Smoke in the basement?

[jmv]

Folks, I'm a new homeowner and am completely stumped by a fireplace issue I'm having, and I'm hoping some of the seasoned pros here can help me understand what's going on.

Our home has a main floor fireplace and a basement fireplace, and we get smoke coming through the basement fireplace anytime we have a fire in the main fireplace. Not unusual, from what I understand.

Here's the kicker, though: the basement fireplace flue is capped off at the top and stuffed with batt insulation at the bottom. Even so, enough smoke enters the basement that it's a problem. Before I stuffed the batt insulation into the bottom of the basement fireplace flue, it was far worse.

The chimney sweep who inspected the chimney is as stumped as I am. My brother-in-law is an architect and suspects a breach between the two flues. I wonder if the cap on the basement flue isn't well-sealed, and that smoke is being pulled back into the house. Or it could be something else. What do y'all think?

I've attached a diagram to make things clearer. Thanks in advance for any ideas you have!

 

[begreen]

This is a fairly common problem. Smoke may be getting siphoned into the lower pressure basement fireplace flue due to lower pressure there. The issue is that all of the tops of the flue are on the same plane. The solution usually is to raise the upper floor flue 12-18", but you could cap off the basement flue if it is no longer in service with a plate glued with silicone to the top of that flue. In this case, a sign in the basement fireplace stating that the flue is closed off is strongly advised.

Stuffing it with insulation is the clue. Insulation is porous, so it will only work partially. Try adhering a plate on top with a fat bead of silicone adhesive. If that does not stop the issue, then your B-I-L may be right, but consider the most likely issue first.

 

[jmv]

This is a fairly common problem. Smoke may be getting siphoned into the lower pressure basement fireplace flue due to lower pressure there. The issue is that all of the tops of the flue are on the same plane. The solution usually is to raise the upper floor flue 12-18", but you could cap off the basement flue if it is no longer in service with a plate glued with silicone to the top of that flue. In this case, a sign in the basement fireplace stating that the flue is closed off is strongly advised.

Stuffing it with insulation is the clue. Insulation is porous, so it will only work partially. Try adhering a plate on top with a fat bead of silicone adhesive. If that does not stop the issue, then your B-I-L may be right, but consider the most likely issue first.

Thanks, begreen. There already is some kind of plate capping off the basement flue, but I don't know how well it's sealed. Could be decades old with degraded adhesive. Maybe that's step #1--to get on the roof and see what the state of that seal is.

 

[kennyp2339]

Does the main floor fireplace have a ash clean out in the floor? If it does lit a few matches and blow them out to see if the smoke is getting sucked into that plate (which will ultimately terminate near the basement clean out)

 

[jmv]

Does the main floor fireplace have a ash clean out in the floor? If it does lit a few matches and blow them out to see if the smoke is getting sucked into that plate (which will ultimately terminate near the basement clean out)

There is an ash clean out plate in the floor of the main fireplace, and I tried what you suggested. The smoke didn't get sucked into it, though.

 

[jatoxico]

The chimney sweep who inspected the chimney is as stumped as I am. My brother-in-law is an architect and suspects a breach between the two flues.

Inspected how? If these are supposed to be three independently lined flues and you're sealed up top even marginally well, I would be suspicious that something is missing or broken within the structure as well. Need a camera.

 

[jetsam]

If the cap is ok, run a camera through all three flues. Maybe there's a hole. Maybe it's not really three seperate flues!

You can get a wifi inspection camera that will do the job for around $30 on Amazon (search for boroscope or endoscope).

 

[jmv]

Thumbs up on the camera idea! I'm pretty sure the inspection was fairly quick and didn't involve a boroscope. I think I'll order one and see what I discover.

 

[Corey]

Sounds like you're on the right track. It seems like the two main keys here are the basement flue 'sealed at the top / fiberglass batt insulation at the bottom', and the possibility of a crack connecting the flues along the way.

