the combustion gases of a burner in a heating furnace are discharged to a vent through a draft hood having three ports . the first and second ports communicate with the furnace and the vent respectively and define a first flow path for combustion gases through the chamber of the hood . the third port is open to the ambient atmosphere and defines a second flow path for dilution air through the chamber . separate temperature responsive , bimetallic elements sense the temperatures in the two paths and cause a vent damper to be moved toward and away from a position in which it closes the second port in response to the sensed temperatures to minimize the flow of dilution air . in a modified embodiment the draft hood is vertically elongate with the bimetallic elements positioned in a zone filling with hot products of combustion for minimizing the flow of dilution air and minimizing spillage of products of combustion during start - up and sudden changes in draft .