My BIL and I were talking over Thanksgiving about him replacing his wood stove. They have an A frame house with a too big wood stove and an 8" pipe. It goes up 3 sections then through ~6' of double wall stainless and then through the roof. So a new stove will be a 6" flue. What is normally done to have a proper draft? Pull the whole works and run the 6" all the way up or if it is reduced @ the double wall or?
KennyG
Super Member
The "right" way to do it would be to change it all out to 6". However, I have a small wood stove with a 6" connection that I then adapt to 8" and it goes into an even larger masonry chimney and it works fine.
gsganzer
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One thing you have to watch for, is condensation if the pipe is too large. The exhaust can cool too much and reach the point where moisture condenses out.
When I was doing forestry work, the owner had a house built and thought he was doing a great thing by upsizing his chimney flue. He had a wood fired furnace in the basement and he was constantly fighting moisture when he wasn't firing hard. The moisture would run out all over the basement floor.
When I was doing forestry work, the owner had a house built and thought he was doing a great thing by upsizing his chimney flue. He had a wood fired furnace in the basement and he was constantly fighting moisture when he wasn't firing hard. The moisture would run out all over the basement floor.
Larry Caldwell
Super Member
People who burn wet wood should not burn wood.
gsganzer
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It's not just associated with wet wood. Even "dry" wood (<20%) has moisture.
With his scenario, you could see the changes in condensing follow barometric pressure changes. Or, if he had the main house fireplace burning he didn't have the issue, because they had side by side flues in the same chimney and would be just enough to get some added heat.
ericm979
Super Member
Most modern (i.e. EPA) stoves use 6". Double wall if it's exposed to the interior of the house, double wall insulated if it runs through a chase. Only the largest ones use 8". You need a really big space for one of those.
Like gsganzer posted, the current thinking is to keep the exhaust hot all the way through. Modern stoves expect it.
The modern EPA stoves not only burn cleaner but get more BTUs from the same load of wood. The only problem is that they don't deal with wet wood as well as older non EPA stoves.
Like gsganzer posted, the current thinking is to keep the exhaust hot all the way through. Modern stoves expect it.
The modern EPA stoves not only burn cleaner but get more BTUs from the same load of wood. The only problem is that they don't deal with wet wood as well as older non EPA stoves.
Rockbadchild
Elite Member
code and manufacture will say to run a 6'' pipe all the way through... I would run the 6'' pipe inside the 8'' double wall insulated.
Rockbadchild
Elite Member
I would lower the new 6'' pipe inside the 8'' put a finishing plate to seal it all... this plate goes in the 8'' and the 6'' goes through no changes needed on the roof...
or this
anomad
Silver Member
I'd keep the "too big" stove and just make smaller fires or be hot. Adding some thermal mass somehow might help the hot/cold of a big stove. I had a big stove in my cabin in Oregon (years ago). It was perfect at full chooch when it was -10 outside. But you had to keep the fires small and short if you just wanted to take the chill down a knotch on a rainy morning. I put a bunch of brick around it for thermal mass. I prefer more short hot fires rather than a constant slow simmering fire. Keeps the chimney clean.
I should make a thread about my stove dilemma as not to derail the OP here.
I should make a thread about my stove dilemma as not to derail the OP here.
Our stove is not EPA approved. Uses 16-in OD double wall insulated. At the time we were building I had the price of the stove factored in and never even gave it a thought to check size of flu required. Needless to say I got a bit of sticker shock once I found out a 30 ft run of flu and elbows was going to be two-thirds the price of the stove itself.
CalG
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Double lining the fire box with fire brick is a WIN-WIN all the way around if the objective is to get more heat out of less fire in a too large wood burner!
For the cooments about "wet wood" and condensation. The products of rapid oxidation/ combustion are CO2 and H2O. There is no way around having water in the smoke stack! (when it's cold).
For the cooments about "wet wood" and condensation. The products of rapid oxidation/ combustion are CO2 and H2O. There is no way around having water in the smoke stack! (when it's cold).
BnRidge
Platinum Member
A larger pipe size slows the velocity and cools the exhaust. The condensation temperature for creosote is 250f. You can burn wet wood, all the wet wood you want, and use a larger pipe size, but the exhaust needs to stay above that temp. A larger flue dia will make it harder to do that (and not draft as well). Wet wood makes it harder to do that. I have a wood stove, with the recommended 6" flue. I had creosote dripping out the back of the stove twice, in 17yrs. Both times early on I decided to start the stove slowly. Long and slow with a cold pipe meant dripping creosote. I put in a 200-1000f thermometer to measure stack temperature. That corrected the problem right there. Get the temps up as quickly as possible, and keep them there.
If it were me, and I got a new to me stove, I would pull whatever doesn't meet installation specs and make it right, top to bottom, all of it. There is a fire in my home. I would be an absolute jerk about making it right. Just my opinion.
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It's an Acucraft 44" see thru insert.
kenmac
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I would say if you can change it to 6'' all the way up until it penetrates the roof and leave that section 8'' you should do just fine. The thing about oversizing the vent pipe is getting the pipe hot enough to establish a draft. But just a short section through the roof should do just fine
ericm979
Super Member
The stove we put in the old house needed about 32' of flue. The house is two story and has steep roof pitches. The required 6" insulated flue for inside the chase cost more than the stove did.
gsganzer
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I think a big issue is the modern stoves have such a big turn-down ratio and your risk of condensing is greater when you do the slow overnight burn. That makes it more sensitive to oversizing.
The OP didn't mention the total vertical rise, that would impact draft. They have on-line calculators for sizing this.
