Long ASHP explainer with questions

I have been hesitating to press the button on this for a couple of years at least. Our current gas boiler we think was installed when the house was built circa 1970. I am staggered it still works but it does. Obviously it is not likely to be very efficient but our gas bills during winter are not particularly high so all in all we have had our moneys worth!

I did consider a like for like replacement a few years ago but since then we have installed solar PV and a home battery.  When I did investigate a replacement boiler the costs spiralled as other items needed upgrading to bring up to spec and were heading south of £5k. Fast forward a couple of years and we have decided to get off gas even if the costs to run are no lower. and with the new grant we have been quoted £7.5k (after the gov contribution) which includes 11 new radiators, new larger hot water tank and the removal of all of the old loft tanks, boiler etc. This seems like a good deal and I am ready to sign….

However… This quote included an 8kW ASHP which we were assured would be appropriate for our house (4 bed detached with double glazing, cavity insulation and loft insulation etc) we agreed location, pipe and cable routing and new location for the larger tank etc, all good and we waited on a couple of questions and then expected to go ahead.

Fast forward a couple of days and a call to say we would need the larger 12.5kW unit and it would have to go in a much less convenient spot.

A couple more days later, another call and they now claim now that the 8kW unit is sufficient if I can increase the insulation in my loft using 100mm PIR (circa £1500) which I am happy to do myself.

My questions/concerns are:

1: I know its impossible to estimate but does anyone have any experience of installing a heat pump of this size on a detached house (151sq m) I am wary that the pump may be a bit undersized and they are shoehorning it in as the larger pump is not available for several months.

2: Will the much larger pump be oversized and lead to other issues such as “cycling” etc which I understand can be a problem?

Interested in any real world experience really.

No ASHP experience, but is there a particular need to use PIR in the loft vs other insulation. PIR gives (nearly) the best insulation for a given thickness, but it comes at a cost. Could you use a thicker layer of mineral wool to achieve the same insulation u/r value at a tenth of the cost? Depends on how you’re using the loft.

Thanks. The loft is already insulated to between 250mm and 300mm with traditional insulation. The addition of the PIR is to increase the performance and reduce even further any heat loss through the loft. Part of the loft is boarded and used for storage. At the edges its useless for storage and I have put additional insulation in those areas >300mm deep.

How ‘big’ is the current gas boiler?

It will have a rating, which you can convert to kW, and then compare that with the proposed ASHP.

That will give you an idea of whether the ASHP is likely to be big enough :o)

How ‘big’ is the current gas boiler?

Its not clear which model we have and unhelpfully the model number (I think it is) varies between 8.8kW and 14kW. Also we have added 3 rooms to the house since it was built.

From my limited understanding an underized ASHP would have to rely on it’s heater coil a lot more than the heat exchanger process, and the net effect during these periods would be like having an electric boiler, i.e. less efficient than just having fan or convector heaters in rooms.

If this is the case, I hope for your sake you have a lot of solar panels, a big battery and live in the Canaries.

If this is the case, I hope for your sake you have a lot of solar panels, a big battery and live in the Canaries.

Yes, yes and Costa Del Wirral.

That sounds quite expensive. I’ve just had Octopus energy in to do a survey for fitting an ASHP in our semi-detached house. It’s now 129 sq m, was a typical 1959 3 bed with two big bedrooms and a box room, but the extension has doubled the size of the kitchen, an we now have a very large master bedoom above the kitchen / garage, and a 3rd story office above that that was classed as a habitable room during the survey. We have double glazing all round, and some very iffy blown foam cavity wall insulation done before we moved in.

We initally enquired in October, and they quoted £2715 all in, to include “any necessary radiator replacements” – that’s quoted, not estimated. We did ask a couple of other firms, and their estimates were a lot higher (around the 5k mark) and very much on the “this is an estimate, the price may go up when we do the survey” side.

Getting the actual survey booked in to be done took ages as they’re very busy but like I said it was done last week – the guy took about 3 or 4 hours to go round and assess everything.

For the intial quote of £2715, they’re going to install a Daikin Monobloc – 9kW (EDLA09DA3V3) pump, plus a Daikin Hot Water Cylinder – 250L (EKHWSU250D3V3). They will also replace 6 radiators in that price, with 5 being big enough already.

For comparison, our existing boiler is a Worcester Bosch Greenstar Junior 28i, fitted in 2007 when we bought this place, and a like for like replacement was being estimated at £2k plus anyway.

