How to Troubleshoot a Vent Damper in a Gas Heater

Ignite the gas heater normally, following the procedure for your particular model. This typically involves pressing and turning the burner control knob to the "Ignite" position and pressing the separate igniter button on the appliance.

Insert the tip of the screwdriver into the slot on the metal rod extending from the vent on the gas heater. The rod controls the damper.

Twist the metal rod so the slot is in line with the vent. This is the open position for the damper. You should hear the sound of the metal damper plate inside as it rotates when turning the rod.

Hold your hand over the open end of the heater vent. You should feel warm air begin to flow through the vent once the heater is running.

Twist the rod so the slot on the end is perpendicular to the vent. This closes the damper and stops air flow through the vent. If the rod spins easily and does not hold its position after you remove the screwdriver tip, the internal damper mechanism is broken and must be repaired or replaced.

Remove the screws on the perimeter of the vent at the damper junction to separate the vent into two parts, using a screwdriver. A bent or broken damper plate must be replaced with a new damper available from an HVAC (heating, ventilation and air conditioning) repair shop. Unscrew the rod from the outside of the vent to remove the damper. Take the removed part to an HVAC parts store to match with a replacement.