker
    samtheman1k thank you for a very informative thread

    Could you please tell me how many radiators in total the Mitsubishi Heat Pump Boiler is heating?

    Also does anyone know if this system could work in conjunction with a LPG boiler?

  • I am currently using it to heat a 3 bed semi which has 7 radiators. I think it is equivalent to a 13kW boiler.

    It would be difficult to get this particular system to work with an LPG boiler as the heat pump is effectively replacing the boiler so you'd be trying to put two boilers on the same system...not impossible, but tricky. There is another option, and that is to use a standard thermal store, such as the gledhill torrent I think it's called, that has multiple inputs, but isn't as integrated as this solution is.
  • adr0ck
    adr0ck Posts: 2,374
    First Anniversary Combo Breaker
    thanks samtheman

    spoken to gledhill who are going to send through some details and get someone from their technical department to call me back with possible solutions....................guy i spoke to seemed to be suggesting that solar panels might work better with an existing lpg boiler

    will let you know how i get on

    thanks again
  • moonrakerz
    moonrakerz Posts: 8,650
    First Anniversary Combo Breaker First Post
    Thanks, very interesting read.

    It is nice to read a post from someone talking about an alternative method of heating who can actually provide some realistic figures to back up their words rather than sounding like an Arthur Daley.

    It was also encouraging to read respected/well known names such as Mitsubishi and Alfa Laval.

    Keep us informed, please.
  • Cardew
    Cardew Posts: 29,034
    Name Dropper First Anniversary First Post Rampant Recycler

    A further question if I may.

    I am aware of GSHP systems that have a 'buffer'(thermal) storage tank for the CH water to enable them to utilize a Economy 7 tariff.

    Was additional fitting cost the reason for not having this facility? or are their other considerations with an ASHP?
  • Hi Cardew, I think the reason there is no buffer is because the HP is able to supply the entire heating demand and thus I can't really see how an E7 buffer/TS is going to help. A tank full of hot water is going to empty pretty quickly when heating a house, and then you'd have to use the E7 peak rate electricity and I suspect that the overall running costs would work out higher than if just on a standard electricity tariff.

    Unless you had two stores, when you turned the heating on, the store would deplete and then you wouldn't have any hot water until the next E7 time (unless you use peak rate electric).

    I guess you could have an additional store for the CH, but that would envolve double the size of the footprint of the system, something that we wouldn't be keen on!

    Let me know if I've misunderstood the question.
  • punamulta
    punamulta Posts: 193 Forumite
    I bought a self install B&Q airforce inverter heat pump/ac split unit for £350 in the sale last autumn, no hot water or rads but it belts out the heat and is ideal to compliment my gas central heating. Not as good as above system of course but you can access this technology at a lower cost
  • Chrislincs
    Chrislincs Posts: 17 Forumite
    Hi, Thanks to cardew for this link, and Samtheman for info, if a bit far on for me at the mo. It is interesting to discover that plumbers/sparks can fit as I am sure that supply and fit deals will be super dear, esp. companies like Ice, etc. who are at so many shows. mind you, finding a guy willing to go down relatively new avenues in backward Lincs is not quite so easy.
    Sam how loud is 49db? What does it compare with - a car engine, lawn mower or expelair? This would definately influence siting. Also, you have now have a winter, with some pretty cool snaps - how well did it cope? Was it sufficient in lounge without extra input. As you need larger rads for asp's do you need a bigger tank, or can I use the one installed when I had my solar panels fitted? Can you turn it off? We wouldn't need hot water from it in the summer, nor heat. This is when the noise element would be most noticable, with open windows and outside living. Do you still recommend it having had a cold season's experience?
    Thanks for all your help.
This discussion has been closed.
Meet your Ambassadors


  • All Categories
  • 341.4K Banking & Borrowing
  • 249.7K Reduce Debt & Boost Income
  • 449.1K Spending & Discounts
  • 233.7K Work, Benefits & Business
  • 605.8K Mortgages, Homes & Bills
  • 172.3K Life & Family
  • 246.6K Travel & Transport
  • 1.5M Hobbies & Leisure
  • 15.8K Discuss & Feedback
  • 15.1K Coronavirus Support Boards