The flue of a boiler (Fig 3, not shown) or warm air heater is modified to incorporate a water chamber which takes heat from the flue gases and is in communication with the conventional hot water circuit. The combined flue/heat exchanger consists of three concentric stainless steel tubes defining an inner water chamber and an outer chamber filled with insulation. Flow and return pipes project from the water chamber through the insulation chamber so that water heated by the flue gases is pumped into the existing hot water or wet central heating system (Fig 3).
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