Can high upper flue temps break down old glazed creosote?

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Gark

Gark

Minister of Fire
Jan 27, 2007
808
SW Michigan
Or did we have a chimney fire and not know it? An exploratory look and cleaning today showed nothing in the liner (sorry no pic's - camera died) but a fluffy, black crispy thin layer of dried creosote foam. It brushed out real easy. Previous two years with un-insulated singlewall liner we got BAD stage-3 glazed creosote buildup in the liner, but only in the upper 9 feet where it's inside the chimney in unheated space (attic). That glazed hard shiny stuff was near impossible to remove. The SS runs through a clay-lined masonry chimney with inside dimensions of 6 1/2 X 10 1/2 inches. This season, the liner is insulated with perlite. Poured perlite down the entire length of the 25' vertical flue. We've only burned a dozen times this year, but have made HOT fires and the wood is seasoned right. Could the higher upper flue temps, because the flue is now insulated, have 'fluffed' the leftover glazed creosote from last March cleaning? Or is it possible there was a flue fire?
 
It sounds like you are getting things tuned in right. What you saw is pretty normal for a warm chimney burning dry wood.
 
I just want to understand the situation a little clearer-
Was the really bad creosote left inside the liner when you insulated it and then after burning some hot fires it disappeared? Or did you clean it out and now, after insulating your liner, the creosote is not returning.
Sorry for not catching the question if it's obvious to everyone else.
 
Last March a little glazed creosote was left from cleaning with a poly brush. Not much - 1/16" thick in about half the upper flue and clean exposed SS (where the brush was effective) for the other half of the top 9 feet. Repeated rigorous brushing just could not get it all out. Then the liner was insulated with poured perlite. After a dozen burns, NONE of the glazed creosote remains. Instead it was fluffy, crystallized brittle creosote real easy to brush out with a single swipe of the brush. It amounted to half cup of light black powder and wasn't the larger solid chunks like used to come out of there. I'm hoping the improved flue temps now prevent smoke condensation on the flue and wonder if the hotter temps may have turned the glazed creosote into fluffy brittle creosote. OR is a chimney fire the only thing that can convert stage-3 creosote into the fluffy brittle kind? We don't use any chemical "creosote killer" in this stove. Thanks for your opinions.
 
Gark said:
Last March a little glazed creosote was left from cleaning with a poly brush. Not much - 1/16" thick in about half the upper flue and clean exposed SS (where the brush was effective) for the other half of the top 9 feet. Repeated rigorous brushing just could not get it all out. Then the liner was insulated with poured perlite. After a dozen burns, NONE of the glazed creosote remains. Instead it was fluffy, crystallized brittle creosote real easy to brush out with a single swipe of the brush. It amounted to half cup of light black powder and wasn't the larger solid chunks like used to come out of there. I'm hoping the improved flue temps now prevent smoke condensation on the flue and wonder if the hotter temps may have turned the glazed creosote into fluffy brittle creosote. OR is a chimney fire the only thing that can convert stage-3 creosote into the fluffy brittle kind? We don't use any chemical "creosote killer" in this stove. Thanks for your opinions.
Click to expand...

AFAIK, glaze creosote won't turn into flaky creosote just from heating it, it needs to be chemically removed. OTOH a chimney fire should have converted most of it to CO2, H2O and probably a lot of black smoke and soot. Was any of the stuff you found later on light and puffy like black cheese puffs? I am told the puffy stuff is basically unburned carbon left over after a chimney fire. Makes sense to me that the newly insulated chimney could have allowed the flue gases to finally stay hot enough to allow ignition of that glaze layer. 1/16" doesn't sound like much, but over a 9' run of 6" flue, that's over half a gallon of Stage 3 creosote.

Whatever it turns out to be, insulating the flue should help eliminate any further development of glaze creo.
 
"light and puffy like black cheese puffs"
Nope, the creosote removed after insulating the liner was more flakey and crispy instead of hard solidified gooey. Maybe 1/16" thick. What confounds me is that the (little bit of) glazed creo from last season is now gone. It will be watched closely. Along with all the other great info on this site comes a strong awareness of safety issues. Has made me nearly paranoid about flue buildup but better that than letting it get out of hand. Thanks.
 
This is something I have been wondering myself.

Could the temps in an insulated flue get high enough to "burn off" creosote like it does the glass on an insert or stove? But without causing a fire?
 
TomWright said:
This is something I have been wondering myself.

Could the temps in an insulated flue get high enough to "burn off" creosote like it does the glass on an insert or stove? But without causing a fire?
Click to expand...
Coal soot does that it bakes off instead of a big poof.
 
Gark said:
What confounds me is that the (little bit of) glazed creo from last season is now gone.
Click to expand...

Who knows where it went, but it sounds like you are on the right track now. The stuff you are getting sounds like a little bit of the Stage 2 stuff we all get, only I get more of it than that. 25' unlined masonry chimney and I always get some above the 15' mark no matter how hot I burn. My sweep says it's fine, but I'm gonna pay for an extra cleaning mid-winter from now on, just to err on the safe side.
 
Gark said:
"light and puffy like black cheese puffs"
Nope, the creosote removed after insulating the liner was more flakey and crispy instead of hard solidified gooey. Maybe 1/16" thick. What confounds me is that the (little bit of) glazed creo from last season is now gone. It will be watched closely. Along with all the other great info on this site comes a strong awareness of safety issues. Has made me nearly paranoid about flue buildup but better that than letting it get out of hand. Thanks.
Click to expand...

Sounds like the stuff that was in my chimney when I had it cleaned which my sweep told me was from a chimney fire, that and the carcked top tile, which he said was typical since it's totally uninsulated.
 
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