Since nobody more knowledgeable has replied so far, I will give it a shot!

Several things come to mind, in no particular order and a bit random until I know more about your thinking and system:

• Have you tried to get the compressor replaced.  Replacing compressors in heat pumps is quite normal and its most unlikely that they used a 'special' so it may be available still if you have the compressor model number.  Dont assume you need a GSHP specialist, the guy who installed my heat pump (very well) also installs boilers and repairs GSHPs, the components are the same once you get past the ground loop heat exchanger.  Maybe @editor can suggest someone in your area.
• Not sure why you want zone valves galore and a big buffer tank, would you consider simplifying.  Open loop, 24x7 operation on weather compensation  is almost always the way to go for max comfort and min running cost.  If you havent tried this you should, its a revelation in comfort.
• GSHPs are certainly rarer than ASHPs because of the need for a ground loop, but several manufacturers do them so I cant quite see why you couldnt get a replacement,  
• Now would be a good time to check the consumption records to find out if you really need 11kW.  Im guessing your barn was converted to high standards of insulation and, unless its very large (which of course it may be) 11kW is a lot.
• A modern GSHP or ASHP will come with much better controls, I dont know if yours has weather compensation but thats a bit of a game changer in terms of comfort.
• I wouldnt assume that oil prices will drop back, they have actually been historically low in recent years.  Anyway it would be a great shame to go backwards in this way.
• If you do need to replace its surely a simple swap of the heat source, you already have the heating infrastructure (albeit that you might want to simplify how you use it).  I would have thought its max a 1-2 day job tops provided its thought through.  You could even do it yourself if you were so inclined.

If I were in your position I would be

1. Checking fully that repair wasn't an option

2. Checking the consumption records to determine the actual loss

3. Actively considering drastically simplifying the system/the way the system is operated and removing the buffer (subject to a more detailed assessment of the hydraulics)

4. Considering both GSHP and ASHP options, discounting Oil

I might even think about DiY!

Just first thoughts.

@taffonthetaff - I agree with James.

I'd prefer to stick with the GSHP, and repair or replace the compressor. It will almost certainly be a generic unit, bought in from a factory in the Far East.

Have a look at this magazine article about the different compressors used in GSHPs.
Do you know which type yours is?

Can you see any model markings on the compressor itself?
If so, then just enter it into Google and see what turns up.

Although there are professional companies doing HP repairs in Carmarthenshire, you might just as easily come across a family business on Ebay who import HP components from China or Indonesia. The tricky bit might be to employ an F-gas registered operator to attend on site and handle the refrigerant exchange part of the process. Do you know what refrigerant is in the duff unit?

@taffonthetaff, I wouldn’t be rushing to abandon that ground loop.

What you’ve got is the hardest, most expensive part of a GSHP system already paid for and, by the sound of it, performing OK. A consistent 4C delta across the loop suggests it’s doing exactly what it should... steady heat extraction, no obvious degradation. The failure you’ve described is plant-side, not source-side.

The mistake I see people make at this point is treating the heat pump as a 'sealed appliance' rather than part of a system. In reality, what you’ve got is a failed compressor on an otherwise very valuable system. IMO, swapping to an ASHP means throwing away your best asset and starting again with a weather-dependent source.

Yes, ASHPs have improved massively, but GSHPs haven't become obsolete. It’s just been commercially sidelined because it’s harder to sell and install. From a physics point of view, a stable ground source still gives you better operating conditions than cold winter air, especially with UFH like yours.

The compatibility issue you’re running into is a bit overstated IMO. You don’t need a like-for-like replacement... you need a unit that can integrate hydraulically with what you’ve got. There are also loads of good new units out there and I can connect you with Stiebel Eltron who can probably advise on a replacement if you want to go down that route.

On oil, personally, I’d see that as a backward step unless this becomes a purely short-term stopgap. We've been forced back to oil this winter because of a failed heat pump and with no protection on kerosene the bills can be brutal. I think you’d be swapping a repairable system with a free heat source for ongoing fuel exposure and volatility. The maths might look tempting on paper, but it’s a different risk profile entirely.

