The photo below shows copper water tubing wrapped around the flue pipe of a gas fired boiler. The water in this tubing feeds into a gas fired water heater. The purpose of this “do-it-yourself” heat transference unit is to warm up the cold water in the tubing before going into the water heater tank.
While this might appear to be a good idea in terms of energy recovery / savings, it could have deadly consequences. The cold water in the copper tubing could reduce the flue gas temperature to the point where the flue gases may not properly draft up the chimney. The flue gases could then enter the living area of the home. Flue gases contain CO (carbon monoxide) which is a deadly gas at high levels.
Photo taken by Dennis Robitaille of Able Home Inspection