Elusive Smoke...

[Northbc]

Hi folks - ten years ago we bought a house with a late 1990’s Harman P38 in the basement. With routine maintenance this stove has been 100% reliable. But we often get a smoke smell in the basement. It makes our eyes water and I can taste it. This is an annoying problem that I haven’t been able to fix.

The stove is routinely cleaned, and I installed an outside air kit in 2014. This week I replaced the rope gaskets on both doors. The nipple looks to be secure, with steel screws holding it tight. Vent is 4” Simpson Duravent and a couple years ago I taped all the joins that I can find. The wall thimble looks like a place that could leak so several years ago I did some sealing with high temp silicone. I can’t detect drafts up there now, and I could before.

The vent seems clean. Last year (for the first time ever) I blew it out with a compressor and there was only a small amount of fines/dust on the ground outside. I routinely clean the temp probe.

Still, on some days/in some weather conditions there is no issue. But more often than not we can smell smoke in the house. This is not a ‘smoke at start-up’ thing, but smoke that develops as the stove is burned for a few hours. I have examined around the stove and vent in a dark room with flashlight but I can’t see any smoke.

I have noticed this most commonly happens a) when the flame is lazy, b) when there are inversion-like pressures outside, c) during very cold sub-freezing / hot indoor temp scenarios, and d) when my wife lights the stove (I don’t understand why, and this one is a sensitive topic

Interestingly, while smoke is sometimes detected in the main stove room, it usually smells first in our furnace room at the back of the house. One explanation could be that smoke is coming in around the wall thimble, travelling above the drop ceiling and then sitting in the cooler furnace room.

I think this is a vent thing. Any suggestions welcomed. I haven’t had a really close examination of the wall thimble because it’s messy up there. And today it is -15 degrees Celsius!

 

[kappel15]

Don't know what type of basement you have, but there could be gaps in the sill between block wall and sill plate. Some windows can leak, as well as doors. kap

 

[Northbc]

Don't know what type of basement you have, but there could be gaps in the sill between block wall and sill plate. Some windows can leak, as well as doors. kap

Thanks Kappel. It is an insulated concrete foundation from mid 90’s. There are no windows or doors along the side where the stove vents, but the rim joist was not well sealed initially. I used a few cans of spray foam years back to get a better seal song that wall.

What is the proper way to seal around the wall thimble? I recall it was a very rough opening with large gaps. I did my best with caulking when I looked at it years ago.

Is this a case where a professional could inspect and diagnose? Any hints on who to call?

 

[kappel15]

They do make a fire proof insulation that could be stuffed in areas. I think there is also a fire retardant spray. I don't know if a CO detector would work on smoke or not. And I don't know of anyone as not from your area. if you caulked around square frame of wall thimble and around pipe going into it , and same on outside, it should be sealed. If you have some kind of negative draft going on, you may need to install an oak. kap

 

[Northbc]

They do make a fire proof insulation that could be stuffed in areas. I think there is also a fire retardant spray. I don't know if a CO detector would work on smoke or not. And I don't know of anyone as not from your area. if you caulked around square frame of wall thimble and around pipe going into it , and same on outside, it should be sealed. If you have some kind of negative draft going on, you may need to install an oak. kap

I did install an OAK a few years ago. It also improved the air quality and burn quality of the stove, but still getting indoor smoke.

Is an HVAC expert likely to be helpful here? There is an OAK for the stove and also 10” air supply to the furnace but maybe there is still a negative pressure in the house?

 

[kappel15]

Is there a chance the oak is sucking in exhaust? And I have windows on side of house adjacent to side that has stove. I can get some smoke in them if not sealed properly. And are there eves on house with vents that could let exhaust in?

 

[Northbc]

thanks again. The OAK is about 10’ below the exhaust on an adjacent wall. Even if the OAK pulled in some smoke would it not stay inside the stove box and be emitted out the exhaust?

If there is negative pressure inside the building envelope I agree that it is possible some smoke could enter via multiple sources such as windows, doors, etc but how do you test this? I have sealed the obvious gaps near the exhaust.

The biggest opening I am aware of is the raw air intake that supplies to the furnace. It is on the opposite side of the house but it is possible some smoke could waft in there...

 

[kappel15]

I am betting on the wall thimble, if it is not sealed properly. I am sure that air is being sucked in around it, and that is the source of your smoke. Esp. when you say it is in another area. kap

 

[Northbc]

I am betting on the wall thimble, if it is not sealed properly. I am sure that air is being sucked in around it, and that is the source of your smoke. Esp. when you say it is in another area. kap

Thanks Kap I will see if I can get at it. For a Simpson Duravent, is it easier to access from inside or outside? Basically I want to have a go at stuffing fireproof insulation and RTV around the outside gaps between the thimble and the rough wall opening.

 

[kappel15]

Like most, it is two pc.'s that slide together on the pipe part. It is good to have dead air space around the pipe part. I don't know why you can't use high temp caulk around the square part that attaches to wall and side of house, and then around the pipe that goes thru it. This way you don't have to take anything apart.kap

 

[JRemington]

I’d make sure the stove adapter is sealed tight around the flue. Also look very carefully at all the seams on any elbows that you have. We’ve had two cases in the last month of tiny splits in the seams of elbows. When you start the stove sniff all around all the elbows and where pipes connect.

