Abstract: A total counterflow heat exchanger comprising a system of partitions dividing an interior chamber into a series of channels which coincide with the path of a duct through that interior chamber. A substance at one temperature is passed through the duct, and a substance at a second temperature is passed through the channels, thereby creating a temperature differential gradient over which thermal energy is transfered from one substance to the other as the substances flow through the heat exchanger. The arrangement of the partitions permits a substance to pass back and forth through the channels in the heat exchanger along the same path as the duct, so that while the substance may flow through the heat exchanger at the same net rate as if the partitions were not present, the length of the heat exchange gradient may be increased many times, therefore enhancing the efficiency of the gradient and the heat exchanger.