Fire inside my Double Wall Stove Pipe?

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rosen431

New Member
Feb 1, 2021
14
Boston, MA
Ok need some help understanding this. Forgive the long post....Started a fire today loaded up beech and ash in my Woodstock ideal steel hybrid, very well seasoned. Filled the fire box. 35 ft stainless liner from the basement wood stove up through middle of my house, so drafts hard. Especially when cold. I had her cruising along nicely in a black box cat burn (no flames). Pipe damper half closed and stove damper almost all the way closed. Stove top temp starts to get near 600 flue gasses at 500 and I close the intake air on the stove all the way (still gets some air, but basically had it on the lowest air setting), and closed the pipe damper all the way to cool it down and keep the burn where it is. I have a probe thermometer in my double wall so that’s where I’m getting the flue gas readings. Now things get interesting - stove temp stays around the same, creeps up a little but the flue gas temperature starts rising FAST. Goes from 500 to 1200 in like 2 minutes and I smell that paint curing smell. Outside double wall at the hottest spot was about 500 degrees! I call a buddy of mine, he says “open up the pipe damper all the way and the stove damper all the way. If that doesn’t work open the door - you need to flood it with cold air”. Sure enough I opened up the pipe damper all the way and the intake air halfway and the flue temp starts dropping immediately. What am I missing here? Only thing I can think of is I kept more smoke in the stove and that just got the cat going even crazier? Second time this has happened in 10 days and last time I warped the radiator cover pretty good. Aside from adding less fuel, what’s actually going on?
 

moresnow

Minister of Fire
Jan 13, 2015
1,848
Iowa
Welcome to Hearth. What is the radiator cover that you warped previously?
 

rosen431

New Member
Feb 1, 2021
14
Boston, MA
It's the cover that goes on top of the catalytic combustor in the woodstock ideal steel... it's removable and sits between the cat and the the stove top. It's open front and back.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,136
central pa
Need someone with more experience to confirm, but I don't think you are sopossed to close the pipe damper all the way.
There is no problem doing that
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
26,136
central pa
It's the cover that goes on top of the catalytic combustor in the woodstock ideal steel... it's removable and sits between the cat and the the stove top. It's open front and back.
Has anyone ever measured your draft? It sounds like you probably have excessive draft and may need a second damper
 

rosen431

New Member
Feb 1, 2021
14
Boston, MA
Has anyone ever measured your draft? It sounds like you probably have excessive draft and may need a second damper
It's funny you mention this - I just called Woodstock this morning and they suggested the same thing. Loren (at Woodstock) seems to think there may have been a fire inside of the double wall stove pipe (500 degrees at the first 90 degree elbow with 1200 degree flue temps seems pretty hot). How much does a draft measuring device cost and where could I purchase one? I would also be totally open to hiring someone, but candidly, I've had a LOT of trouble finding a qualified person here around Boston. Most chimney sweeps aren't particularly knowledgeable on wood stove installations. I need to find someone really good to help rectify this, I suspect. Thoughts?
 

WoodBurnerInWI

Feeling the Heat
Feb 2, 2020
275
Madison, WI
I have an Ideal Steel with 15' of pipe. It's double wall pipe inside to the class A chimney. I've never had anything like that happen with my stove, when my catalyst temps approach or cross 1200 degrees, the smoke poofs into flame and I get a nice secondary burn at the top of the firebox for a few hrs before it goes away. Because of this, I don't see how the stove could be overfired but then again my draft is nowhere as strong as it would be on a 35' system.

The pipe temps reaching 1200 is concerning. It makes me wonder what the catalyst temps were at that time. Do you have a probe to measure cat temps? If not I would highly suggest getting one. Condar makes a nice one that many use to measure cat temps.

