I came across this on a recent inspection and don’t know what it is, nor can I find it on the net.

Model #GVD 1-6
Serial #66723

Thermally Actuated Vent Damper

Where was it located what was it on???

There are two types of vent dampers: electric and thermal.

Your web search could not return a single source for thermal dampers. Although thermal dampers may still be available and many are still in service, they are not as efficient as the electric dampers and were often the target of critics for the amount of “flue gas spillage” they could cause. The thermal dampers were much less expensive and could be installed by a do-it-yourself person in residential applications. They are heat activated and very simple in design and operation. However, they are slow in opening and closing and some designs resulted in a permanent flue restriction.

Electric vent dampers have a small electric motor that drives the damper. It is spring loaded such that if the electric power or the motor fails, the damper will fail open. The damper motor is wired in with the gas value. When the gas valve opens and the burner ignites, the vent damper opens. When the burner is shut off, so is the vent damper. The highest value heat is saved soon after the burner shuts down, when the heat exchanger and the flue are the hottest and the draft is the most severe (when the chimney is sucking out most of the hottest air).

Thank you for the reply. When a thermally controlled vent damper is found during an inspection should it noted or written up, or suggested it is removed. My thoughts were it will just sit there with nobody watching it, get stuck in the closed position or partically closed position and fill the house with carbon monoxide.

I agree!

Was this on a coal furnace, by the way? You see a good many of these old thermal dampers on coal furnaces, since having no damper at all can fight with the stoker control.