An exhaust duct orifice adjustment device for a gas-fired heater used to indirectly heat an oven communicating with a heat exchanger, wherein the exhaust duct includes a fixed annular orifice plate and a switch having a pressure sensor on opposed sides of the fixed orifice plate, turning off the burner if the pressure drop falls below a predetermined minimum. The orifice adjustment mechanism includes a moveable plate upstream of the fixed orifice plate having an opening communicating with the opening in the fixed orifice plate and an adjustment mechanism moving the orifice adjustment plate from a retracted position, wherein the opening through the fixed orifice plate is substantially unobstructed, to an extended position which restricts the opening through the fixed orifice plate, thereby adjusting the pressure drop.