Abstract: A heat exchanging to exchange heat between a first fluid and a second fluid has a thermally conductive conduit with a double wall construction with an inner wall and an outer wall fixed thereto and defining a plurality of leak channels therebetween. Any fluid entering the leak channels is conveyed and detected to indicate a potential breach of the inner wall or outer wall. One of the fluids may be warm waste water and the other fluid may be clean water under pressure to recover heat from the waste water. An interleaving baffle located in the fresh water conduit increases heat transfer.