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Hi AllSomewhere in the early 80's we had a guy, ahead of his time, who built a house here in South Africa where he had placed rocks in a cavity under his floor slab(this was featured in a House and Home magazine at the time). He then pumped the hot air from the ceiling into the cavity during the day. At night he could re-use this heat as underfloor heating. This idea was correct because heat in the ceiling is lost very quickly once the sun goes down but is retained much better in rock that is not exposed to the elements