Down Draft from Second Chimney Flue

[Karottop]

Hey Everyone,

I'm new to wood burning and just got finished with a renovation that included two wood burning inserts on the top and bottom floors of the house.

I lit the basement wood stove yesterday and everything went very well except we had a bunch of smoke pouring into the upstairs. I couldn't figure out the source and nothing seemed to be hot but I was a little panicked and almost called the fire dept. Then the smoke stopped and I was able to enjoy the rest of my fire

Today I burned again to see if I could replicate the problem. Same thing happened - smoke coming in upstairs. This time I was a little more calm and collected so I was able to determine that the smoke was coming down the upstairs chimney liner and out of my upstairs wood stove. I closed the damper on the the upstairs stove but that didn't seem to stop it.

I've been reading online that this is a common problem when you have two chimney flues right next to each other. Apparently the fire sucks in air from outside to fuel it and the easiest place to get it is from the chimney.

I tried opening a window, but that didn't seem to fix it unless I opened a bunch of windows - which will not work for us in the winter time. I could raise the basement flue so that the smoke is less inclined to go down the other flue opening, but what if I want to light a fire upstairs? Is there something I can install at the chimney cap to prevent down drafts? any other ideas on how I can easily prevent airflow into my house when burning?

 

[begreen]

Yes, extending the flue often cures this issue. It's interesting that you are experiencing this with the upstairs flue. Usually it is the opposite. It's still pretty warm in Baltimore. When temps drop below 50 things may begin to behave better.

 

[DAKSY]

Another option would be to put an OAK on the downstairs unit, if possible.
The air (with smoke) that is being drawn in is replacement combustion air. An OAK may mitigate the effect.

 

[Karottop]

Yes, extending the flue often cures this issue. It's interesting that you are experiencing this with the upstairs flue. Usually it is the opposite. It's still pretty warm in Baltimore. When temps drop below 50 things may begin to behave better.

Ok that should be a simple fix. How much should I raise it? I have a 12" section of rigid SS stove pipe, is that enough? I've mortared the flex liner into the top of the 13" x 13" terracotta a few inches deep so could I just extend a section off the flex liner and put a cap on the top of it? or do I need to lay brick around it for support?

 

[Karottop]

Another option would be to put an OAK on the downstairs unit, if possible.
The air (with smoke) that is being drawn in is replacement combustion air. An OAK may mitigate the effect.

What is an OAK? where does it get installed on the insert?

 

[showrguy]

What is an OAK? where does it get installed on the insert?

OAK= Outside Air Kit

 

[DAKSY]

What is an OAK? where does it get installed on the insert?

OAK = Outside Air Kit...You will have to look at your owner's manual to see if that is an option for your unit

 

[bholler]

Ok that should be a simple fix. How much should I raise it? I have a 12" section of rigid SS stove pipe, is that enough? I've mortared the flex liner into the top of the 13" x 13" terracotta a few inches deep so could I just extend a section off the flex liner and put a cap on the top of it? or do I need to lay brick around it for support?

As a test extending with single wall is fine. But if that fixes the problem it should be extended with a section of insulated chimney. The top of the liner also should not be mortared in it csn cause lots of problems.


As far as the oak goes it may help but many times it isnt enough. Especially if it took multiple open windows to fix it.

 

[Karottop]

As a test extending with single wall is fine. But if that fixes the problem it should be extended with a section of insulated chimney. The top of the liner also should not be mortared in it csn cause lots of problems.


As far as the oak goes it may help but many times it isnt enough. Especially if it took multiple open windows to fix it.

Bummer, I though I was being clever mortaring it in since they said the bottom part of the chimney cap is supposed to be siliconed down anyways. I should be able to remove it pretty easily.. I think

I tried researching issues with mortaring the liner in but couldn't find any information about it. What kind of problems will this cause?

 

[Karottop]

OAK = Outside Air Kit...You will have to look at your owner's manual to see if that is an option for your unit

Ok I'll look into this, thanks. I have an interior chimney so it might be difficult to get outside air but def worth looking into

 

[bholler]

Bummer, I though I was being clever mortaring it in since they said the bottom part of the chimney cap is supposed to be siliconed down anyways. I should be able to remove it pretty easily.. I think

I tried researching issues with mortaring the liner in but couldn't find any information about it. What kind of problems will this cause?

When that liner heats up it is going to expand. It needs room to do that if it doesnt have room something will give. Either the liner will buckle or the masonry will crack.

 

[bholler]

Ok I'll look into this, thanks. I have an interior chimney so it might be difficult to get outside air but def worth looking into

it may not fix your problem entirely but it sounds like you have negative pressure issues with your house so it would be a good addition if it can be done correctly.

 

[Karottop]

When that liner heats up it is going to expand. It needs room to do that if it doesnt have room something will give. Either the liner will buckle or the masonry will crack.

Thank you for all your help bholler! One more question - is 12" enough to raise the basement chimney to prevent the downdraft upstairs? Or is trial and error the only approach?

 

[bholler]

Thank you for all your help bholler! One more question - is 12" enough to raise the basement chimney to prevent the downdraft upstairs? Or is trial and error the only approach?

12" may work fine. 18" is usually recomended but if you have 12" i would try it.

 

[Karottop]

12" may work fine. 18" is usually recomended but if you have 12" i would try it.

Great, thank you! Good advice is hard to find - but it appears to be in an abundance on this site

 

[begreen]

Is this a 1 or 2 story house? Negative pressure on the 1st floor is less common. I'm wondering if there are other issues that are causing negative pressure in the fireplace. Examples might be a 2nd floor window or attic vent open, attic fan running or large kitchen exhaust fan running.

