Why is a draft proving switch necessary if the furnace has a pressure switch?
The draft-proving switch is set to detect the negative pressure created at the inlet to the power venter. The pressure switch in the furnace is set to detect the negative pressure at the inlet of the furnace inducer fan. If the power venter were to fail, the draft-proving switch would open and cut off the appliance. If the power venter were to fail and didn’t have a draft proving switch, the furnace could continue to run, because the furnace inducer fan would hold the furnace pressure switch closed. This situation would put the vent system under a positive pressure and flue gases would leak into the structure through the draft control and pipe joints.
