A boiler flue damper increases the efficiency of a heating system by closing the vent pipe when the system stops burning. The boiler flue damper opens when the boiler uses flames to heat water, so noxious exhaust fumes can escape through a pipe or exhaust vent.
The damper should work automatically with the boiler's operation. Older boilers can be retrofitted with mechanical vent dampers to decrease backdrafts of cold air into the system. As of 2015, boiler flue dampers can be added to older systems for $100 to $400, depending on the complexity of the vent system and the model of the boiler. Older boilers, such as those made near the beginning of the 20th century, stand to gain the most efficiency by having a flue damper installed.
Heating and cooling systems that require these types of devices have electrical-based or temperature-based dampers. Electrical dampers switch to open when the gas valve opens and burners ignite to heat surrounding air or water. Thermal dampers open when the air temperature increases and close when the air gets colder. Thermal dampers are usually less efficient than electrical ones, and these types of systems may permanently restrict air movement within the flue. Modern systems usually put flue dampers between the heating unit and the chimney of a house.