Thanks. The loft is already insulated to between 250mm and 300mm with traditional insulation

Loft is already reasonably insulated then, sounds like they’ve put your data through their spreadsheet/tool and you’re right on the cusp of needing the 8 or the 12 kw.

bit like those threads where people are right between bike sizes according to the guides.

I’ll defer to someone that actually knows what they’re talking about – is it better to size up or down when on the cusp with an ASHP?

We initally enquired in October, and they quoted £2715 all in, to include “any necessary radiator replacements” – that’s quoted, not estimated

That’s… eye-opening. I’ve been quoted £11-12k for supply and installation of a 3kW ASHP in a small new-build office, where I’ve already designed, installed, screeded and pressure-tested the UFH wet loop and have the manifold and controls all ready to go. And the company who quoted me called me up to chastise me for suggesting it should be less than half that price, given how much a heat pump costs wholesale.

Think I’ll be giving Octopus a call.

The octopus price includes a £7500 discount thanks to the subsidy from the gov’t, which may not apply to non-residential properties

The octopus price includes a £7500 discount thanks to the subsidy from the gov’t, which may not apply to non-residential properties

this is the kicker with these prices – yours is really a £10k for 130sqm and 6 rads while the OP is £15k for 150sqm and 11 rads. Yours appears slightly better value, but it could just be due to access or whatever – you can’t really compare the post-discount prices

But the OP may (should?)  also be eligible for the subsidy, as it’s a dwelling, which would make things a heck of a lot more affordable

Yes I am eligible. The price is above is what I pay so total before grant circa £15k

@nbt thats interesting. BTW Octopus dont install a unit big enough for us yet apparently. We are with them for our Elec/Gas.

Mine is the final quote and they did spent hours here doing the detailed survey. They are also happy to suck up the extra cost for the larger pump however it is not available until September. I am happy to wait, its more important that its sized correctly and they are not just trying to bring the work forward to avoid losing the order.

Interesting read/ question. I’m surprised at the size you’ve had suggested – what’s the floor area of your house?
FWIW, I’ve had ASHP heating installed in my house as part of a full renovation/ refit. It’s 1860s detached, so (very) breathable and damp, also quite large, with 4m ceilings all round. We had a bunch of insulation fitted, but an HVAC engineer calculated we needed 12kW including design margin for the heating, with another 9kW for the hot water.

So with cavity wall, double glazing and good attic insulation I can’t imagine you’d need 12kW just for heating, but it’s possible if you roll in HW you may need that large?
It’s also clear from going through the process that there’s an awful lot of people who only know a little about ASHP, including the installers (which is alarming). So folks saying it runs all the time by design (it doesn’t – it just takes longer to hit your target temp), and so on. I’ve not come across cycling on/ off before – nobody mentioned it as potential concern, but that may just mean they don’t know enough about it.

At least in the water heater market, I’ve encountered customers who are disappointed with their HP. A lot comes down to how the product was sold to them. It seems that the installers either haven’t given the customers the correct info or don’t know it themselves. And that rebates that were available stopped at a specific HP size. The point I’m making for the OP is that sizing is important and confirm that the company you use knows how to spec the unit.

what’s the floor area of your house?

151 sqm

but it’s possible if you roll in HW you may need that large?

Yes, needs to provide HW

“How ‘big’ is the current gas boiler?
It will have a rating, which you can convert to kW, and then compare that with the proposed ASHP.”

Actually, it wont, unless the gas boiler was sized for the property, which is very unlikely. Gas boilers have been oversized for 40+ years, as the lazy git fitters dont bother doing the heat loss calculations, and just fit the same size as every other house, which is oversized.
The OP is right, he needs a HP that is within 10% of the house heat loss. I’m surprised by their statement that added loft insulation will lose 4.5kW of HP. There’s no way even an uninsulated house will lose 4.5kW through the attic, and being as it already has 300mm of rockwool,I’d find it near impossible to insulate more to reduce the heat loss through the roof by anything above 1kW. I’d be very careful about anything else they say, and if me, I’d be asking someone else to do an heat loss calc, as it seems they are making it up with some starnge figures.

