Might An 8" Chimney Be More Tolerant of Abusive Overburn Than a 6"?

[Renovation]

Yes, another chimney question here. Just call me Curious George. %-P

This time I'm wondering if, all other things being equal, an 8" chimney might be more likely to survive a given abusive overburn without igniting the house than an 6".

For example, if there were two chimneys, both clean, same height, correctly installed, with proper clearances, etc., and we hooked up identical stoves to them, got a nice bed of coals in both, filled up their fireboxes with nice seasoned, wood, and accidentally left the door open, would the 8" chimney be more likely to survive a longer amount of time before burning down the house? In other words, a larger margin for error?

I'm thinking it might, because the larger diameter gives more surface area, so the 8"'s surface might stay cooler, and survive longer before failing and/or igniting nearby combustibles?

If so, this might help answer a question that's been nagging me for a while...

I know there are plenty other factors in choosing the right chimney, but I'm curious about this one.

Thanks for any insights, and have a great one!

 

[Slow1]

Given your scenario the both chimneys will perform substantially the same - given that you say they are clean then there is nothing inside to ignite and burn. So - only thing going up the chimney is the exhaust from the stove which while very hot in the scenario you have given will almost certainly be well within the specs of a proper woodstove chimney.

Now the stove may be in bad shape - but again, the chimney diameter won't really affect that.

I believe the 'right' thing to do is have the chimney (diameter and height) matched well with your stove for maximum performance. Running properly it will stay clean and you won't have to worry about it turning into a ground-mounted rocket.

 

[Renovation]

Given your scenario the both chimneys will perform substantially the same - given that you say they are clean then there is nothing inside to ignite and burn. So - only thing going up the chimney is the exhaust from the stove which while very hot in the scenario you have given will almost certainly be well within the specs of a proper woodstove chimney.

Running properly it will stay clean and you won't have to worry about it turning into a ground-mounted rocket.

Thanks S1,

So, if the chimney is clean, it should be able to handle someone accidentally leaving the door open with a full load? That's good to know. I was thinking that a stove with a big firebox, and Equinox for example, might be able to push a chimney beyond its design limits on an uncontrolled burn. Of course I suppose the stove itself might be junk after such an adventure?

Of course I have no intention of doing that, but I'm curious about what the limits and dangers are, so I can think ahead and hopefully respond better in an emergency. One guy was talking about how pilots do that--I know sailors do--and it seems like a good idea.

Thanks again, and have a great night.

 

[Pagey]

If both are UL 103 HT Class A chimneys, they should be rated for 1,000F continuous operation and tested for three 10-minute simulated chimney fires at 2,100F.

 

[Battenkiller]

I'm thinking it might, because the larger diameter gives more surface area, so the 8"'s surface might stay cooler, and survive longer before failing and/or igniting nearby combustibles?

Theoretically...


At the identical temperatures, 8" pipe gives off 33% more heat than 6" pipe because it has 33% more surface area.


However...

Hot flue gases will expand and cool faster in the 8" pipe than in the 6" pipe, so the pipe will initially be cooler.


However...

In the event of a neglected stove (open door/ash pan), the increased capacity of the 8" flue will gradually allow more and more air to feed the stove as the flue continues to heat up, leading to runaway combustion and extremely hot pipe temps.


However...

An 8" flue on a stove designed for 6" will be more sluggish in the beginning, so you will have a longer window of time before runaway begins.


How this plays out in a real life runaway fire will depend on many other variables, but the main fact to remember is that larger radiating surfaces give off proportionately larger amounts of heat, so at identical temperatures, your are safer with smaller pipe.

 

[Renovation]

I'm thinking it might, because the larger diameter gives more surface area, so the 8"'s surface might stay cooler, and survive longer before failing and/or igniting nearby combustibles?

Theoretically...


At the identical temperatures, 8" pipe gives off 33% more heat than 6" pipe because it has 33% more surface area.


However...

Hot flue gases will expand and cool faster in the 8" pipe than in the 6" pipe, so the pipe will initially be cooler.


However...

In the event of a neglected stove (open door/ash pan), the increased capacity of the 8" flue will gradually allow more and more air to feed the stove as the flue continues to heat up, leading to runaway combustion and extremely hot pipe temps.


However...

An 8" flue on a stove designed for 6" will be more sluggish in the beginning, so you will have a longer window of time before runaway begins.


How this plays out in a real life runaway fire will depend on many other variables, but the main fact to remember is that larger radiating surfaces give off proportionately larger amounts of heat, so at identical temperatures, your are safer with smaller pipe.

That's the spirit!

Thanks BK, that's just the stuff I was pondering. I know it's theoretical, but it helps me get my mind around the concepts, and maybe prepare for/prevent an emergency.

I was thinking about the larger surface/radiating area, but not the other factors you mention. And I drew the opposite conclusion--that the larger pipe would be safer because radiating more would save the pipe. But if an overburn doesn't push a pipe past it's design limits, I suppose that's less of a concern.

How hot might flue temps get in an open door, full firebox, accidental runaway burn, given that the chimney is clean, there's no chimney fire, etc?

I was imagining glowing, melting pipes and fire for sure, but, in theory at least, is the chimney system designed to handle such abusive mistakes?

How about the stove--would it probably be junk at that point? I've read here about people completely rebuilding stoves after overfiring.

Thanks for feeding my curiosity, and have a great one!