Abstract: Double cylinder walls are adjustably sealed to an exhaust gas conduit of any of a variety of hot water heating systems. An inner cylinder wall houses a water preheater pipe coil with an inner turbulator coil to circulate source water to be preheated by the exhaust gasses prior to feeding into the water heating system. An outer cylinder wall sealed to the inner cylinder wall prevents exhaust gas leakage from the inner cylinder wall from escaping. Insulation between the cylinder walls retains the heat within the inner cylinder for maximum preheating and an outer cylinder wall that is safe to the touch. A series of valves and pressure and flow gauges control the flow of water and diverts the flow or shuts down the preheater system or the entire hot water heating system as needed for failsafe operation.