No need to dig up the Wisconsin sand bed - just cut off the pipes if it doesn't work! But it would be better not to spend the effort if it isn't going to work. Calculation is difficult but cheaper if you know how.

How much would 16.6sq m of solar panel cost, £10,000? Never mind combined PV/ST. Say the collection of heat a la Wisconsin raises the floor temperature too high until Dec/Jan in a mild autumn/winter so you have to "loose" the heat. Without calculation I don't see how one can be sure about this.

One GBF members house with interseasonal store:-
http://www.tonyshouse.info/

The idea of insulating a block of ground under the house as an inter-seasonal thermal store seems easy enough, unless you have ground conditions where water seepage may be a potential problem. I looked at the idea closely when Viking House kindly published his work here, but for me the idea was ruled out by having water only a few feet down. Even a small flow of water through the underground thermal store would leach out all the heat pretty quickly. This would have meant making a watertight barrier around and under the insulation, in effect creating a sealed, insulated, soil-filled basement. It was at this point that I realised it probably wasn't a practical solution for me, although I think the idea is a good one when used in the right location.

If you are looking at between about £2600-£3500 for that approx. panel area, add the cost of pumps, controls, tank, PEX pipe and installation and I guess one would be quite pleased at a total of £10,000 - so long as it was well spent.

And if "other approach is doomed due to the relatively high conductivity of the earth, the huge surface area of all the possible heat paths & the length of time available between seasons" are you saying that putting the summer heat down under the dwelling a few metres like Tony and the Viking is just an expensive way of creating a heat sink? I imagine that the Viking House dwellings have had a full analysis of heat losses/gains/temperatures over the season and that this predicts rises in the temperature of the subsoil as well as his slab. What the Viking does above the insulation is something very like the Wisconsin plumber's approach but the insulation is 4x thicker and he insulates above as well to prevent the surface getting too hot. The Wisconsin plumber relies on heat just leaking into the dwelling and presumably leaking into the surrounding earth at a greater rate all with minimal controls. The question is does this low tech approach work?

Posted By: bellaAnd if "other approach is doomed due to the relatively high conductivity of the earth, the huge surface area of all the possible heat paths & the length of time available between seasons" are you saying that putting the summer heat down under the dwelling a few metres like Tony and the Viking is just an expensive way of creating a heat sink?

I've not seen drawings of Tony's set-up, but where Viking House shows a coil heating the soil below the insulation, it is just that, a heat sink. The thermal store's heat exchanger is entirely above the lowest section of insulation.

Think about all the possible parallel paths that the heat can take from below the slab to the outside. There is the most direct almost horizontal one, but there are a large number of more curvy paths which go progressively deeper & deeper before coming up outside the house. In cross section, the paths look like layers of half an onion, with the flat side against the underside of the floor insulation.

Because the conductivity of the earth is relatively high, these other paths cannot be neglected. So you need to calculate the thermal resistance of each of the paths & integrate these together in parallel to get the total thermal resistance.

To cut a long story short, if the house is well insulated, the ground will lose the heat long before the house gets to benefit from it.

David