If it were me, I’d be exploring:

• replacing the GSHP unit only, keeping the loop
• potentially simplifying the system design at the same time

The one thing I wouldn’t do is make a rushed switch to an ASHP or oil just because the original unit has failed. 

You’re actually sitting on a very good foundation... it just needs the right next move rather than a reset.

Thank you very much for the responses!

Regarding swapping out the compressor, there are signs of the GSHP not going to last too long. For example, a few years ago we installed a Magnetite filter which we cleaned out after a year. The next year I attempted to loosen the filter housing but another joint started to leak. Even gripping the pipe quite hard wouldn't let me undo the housing. There are also signs of corrosion in quite a few places. We are next to an estuary but I cannot see salt spray getting in the garage. 

I have been told that the refrigerant used is no long allowed! Even a quote we had for a replacement had a refrigerant (R407C) which is being phased out.

The refrigerant in the potential replacement ASHP is propane. Never thought that would be used as a refrigerant when it powers our cooker!

I think the old GSHP is now in a state of some of my old cars. Fix something and something else will go kaput soon.

My own gut feeling was just to replace the compressor originally but even when not kicking in additional heat that is a thirsty bit of kit. Some modern GSHP's have variable speed compressors which would be more energy efficient.

Regarding zone valves. I had an interesting debate with a decent heat pump engineer when he first came to look at it. He suggested taking the three valves out (one for each floor) and letting the heat pump sort it out with weather compensation. (By the way, originally it had a internal temp sensor fitted but that was disconnected after it was seen to be competing with room set-back thermostats!)

However, this is a sizeable house in a windy area and has no airlocks and the like. Also, the basement has a self-contained flat which we would not wish to heat if unused. 

The quote for a new GSHP is much more than the equivalent ASHP which I can understand. The COP is also much higher. I also understand how the ground temp will always be higher and more stable than the air temp in the winter. 

By the way, it makes me wonder about the IVT we installed in Bannau Brycheiniog (as The Brecon Beacons is now known). That is 16 years old too. 

We are currently supplementing the log burner with multiple electric heaters. An outside temp of 6c resulted in a bedroom temp of 12c this morning. It would be much, much colder in The Beacons!

We have discounted using oil as that would be a very backward step and dependent on various overseas factors!

Thanks again for the quick and informative replies. If anyone has any information on the Grant Aerona 290 ASHP, I would like to see it...
https://www.grantuk.com/products/air-source-heat-pumps/r290/

Bye for now,

Gerald.

Posted by: @taffonthetaff

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Regarding zone valves. I had an interesting debate with a decent heat pump engineer when he first came to look at it. He suggested taking the three valves out (one for each floor) and letting the heat pump sort it out with weather compensation. (By the way, originally it had a internal temp sensor fitted but that was disconnected after it was seen to be competing with room set-back thermostats!)

However, this is a sizeable house in a windy area and has no airlocks and the like. Also, the basement has a self-contained flat which we would not wish to heat if unused. 

The heat pump engineer was almost certainly right.  The key to operating heat pumps efficiently (ie cheaply) is to keep the flow temperature low, and you do that by keeping it on 24x7 and, unless the house is a funny shape or there is insulation between rooms that is as good as the insulation to the outside world, heating all of the house.  The savings in energy lost from the house by not heating part time and/or only heating part of the house are much smaller than most people think - houses continue to lose heat when the heating is switched off and that heat must be replaced when you switch it back on.   This saving is usually more than outweighed by the efficiency gains from operating at a lower temperature, and the house is more comfortable.  I strongly recommend you try it!

Several here have the Grant ASHP, but not me so I cant comment.

GSHP are quite similar to ASHP, differences being mostly the heat exchanger on the air or loop side, the fan, and software.

