A gaseous fuel fired furnace having a vapor condensing flue gas vent conduit, a flue gas flow restrictor disposed in the vapor condensing flue gas vent conduit having a flue gas upstream side and a flue gas downstream side, and a condensate bypass conduit extending around the flue gas flow restrictor having a condensate inlet in fluid communication with the flue gas downstream side of the flue gas flow restrictor and a condensate outlet in fluid communication with the flue gas upstream side of the flue gas flow restrictor.