cold smoky air backdrafting down adjacent flue

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twd000

twd000

Feeling the Heat
Aug 28, 2015
448
Southern New Hampshire
I had my chimneys clean a month ago and declared ready to burn. There are three separate flues sharing the same chimney, but they are all independent. So far I have had two fires in the open fireplace on the first floor. Both times, I noticed a large draft of cold, smoky air drafting into the basement. It is coming through the collar of the woodstove, which I have never used. What is causing this effect, and how can I prevent it in the future? Do you think there is enough conduction from the hot flue to the cold flue, to drive a pressure difference? And would that be enough suction to pull smoky air from the open environment, down the cold flue?

I tried to attach the chimney sweep's inspection report, but the forum tells me it is too large (3.5 MB PDF)
 
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I had the same problem. Smoke from the hot flue was being sucked down the cold (unused) flue and getting into the house. I think the problem is that the cold flue has a negative air flow, meaning air wants to go down rather than up. My 2 flues coming out of the crown of the chimney were almost the same height so the smoke coming out of one flue was being sucked down the other. If you extend the hot flue 6 or 8 inches higher than the unused flue I think the problem will go away. You can do a temporary fix to make sure that it is going to work first.
 
Perhaps if you sealed the collar and turned down the air on the stove not as much air would be drawn in. If you extend the first floor flue you have to remember the you might have negative draft there too and draw in smoky smell from the basement stove.
 
i would guess the problem is that your fireplace is sucking a large amount of air out on the house which is causing a negative pressure in your basement. Try cracking a window near the fireplace to see if that fixes the issue. If so you need to get more makeup air to that fireplace
 
velvetfoot said:
Perhaps if you sealed the collar and turned down the air on the stove not as much air would be drawn in. If you extend the first floor flue you have to remember the you might have negative draft there too and draw in smoky smell from the basement stove.
Click to expand...
Yeah setting one flue higher negates using the other fire-burning unit. The higher one will always suck down.

As far as sealing the collar, those aren't supposed to be airtight, right? I mean, it's a series of sheet metal fittings that go from room temperatures to several hundred degrees. There's no way they could be airtight across that range. I would have to plug the flue hole directly.

I have also considered a top-mounted damper on the in-used flues. I understand you can drop a pull chain down for the open fireplace, but is there a way to install a top-mount damper on a clue connected to a wood stove or insert?
 
bholler said:
i would guess the problem is that your fireplace is sucking a large amount of air out on the house which is causing a negative pressure in your basement. Try cracking a window near the fireplace to see if that fixes the issue. If so you need to get more makeup air to that fireplace
Click to expand...


I'll test that theory next time. Opening a window defeats the purpose of using an already inefficient open fireplace, but it would be a good science experiment. I'm more concerned about the opposite problem, where I'm running the stove in the basement and back drafting into the first floor fireplace due to negative pressure.
 
twd000 said:
I'll test that theory next time. Opening a window defeats the purpose of using an already inefficient open fireplace, but it would be a good science experiment. I'm more concerned about the opposite problem, where I'm running the stove in the basement and back drafting into the first floor fireplace due to negative pressure.
Click to expand...
That is possible but not nearly as likely. Honestly opening a window right near the fireplace could actually mean less preheated air is sent up the chimney. It can at times actually not mean loosing any extra heat. But i usually suggest it as a diagnosis tool and move on to better air supply solutions from there
 
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sal77 said:
I had the same problem. Smoke from the hot flue was being sucked down the cold (unused) flue and getting into the house. I think the problem is that the cold flue has a negative air flow, meaning air wants to go down rather than up. My 2 flues coming out of the crown of the chimney were almost the same height so the smoke coming out of one flue was being sucked down the other. If you extend the hot flue 6 or 8 inches higher than the unused flue I think the problem will go away. You can do a temporary fix to make sure that it is going to work first.
Click to expand...
That's exactly what I had to do. Put another 2ft section on my class a. When they were at the same height had exactly same problem as op
 
For some reason, the three chimneys in my seemingly tight, late-model house tend to downdraft. Maybe that's like yours. It's been a problem I've thought about a lot.

Sure, I can open a window in the basement and that would help, but who wants to do that?

A smokey smell used to come through my oil boiler's barometric damper. A Fields automatic damper in the flue helped a lot with the smokey smell. I haven't sealed any of the stove pipe joint's, which I guess I could do, but I'm not sure it needs it.

I staggered the chimney heights, with a look to prevailing winds. I think that helped to some degree as well.