 

[Karottop]

Is this a 1 or 2 story house? Negative pressure on the 1st floor is less common. I'm wondering if there are other issues that are causing negative pressure in the fireplace. Examples might be a 2nd floor window or attic vent open, attic fan running or large kitchen exhaust fan running.

definitely still open to other causes - someone else also mentioned that it wasn't very common for the upstairs chimney to pull down air too. its a 2 story house with a large attic that has an attic fan and a carpenters gap all the way around the attic so there is a lot of ventilation in the attic. I can't imagine I have a very tight house - it was built in the 50s and the fins where the attic fan are cannot be very air tight but I guess its still possible. I didn't have an attic fan running, kitchen fan running or 2nd floor windows open until the smoke started coming in.

The draft in the downstairs fireplace is very powerful. I only seem to notice the issue when I'm first starting the fire trying to get the cat up to 500 degrees as fast as possible so I can engage it. during this time I have all the dampers open on the wood stove so combine that with a strong draft and I figure a pretty good amount of air is being pulled out of the house through the downstairs chimney. Once I damper the stove down and engage the cat, the smoke stops coming in upstairs

 

[begreen]

What may be happening is that the house is acting like a chimney with a large volume of air being exhausted via the attic and/or 2nd floor windows. The 1st floor fireplace chimney is reversing to supply makeup air. Before changing anything, on a cool night, try again with the attic vents and all windows closed, and any exhaust fans off. See if the problem still happens.

 

[Karottop]

What may be happening is that the house is acting like a chimney with a large volume of air being exhausted via the attic and/or 2nd floor windows. The 1st floor fireplace chimney is reversing to supply makeup air. Before changing anything, on a cool night, try again with the attic vents and all windows closed, and any exhaust fans off. See if the problem still happens.

Ok I'll try that. All attic vents were closed when I was having the problem but since I have that attic fan I think the air freely leaks through the fins even when the fan isn't running. Maybe I'll try to put something over the fins to resist air flow but I'd have to come up with some sort of a long term solution if that's the problem

Just to make sure I'm reading you right, when you say 1st floor, you mean basement and 2nd floor is the upstairs? bc the fire is in the basement (1st floor) and smoke is coming in upstairs fireplace (2nd floor)

 

[begreen]

Ok I'll try that. All attic vents were closed when I was having the problem but since I have that attic fan I think the air freely leaks through the fins even when the fan isn't running. Maybe I'll try to put something over the fins to resist air flow but I'd have to come up with some sort of a long term solution if that's the problem

Just to make sure I'm reading you right, when you say 1st floor, you mean basement and 2nd floor is the upstairs? bc the fire is in the basement (1st floor) and smoke is coming in upstairs fireplace (2nd floor)

Is there a ceiling vent to the attic? If yes, that should be sealed for the winter.

You indicated this is a 2 story house. The basement usually does not count for a story, but maybe this is a daylight basement.

 

[Karottop]

Is there a ceiling vent to the attic? If yes, that should be sealed for the winter.

You indicated this is a 2 story house. The basement usually does not count for a story, but maybe this is a daylight basement.

ah! got it, sorry for the confusion. Its a rancher with an attic and a basement where half of it is daylight so we call the basement the downstairs and we call the main floor the upstairs, but I think your right, this must be considered a 1 story house in that case.

I will look into a way for sealing off the attic fan vent seasonally first since that would be an easy permanent fix. If that doesn't work I'll test extending the chimney and let you all know how it turns out

 

[Karottop]

Ok so after some changes, the downdrafting issue has improved a lot. I blocked off the attic fan using a thin piece of underlayment with strong magnets and some weather stripping so I can take it on and off easier. I also sealed around a lot of the holes on the 1st floor ceiling (not the basement) using spray foam, particularly around the chimney! I can't believe how much air was leaking into the attic from the basement two stories below by sneaking around the chimney. I suspect this will help keep my house warm in the winter as well.

The chimney still downdrafts but not as much, so thank you begreen for the suggestion - definitely would not have considered this otherwise. I also noticed that the downdrafting effect is greater when I have my central air running. When its off, I get much less smoke on the main floor.

I just bought a flue extender for the basement chimney to rise up above the other chimney cap as suggested by a few of you folks. I'll let everyone know if that fixes the problem once and for all. Thanks again for everyone's advice!

 

[bholler]

Ok so after some changes, the downdrafting issue has improved a lot. I blocked off the attic fan using a thin piece of underlayment with strong magnets and some weather stripping so I can take it on and off easier. I also sealed around a lot of the holes on the 1st floor ceiling (not the basement) using spray foam, particularly around the chimney! I can't believe how much air was leaking into the attic from the basement two stories below by sneaking around the chimney. I suspect this will help keep my house warm in the winter as well.

The chimney still downdrafts but not as much, so thank you begreen for the suggestion - definitely would not have considered this otherwise. I also noticed that the downdrafting effect is greater when I have my central air running. When its off, I get much less smoke on the main floor.

I just bought a flue extender for the basement chimney to rise up above the other chimney cap as suggested by a few of you folks. I'll let everyone know if that fixes the problem once and for all. Thanks again for everyone's advice!

That tells me your hvac system is unbalanced

 

[Karottop]

That tells me your hvac system is unbalanced

You mean its drawing more air from upstairs than it is downstairs? should I block off a return upstairs to see if that fixes the problem?

 

[bholler]

You mean its drawing more air from upstairs than it is downstairs? should I block off a return upstairs to see if that fixes the problem?

You could try that it certainly could help.