This is the one I use to measure temps, the 4" probe length is perfect for the Ideal Steel:

Amazon product
 

gthomas785

Minister of Fire
Feb 8, 2020
530
Central MA
I've also seen that happen when closing the pipe damper. Don't forget that when you reduce the draft, you're also reducing the amount of cooling air hitting the fire. So temps will go up initially.
 

rosen431

New Member
Feb 1, 2021
14
Boston, MA
I just bought a cat probe thermometer. @gthomas785 that makes sense, especially considering when I opened the damper all the way the flue temp came down very, very fast. Given what you've said, do you believe I had a fire in the double wall stove pipe, or do you believe the flue gas was just that hot? I'm torn here.... from what I've heard, the fact that the temp came down so fast means it wasn't likely a fire. Candidly I'm kind of scared to even run the stove right now without taking it apart and inspecting it, which sucks because it is freezing and we have a blizzard right now.
 

gthomas785

Minister of Fire
Feb 8, 2020
530
Central MA
I just bought a cat probe thermometer. @gthomas785 that makes sense, especially considering when I opened the damper all the way the flue temp came down very, very fast. Given what you've said, do you believe I had a fire in the double wall stove pipe, or do you believe the flue gas was just that hot? I'm torn here.... from what I've heard, the fact that the temp came down so fast means it wasn't likely a fire. Candidly I'm kind of scared to even run the stove right now without taking it apart and inspecting it, which sucks because it is freezing and we have a blizzard right now.
Doesn't sound like it to me, but if there was a fire in there, it wasn't too serious. I think you just overfired the stove. If you crank it hard on a cold night you could have flames shooting into the pipe from the stove without having an actual stovepipe fire. I would burn it again but just keep a close eye on the temp and adjust it gradually, maybe start shutting it down sooner than you did last time. Worst case, you know what to do if it gets too hot again.

If it makes you feel better, take the pipe apart and check for creosote accumulation before lighting it.
 

begreen

Mooderator
Staff member
Nov 18, 2005
90,604
South Puget Sound, WA

Highbeam

Minister of Fire
Dec 28, 2006
19,322
Mt. Rainier Foothills, WA
I've lit off the junk in my double wall before and it did just as you describe. Flue temps on a regularly slow moving condar probe meter spin up fast. Even though stove and cat temp just sit there cool. Tinking sound.

The absolute last thing I would have done is open up the dampers or stove door. Duh. That just makes a blast furnace out of it. I shut everything down as tight as possible and held my breath. Temps came down pretty fast after the flue fire was snuffed.

Make no mistake, that was a chimney fire. Some are bigger than others. In the old days, it was done every day to keep things clean!
 
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begreen

Mooderator
Staff member
Nov 18, 2005
90,604
South Puget Sound, WA
Yes, I had this condition inside our double-wall many years ago when I had to burn some damp maple. That tinkly krinkle sound is pretty distinctive. It did light off once before the flue was cleaned. That was the year of the plugged cap screen. Since then there has been no such excitement.
 

rosen431

New Member
Feb 1, 2021
14
Boston, MA
I've lit off the junk in my double wall before and it did just as you describe. Flue temps on a regularly slow moving condar probe meter spin up fast. Even though stove and cat temp just sit there cool. Tinking sound.

The absolute last thing I would have done is open up the dampers or stove door. Duh. That just makes a blast furnace out of it. I shut everything down as tight as possible and held my breath. Temps came down pretty fast after the flue fire was snuffed.

Make no mistake, that was a chimney fire. Some are bigger than others. In the old days, it was done every day to keep things clean!

Interesting. If the open flue accelerated the fire, why did the flue temps fall almost immediately? Aside from disassembling the double wall to inspect it (not sure what I'm going to see when I do), I'm at a bit of a loss at what to do next here. Seems like I can just keep running her and put in less wood, I guess?
 

gthomas785

Minister of Fire
Feb 8, 2020
530
Central MA
Whenever you feed excess air to a fire, there's a double effect. It gives the fire more oxygen which makes it burn faster / hotter, but it also cools the fire by blowing the heat away up the chimney.

Now if you're having a real chimney fire, this could be a double whammy because that excess heat going up the chimney may increase the temp in the upper flue, in turn accelerating the chimney fire even more. Or, you may get enough excess air to cool it off to the point where it goes out.

It's really a gamble as to which effect is going to dominate, depending on all sorts of factors like how well established the fire vs. chimney fire is, how restricted the flue is, how much fuel/creosote is available, and highly localized temperatures at the moment you open the door.
 

Highbeam

Minister of Fire
Dec 28, 2006
19,322
Mt. Rainier Foothills, WA
Interesting. If the open flue accelerated the fire, why did the flue temps fall almost immediately? Aside from disassembling the double wall to inspect it (not sure what I'm going to see when I do), I'm at a bit of a loss at what to do next here. Seems like I can just keep running her and put in less wood, I guess?