OP. £15000 is a bit more than I’d expect, but I dont know what the job entails. Just the parts will be £8000+. I’d be expecting them to replace the bulk of the piping too, if its from the 70’s with a cast iron heat exchanger, the pipes will be full of crud, and HPs really do not like mucky system water.Power flushing cleans some of it out, but its never perfect.
As my previous post, make sure the heat loss survey is correct. It can make a big difference in your electric costs in the future. Be aware of Octopus too. Rumour is (anecdotal, though quite reliable usually) they design their systems to run at 45 degrees C. This is a poor design, as it’ll cost you 25% more electric to run it at 45 compared to 35 deg C.
you ar eright thta an oversized HP will not run efficiently, you need it to run at a constant low level, with as few starts/stops as possible. Good HPs can modulate down their output, but a 12kW HP isnt going to modualte down enough if its 15 deg outside, it’ll be on off at least twice as many times as a 8kW unit. (Starts are quite energy sapping, if seeing an electric reading, there is a spike when they start up, so use mor eelctric than one that trickles along at a low setting).
If you want a quick heat loss survey, send me over all the dimensions and insulations materials/thickness etc.

If you have the time/information about your house play around for yourself on here and see what you come out with:

https://heatpunk.co.uk/home

I paid £22,500 for NIBE Air Source for a 5 bed, 3 bathroom 220 m2 self build. This included heat pump 12kw, wet underfloor on ground floor, rads and plumbing / waste for all bathrooms, kitchen and utility. I got a £5k government rebate. The house is B on the EPC due mainly due to a lot of glass.

Advise would be improve insulation wherever possible and oversize rads and pump plus keep the pipe run from pump to tank as short as possible. The gotcha with Air Source can be when the temp goes well below zero it works hard to get the heat and so kicks in the heating element. When this happens the run costs go north very quickly. No gas and total electricity around 7500kwh PA. House is toasty at 22C in deepest winter.

Whats the current EPC rating of the house?

If its a 1970’s build its not going to be great and unless its super insulated, it might not produce the heat you want. ASHPs work on a lower but more continual heat output, so you need to be minimising the heat loss.

The other thing to mention is that Elec is x3 the price of Gas, so your energy bill is going to increase.

Do you know your most recent annual gas bill? Someone has come up with a “rule of thumb” of dividing your annual gas use in kWh by 2900 to come up with a heat pump size in kw. I don’t understand it but it’s based on “degree days” and seems to check out fairly well from what I’ve read.

Ours comes out at just over 4kw that way, which was lower than I would have assumed, but having since had some A2A heat pump / AC units installed and measuring their power input it feels pretty close.

It might be nonsense but it’s quick to check.

Another Heat Pump Spreadsheet: Beyond the Rule of Thumb

“If its a 1970’s build its not going to be great and unless its super insulated, it might not produce the heat you want. ASHPs work on a lower but more continual heat output, so you need to be minimising the heat loss.
The other thing to mention is that Elec is x3 the price of Gas, so your energy bill is going to increase.”

Unfortunately, the statement above is what is usally said about heat pumps, but is total rubbish, and should be disregarded.
HPs are just another source of heat. If the house is uninsulated, then you need a bigger heat source, doesnt matter whether that is gas, electric or a HP.They run at a lower temperature to eb more efficient. A gas boiler if ran at lower temperatures is more efficient. It should be designed to run at these lower temperatures, and it will keep the house at 19-22 degrees at any time.
Electric is 3x the price of gas. Yes, it is, but a decent HP install will have a COP of over 3.5, so it will be cheaper to run that a gas boiler.

1: I know its impossible to estimate but does anyone have any experience of installing a heat pump of this size on a detached house (151sq m) I am wary that the pump may be a bit undersized and they are shoehorning it in as the larger pump is not available for several months.

I’m in no way qualified but I did happen to watch this last night which is a 12kW installation to a 300sqm property, which they were complaining about getting too hot.. not only that, they’re using very inefficient skirting board radiators.

https://youtu.be/U8tWxn9MBXA

but a decent HP install will have a COP of over 3.5, so it will be cheaper to run that a gas boiler.

In ideal conditions that may be the case, but that number drops dramatically in winter when you’re using the system the most. It won’t be 3.5 times the price of a gas system to run, but it might be twice the price in reality – additionally because the heat basically needs to be on 24×7 (rather than optimizing it for busy periods) you lose more heat through the walls compared to an optimized gas setup.

If you’re off (gas) grid or have a large amount of wasted/sold generated electricity it’s likely to be cheaper, otherwise no way

Lots of helpful information, thanks. I suspect some of it may help others as well.

Our house has an epc rating of C.

Another thing to mention although the boiler is ancient I installed a Tado unit some years ago and I have found setting it around 16-17C during the day and around 18C evenings the house feels generally quite comfortable. It really does seem to retain the warmth. During cold spells we increase a couple of degrees but how people have their homes above 20C is beyond me.

We are fortunate that our roof seems to be very well placed to generate solar and we do feed a lot in even after using a solar diverter to heat our water. As I mentioned above I am not looking for savings although I would hope the running costs would be similar. I also understand that they don’t run all the time and this is another myth.

