furnace flue damper

[Greg Jenkins]

What is this damper (and its purpose) on the furnace flue. I never see these and I am not familiar with the device.
Is it a thermally activated damper?
I realize the picture is lousy but I was taking a picture of the flue connection not the damper.
This is a mid efficiency furnace with induced draft.
Greg

[Nick Ostrowski]

Greg, it looks like a barometric damper door. Is that a gas or oil-fired heating system? I have never seen one on a gas-fired system so I assume this furnace is oil-fired which would make the presence of the damper normal. It's purpose is to teeter open when the furnace is firing and allow air to enter the flue to assist in drafting the combustion gases to the exterior.

[Greg Jenkins]

Natural gas rheem furnace about 20 years old and well maintained. (Except flue connections were loose and in direct contact with wood framing.)

[DavidR]

It's a double acting barometric damper, was there a water heater common vented with this furnace by chance with the same type of damper on it?

[Greg Jenkins]

Water heater in same room but has its own vent. No other connections to the furnace flue were observed.

[Stuart Brooks]

I just ran into a gas furnace with B-vent and a damper on it. No other gas appliances. The B-vent runs to rear of the house with a direct-vent type enclosure. Wood deck and vinyl siding is close. This wasn't a home inspection so I don't have details. I just saw it while doing a HAP verification inspection and curious. The gas furnace is a Lennox and may be from 1994

[Harry Janssen]

It is a barametric damper,found on oil fired furnaces,never have seen it on gas fired appliances.

[Jerry Shipman]

They are common on natural gas furnace flues in installations such as shown by Stuart.
The termination in Stuart's picture is a side-wall power venter.
Because a power-venter starts before the draft inducer motor on the furnace, there is a chance that the unpowered draft inducer motor on the furnace could begin to turn backwards. When a motor is turning backwards when power is applied, there is a chance that it can continue to run backwards after. Barometric dampers are common on those installations to allow control of the draft created, to avoid turning the venters blower wheel backwards before the furnace actually starts.