Dead-end duct coming out of furnace - what is it?

There is a round, possibly aluminum duct, about 8" diameter, coming out of my furnace that wraps around the back of the furnace against the wall and has a circular louver at the end that seems to have been permanently shut with some sort of caulk or putty. Judging from aging, it looks like this was done a long time ago. The duct does not immediately appear to be connected to either the supply or return air flows. Any idea what this thing is? It’s really in the way and I’d love to remove it if possible. I’ll take a photo when I get home tonight.

Question: approximately when was your house built (what era)?


I have a similar such part on my 1948 condo unit. THe furnace is in a “closet” that is also accesable from the outside. There also is a hot water heater in this closet. THink tiny 4 ft by 6ft room with two doors on either end (one to the inside; one to the outside). There are water pipes and electrical boxes there too. I know the furnace duct that comes out from behind the furnace and dead ends in the closet room is to keep the closet warm and keep the water pipes from freezing (since the door to the outside was originally thin, unisulated and had louvers directly to the outside air). I’ve since closed that duct (like yours is) and insulated the door to the outside. Heat was cheap in '48

Darn, now you have me wondering too. I think it might be a damper of some sort, but I’m really not sure.

In our 1952 or 1953 home we had something similar on our furnace in the basement. As I recall, there was a little stub (8" dia. sounds about right), with a circular door that was balanced on a “hinge” that ran horizontally through the center. (That is, you could push the top and the bottom of the door would come toward you, and vice versa.) At the bottom of the door there was a counterweight on a bolt attached to the door. You could adjust the weight by turning it clockwise/counterclockwise. I would walk by this door and would see it gently bobbing in and out when the furnace was on.

Dear Plynck

Your early '50’s house had a barometric draft control on the furnace. is for sale - The description given in the OP is too vague for me to figure out just what they have, although I don’t believe that they are talking about a draft control.


Thanks Z, that’s exactly what we had. And now that I come to think of it, it was located just before the pipe connected to the chimney flue. Ignorance fought.

My setup sounds similar to this. The duct ends right next to the base of the chimney in the basement, but it’s not connected. Perhaps it was at one point.

So I feel like a total idiot. I took another look at this thing yesterday and sure enough, where it runs flush against the chimney, there’s a ‘T’ fitting and part of the duct goes into the chimney. I really had to crane my neck around to see it though, as it’s in an awkward spot. I also noticed that the bottom of the duct seems to be corroding, so I guess I should replace it or have a tech take a look at it. Is it something I could likely do myself or is it more complicated than just replacing a piece of duct?

You could do it yourself. It will be a PITA of course. Part of the pain could be getting the right gauge ducting. Corrosion could indicate an incorrect pitch for condensation drainage, or perhaps need for a chimney cap.