I think you just didn't have much in the flue to burn. That's the magic of the daily chimney fire from the olden days. You burn off just one day's accumulation. When the whole thing is full of tar from a winter's smolder, that chimney fire can blow molten lava from the cap and burn your house down!

Since you really don't know how much gunk is in your flue, once it starts burning I would slam all air inlets shut to try and slow the thing down at least. At best, you may even snuff the chimney fire.

How to prevent? Well, that's tougher. I make an effort to not allow excess fire to shoot up the flue and of course monitor flue temperature to keep accumulation down and stay within the 1000 degree rating of the system. With a bypass on a cat stove, the fire has a pretty direct shot up to the flue so stacking the wood to prevent this is a big help, keeping door closed once the fire is ignited sufficiently, and even turning down the intake air if you still see and hear the the fire getting sucked out of the stove into the flue.

It is important to not have a lot of accumulation in the chimney to cause a disastrous chimney fire should one start. That's why folks recommend sweeping when accumulations at the worst place are over 1/4". That accumulation isn't hurting the flow or performance, it's just a powder keg.

Keep running it, pack it full per the directions if you want, shut the door as soon as you can without snuffing the fire, keep flames out of the flue with stacking and intake control, shut the bypass as soon as the cat meter says "Active", and keep your chimney swept. I see no reason to remove your double wall to look at it unless it is for regular cleaning.
 

begreen

Mooderator
Staff member
Nov 18, 2005
90,604
South Puget Sound, WA
Interesting. If the open flue accelerated the fire, why did the flue temps fall almost immediately? Aside from disassembling the double wall to inspect it (not sure what I'm going to see when I do), I'm at a bit of a loss at what to do next here. Seems like I can just keep running her and put in less wood, I guess?
Small accumulation that burned off quickly? If there had been a major accumulation that could burn for 15-30 minutes the situation would have been quite different.
 

rosen431

New Member
Feb 1, 2021
14
Boston, MA
Keep running it, pack it full per the directions if you want, shut the door as soon as you can without snuffing the fire, keep flames out of the flue with stacking and intake control, shut the bypass as soon as the cat meter says "Active", and keep your chimney swept. I see no reason to remove your double wall to look at it unless it is for regular cleaning.

Interesting.... I think one issue that is compounding my confusion though is that there were no flames at all in the box as the flue temp rose so quickly. It was a pure, black box, cat burn. With no noise of a fire, nothing. Just a rapid increase in flue temp, with no attendant increase in stove top temp.... that was it.
 

MMH

Feeling the Heat
Jan 21, 2019
406
NV
Interesting.... I think one issue that is compounding my confusion though is that there were no flames at all in the box as the flue temp rose so quickly. It was a pure, black box, cat burn. With no noise of a fire, nothing. Just a rapid increase in flue temp, with no attendant increase in stove top temp.... that was it.

These seasoned pros in here are likely right; I did this once also in my BK, but I don’t think I had an actual chimney fire. However, it’s hard to say now. I think this is either a draft issue like others have said (35 ft is a lot of pipe), or you did have a small fire in there. Remember you don’t necessarily need fire in your box, perhaps something in your chimney reached temp, or started a smolder and poof off it goes, quick on quick off as there wasn’t much build up etc. you could inspect your pipe, if it’s clean then likely a small fire, if not likely a large draft issue. As others said pay attention, I tinkered with my loading/starting styles and monitor my temps if it hits my trigger point I shut my damper regardless and start tinkering with air settings. I haven’t had the problem since.
 
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cabinwarmer

Member
Sep 11, 2020
192
SE PA
Just to add....this morning I had a similar issue as well. 75% of the way thru a top down light, my flue reached 550F. I closed the intake all the way and watched as the flames danced and then died down to nothing. The flue temp starting rising extremely fast, jumping 8-10 degrees at a time on my digital therm. I opened the intake and crack the door, flames inside the box relit and things settled quickly. Seems my GM60 likes it better when there is always a flame lit in the fire box. The gases must have built up........?
 

