Abstract: A heat exchanger comprising an elongated tubular housing having an inlet at one end and an outlet at its other end is secured around and spaced from the furnace flue by a plurality of plates secured around the furnace flue. The plates are provided with apertures to permit air flow past the plates. The apertures in each plate are angularly displaced with respect to the apertures of an adjacent plate such that the air circulates around the furnace flue as the flow continues from the inlet to the outlet of the elongated housing. An air pump produces a flow of pressurized air into the inlet of the housing to create a flow of pressurized air through the housing from the inlet toward the outlet of the housing.