Without insulation its more like a solar heat dump then an interseasonal thermal store. However, it can only help to increase the ground temperature & dry out the walls.

David

With massively xs summer heat production, it doesn't matter if the AGS pies act mainly as a dump, provided that as a minor but significant by-product, a slowly expanding volume of the surrounding subsoil permanently adopts a higher-than-otherwise temperature. If that temp is higher than the target temp of the old cottage's winter interior, some heat at least will flow in that direction, along and up thro its uninsulated floor.

Heat will also flow away along under the new extension's insulated floor, to outside air - so insulative obstructions (downstand, wing) to that flow will retain more of the heat in the subsoil block.

The other place heat will flow away to is downward 6400km to the earth's core furnace! Or more accurately, to the 40km thick egg-shell of rock that lies beneath our surface. That eggshell is mainly much hotter than the AGS pies. Unfortunately, its topmost few 100metres has long been cooled by surface mean temp. Given enough heat input to the AGS pies, and given long enough time, an ever-expanding volume of that cooled rock will be re-warmed. Put another way, the downward temp gradient, from the AGS pies downward, will steadily flatten, so the rate of heat loss downward will decrease.

However the hemispherical perimeter of the re-warmed volume will also be increasing, so an argument says that tho the temp gradient has flattened, the frontal area has increased. This seems to be the stumbling block to hopes of seriously useful heat storage in uninsulated subsoil. Or is it actually not a hemispherically expanding front, more like a uni-directional (straight down) non-expanding front? Or something in between?

ST's and mike7's comments awaited!

Posted By: fostertomST's and mike7's comments awaited!

More of a cone shape with curved sides that follow an exponential curve I think

I like the idea of dumping excess solar heat below a house rather than to a rad then the air, Viking house has a similar approach with his with his solar slab:-
http://www.viking-house.co.uk/hydro-thermal-energy-store.html

Andy, looking at our new pr of 2000 litre black orange juice barrels (which will store 4 tonnes of rainwater of the polytunnel), how about substituting these (£82 from Mole Avon Farmers) for the buried mesh cage that you wind your pipe around? Full of water, they'll have something like 2.5x (anyone?) the volumetric heat capacity of the soil they replace.

Your pipes need to be well spaced - decreasing pipe spacing won't increase heat transfer rate pro rata - you may find you need your pipe to be spread, more widely spaced, over a larger number of pies, to get the input rate you're expecting. Good thermal contact to barrels might allow closer spacing, and the whole barrel would then act as a large-surface heat dissipator.

Better still, drop your pipe coil right into the barrel!

Posted By: fostertomAndy, looking at our new pr of 2000 litre black orange juice barrels (which will store 4 tonnes of rainwater of the polytunnel), how about substituting these (£82 from Mole Avon Farmers)

Great idea and cost comparable with sand, however once i build over the top i cant refill tank with water. and i think they might be too big to bury next to old existing cottage without weekening substructure.

However since your thinking- can you come up with anythink better than sand as a backfill?

Posted By: fostertomGiven enough heat input to the AGS pies, and given long enough time, an ever-expanding volume of that cooled rock will be re-warmed.

I don't have any simulation results or measurements, but I would be surprised if the house or the solar heat dump had much affect at depths greater than the house width. Below that depth the parallel sum of the thermal resistances of all the possible heat paths to the surface makes the temperature increase under the house less & less relevant.

David

I want it to work but can't get the numbers to stack up.
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=9021&page=1#Item_24

Posted By: an02ewBut we are missing the point

Think I was

The main point about heating anything with solar is that you need a good resource, or in English, it needs to be sunny.
So forget winter, spring, summer or fall, to quote Carol King, and start thinking how often and when you get enough good times to heat enough water to the temperature you need.
You may find that a bit of PV direct to a resistance heater helps a lot here rather than a small gas boiler. Probably cheaper too.

<blockquote><cite>Posted By: an02ew</cite>Thanks all for comments so far. Not the firing squad i was expecting

David- i am using two cylinders to separate the DHW from the solar buffer and they can be very cheap/simple standard cylinder. Once my DHW has charged then a simple valve to switch all solar heat over to the second cylinder and then away to the solar dump. But DHW takes priority always

Tom- i always appreciate your teachings, any chance i can have your brain when you’re finished with it.

ST- A new highly insulated extension is being place adjacent to our very old cottage with the AGS coils under the extension, of the two building the heat will pass more easily into the old because of the insulated floor(down stand insulation being applied externally with a EWI system)</blockquote>

Read this all with interest as there are similarities in the way we're going:
We're also probably keeping the DHW cylinder small, possibly simply re-using our existing standard cylinder, and dumping "excess" to the ground.
This is partly because we're cash-poor and space-mean so the thought of a massive heat store doesn't sit well.
The extension is timber frame with EWI and is adjacent to a building that loses lots of heat to the crawl space.

Where we differ is:
The extension footings have been insulated vertically to about 2m and will eventually get a horizontal "wing" to about 2m.
The floor slab of the extension is not insulated immediately below floor.
Instead insulation has been laminated with reclaimed brick and soil so as to force a 6m path from a heat delivery coil set about 1.5m deep under the centre of the new slab.
In theory ( ) it will take 6 months or so for the heat to reach the interior floor surface.

The roof collector is Alu-pex pipes run along metal roofing battens under 25sqm of single lapped slate.
We're not expecting to achieve more than 40 degrees from this ... using it as a pre-heater for a 9kw electric boiler set up right next to the tank
This should also leave us some flexibility for using the existing 3kw immersion socket to fit a heating coil from any other collectors we eventually get round to making.
Our priority for the time being is AGS UFH but if temperatures off the roof exceed expectations we would shift the controls to allow more draw off to the DHW.

Don't know if this is of interest or any help but "better out than in"!

Maybe there is an opportunity to have a vertical borehole and dump your excess solar into in the summer and retrieve it with a ground source heat pump in the winter?

Posted By: an02ewa dump for excess heat from ST

i don't think you shd take that personally, SteamyTea

No no, it was just my little joke - pretending I thought ST (solar termal) meant SteamyTea. OK, it's weak.

Posted By: fostertomNo no, it was just my little joke

Catches me out all the time

PS ... from what I v'e heard both here and on the Navitron forum a drain down system has its own problems with water/air mixing, and wear & tear on components ... Anyone else more knowledgeable care to expand?

... and how did the end of my first post end up as
"posted by steamytea" ...?