Jump to content

Does Geothermal heating really work?


Recommended Posts

[quote user="bevvy"]

What I really need to know is whether the system will really keep the house warm without the need for back-up heating.  We are going to be using underfloor heating both downstairs and up and will probably have an open fire in the lounge during very cold spells.  I would hate to spend all this money and find that we are having to supplement it as obviously I am hoping to recoup some of this initial large outlay!



Our neighbour had geothermal heating installed in a new build about 5 years ago.  We were very sceptical that it would work, and frankly couldn't believe it.  I don't know the ins and outs of the finances but I know it was expensive to install but he expects to stay in the house for the rest of his life (he's about mid 40s) and easily get his money back.  He says it is cheap to run as expected from the installer's sales promotion.   His house has provision for a fireplace or woodburner which has never been used so I guess that answers the question about back up heating.  It is always pleasantly warm and I would certainly go for it in a new build.


Link to comment
Share on other sites

I have an old french farmhouse in the Vendee which is not well insulated (the previous owner did the job, and over the next few years I will revisit his work).

We have just taken out his old boiler (oil) and installed a heat pump, using the existing radiators (ie pumping 35 degree water rounmd a system designed for 60 degree water); this uses 800 metres of pipes buried 80 cm down over a 400 sq metre area.

Running this in its basic form costs about 3 euros a day for the electricity and keeps the house about 17 degrees (lower in the kichen which is on the end) which we boost with a woodburner in the sitting room.

The system also has an electric 'booster' which cuts in when the temperature outside goes below 5 degrees - this costs another 2 euros a day to run.

If we were to enlarge the size of the existing radiators and improve the insulation (cut the draughts!) I think it would be pretty good.

We hope to get 40% of the heatpump costs back this year in a tax credit.

I'd say for a new well insulated house, with underfloor heating designed for the heat pump you'd be onto a winner

Link to comment
Share on other sites

Hello Paul

We must stop meeting like this ..... the air to air system is great in a medium / small property ........ without going into too much detail / depth each room has its own system one indoor unit, one outdoor unit ( no trunking) so you could start with just one room or a multi unit ...... each room has indoor unit with only one large out door unit feeding up to 6 indoor units & each indoor unit has its own infra red remote control ....... normally the units work in heating or cooling mode (you cannot have heating in one room and cooling in another) there are a couple of systems that will do both but they are a lot more expensive .......... I know you can pull holes in the idea ..... its NOT perfect ...... it gives you heating and cooling ...... and depending on where you are, will not suit everyone ...... also if you do not need it in a certain room you can turn it off in that room !!!!! ....... looking at some of the claims / figures and knowing what I know and putting into plain English ........ in heating mode for every euro used you get on average 4 / 5 euros of heat, depending on outdoor temperatures .......one could argue for years over what is the best system in the same why that you could over wine and I am not saying this is the best system ...... its a tried and trusted system at a reasonable price

Link to comment
Share on other sites


Your post about air to air heat pump units was very very interesting,

are they about the size of a 'window unit' aircon pack ?. What sort of

prices are we talking and is noise a possible problem.

I am looking for heating ideas to supplement the main woodburner in the

sitting room and can't go down the underfloor route ( wet or dry ) and

the thought of using LPG (expensive) or oil (not going to be cheap in

future) does not appeal.


Link to comment
Share on other sites

Hi Powerdesal

The indoor wall mounted units are approx 1 meter x 260mm depending on make and output.

Prices will depend on size of unit / units, length of run (from indoor unit to outdoor unit)..... as an approx  figure I would say € 1200 + tva,  Fully installed and commissioned for a single unit. I have seen some horrendous prices, also buy this unit for ONLY €1600 and we will install for FREE

Today's split units are very quiet,  window units in the trade are called "window rattlers" and they do especially if they are installed badly.

Hope this helps, if you need any more info just ask or PM me


Bonne Chance

Link to comment
Share on other sites

<bejay wrote:


"At 100m I think its earning the right to be called geothermal energy. I presume there wil be a constant temperature at this depth. As a matter of interest and to save me from doing the work and finding out for myself, do you know what temperatures are found at 100m"


I was rather hoping some geologist was going to help us all out here, but no answer as far as I can see.  So here some facts I have gleaned and some personalobservations which may or may not be relevant.


The deepest mine in the world near Jo'burg is 3600m deep and is 28°C.  The deepest mine in the UK (Boulby potash Mine in N Yorkshire) is 1500m deep and is 104°F = 40°C.  So it seems there is no direct correlation between depth and temperature and the result probably depends more on local geological conditions.


I have for about 30 years been a more or less active potholer.  Within about 100ft of the start of the dark zone (verticle or horizontal) , the temperature in the cave remains at a constant temperature pretty much throughout the year under normal conditions.*


*Normal conditions means no strong air currents (usually caused by fast flowing water or waterfalls), no snow and ice in the entrance and no large amounts of snowmelt flowing into the cave.


This temperature remains the same year in and throughoutthe cave irrespective of distance travelled from the entrance or depth below the surface.  This implies that once out the the surface influenced zone there is no significant change in temperature with depth over the range we are looing at - up to 1100m deep from my observations and measurements.


However there are different temperatures depending where you are.  Moving South:

In N Yorkshire the ambient cave temperature is around 8°C

In the Mendip (Sommerset)  10°C

In the southern Ardèche 13°C

and In Majorca (Caves of Drach) 15°C


This implies that the temperature increases with mean annual temperature and suggests that the rock around the caves is still effectively being solar heated -  even to a depth of 1100m.


Can anyone pick any bones out of that?



Link to comment
Share on other sites


Nice to see this thread come to the surface again, if you will excuse the pun.

