Automatic Flue Dampers

When a gas-fired furnace or water heater is operated in a home or other building, room air is drawn through the furnace or heater to help burn the gas. The warm exhaust gases are then vented up a flue pipe through the roof. When the unit shuts off, residual heat continues to warm the air and draw it through the vent to the outdoors in a "chimney effect." This robs a building of heated air. An automatic vent damper not only restricts the flow of heated air up the chimney, but can also reduce the amount of cold outside air leaking in to replace the escaping warm air. It is placed in the flue pipe between the heating unit and the chimney to block the passage of heat after the unit has been shut down.

By preventing the escape of heat during the "off" cycle, a vent damper can also increase the furnace's operating efficiency by minimizing "cold" starts. At the beginning of each "on" cycle, the unit's efficiency gradually increases until it reaches steady-state. The time needed to reach this point is reduced as a result of the vent damper. Vent dampers can also help to retain humidity in the building by reducing infiltration. This can improve comfort in the winter months.

There are two types of vent dampers available: electromechanical and thermomechanical. The electromechanical vent damper is coupled with the gas valve. It opens when the thermostat signals that the heating unit is on and closes when the thermostat signals the unit is off. These have fail-safe mechanisms to prevent them from locking in the closed position.

A thermomechanical damper, unlike its counterpart, has no electrically activated parts. It consists of bimetallic fingers that extend into the vent pipe. When the heating unit fires, the hot gases heat these fingers and force them to separate. After the unit shuts down, these fingers close and block the flow of heat.

Vent dampers are most cost-effective when installed on heating or water heating equipment located within the heated space. Air passing through these heating units has already been heated, making it even more important to prevent its escape up the chimney.

CAUTION: Thermomechanical dampers should not be used on oil-fired equipment and must be approved by the appropriate inspectors and equipment manufacturer. Vent dampers should never be installed in a flue pipe serving more than one appliance.

A malfunctioning vent damper can cause inadequate venting of flue gases, causing health and safety hazards. Any type of vent damper should be installed only by a trained, qualified service person.