[tagwex]

I have done some investigation into this matter, admittedly not enough, and looked back on a few old threads that didn't go into much detail. So I thought I would throw it out there again. As you can see from the photo I have paving slabs (6No.) at 0.45m square, so 2.7m long. You should also be able to make out that I have used sleepers as the frame. Under the slabs are 50 x 50mm battens fixed to the sleepers which support the slabs. Under this is a void of 250 to 300mm. I have loads of empty bottles which, if filled with hot water would be a step in the right direction. I can also easily insulate this area with rigid polyurethane wall insulation. Now the crux, how can I move this warm air up and get cold air in? There is no way of getting a pipe into this void from the outside (this is an afterthought and, in hindsight, an oversight). Actually just been thinking about that and there is a way but it would be a lot of hassle. Alternatively, how do I just simply get this air circulating and keep the water in the bottles warm? A piece can be cut from a corner of one of the slabs to pass a pipe through vertically.

[Blowin]

As no-one else has chipped in, I'd probably make sure the bottles got as warm as possible - maybe by lining the pit with oven foil or similar - but, given the existence of that heat source, plus the fact that hot air will always rise, surely it will be replaced somehow by air creeping in from somewhere? After all, a totally airtight greenhouse is pretty much unheard of?

On the plus side, I imagine you'll conserve heat for longer this way. If, as you say, you'd installed a pipe from outside to bring in cool air, the through flow would inevitably be faster and the heat would dissipate quicker. Once it's all gone you're at the mercy of the elements. Nil desperandum!