Elusive Smoke...

Northbc

Member
Dec 5, 2016
9
Prince George bc
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!

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Last edited:

kappel15

Minister of Fire
Sep 14, 2014
2,335
Iowa
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

Member
Dec 5, 2016
9
Prince George bc
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

Minister of Fire
Sep 14, 2014
2,335
Iowa
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

Member
Dec 5, 2016
9
Prince George bc
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

Minister of Fire
Sep 14, 2014
2,335
Iowa
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

Member
Dec 5, 2016
9
Prince George bc
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

Minister of Fire
Sep 14, 2014
2,335
Iowa
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

Member
Dec 5, 2016
9
Prince George bc
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

Minister of Fire
Sep 14, 2014
2,335
Iowa
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

Minister of Fire
Nov 4, 2017
533
Belleville New York
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.
 
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Northbc

Member
Dec 5, 2016
9
Prince George bc
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.