 

[Northbc]

I’d make sure the stove adapter is sealed tight around the flue. Also look very carefully at all the seams on any elbows that you have. We’ve had two cases in the last month of tiny splits in the seams of elbows. When you start the stove sniff all around all the elbows and where pipes connect.

Thanks for the suggestions all. I checked this weekend and I really can’t find any gaps that I haven’t sealed in the past. I suspect I could be looking at a house negative pressure issue that happens in some weather conditions. No smoke for the last few burns...

I have an appointment with a stove installer to come and inspect it in 2 weeks. I really hope to fix the issue.

 

[Northbc]

Hi again. I am responding to a post I made nearly a year ago, to share a bit more information and ask for help. I still have smoke leaking into the house and I am confident it is coming in somewhere between the elbow and the thimble in photos. I can smell smoke there, which seems to build when the stove is burning/hot. I have tried shining a light on it in the dark but still can't see where it is leaking. As you can see I have taped all of the Simpson Duravent joins, put high temp silicone where the vent penetrates the wall, and spray foamed around rim joist and floor joists. No fire alarms going off, but a REALLY annoying smoke smell. At least I've narrowed it down to this location... Any advice on finding the source of the smoke?

 

[JRemington]

Something about that setup makes me want to cut it all off with a sawzall and start over. Can you get a picture where the pipe connects to the stove?

 

[Northbc]

I hear you. I wasn’t the installer but I surely would have done a cleaner job. Here is the pic you asked for

 

[SciGuy]

Hi again. I am responding to a post I made nearly a year ago, to share a bit more information and ask for help. I still have smoke leaking into the house and I am confident it is coming in somewhere between the elbow and the thimble in photos. I can smell smoke there, which seems to build when the stove is burning/hot. I have tried shining a light on it in the dark but still can't see where it is leaking. As you can see I have taped all of the Simpson Duravent joins, put high temp silicone where the vent penetrates the wall, and spray foamed around rim joist and floor joists. No fire alarms going off, but a REALLY annoying smoke smell. At least I've narrowed it down to this location... Any advice on finding the source of the smoke?

I may be wrong but it appears that one of the joints isn't taped. When in doubt seal it up. I've added an arrow to point to the area my ancient eyes are drawn to.

I'd also check to see that all the aluminum tape is well stuck down. I've seen it lift off if the original surface wasn't cleaned up enough. Skin oil is sometimes enough to prevent adhesion.

YMMV,

Hugh

 

[JRemington]

I hear you. I wasn’t the installer but I surely would have done a cleaner job. Here is the pic you asked for

What is with all the tape? Did you pay for that installation? I’m guessing there’s no high temp sealant on the stove flue. Take the tape off the stove flue and see if it has high temperature sealant on it. If you’re getting smoke that’s my guess.

 

[Northbc]

Thanks Hugh. All seams/joins are well taped and I have examined all of the vent and can’t see any obvious voids. I think the wall thimble could be an obvious spot for smoke to enter. Have a look at this pic of the exterior. Should the siding have been cut away and the thimble sealed to the sheathing? The rough opening is really rough with a large gap through the sheathing. I have sealed it up on the inside but...

 

[SciGuy]

What is with all the tape? Did you pay for that installation? I’m guessing there’s no high temp sealant on the stove flue. Take the tape off the stove flue and see if it has high temperature sealant on it. If you’re getting smoke that’s my guess.

Jeff,

The thing about silicone is that once you've juiced a joint up it becomes incredibly difficult if not impossible to ever get it apart should you want/need to. Aluminum tape on the other hand can be pulled off more easily if need be.

YMMV

Hugh

 

[Northbc]

What is with all the tape? Did you pay for that installation? I’m guessing there’s no high temp sealant on the stove flue. Take the tape off the stove flue and see if it has high temperature sealant on it. If you’re getting smoke that’s my guess.

I did not do the install. It came with the house and leaked smoke badly. The flue is high temp sealed and I taped it as well, going crazy trying to eliminate all smoke sources.

 

[JRemington]

The pipe has to be a minimum of 12 inches from combustible materials on the outside of the home. You could add a 6 inch piece to that and then cap it. If it’s siliconed where the pipe goes through the thimble and around the thimble you’re not going to get any smell from the outside.

 

[gfreek]

The pipe has to be a minimum of 12 inches from combustible materials on the outside of the home. You could add a 6 inch piece to that and then cap it. If it’s siliconed where the pipe goes through the thimble and around the thimble you’re not going to get any smell from the outside.

Agree, think vent cap should be further from the house

 

[JRemington]

I did not do the install. It came with the house and leaked smoke badly. The flue is high temp sealed and I taped it as well, going crazy trying to eliminate all smoke sources.

Did you check for smoke in the dark with a flashlight while it was already running or on start up? It has to be done at start up. Any smoke will show up huge in the dark with a flashlight. If you don’t see any at start up there isn’t any. Is there any chance what you’re smelling is high temp sealant?

 

[Northbc]

Yes - have checked for smoke in the dark but can’t find any. I have sealed up everything obvious on the inside of house but never really looked at the outside of thimble. I think I found my problem. Huge gaps around the outside of the thimble and the outside face of the thimble wasn’t sealed or even close to the wall sheathing.

I closed up the gap with fire-rated foam and trimmed away the siding and tightened up the thimble against the wall sheathing.

My theory is that the smoke was getting sucked from the vent, around the thimble and back into the house. Time will tell if I was correct.

thanks for all the great ideas and suggestions!