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Smoke problem with a two-flue chimeny

Posted by wlai (My Page) on
Sun, Dec 2, 07 at 0:15

I believe I have what's termed a two-flue chimney, by which I mean one brick chimney within there are two passageways. One is used by a woodburning fireplace upstairs. The second flue is downstairs, where there is a gas burning fireplace as well as the furnace/water heater.

When we are burning wood/firelogs upstairs, it's drafting properly. But the basement get smoky, as smoke seems to be descending into the basement via the second flue. I checked the damper, and a screw is installed so that I can't close the downstairs damper completely; the best I can do is to close it half way. I believe the smoke problem is due to the smoke going up the upstairs flue, only to descend back down via the second flue smoking out the basement.

Is there a reason why the downstairs damper is the way it is? The previous owner must have had it installed for some reason, I'm just trying to see if I can remove the screw and closed it up completely, unless I'm using the gas fireplace downstairs.

Thanks for any advice

Follow-Up Postings:

RE: Smoke problem with a two-flue chimeny

I don't know how all chimneys are built, but I have a 2-flue chimney and this wouldn't be possible for me. My flues are completely separate. There would be "zero" chance of smoke from one flue getting into the other one.

I really don't know if they would have tied both the flues together inside the chimney. On top of the chimney...are there (2) flues coming out the top or just one?

RE: Smoke problem with a two-flue chimeny

What is happening is that when you light a fire upstairs you are creating a negative pressure inside the house, if the house is fairly airtight then the easiest way for the fire to get air is through the open flue of the downstairs fireplace in other words the smoke goes up the chimney and gets sucked back down the other flue along with the necessary fresh air. Theres a few things you can do.
1 extend the height of the upstairs flue tile enough this makes one flue higher than the other so that the smoke escapes and doesnt get pulled down with the fresh air . You can try this by temporarily perching a new piece of flue tile of the same size on top of the woodburning flue tile on top of the chimney
2 install a fresh air kit in the upstairs fireplace .
3 Try & get the damper working properly downstairs. this may not be possible if the gas fireplace has a ventpipe going through it
4 IF & thats a big IF the downstairs flue is not being used in any way at all either now or in the future, you could block it off at the top. ( personally I don't recommend doing this )
5 install a locktop damper on top of the downstairs flue
6 simply crack open a window near the fireplace upstairs when using it.

RE: Smoke problem with a two-flue chimeny

When gas logs are installed in a fireplace, the damper is typically fixed in an open or partly open position. I believe this is required by building codes. It is here in central North Carolina, and I suspect in most localities. It would be unsafe to close the damper altogether, that is, to use the gas logs with the damper completely closed. If you decide to close the damper, to correct the downdraft smoking problem, you shoudl probably disable the gas logs so that no one accidentally uses them.

RE: Smoke problem with a two-flue chimeny

Dear wlai,
I'd love to know if you figured this out. I'm having similar issues and I see it's been a few years since this post. Thanks.

RE: Smoke problem with a two-flue chimeny

It's quite simple, the chimney is not tall enough for the home. you can complain about tight houses, crack a window, seal the damper, and apply bandaids wherever you may, but actual problem is the stack (probably an exterior one) is too short. I've spent a lot of time troubleshooting and fixing problem chimneys, and sadly, the solutions are really quite simple (although often not inexpensive) Here is a post on a similar question that I answered:

"The problem has little to do with the similar height of both flues; you shouldn't be getting a downdraft down either one of them if the chimney was built properly. If you have an exterior chimney with the flue opening (top) lower than the lowest interior part of your home, you will definitely have problems. The stack should actually be a bit higher than the highest exterior (ridge line of roof) portion of the home - especially on an exterior chimney. It is sad that so many builders and masons don't know and often don't care about the proper functioning of a chimney. the 10/2 rule is a minimum requirement for a builder to pass code, NOT a recommendation on what SHOULD be done. Nothing other than extending the stack to its proper height will solve your problem permanently - and if the mason or "chimney professional" you hire says othewise, especially if they advise you that installing a stainless liner will fix it, FIRE HIM."

RE: problem with a two-flue chimeny

btw, it is required that the damper be held permanently open on a retrofit of gas logs to a masonry fp. This way, if you forget to open the damper with the new gas logs (which produce little smell or smoke as indication of a damper left closed) you won't kill yourself with oderless CO.

RE: Smoke problem with a two-flue chimeny

Sorry to resuscitate this post, but I too have a similar problem - smoke down-drafting into an adjacent furnace flue. Chimney is well above highest point of the house. Berlin, if you're still around, you seem to know what you're talking about, and several of your posts on this topic recommend raising the fireplace flue higher than the adjacent furnace flue. I had a well-respected chimney sweep recommend a "flue stretcher" (see link below) - have you (or has anyone) seen those, and have any thoughts as to their efficacy? Seems like it could effectively raise the flue in a cost efficient manner. Thanks all!

Here is a link that might be useful: Flue stretcher

RE: Smoke problem with a two-flue chimeny

the height of the chimney would NOT have anything to do with smoke coming down a different flue than the one the smoke is exiting!
the house is tight and the chimney flue in use is looking for air to breath
it is getting that supplement air from where ever it can find a source which is the opening to the second flue
here is a test you can the fire as to cause you problem when the smoke is entering the house thru the second flue opening try cracking open a window a few inches in the room where the fire is running
have another person watching the smoke entering as you do this
give it 5 minutes the smoke will stop coming in

RE: Smoke problem with a two-flue chimeny

Better just to buy the heater with the isolation system. Metal doors with vacuum gasket or something like that.

Here is a link that might be useful:

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