Abstract: A high efficiency heater system made up of a longitudinally extending combustion chamber located within an external enclosure which is spaced apart from the combustion chamber to provide an air space. A burner assembly provides combustion gases into one end of the combustion chamber which travel down the chamber and are exhausted through flue gas vents at the other end of the chamber. An air path divider separates the air space between the upper portion of the combustion chamber and the enclosure from the air space between a lower portion of the combustion chamber and the enclosure. A blower supplies ambient air which flows into the upper air space, flows along the hottest part of the combustion chamber and then, upon reaching the flue gas end of the combustion chamber, reverses direction and travels back along the lower air space before being exhausted into the ambient air.