yes possible but not easy -- a vertical bore hole will often stand up whereas a diagonal are not so stable-- boreholes can be drilled diagonally.

I would forget it under the glass room.

ppmc: If you have a dirt floor in your basement (it's not clear to me from your description) then dig a trench down the middle of your basement, about 3-4' wide, lay 3 or 4 runds of PE pipe (3/4" or 1"; the stuff sells for ~20c/ft here in Canada), cover it with concrete grout, and then lay 1" of polystyrene foam over it.

Posted By: pmccralph, that's sort of what I'm thinking of doing.

Unfortunately the house is 1974 timber-frame construction with concrete foundation and no basement. The under-floor space is on average 3 feet high but is purely for ventilation of floor joists. It looks quite challenging to dig a reasonable series of trenches in that, made worse by structural walls every couple of metres. AGS in its pure form as described by Don Stephens is pretty hard to retrofit on my type of house.

I had to dig a hole under my concrete footing to provide drainage during construction. If I had a source of pressurized water I think it would have been much easier. So I think it is feasible to dig holes big enough to slide a pipe through under your load-bearing walls.

-Ralph

Going external would require excavating around the house. I suspect it would cost about £75/hr for the machine and operator.

Posted By: Jeff Norton (NZ)What about looking at it differently and focusing on the insulated wing/umbrella fitted externally (fitted vertically or horizontally) to create the thermal store? This would allow heat to be stored in the ground passively and no need to install pipes etc.. Having the internal space in contact with floor and floor in contact with ground creates the thermal pathway in both directions (as originally designed).

Jeff, how many retrofits (at least in the UK) will be able to dig all around the house and have a solid floor in contact with the earth? I suspect not many.

My house fails on both counts. As mentioned above there is a ventilation space under the ground floors. Perhaps this can be overcome simply by blocking off the ventilation points and assuming that there will not be any condensation because the air under the house is warmer than the joists.

However, there is also a built-up patio area along the south side of the house which would be very messy and costly to rip up and replace.

Posted By: ralphdI had to dig a hole under my concrete footing to provide drainage during construction. If I had a source of pressurized water I think it would have been much easier. So I think it is feasible to dig holes big enough to slide a pipe through under your load-bearing walls.

How could that be done? Dig a big hole at either side, use pressurised water to bore the channel, then slide in the pipe? What happens if there are big rocks in the way? I would worry about 2 things: depth and channel length. Pipe needs to be long and deep enough to get useful heat out during winter.

After doing some calculations, I reckon that my annual space heating load will be around 10,000kWh. A small wood burner will contribute 30% of that, leaving around 7,000kWh to come from AGS. Required ground volume to store enough heat is 8x8x6m (after looking up relevant values for type of ground, etc). That's not too far away from my 6x6m garage, so with a bit of overlap to the surrounding ground I reckon it can be done. However, there are 2 key practical issues:

1. Heat input/output has to be deep enough to avoid excessive losses through surface insulation and allow sufficient flux of heat out of the ground during the winter. I've not finished the calculations, but at this stage guess that anything less than 3m will not work well enough.

2. Conduction of heat through garage walls. This will be an issue for Jeff's under-house suggestion too, where external walls of the house will conduct away heat which will be lost to the environment, unless the walls are externally insulated.

Going back to the general question posed by the thread, I can see pipe depth & length (to get necessary heat flux from the ground) and conduction through walls as potential limiting factors to successful retrofit of inter-seasonal ground heat storage. Although these are practical problems, they don't look insuperable. The prize for finding workable solutions is so high I'm surprised there is not more interest (perhaps there is and I've not found it yet!).

Investigate directional drilling - steerable horizontal drilling http://www.chilternthrustbore.co.uk.

See Rehau 'Awadukt Thermo' http://www.greenspec.co.uk/html/product-pages/rehau_awadukt.php which finds that the ground transfer pipes have to go in as close as 1m c/cs and be 200 diam to get the reqd transfer surface - also theirs are manufd from polypropylene not uPVC because the latter (ordinary drain pipes) actually have a foam structure within the body of the plastic, which is quite resistive, thermally. Also their pipes incorporate silver bactericidal, otherwise nasties tend to grow within, as they're frequently liable to condensation. Which explains their probable great cost compared to ordinary drain pipe! Also, consider clay drain pipes, or the duct grade thereof.

I messed up the quoting; half of 75 pounds would be about $70 per hour.

Posted By: Paul in MontrealI paid half that for a 13 T machine.

Posted By: fostertomhttp://www.greenspec.co.uk/html/product-pages/rehau_awadukt.phpwhich finds that the ground transfer pipes have to go in as close as 1m c/cs and be 200 diam to get the reqd transfer surface

Thanks Tom, very helpful. Their bumpf reads like the design wishlist I've been putting together :).

Closeness of the pipes looks very important, depending on the type of ground. I'm modelling 40% damp clay. Over 6 months the heat wave travels 4 metres (ie about two thirds of it still remains within 4 metres of source).

Heat loss from the house is 3120W. Heat flux from 1 metre of 100mm pipe (assuming various things) is around 28W. So around 110m of pipe are needed. In my 8x8 area that means 2 arrays of pipe 1m apart, with around 3m of vertical separation between them.

So... hole for the ground store needs to be 8x8 in area. Depth depends on losses through the insulation under the garage surface. In ideal AGS this is irrelevant because it just goes into the house. In the more controlled external variant where it's in an out-building, this will force the hole to be a bit deeper. Let's say the first set of pipes are at 3m and the next set at 5m (need to model this a bit better).

That's a big hole! Might still be a fair bit cheaper than getting specialist drilling rigs out to make that amount of holes under the house though (local mini-digger + driver for a couple of weekends?).

Quite likely that the sums behind the numbers above are wrong somewhere :). However, they 'feel' reasonable. If right then meeting most space heating needs for a detached house using an inter-seasonal ground store under an out-building will take a fair bit of pipe and surface insulation (to avoid too deep a hole), but looks quite feasible.

Next steps are 1. check sums 2. do some preliminary costings.

Modelling semi-manually using pencil, paper, spreadsheet and google - with distant echoes of a long-ago education :). Big digger looks like the right approach, but I need to think about how to dig the hole without destroying the garage! Probably start digging the hole outside the double doors to give the digger headroom as it moves inside.

Just found this thread which discusses and models heat store losses http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=921&page=2. I suspect heat losses to the environment will end up not being the limiting factor, although will need to model it in more detail to be sure. Intuitively, the temperature difference between store and pipe will be much greater than the incremental difference between adjacent ground, so the heat flux should be mostly from ground to pipe during the winter.

Repeating Mike7's heat loss calculation gives annual loss for my store of 17,000kWh, for projected return of 7000kWh into the house. That looks like fair game to me, despite obvious risk factors such as groundwater and rocks.

Even in dreich old Scotland it should be easy enough to collect 24,000kWh from the sun during summer. I'll have another look at the collector calcs later.