Woodfire2019

New Member
Dec 27, 2019
33
DC
Ok need some help understanding this. Forgive the long post....Started a fire today loaded up beech and ash in my Woodstock ideal steel hybrid, very well seasoned. Filled the fire box. 35 ft stainless liner from the basement wood stove up through middle of my house, so drafts hard. Especially when cold. I had her cruising along nicely in a black box cat burn (no flames). Pipe damper half closed and stove damper almost all the way closed. Stove top temp starts to get near 600 flue gasses at 500 and I close the intake air on the stove all the way (still gets some air, but basically had it on the lowest air setting), and closed the pipe damper all the way to cool it down and keep the burn where it is. I have a probe thermometer in my double wall so that’s where I’m getting the flue gas readings. Now things get interesting - stove temp stays around the same, creeps up a little but the flue gas temperature starts rising FAST. Goes from 500 to 1200 in like 2 minutes and I smell that paint curing smell. Outside double wall at the hottest spot was about 500 degrees! I call a buddy of mine, he says “open up the pipe damper all the way and the stove damper all the way. If that doesn’t work open the door - you need to flood it with cold air”. Sure enough I opened up the pipe damper all the way and the intake air halfway and the flue temp starts dropping immediately. What am I missing here? Only thing I can think of is I kept more smoke in the stove and that just got the cat going even crazier? Second time this has happened in 10 days and last time I warped the radiator cover pretty good. Aside from adding less fuel, what’s actually going on?
You got the stove hot, then shut it down with the stove damper, and then shut it down even more with the pipe damper. This created a very under-ventilated fire that lit off post-cat. Your friend gave you good advice to open it up. This is why stove pipe dampers can be dangerous.
 

stoveliker

Minister of Fire
Nov 17, 2019
1,953
Long Island NY
If there is fire in your stove pipe, it means there is fuel in your stove pipe. That should not be there. This means your burning practices may need changing, whether it is the air/damping, or the fuel. Please be safe.
 

Corey

Minister of Fire
Nov 19, 2005
2,608
Midwest
600F stove top and 500F flue - sounds like you had a pretty good rippin' fire going, then snapped everything closed and the cat lit off?

Can't say on your specific stove, but in mine, to get 500F up the flue and 600F on the stove top, that has to be a big roaring fire and very inefficient. When you suddenly close everything, all that fire then has a chance to actually transfer heat to the stove and pipe and temps rise considerably - at least in the short term. If that temp rise generates enough draft to offset the amount of air you have cut off by shutting down then it starts to 'run away' again. May or may not necessarily be a 'flue fire', but dumping a lot of heat in the flue.

As for what to do. Possibly repeating some suggestions above... bigger chunks of wood, don't let the fire get so big before shutting down, shut down in stages so its not a sudden spike in heat, obviously double check the flue for creosote, etc.
 

rosen431

New Member
Feb 1, 2021
14
Boston, MA
Just to add....this morning I had a similar issue as well. 75% of the way thru a top down light, my flue reached 550F. I closed the intake all the way and watched as the flames danced and then died down to nothing. The flue temp starting rising extremely fast, jumping 8-10 degrees at a time on my digital therm. I opened the intake and crack the door, flames inside the box relit and things settled quickly. Seems my GM60 likes it better when there is always a flame lit in the fire box. The gases must have built up........?

Yeah - so this is my contention as well. When I closed the flue damper during the cat burn (no flames), what I now believe happened is that the exhaust gasses were simply staying inside the stove longer. The catalyst was already lit off and doing its job and was being fed a very large volume of gasses to combust. Closing the flue damper increased the volume of combustible fuel the catalyst could burn. As a result, I had a lot of heat produced very quickly. This would explain the warping of the radiator cover above the catalyst. This would also explain why, when the flue damper was opened, and the stove damper was opened partially to allow air into the firebox to ignite the flame, the flue temp came down just as quickly.

I have difficulty believing there was a fire in that double wall stove pipe. It's certainly possible, but my bet is when I take apart that double wall I'm going to see some light buildup. The reason I think it's less likely is that the exact same thing happened to me 7-10 days ago (when I melted the stickers off the outside of the double wall stove pipe). So two fires inside that double wall, with the same characteristics, within 10 days? Doesn't feel likely to me since incident #1 would arguably have burned off the fuel in there.

Again, I don't know this is all conjecture. But it appears to me that the flue damper is not the perfect answer, and I may need smaller loads of wood that are controllable with the stove damper....