Thats what I'm thinking Ray, but i think you have the pipes backwards. The shower water goes through the coil & smaller dia. pipe. The radiator fluid dumps into the main chamber with a larger inlet/outlet. Some of them I've seen have baffles in to slow the fluid flow down.

My biggest question is as Wyleone states - how big/long for each? Too big or two small could make a huge temp difference.

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