Abstract: An improvement in heat exchangers for hot air furnaces and water heaters. The improvement is a rotating damper having an opening above each flue that momentarily closes off the outlet of each flue in turn. This holding of hot gases provides more time for the transfer of heat before the gases are discharged to a chimney. The damper openings are sized and arranged so that at least one of the flues is open and one or more closed at any particular moment during the cycle. When a particular flue is opened by the rotating damper the held gases which have now cooled are discharged to the chimney and replaced by a new supply of hot gases. When this flue is closed by further rotation of the damper, it becomes a holding chamber for the hot gases. The other flues operate in turn in the same manner.