You can try to bore scope it if you want, but in a chimney of that age, I'd expect there are going to be 'some' cracks and likely other areas that look highly suspicious. So at best the video is going to suggest possibilities of cracks anyway. You already know you have smoke smell, so don't know that the inspection adds much more evidence?

So then the question becomes what to do about it. Two main approaches I see there. First is obviously to fix / reline the flue - especially if damage is bad and you intend to continue using the fireplace. The ideal option would be to install a wood stove and liner so you get actual usable heat out of your hard won wood. Though obviously that is subject to personal taste and individual use.

The second option would be to bandaid the problem. I suspect the main reason for smoke smell is that the fiberglass batt in the bottom isn't really an air stop. So you have two lengths of flue next to each other, one exhausting smoke and one trying to bring in fresh air to replace it. If you can make the bottom seal air tight, too, at least that stops the direct flow of air from the flue out into the hosue. Though old masonry chimneys can be a labyrinth of minor cracks and channels in the mortar and bricks so that still might only be a partial fix.

Good luck!

 

[kennyp2339]

If you pull a fiberglass bat out, does it stink like old chimney smoke?

 

[MagdalenaP]

Do you have a dryer in the basement?

 

[jmv]

Sounds like you're on the right track. It seems like the two main keys here are the basement flue 'sealed at the top / fiberglass batt insulation at the bottom', and the possibility of a crack connecting the flues along the way.

You can try to bore scope it if you want, but in a chimney of that age, I'd expect there are going to be 'some' cracks and likely other areas that look highly suspicious. So at best the video is going to suggest possibilities of cracks anyway. You already know you have smoke smell, so don't know that the inspection adds much more evidence?

So then the question becomes what to do about it. Two main approaches I see there. First is obviously to fix / reline the flue - especially if damage is bad and you intend to continue using the fireplace. The ideal option would be to install a wood stove and liner so you get actual usable heat out of your hard won wood. Though obviously that is subject to personal taste and individual use.

The second option would be to bandaid the problem. I suspect the main reason for smoke smell is that the fiberglass batt in the bottom isn't really an air stop. So you have two lengths of flue next to each other, one exhausting smoke and one trying to bring in fresh air to replace it. If you can make the bottom seal air tight, too, at least that stops the direct flow of air from the flue out into the hosue. Though old masonry chimneys can be a labyrinth of minor cracks and channels in the mortar and bricks so that still might only be a partial fix.

Good luck!

I'll see what the boroscope reveals, but I'm beginning to think that relining the flue is a possibility here. We'd like to be able to use the fireplace regularly without having to keep applying band-aid solutions. And a wood stove insert is a tempting prospect if we end up relining anyway! Seems like if I can verify whether there are cracks connecting the flues and also assess the state of the seal on the plate capping off the basement flue, I'll at least know the path the smoke is traveling and can then choose an appropriate solution.

The fiberglass insulation definitely smells like smoke, and we do have a dryer in the basement, though I've made sure it's not running while the fire is going. Same with the bathroom fan.

I'm getting a much clearer mental picture of what's going on, as well as a range of potential solutions. Thanks!

 

[DaHadster]

Out of curiosity, did you resolve this issue? I have a dual flue - one large one for the fireplace and a smaller one for an oil burner. Whenever I use the fireplace, the utility room (under the fireplace) gets smokey. We switched over to central air/heat pump and the oil burner vent was capped in the basement prior to moving in. We had major water intrusion issues and had a gutter system installed along the perimeter of the basement and they sealed the fireplace clean out box with a cinderblock and mortar (water would pour in from there during heavy rains) and covered it with liner that feeds into the gutter. I was going to call a chimney company to come cap the oil burner flue at the top of the chimney stack but found a post saying it may make things worse. We had the chimney inspected and cleaned before moving in and they said it looked fine (did not use a camera). My neighbor suggested it was probably missing/cracked mortar joints in the box that need to be re-pointed, but everything looks okay to me.

Have all this wood and would love to use it :-/. Do people normally crack a window when using the fireplace to allow air in?