I also understand that they don’t run all the time and this is another myth.

The heat circuits pumps 24×7 when the system drops into winter mode (based on outside temp and inside temp calculation).  Pump heats the water for the tank managing the heat exchange but only when flow temp drops below heat curve.

16-17c would lead to divorce.

Don’t installers look at more than just EPC? There a better way of working out how well insulated the house is? That is the key.

I’d suggest my former employers’ service, but it’s Edinburgh only for the moment (later Scotland).

EcoCosi Home Retrofit Service

Don’t installers look at more than just EPC?

The installer does appear to have a very structured process, there is no indication that they rely on the EPC, I posted that in response to a question on the thread.

IMO a 1970 construction house with 129m2, will be too close for comfort on 8kw unless its been well retrofitted for energy efficiency.

For comparison, I have a 1980, 121m2 detached house with a gas boiler that can modulate almost as well as a heat pump. Its flow temp is 47° so is pretty close to a heat pump too, and it runs comfortably on 10kw peak (will go to 15kw if required). I have modernish argon filled DG from 2013, 300mm glass fibre in the roof and 85mm graphite poly beads in the cavity.

Do you know your most recent annual gas bill? Someone has come up with a “rule of thumb” of dividing your annual gas use in kWh by 2900 to come up with a heat pump size in kw. I don’t understand it but it’s based on “degree days” and seems to check out fairly well from what I’ve read

Interesting. Our Gas use last year was 16184kW. Using that calculation suggests a HP size of 5.58kW. Which indicates that an 8kW unit may be a good fit.

Likewise finding that interesting, our gas usage over 12 months was approx 12991 Kwh (404 cubic meters), which works out at a HP size of 4.47kW, so the 9 kW unit specced has plenty of capacity.

For comparison, I have a 1980, 121m2 detached house with a gas boiler that can modulate almost as well as a heat pump. Its flow temp is 47° so is pretty close to a heat pump too, and it runs comfortably on 10kw peak (will go to 15kw if required)

I may be wrong but I didn’t think it was possible to compare efficiencies of GB and HP in this way. They claim to be 3-4 times more efficient.

We have owned 2 houses with ASHP’s. The first was a drafty barn conversion on bottled gas & a combi boiler (8.5kW), we built our current one to be all electric with Pv, an ASHP & a battery (4.5kW), it is designed to be super efficient – the EPC is 111.

The two key things to bear in mind are:

that a heat pumps peak power output is relatively low vs a conventional boiler so temperature changes occur more slowly.

the greater the temperature gradient you want to create the less efficient it becomes.

the knock on effects of these are that you will need larger emitters & hot water storage than with a conventional boiler & warming the house takes longer. AIUI every thing else comes down to designing/sizing the system well & cost.

Don’t ignore any additional heat sources you might have. In the barn we were at the upper limit of the heat loss calcs for the 8.5kW unit & installed a woodburner to compliment it for when the cold weather came in. We were fine when the ‘beast from the east’ hit.

If space for DHW storage is at a premium or you need quicker response times there are higher temperature ASHP’s which are reasonably efficient up to 70°C. We have a Vaillant aerotherm plus.

On the down side there is a real shortage of competent engineers to design, install & service them (at least in Cornwall) & parts can be hard to get hold of/ heinously expensive.

On the subject of heat sources, don’t discount passive stuff – climate, orientation, surrounding geography, construction etc. Comparing two houses that are completely different is a fools errand.

I may be wrong but I didn’t think it was possible to compare efficiencies of GB and HP in this way. They claim to be 3-4 times more efficient

I’ve not mentioned efficiency. Kw are Kw irrespective of source. Its a measure of power, or ability to do work, not efficiency.

COP doesn’t crash dramatically in the temperatures of an English winter. Here is the graph for our HP and it has been no problem at all in a  relatively hilly and exposed location. On the Wirral the OP will be fine. Similarly no problems with icing up / excessive defrosts in our “damp cold” winters. It just works, and if we had mains gas I wouldn’t be going back to a regular boiler.

Adding extra insulation in an already well insulated attic seems like the easy suggestion to try and fiddle the calcs / clutching at straws  by the installer. What about spending time and money in other areas where you might get some real benefit like floors, windows, detailling around window openings?

I’ve not mentioned efficiency. Kw are Kw irrespective of source. Its a measure of power, or ability to do work, not efficiency

But if something provides more work for less kW then doesn’t that make it more efficient?