In fact GSHP should be cheaper due to being smaller and easier to ship, but due being low quantity "boutique" products they are ridiculously priced. 

My parent's house had a Leroy Somer GSHP from 1988 running on R22 and groundwater open loop. It lasted 45 years, with two compressor swaps and two coaxial heat exchangers, the latter because the groundwater here is a bit acidic so it corrodes copper heat exchangers. On this kind of machine, it was possible to replace pretty much any part and keep it running, because it had no electronics at all. Just a compressor, pressure regulator, two heat exchangers, a bunch of 1980's era contactors that still worked fine, and pipes. It could probably run for 100 years if spare parts and R22 were available, which was no longer the case as Leroy Somer went under and R22 was banned.

Modern heat pumps contain lots of electronics and plate heat exchangers which are both nice to have but much more vulnerable and hard or impossible to fix. In particular plate heat exchangers are quite vulnerable to corrosion so if you have magnetite and other crap in your heating circuit you should investigate that first.

These machines are "throwaway devices": they must be bought at a price such that they pay for themselves over their warranty period(5 years) compared to a cheaper to buy but more expensive to run heat generator like a boiler. After this, they run for as long as they run, then they die, you get another. Considering this, there are almost no circumstances where a 30k heat pump makes sense. I'm talking about the price of the machine. Installation and piping work is a different matter: the pipes last a lot longer.

On the other hand, R290 is propane. This has one drawback: it is explosive. This also has one massive advantage: the bureaucratic eggheads cannot ban it and cannot mandate F-gas certification to buy it. This means R290 machines are infinitely more repairable than R32 or R410, including black market repairs. You can also install a unit yourself without need for F-gas certification, because it does not contain any regulated high global warming gases.

You can also buy it used, or new old stock. When the Leroy Somer heat pump died, I bought the replacement heat pump used for about 300€ from a guy whose underground loops were leaking.

I spotted one Stiebel at half price, OK it's a bit far from where you live, but new in its cardboard box at half price, worth it to spend a bit of time looking for it.

If you don't want to pay a lot, there are some on aliexpress too, no warranty obviously...

Note if you install an inverter (variable speed) GSHP it can be a good idea to change the loop circulator to variable speed too, that depends how much power it draws, as it can be a significant part of the total when running at low power.

@bobflux 45 years! Bloody wow!

We thought that we were pioneering when we installed the GSHP in the Beacons. I can remember a discussion with a very scientific mate of mine, in the context that we lived about 28 years on the south coast of England...

Me: "But Simon, it is even cold in July here - usually a good 5c colder than Cardiff on any single day." (We did have the effect of altitude too - around 1000 ft.)

Simon: "Gerald, if it is warmer than zero Kelvin, there is heat in there."

Thank you so much for the detail in your response. At least the outside temp has increased this evening and the strong north-westerly has gone. One think I shall miss is the Home Assistant interface on the Raspberry Pi. They have their own publicly visible portal. Here is an example from a similar heat pump installed in Finland in 2008.I wonder how many compressors they have had.

And here is the HA screenshot of our knackered one...

I'm very much in agreement with @bobflux on this.

Whether you change the entire GSHP or just the compressor, you're still retaining the valuable parts of the installation. Those are the ground-loop and the in-home pipework.

I'd certainly be looking online for a cut-price offer online... or considering Alibaba. I have several years experience of using Alibaba/AliExpress and can offer more advice if you'd like. It's now common for Sellers on Alibaba to hold stocks in Europe. Poland or Turkey are favourites. They use spare capacity in containers to pare carriage costs to the bone.

That means you don't wait 6-weeks or so for an item to travel by ship. Instead it gets picked up by a national carrier such as DHL who use their own logistics and tracking to deliver it to you.

There are some guarantees inbuilt to the Alibaba purchase system. They hold your money in Escrow. It doesn't get released to the seller until you acknowledge that you've received what you paid for. For many products there's also a 30-day or 90-day Returns process if the item proves to be unsatisfactory.