 

[jmv]

To be honest, I haven't moved on this since I last posted. When I do, though, I'll be sure to update this thread. Best of luck with your situation!

 

[jmv]

Hi everyone--I wanted to be a good internet citizen and share how we fixed this problem, a year later.

I did attempt to use a boroscope to check out the flues, but I couldn't see much. In order to figure out if the basement fireplace flue really was sealed tight on top (or whether it was poorly sealed, allowing smoke to draft back down), I had the chimney sweep back out a couple weeks ago to climb up on the roof and take a close look while I lit a small fire in the main fireplace. He confirmed that the basement fireplace flue was completely sealed with a plate and a silicone bead. He did remark that the cap shared by the other two flues was pretty low, and if he removed it for the winter season, it might help the basement smoke issue a bit by improving the draft on the main flue. I told him to go ahead.

Problem solved. We've had 3 fires in the main fireplace, and none have resulted in any smoke coming back in through the basement fireplace. I don't understand the mechanism here---if the basement flue was well-sealed, what does it matter how the flue next to it is drafting?---but I'm not going to argue with success. I welcome any commentary, but mostly I just wanted to share what worked for us in case others in similar situations find this post in the future.

Thanks again for the help in thinking through this last year! This is a great community.

 

[begreen]

Hi everyone--I wanted to be a good internet citizen and share how we fixed this problem, a year later.

I did attempt to use a boroscope to check out the flues, but I couldn't see much. In order to figure out if the basement fireplace flue really was sealed tight on top (or whether it was poorly sealed, allowing smoke to draft back down), I had the chimney sweep back out a couple weeks ago to climb up on the roof and take a close look while I lit a small fire in the main fireplace. He confirmed that the basement fireplace flue was completely sealed with a plate and a silicone bead. He did remark that the cap shared by the other two flues was pretty low, and if he removed it for the winter season, it might help the basement smoke issue a bit by improving the draft on the main flue. I told him to go ahead.

Problem solved. We've had 3 fires in the main fireplace, and none have resulted in any smoke coming back in through the basement fireplace. I don't understand the mechanism here---if the basement flue was well-sealed, what does it matter how the flue next to it is drafting?---but I'm not going to argue with success. I welcome any commentary, but mostly I just wanted to share what worked for us in case others in similar situations find this post in the future.

Thanks again for the help in thinking through this last year! This is a great community.

That's what I suspected. When all the flues terminate at the same height, smoke can get siphoned to the lower (basement) fireplace. A flue extension cap can remedy the situation when placed on the main floor's flue.

Chimney Flue Extension | Chimney Extension Kit - Rockford Chimney

The Specialty Flue Extension will increase height and draft of your chimney and features an attached chimney cap. Easy to install and ships free.

www.rockfordchimneysupply.com

 

[stoveliker]

That's what I suspected. When all the flues terminate at the same height, smoke can get siphoned to the lower (basement) fireplace. A flue extension cap can remedy the situation when placed on the main floor's flue.

Chimney Flue Extension | Chimney Extension Kit - Rockford Chimney

The Specialty Flue Extension will increase height and draft of your chimney and features an attached chimney cap. Easy to install and ships free.

www.rockfordchimneysupply.com

but how does taking the cap of a capped flue *solve* the problem then...?
It seems the opposite of what you said...

 

[jmv]

That's what I suspected. When all the flues terminate at the same height, smoke can get siphoned to the lower (basement) fireplace. A flue extension cap can remedy the situation when placed on the main floor's flue.

Chimney Flue Extension | Chimney Extension Kit - Rockford Chimney

The Specialty Flue Extension will increase height and draft of your chimney and features an attached chimney cap. Easy to install and ships free.

www.rockfordchimneysupply.com

I follow you (I think), but what's so strange is that the basement fireplace flue terminates in a plate at the top, which is sealed with silicone. So there should be no way for the smoke to be siphoned into that flue---it's sealed! Yet smoke was coming out of the basement fireplace nonetheless, and removing the double chimney cap (similar to the one below) from the other two flues solved the problem.