The chimney of my pellet boiler, which is in the basement, also suffers from downdrafts when totally cold, and it too has a barometric damper. It's barometric damper has a seal on the flap, and that helps some. It's tough starting up the pellet boiler when it's stone cold, because there is still some leakage through the baro damper before the direction in the flue reverses. Note that I've also put elastic silicone over the joints of the double-walled stove pipe, which stays remarkably cool. When starting it up from stone-cold now, say after I've burned the oil boiler for a while, I've taken to putting a plastic bag over the baro damper. IMGP3444.JPG

edit: Old picture. I've since had the extension of the middle wood insert chimney "double flue-ized".
 
is this the kind of barometric damper you used? Does it replace one section of the stove pipe?

http://www.homedepot.com/p/Vogelzang-Barometric-Damper-BD-06/203630384?cm_mmc=Shopping|THD|G|0|G-BASE-PLA-D28I-Fireplaces|&gclid=Cj0KEQiAycCyBRDss-D2yIWd_tgBEiQAL-9RkgYs3g5lLRrXtrF5MtvdbYTIlL9Z8dB4Ja4VexXxXYIaAkGV8P8HAQ&gclsrc=aw.ds

What about installing top mount dampers in all three flues, then closing off the un-used ones completely when not in use? I know they're usually for an open fireplace - is there any way to use them with a stove, how would you run the pull chain down with the liner/pipe?

http://www.woodlanddirect.com/Chimney/Top-Sealing-Chimney-Dampers/Seal-Tight-Chimney-Top-Damper
 
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Usually the chimney heights are set so that the stove (or fireplace) that is lower, say in the basement) has the lower flue. Negative pressure is more apt to happen in the lower stove or fireplace than the upper. It's now always a perfect solution. There are days of very low pressure and calm winds when smoke can sink but usually that is many feet away from the chimney as the smoke cools down.
 
twd000 said:
I had my chimneys clean a month ago and declared ready to burn. There are three separate flues sharing the same chimney, but they are all independent. So far I have had two fires in the open fireplace on the first floor. Both times, I noticed a large draft of cold, smoky air drafting into the basement. It is coming through the collar of the woodstove, which I have never used. What is causing this effect, and how can I prevent it in the future? Do you think there is enough conduction from the hot flue to the cold flue, to drive a pressure difference? And would that be enough suction to pull smoky air from the open environment, down the cold flue?

I tried to attach the chimney sweep's inspection report, but the forum tells me it is too large (3.5 MB PDF)
Click to expand...
The main problem your home is experiencing is your home is under negative pressure which is what is causing the smoke to be drawn into your other flue. Here in our neck of the woods, we will extend the main flue that is used higher than the opposite. Make sure your partition separating your flues is intact and not allowing smoke to enter. Another issue that creates this issue would be a running furnace or source which needs to circulate your interior air with a cold air supply which draws air from the path of least resistance which is a open window,door or opening like down a chimney. Another cure for separating the flues would be putting on a Custom Chimney Cap that has a partition between the flues or tiles. This will normally create enough separation to resist a downdraft occurrence. Another concern to be aware of is downdraft occurring due to high winds. This is kind of obvious if things have been overly windy in your area as of late. So if the inspection report is thorough and accurate, it sounds like a issue that can be addressed by your local chimney supply company which should be able to help you with all the above. Best wishes
 
Same issue with our latest install for a EKO gasser in the basement.
Oil furnace with a power vent sucks so much air in the basement that the normal barometric damper leaks - we had to take it off when the basement was full of smoke during the night - not fun.
We know we also have a high wind issue at our location. We can easily see the draft at 0.12 "h20 and above in high winds. And other times its as low as 0.02"h20. Thats when the EKO40 starts to "puff"...

So does anyone sells or know someone who distributes these (or similar) positive seal barometric regulators in Canada (Ontario) ?
K&W model ZUK150S
I've just contacted the manufacturer so I'll wait and see. I know some of these positive seal regulators are shipped with Windhager pellet boilers. Saw the pictures around here and on John's Siegenthaler's biomass heating course.

Thanks
Trex83
 
Thanks for all the suggestions. I have switched from using the fireplace in the main floor, to using the Woodstove in the basement. The immediate problem has gone away but the phenomenon still bothers me. Extending the flue 6-8" may stop the smoky air problem, but it doesn't fix the cold air problem. I have been working on sealing up the house and saving money on energy bills. It bothers me that there is a strong pressure differential pushing cold air down the unused flues every time I run a wood burning appliance in a different flue. Those air leaks can really add up, and negate the cost savings of heating with wood
 
I have one of those baro dampers with my Windhager. It's better sealing than a Field, but still not a total seal.

My opinion is that if the pipe to the basement wood stove, including the collar, would be better sealed, there would be less smell. I don't think the downdraft air will escape through the wood stove that easily, plus the air control can be turned down so that only a small, maybe high resistance?, opening would remain for the air to escape from.

As far as "why", I haven't found a good reason, and I believe my house is pretty well insulated and sealed.
 
I was looking at the stove pipe and collar and trying to figure out how to seal it up better. The thermal expansion of that sheet metal is going to make it impossible to make an airtight seal. HVAC tape over the joints would create a near perfect seal, but the smell of that mastic glue melting would not endear me to the wife ;-)
 
velvetfoot said:
That's the non flexible stuff, right? I think it'll come off, but I'm not sure.
Click to expand...
From reading the info on the tube, it cures hard, not flexible. It talks about grinding or sanding to remove it. Not something I would look forward to, when removing the pipe sections for cleaning. Is there another product out there that would peel off like a silicone bead when I need to remove it?

Anyone tried this stuff? http://www.acehardware.com/product/...43&cp=2568443.2568453.2627947.2627970.2627967
 
I think that's like glue, plus, I don't think it'll hold up too well under wood stove heat, esp closer to the stove.
 
If that's like the RTV I used once on our stove gasket take them seriously. The stuff I used (Permatex Blue) turned to rock. It was a real bear to remove.
 
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