Also nice to see some numbers. My knowledge extended to the point where I was aware that it is "surprisingly warm down a coalmine" and crucially that this temperature is constant . This is what makes it such an interesting form of alternative energy for heating purposes (and for that matter, cooling ).

My private grumble is the use of the word 'geothermal' for some plastic pipes buried two or three feet below ground level when the temperature of the ground will change seasonally and also of course be at its coldest when you most need heat from it. So what name do you give to true geothermal energy?  ( I do understand that not everybody feels the need to worry about such details, this fact has on occasion  been pointed out to me by close members of my family).

We still don't know what temperature might be found down a 100m borehole but it seems likely to be a bit more unpredictable than I imagined particularly if water tables are involved. Clearly its complicated down there.

In reference to caves becoming warmer further south, this may occur because the surface temperatures will on average be higher and this will slow down the rate of heat transfer to the surface rather than any direct solar heating effect.




Link to comment
Share on other sites

  • 2 weeks later...

I make absolutely no claims to expertise, but I've been reading up on French heating practice as background knowledge for when we build our planned house.


In a magazine (I think it's an hors serie of 'Bains & Cuisines' (or summat, it's at home) dealing specifically with all sorts of heating and boiler systems) there is a section about 'PAC eau-eau' which appears to use heat extracted from an artesian well.They even say that it can also provide an uinmetered water supply. With your water table this could be your solution.


I'll try to track down the article this evening.

Link to comment
Share on other sites

I asked the experts who visited (last year to investigate

renewable energy possibilities for my mill) about extractive heat from the

river.  My (“O-level physics”) theory was

that whist the water might be cold, it was moving.  In the ground the heat transfer pipes must set-up a heat gradient

which would limit the effectiveness. 

However, in a river, although the water might be colder there would be

no heat gradient and thus the system may work (well ?).  They said that it can be done provided the

water was more than 1m deep (though I did not go into why as my millpond is

only 1m at the deepest point).


Link to comment
Share on other sites

BJ wrote

"In reference to caves becoming warmer further south, this may occur because the surface temperatures will on average be higher and this will slow down the rate of heat transfer to the surface rather than any direct solar heating effect."


Sorry for a late reply on this.  BJ, I don't think this can be the case because you would expect the cave temperatures to vary throughout the year as surface tempertures varied - and this is not the case.  At the moment I can't quite square the circle that the year round constant temperature implies that the heat is geothermal, but the latitude temperture gradient suggests it is solar.  If thermal gradient were such a strong influence you would expect the temperatures to increase with depth - and they don't.


Ian, I suspect 1 m depth is required because of a: droughts in summer and b: freezing in winter - both of which would significantly reduce the effectiveness of any heat transfer.  Water being a good conductor of heat means, I think, that whether it flows or not is of less importance than the total heat sink available to be tapped.  So a big lake would be just as good as a flowing stream.

Link to comment
Share on other sites

Deimos wrote 'I asked the experts who visited (last year to investigate renewable energy possibilities for my mill) about extractive heat from the river.'

The system I'm reading about actually works with underground water in les nappes phreatiques. Apparently you need two boreholes and pump water up one and back down the other. Apparently it can achieve a COP of 5, which is pretty good, because the underground water averages about 10 degrees all year round.

Link to comment
Share on other sites

Andy. Hi there.

In the meantime I have found out a little more detail.  I was quite surprised to discover that ground temperature is really quite  constant throught the year from depths as little as 2 m down to about 100 metres giving a range from about 10C- 13C. These temperatures are described as a solar heat store. These are UK figures and it seems reasonable to suppose that they will be higher in more southerly latitudes. I think I am inclined to stick to my original viewpoint.  However I am really talking about borehole conditions and you are referring to cave systems which are bound to be more complex.. It may be that air circulation will bring a cave system to an equilibrium temperature even if this is not immediately apparent. We are after all talking about a quite narrow temperature range.

It seems clear why some mines are hotter than might be expected  They lie in areas of 'hot rock', rock which is highly  fractured which allows the circulation of  of water to very great depths (1000's of metres). True industrial scale geothermal energy plants are attempting to reach these sources.

Interesting stuff.


Link to comment
Share on other sites

".... ground temperature is really quite constant throught the year from depths as little as 2 m down...."

So would I be right in thinking that the people who tell you that the ground temperature is both stable and useable from 60cms down are stretching the truth a bit ?

I'm begining to think it's a minefield..

I recently attended a sales pitch by a rep from a comany selling air-to-air systems who claimed a 4:1 coef for his system (with no mention of the effect of changes in external air temperature) then later admitted that from 0c one needed an auxilliary source of heat!


Link to comment
Share on other sites

It seems te me that heat pump technology only just works in financial terms. In fact in the UK mains gas central heating is cheaper to install and to run  than a electrical heat pump system.which makes it a bit of a non-starter for most people. But, of course, the situation is different in France. Consequently there are compomises all down the line. However I am surprised  just how quickly temperatures smooth out at quite shallow depths, 60cm does seem a bit shallow but at depths of about 80cm to 1m  companies do seem to become quite confident.of its effectiveness.

It is perfectly possible technically to install a totally renewable energy source heating system using a mixture of photovoltaic solar panels small scale wind turbines heatpumps batteries etc but at present the cost is astronomic. My rough calculations seem to suggest a figure of about £30,000 as a minimum.

I can't raise much enthusiasm for air to air systems. They don't call it air-con for nothing!



Link to comment
Share on other sites

We had an air/air salesman visit.  He told us that as there was thermal energy in the air down to absolute zero, the system would have no trouble working well at minus 10, which was 263 degrees C warmer.  I frankly didn't believe him - not the physics, but the claims of effectiveness of the system in cold weather.
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...