Mechanical draft controller with vent damper

Abstract

A mechanical draft controller integrated in the draft hood of a furnace and coupled to a flue damper is responsive to the draft in the flue for controlling exhaust stack heat loss and eliminating heated room air loss via the draft hood under normal operating conditions. The pressure-sensitive controller is variably positioned as a face damper on a surface of the draft hood and is coupled to the flue damper by means of mechanical linkage. Increasing pressure in the draft hood caused by a reduction in the draft value due to partial flue blockage, large down draft, etc., displaces the pivoted controller resulting in a corresponding opening of the flue damper to re-establish the desired draft value. Similarly, excessive draft causes the displacement of the controller in an opposite direction resulting in flue damper displacement so as to reduce the draft value and exhaust stack heat loss. Between these two operating conditions the controller is variably positioned in the surface of the draft hood so as to block the escape of heated room air via the exhaust stack. A completely blocked flue resulting in an abnormal increase in pressure in the draft hood causes the controller to be further displaced resulting in the further displacement of the flue damper to a nearly completely open position to permit re-establishment of the desired draft value. Another safety feature provides for the breakage of the linkage coupling the draft controller with the flue damper in which case both dampers are automatically opened allowing the system to revert to normal draft hood operation. The draft hood control damper and the flue damper may be utilized in combination with a conventional bimetallic damper positioned downstream from the flue damper in reducing heat loss during burner OFF periods.