The owner of the office space I rent had a huge industrial air/air heat pump installed outside (the size of a small electric substation transformer). It regularly breaks down when the temperature drops below -7°C, which is a frequent occurrence in the Massif Central. Additionally, it is reversible, so it is used as an air conditioning unit in the summer. Again, above 32°C (also fairly frequent), the unit stops being functional and is incapable of supplying air cool enough to keep the offices at a temperature anywhere half-decent. This shining example has convinced me of all the reasons why investing in a similar system for my domestic heating requirements is a huge no-no - Expense. Inefficiency. Size. Noise. Enough said. Some might argue that unsightliness is also an issue, but the same could easily be said for a fuel oil or gas tank !
Ours has been OK. Of the two problems we had the first was definitely user error (I set the flow temp too high) and the second was kind-of user error; I naïvely thought “standby/frost protect” was somewhat cleverer than it actually is and, with the system off the heat store temperature dropped too low and it sulked. Fortunately in the interim I got the full documentation and was able to get into the service menu to clear the error and the system survived despite no flow at all and outside temps of -10.
I now leave the system on through January and into February with the room thermostat (now that I installed one) set low - 12/13°C and plan on getting remote control over t’ internet although I need to cook up some DIY electronics to do so and I haven’t had time yet. The system is “old enough” (sadly this means installed in 2008) that Stiebel don’t sell the IR based interface any more.
So not all systems are unreliable.
Still possible Stella, but much more expensive as the bores have to be core-drilled through the rock. The deeper the bore, the more efficient the heating system - hence why we bore down circa 30 metres on our developments.
Air source heat pumps are efficient - if they are properly specified - but the disadvantages are that they take up room in the garden, are noisy when working hard and can require tanks and pumps in a dedicated plant room (much like ground source systems) which all works out quite expensive.
One of my neighbour’s is a builder. He has done a cracking restoration on his house and heats it by warm air drawn up through holes bored down into the granite that surrounds us here, just saw the word elsewhere …geothermique!
No idea of what pumps etc; he uses but there are no large installations outside.
Ha ha… 30 metres… that is a definite no-no…
I’ll wait and see about wood-pellet burners…when the time comes.
Why so?
Paul… the rock we are on does not lend itself to boring… or anything else much… there is a shallow earth covering which allows some flowers and grass, but other than that… “solid as a rock”…
Maybe there are specialist tools to do the job… but, so far, any work which involves going down… has been swiftly changed to… going- along-the ground . After a number of expensive false-starts over the years… the Community of Communes is not prepared to foot the bill for any procedures which involve trying to get into the rock…
and if they cannot afford it, neither can I… 
Something nice and hard then like a granite or basalt. I’m not aware of any rock which is so hard it cannot have a hole drilled in it but, as you say, the price probably goes up.
The only way to be certain would be to discuss with a boring company with experience in the local area.
I’m not sure I would discount it at this stage though, if you are serious at renewable energy heating.
That’s about the only point you can’t say. 
I’m looking at eventually replacing a boiler which currently does radiators and hot water.
Probably take on a wood-pellet boiler (in some form) for the rads and then a water cylinder with immersion heater…
If a super-deal on solar panels for water heating… comes along, I will possibly jump at it, as a back up…
Ann, I would be interested to know whether radon leakage is an issue with your neighbour’s installationn, or a potential issue, in such a scenario. Like Stella, our house sits on a granite outcrop covered with a layer of what I would loosely describe as accumulated earth from precipitation and humus, rather than soil proper. Apart from the expense of coring through granite, which we already know would be a non-starter, I feel I would rather let nature take its course and have the radon vent naturally rather than inviting it directly into, beneath or around my home. Of course, that might not be an issue where you live. I’ve always been amazed at how glib the French authorities appear to be when it comes to radon build-up in this area, as the last study I can find dates back to 2007.
I suppose you could claim a heat pump which isn’t working has zero efficiency 
Wood pellet boilers have com a long way Stella, i’m seriously considering it. Nowadays the pellets can be delivered in bulk into large hoopers/silos which can last months at a time.
In another commune… I’ve been on a tour of a superb project which heated the Mairie, the Post Office, the Old Folks Home and a couple of Council Houses…and yes, the silo was huge, but horizontal rather than vertical…so it was not “in your face” but blended in with the surroundings…just another attractive building…
Of course, not every situation is as ideal as that… but, it gives me food for thought.
Sounds good Stella. My sous-sol is quite extensive so installing a silo wouldn’t be a problem. I’m now awaiting a devis from a local company. The guy checked the size of my radiators to find a boiler which could do the job.
I’ll be following your progress… with great interest… 
In our local village/nearly a town, there is a communal heating system that uses the shredded waste from all the forestry harvesting.
biomass boiler?
That’s the thing, couldn’t remember the word…