Abstract: A monitor for a gas appliance vent includes a conduit section that is mounted in line with a vent leading from a furnace or other gas appliance. The conduit section has an interior tubular plenum member which extends across the vent interior. The tubular plenum member has upstream facing openings therein. An outer end of the tubular member passes through an aperture in the conduit section. The outer end is open to allow flow therethrough from the upstream facing openings. A switch is mounted in proximity to the outer end. The switch is thermally responsive to respond to the heated flue gases that flow out the outer end. The switch is operably connectable to either an alarm or a fuel line shut off valve.