The attached youtube link shows a well designed water to water heat exchanger designed to recapture heat from drain waste water as a way to preheat cold water entering your hot water heater. The whole pipe is copper tube, wrapped with copper coil. Looking at it, I thought you could incorporate this into a coal stove by using this thing to replace part of your flue. It's only 3 inch diamter so it would only work if you had a power venter setup, but wouldn't it do a good job of recapturing a lot of the heat before the flue gases vented outside?
