We had an installation done last year and I attach a quote which we received for a 15kW installation with boreholes in deepest, darkest Cornwall.  In the end, we went with a different solution with trenches not boreholes.  This reduced the price considerably - the trenches cost £4k not £10k.  This was all done before the Cost of Living Crisis, so I suspect that prices will have increased quite a bit.

This was in the good old days of the Renewable Heat Incentive which made the economics of the project very attractive.  But it did force us to install at least 15kW of capacity - way more than we need.  We installed Kensa kit and their 15kW heat pump doesn't heat DHW to 60C so we have two 9kW pumps, but we could have got by fine with just the one.

So, a few thoughts:

1. Do you really need a 40kW pump?  (If so, you must have a very badly insulated mansion.) If you don't need that amount, it would save you money on the pump but also on the boreholes because the rule of thumb is 10m of borehole for each kW of capacity.
2. Even if you need the full 40kW, £45k for the boreholes does seem a lot.  Is there no way you can go trenches?
3. The cost of the heat pump if anything seems low.
4. Two heat pump setup has other advantages other than getting them through a gap.  If one of them failed, we would still have CH.  We also have dirt cheap electricity for 5 hours at night so we run both as hard as possible through the radiators for that 5 hours, with one of them taking time off CH to fill our HW tank. If we only had one pump, we wouldn't be able to get so much heat into the house.

@iainw for those prices I would just hire (or buy a used) digger and do the job myself. You can always sell it at the end if you buy it. The majority of that cost will be in the hole (I imagine).

The cost for the heat pumps and installation are also, frankly insane. That said, your heating demand is 60MWh per year? Is that pre heat pump at a COP of 5? meaning you would need 12MWh? What the hell are they installing and how that they could possibly demand nearly 100k. Why do you need a 40kw pump?! Mine is 12kw and it uses around 7.3MW per year. 

I would suggest move to trenches and do it yourself, or hire in groundworks for 2 days. Move the location of the pump from a cellar (its adding cost) and finally check and do your own heat loss calcs to determine what you need. There is one in my signature which will get you 90% of the way there.

Posted by: @windyinsancreed

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you must have a very badly insulated mansion.

Yup... about 330 sqm of 1830 "maison de maître". We can't do anything about the windows without significantly altering the look of the house, although better roof insulation is certainly on the cards.

But thanks for the advice, both of you, our heating engineer estimated our heating needs at 59.2 MWH, plus a bit for hot water. This corresponds quite nicely with the estimate provided in the independent energy rating when we bought the house (last month), of 61MWh, so I guess it's correct. I'm afraid a lot of the technical stuff is beyond me, but our quote was very detailed, two figures for COP (whatever that may be...), see below, and they also did a room-by-room calculation akin to the link in your sig batalto.

But what I don't understand is

1. the extortionate borehole price - £10k seems much more like it.

2. The cost of installing the pump itself, which is more than double the cost of the pump (€26k to install a single, €19k, pump, for a total of €45k).

Happy to post the quotes, but they're in French.

I may well go for the horizontal, it rarely drops below freezing where we are (near Bordeaux) and we have plenty of space. However, one reason in favour of the vertical is that we have a well and could also use the water. But perhaps not at that price...

I've posted in a French forum, too, but no responses yet.

@iainw 

The borehole price is high, but not that extortionate.  I was quoted £10k for a £15kW pump so £27k would be the equivalent amount for your installation size.  But it is way, way more than doing trenches.  As @batalto said, if you have the space I would be very tempted to go with trenches.  The place will look like the Somme for a couple of weeks, but it will grow back.

We are maybe 500 miles north of you.  We are in the Cornish Riviera(!), so it very rarely freezes here either.  However there was no apparent reduction in performance of the system over the winter, so I am sure that trenches would be safe where you are. It will be far cheaper to drill for water and use trenches for the heat pump than to drill for both.

More than happy to look at the quote.

The two COPs are for two different water temperatures.  The hotter the water you want from the system, the less efficient the pumps are.

@iainw

COP stands for Coefficient of Performance, or efficiency in other worlds. Heat pumps absorb energy from the air or from the ground, or even from water, but require electrical power to operate. COP is a measure of how much heat energy comes out for how much electrical energy is put in.

In your quotation it is stating that you should expect a COP of 5.47 when the ambient air temperature is 7C and the temperature of the warm water coming out of the heat pump is 35C. Therefore the heat pump would consume 1kWh of electrical energy for every 5.47kWh of heat energy that it gives out.

The lower figure giving of predicted COP of 3.57, is at the same ambient air temperature of 7C, but with the water coming out of the heat pump at 55C.

The Leaving Water Temperature (LWT) from the heat pump varies to meet the heat demand, which of course is due to the heat loss, with that being dependent upon the temperature difference between the indoor and outdoor. To maximise efficiency, and minimise running cost, it important to reduce heat loss as much as possible with insulation. Having larger heat emitters (radiators or under floor heating) can also lower the required LWT and improve efficiency.

What are the limitations on making changes? Is this just to the outer fabric of the building or does it also apply to the inside?

