Flue Temps - A bit confused. Clarify?

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FlyFish'n

New Member
Oct 23, 2021
62
OH
I've been reading quite a bit on flue temps and am a bit confused, more so on the low end.

I've heard flue temps above 400deg are best. However, when researching things here I found several references to temps as low as 250deg in that above that is recommended for keeping creosote from condensing.

As for water vapor - being above the boiling point of water, at 212deg F, would seem to make sense and this lines up closer to the 250deg F temp - that 250 is hot enough for water vapor to flow right up and out rather easily.

Are the references to 250deg F in fact in reference to limiting creosote condensing? Or does that temp really need to be up over 400deg?

On the high end it sounds like running temps may be 800-1000deg F during the high end of a burn cycle. Anything over 1000deg F should be avoided. Is this true for all flue pipes? Or double or triple wall stove pipe?

Depending on which way the numbers go - the normal operational range sounds like it could be 250-800deg, 250-1000deg, 400-800deg, or 400-1000deg.

The temps I am referencing are gas temps in the flue, not surface temps of single wall pipe. It appears there is a 30-50% reduction in temp for a single wall flue pipe surface temp compared to what the exhaust gas temp is going through it.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
28,179
central pa
I've been reading quite a bit on flue temps and am a bit confused, more so on the low end.

I've heard flue temps above 400deg are best. However, when researching things here I found several references to temps as low as 250deg in that above that is recommended for keeping creosote from condensing.

As for water vapor - being above the boiling point of water, at 212deg F, would seem to make sense and this lines up closer to the 250deg F temp - that 250 is hot enough for water vapor to flow right up and out rather easily.

Are the references to 250deg F in fact in reference to limiting creosote condensing? Or does that temp really need to be up over 400deg?

On the high end it sounds like running temps may be 800-1000deg F during the high end of a burn cycle. Anything over 1000deg F should be avoided. Is this true for all flue pipes? Or double or triple wall stove pipe?

Depending on which way the numbers go - the normal operational range sounds like it could be 250-800deg, 250-1000deg, 400-800deg, or 400-1000deg.

The temps I am referencing are gas temps in the flue, not surface temps of single wall pipe. It appears there is a 30-50% reduction in temp for a single wall flue pipe surface temp compared to what the exhaust gas temp is going through it.
400 is the absolute min it should be at 18" off the stove. It will be much cooler by the time it reaches the top of the chimney. The goal is to keep it around 250 internal temp at the top of the chimney. I don't use a probe I use a surface thermometer so I aim for 250 as a min which means roughly 500 internal at about 18" off the stove.
 

ABMax24

Minister of Fire
Sep 18, 2019
1,445
Grande Prairie, Alberta, Canada
250F is to limit creosote formation, if water vapor condenses in the chimney it rapidly causes creosote accumulation. That being said most non-cat stoves don't burn very clean at 250F flue temps, the firebox is just to cold to ensure complete combustion and is where the 400F number you see comes from. Granted during the coaling stage its not uncommon to see temps below 250F, and this isn't a concern as water vapor and the wood gas that cause creosote are at a minimum.

800F is pretty hot to operate a stove at, probably considered overfiring, at those temperatures its quite likely points on the stove top would glow a very dull red in a dark room. Short excursions to this flue temperature shouldn't overheat the stove though. Not to mention the wasted heat up the flue, but most double wall chimney should be rated to 1000F continuous operation, mine is good to 1200F.

Personally I shoot for between 400F-600F internal flue temps on my stove, but sometimes I'll push that to 650F for more heat output in really cold weather.
 

Poindexter

Minister of Fire
Jun 28, 2014
2,562
Fairbanks, Alaska
Every install is going to be different. It would be lovely to have a (bluetooth?) sensor at the top of the chimney so we could run as low as possible with minimal creosote accumulation at the top.

Real world, sticking a flue gas probe 18-30" above you stove collar is cheap and easy. What you have to figure out every time you move to a new house or change your chimney height is how low can you go and still have moderate/acceptable crud accumulation at the top. Just about everyone finds "most" of the crud accumulation in the top 2 feet or so of whatever pipe they are running.

Running flue gas temps (measured 18-30 inches above the stove collar) above 1000 degrees F is supposed to be bad for the pipe. Otherwise, once you are up and running on your cat or secondary burn chamber, the lower your flue gas temp the more heat you are keeping in the house. The question is now how often are you willing to sweep the pipe to keep the creosote buildup at low risk for a devastating house fire?

With a new stove or new chimney height my personal preference is to sweep once for every cord burnt while I find my comfort zone, then go to two, and then go to four cords. I still sweep every four cords now just to make sure everything is running right. Chimney sweeping is like when you turn 50 years old and realize how big your doctor's fingers are, but your chimney doesn't tense up on you. I like docs with little fingers at my age, so 4 cords max between sweepings works good for me.
 

moresnow

Minister of Fire
Jan 13, 2015
1,958
Iowa
Without doubt the most unique analogy/reference to wood burning and creo control yet^

;lol
 

begreen

Mooderator
Staff member
Nov 18, 2005
92,798
South Puget Sound, WA
The temperature drop in the flue system is going to be relative to the system design, system height, & outside temps. In our climate running a stove with a 1 story straight-up flue that is double-wall stovepipe connected to a relatively short chimney is going to have notably higher exit flue gas temps due to our ambient outdoor temp being benign throughout most of the winter. The opposite is a single-wall stovepipe fed into an exterior 2 story masonry chimney with an 8x8 flue tile in Maine. Alaska or Minnesota.

Running our 2 story, straight-up flue with a flue probe 24" above the stove with temps in the 4-600º range means virtually no soot or sote buildup. Even after 6 cords burned I usually only get a cup or less of soot&sote. It helps a lot that the system is in the home envelope except for the last 7 ft and our ambient winter temp is in the 30s.
 
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Highbeam

Minister of Fire
Dec 28, 2006
19,679
Mt. Rainier Foothills, WA
These are all excellent responses.

I will say that it is important to measure and maintain the appropriate flue temperatures. The mechanical flue probe meter or the cool electronic probe meter from auber are both excellent.

In both of my stoves, during the bulk of the burn, I keep flue temperatures at 400-700. That matches the dial recommendations on the mechanical condar meter but I don't see the need to go up to the top of the "normal" range. There's nothing "new" in physics that makes this normal range any less true than it has always been.