An automatic flue damper is an energy-saving device that closes off or restricts an appliance flue when the appliance is not operating and is in its “off” cycle. It traps residual heat in the heat exchanger after the burners shut off, and prevents the escape of conditioned air up the vent pipe.
An automatic vent damper should be labeled and of a listed type. The installation of an automatic damper must be in strict accordance with the manufacturer’s installation instructions. A malfunctioning or improperly installed damper could cause malfunction and a discharge of combustion byproducts into the dwelling's interior.
A manual damper should be used only with a solid fuel-burning appliance, and not with a gas-fired appliance. The user of a solid fuel-burning appliance would be aware of a closed or malfunctioning manual damper because of the smoke. The user of a gas-fired appliance may not know of a partially closed or malfunctioning manual damper, and a hazardous condition would develop.