Thanks again for the advice both, it seems like trenches are the way to go. As for the insulation and so on, the radiators are pretty good, I think, they're the old fashioned cast iron ones with (if my memory serves me correctly) five element (if that's the correct term) and close to a metre long. The windows are nice old blown glass french style so we wouldn't want to change them, although on the first floor we have double windows, which helps a bit I suppose. Not sure what we could do to insulate inside, as I say the roof is the best candidate, although according to the energy report the greatest losses are (about 25% each) walls, window and "ventilation", whatever that means. I think this is just what you get with an old house. The walls are solid stone, we'd be unlikely to be allowed to do anything outside, and probably wouldn't want to.

Posted by: @iainw

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Thanks again for the advice both, it seems like trenches are the way to go. As for the insulation and so on, the radiators are pretty good, I think, they're the old fashioned cast iron ones with (if my memory serves me correctly) five element (if that's the correct term) and close to a metre long. The windows are nice old blown glass french style so we wouldn't want to change them, although on the first floor we have double windows, which helps a bit I suppose. Not sure what we could do to insulate inside, as I say the roof is the best candidate, although according to the energy report the greatest losses are (about 25% each) walls, window and "ventilation", whatever that means. I think this is just what you get with an old house. The walls are solid stone, we'd be unlikely to be allowed to do anything outside, and probably wouldn't want to.

I don't think the radiators will have been installed when the house was built. 🙄 Give one of the radiators a close inspection and see if there is a manufacturer's name visible. If there is then see if they have a website, if not then look for similar designed radiators. If you can find some information then look for the specification details and find the standard output rating, it will be something like 2035W. Add up the values for all the radiators in your home, which should hopefully come to some figure in excess of the 40,000W rating of the proposed GSHP. The larger the radiator heating capacity the better, since the heat pump will be able to operate at lower water temperatures, which is more efficient.

Did you say your present heating system is an oil boiler? If so, try turning down the water temperature to say 50C, and see if all the radiators get warm and if your home reaches the desired temperature. If not then a heat pump would be very expensive to run without replacing the present radiators with larger ones.

Adding insulation to the outside of the building is best, since the building fabric can act as a thermal mass and help regulate the indoor temperature. If external insulation is not possible, you could consider internal wall insulation. Whilst there will be a price to pay, it may reduce the size of heat pump required and hence the overall cost. It also reduces energy demand and will pay dividends long into the future.

As a trial you could try installing some secondary glazing on the inside of the windows in some of the rooms. Stormguard produce a 'secondary glazing film' that is quite cheap and fairly easy to install. This should not only reduce drafts, but also act as a cheap type of double glazing. It is cheap enough to fit during the Winter and then remove when Spring arrives. If it proves successful you could then consider having some more substantial secondary glazing made from Perspex sheet, that again can be installed during the Winter period and then removed but then stored during the warmer months, for use again the following Winter.

No, not 1830 radiators... 😀  But they were already there when the previous owners bought the house in 1958, so old enough. As for the rest, I'll be home for the weekend and will check it all out then. And we have an appointment with an energy advisor, govt funded and free (the French are good at these things) on Friday, so we'll see what he says, too.

@iainw 

A few more thoughts:

Just going back to the size of heat pump needed.  As I say, our heating report said we needed 15kW.  But that was assuming that every room in the house is heated on the freezing day.  Neither assumption is realistic as we don't get many guests during winter and it almost never freezes here.  If some rooms of your mansion won't be used in winter, why over-spec the pump?  We could easily have got by with a 9kW pump.

We also have cast iron radiators (1936 vintage) and we were warned that they wouldn't be enough at the lower water temperature supplied by the heat pump.  They are ample at 45C.  (The only rooms where we added/upgraded the radiators were where the radiators were already struggling to heat the room using an oil boiler.)  Follow @IainW's excellent suggestion.  Try it on cold days/nights and play around with the water temperature and see when the radiators start to struggle to heat the room.  But remember, heat pump CH is meant to be run basically 24 hours a day, not blasted on to raise the temperature quickly.

That said.....

Are there cheap overnight tariffs available to you?  We have Octopus Go Faster, which gives us a tariff of 8.25p/kWh for 5 hours overnight, 41p/kWh the rest of the time.  As you would expect, we move as much of our grid demand into these 5 hours as we can - over 80% of our total grid usage is during these hours over the course of the year.  Both heat pumps are blasting out during the night.  So we wake up to a very toasty house.  The heat slowly dissipates during the day but usually takes us through to dusk.  By this method, our total energy usage (CH, DHW, cooking, lighting, 6,000 miles in the car) of c 8 mWh will cost less than £1,500 pa.
On the subject of insulation:

1. We have suspended exposed wooden floors on the ground floor and when I went down there I could not believe how cold it was - and that was in September.  We insulated the underside of the floors and it made a huge difference.
2. Where we live, there is a strong wind off the Atlantic which normally comes basically from the West.  During winter, we close translucent blinds on the windows facing NW which has also made a major difference.
3. "Ventilation" might include the draught around exterior doors.  A curtain placed in front of these will help with this.  Get a Switchbot Curtain, and you can even get it to close behind you.

Indeed, and it's not very cold at the moment so most of the radiators are on pretty low - we do need a minimum flow through to keep the whole system working, but as you say most of the rooms aren't in use most of the time. But playing around with the temperature is a good idea, see just what the system needs, I'll do that this weekend.

As for the tariffs, we're also looking at solar panels to help power this. I reckon we can get a 24kW array on the roof.

And floors: I'll go down and have a look.