Duct from outside into air return

I have a downdraft furnace and my question is this. On top of the furnace where the return duct work for the house collects, there is a 6" round duct that goes through the wall to the outside, with a damper of some type which is spring loaded. What is this for??

 

If I get what you have. If its tied to the return duct then it is a air intake from out side If not it can be for combustion air to the unit. Is there a code there that calls for fresh air intake to the furnace room.

ED

 

If I get what you have. If its tied to the return duct then it is a air intake from out side If not it can be for combustion air to the unit. Is there a code there that calls for fresh air intake to the furnace room.

ED

Yes, it is an intake from the outside to the top of the return air duct on top of the furnace, and like I said it does have a damper that is spring loaded with a counter weight, and is not controled by the furnace (no wires coming from it that might be calling for intake air). Could this damper not be working, and if so how do I check it?
As far as the code you speak of, if you are talking about the LED fault code, I am getting a code error of 3-3 on my Bryant Plus 80 unit. 1: "Inadequate Combustion Air Supply flame roll out switch only" The resets are not tripped nor never have. 2: "Dirty Filter or Restricted Duct System"
I recently disabled the Honeywell Air Cleaner and installed a 4"media filter.

 

Sorry about that CODES. Not the tstat but where you live. Do you have to have a outside air intake on the home. Some places do. From what you say. Turn the furnace blower to on. With it running see if that damper opens. Or move the weight a little and see if it opens. It can be that some one made it so it is closed as it dont take in all that cold air in the winter. And hot humid air in summer.
If you went from a EAC filter to a good one you might want to check the AC coil if you have one .make sure its clean.

ED