Abstract: A heat exchanger for extracting and recovering waste heat from the flue of a heater, such as a furnace. The present heat exchanger includes a housing having a plurality of sealed, oil filled, parallel metallic finned tubes arranged around the periphery of the heat extractor chamber of the heat exchanger. Hot flue gases from the furnace travel between the inner housing and the outer housing. Thus, the hot flue gases pass through and around the finned tubes thereby heating the oil inside, so that a very large proportion of the heat of the flue gases is extracted by the finned tubes. The plurality of finned tubes are all connected together as one unit exposed to hot flue gases, thus once oil is heated in the tubes, the unit retains the heat even when furnace is not operating. The tubes are made of copper and act as an oil reservoir and heat sink for the heat absorbing fins. A blower exhausts clean heated air out of the rear duct for useful heating.