Where is the best place to reduce a flue size from 8" to 6"?

[Kong]

I've got an 8" flue, horizontal ceramic pipe through a block wall into a ceramic lined chimney, 12"x8". Everything is sound. My new stove has a 6" flue outlet so I need to use a reducer.

My stove sits just a little under 24 inches below the 8" diameter inlet to the chimney so its not problem to trim a standard section of pipe and turn it with a heavy fixed elbow before exiting the house.

I could reduce the size right at the stove, simply buy a reducer with the 6" side crimped and then use 8" pipe and elbow above that into the flue. Or I could come out of the stove with 6" pipe, run that up to a 6" elbow, and then put the reducer as it enters the 8" flue.

I can think of what I suspect would be advantages either way. Small pipe keeps the velocity of exhaust gasses high - good thing. Gasses cool rapidly at the adaptor, I know that too - bad thing.

So which is best, reduce it at the stove or reduce it two feet higher up and one 90-degree turn away from the stove but right at the flue?

 

[CrappieKeith]

Your flue is too big for a 6" outlet on that stove.The entire flue should be 6".
Don't line your flue and your gonna have major drafting issue including excessive creosote.

 

[Kong]

According to the Jotul manual for the stove (F600) the largest acceptable size for the flue is 8"x12"; I am presuming they know what they are talking about and I am within their specifications. My question is this - is it better to make the pipe part of the run 6" or 8" because that's as far as I'm going to deal with. If I have to run a brush through the chimney, then that's what I'm going to do - but it couldn't be any worse than the 35 year-old Earth Stove I've been cleaning up after for decades now.

 

[laynes69]

You may not see an issue with creasote as much as you will with draft. Which in return causes issues with the way the stove burns and effecting your heat output. Newer stoves run with cooler flue temps. You put that on top of a large chimney and its a recipe for disaster. I would go with whatever diameter your flue is to the thimble. You don't have to, but you will see a huge difference in how the stove will operate if you line. Just something to think about.

 

[stint]

Took me a re-read to get the picture.

You are not "reducing" anything; you are *expanding*.

Never heard the terms used that way, i.e opposite the direction of smoke travel

Also never heard any significant advantage as to where you allow the smoke to *expand", (including the 'velocity' theory).

Expand wherever you want for aesthetics, ease of cleaning, etccc.

Efficiency or safety is not determinant of "where" if you are just giving smoke more room to move

 

[sawjo]

I have a 6" outlet on my stove and a 7" thimble. First time around I reduced at the stove and went with 7" stove pipe to the thimble. This season I reduced at the the thimble and went with 6" from stove to reducer, that setup works much better, less area for flue gases to cool and a slightly better draft - plus it looks better.

 

[Kong]

You all's comments have really got me thinking. If I have to shove twenty feet of pipe down my chimney with a 90 on the end I'll do it, its not like it would be that hard to do or cost all that much. I just don't think it will be necessary.

Maybe my thinking is wrong but it seems to me all I have to really worry about is keeping the velocity of the gasses up inside the stove. Some height of vertical flue above the stove outlet will be required to accomplish that. I'm not sure but it seems to me that the length of that pipe is dependent on the difference between flue gas temperature and inlet air temperature, or maybe that should be flue gas temperature and air temperature outside the flue, but one way or the other its going to be a temperature differential between that flue gas and something that pulls the draft.

So if it takes 2 feet of vertical pipe to get the gasses moving, well, I've got that much. So now what? Well, I'm not worried about creosote building up in the chimney, because if air is moving through the stove fast enough all that stuff gets burned up before the chimney. What I can't see is what it is about a wider upper chimney that the vertical pipe is going to dump out into that will have anything to do with slowing down the movement of gasses in the pipe that is under it. See what I mean? Its sort of backwards logic to say that the gasses will move fast enough through the pipe unless the pipe dumps into a bigger pipe, in which case then it wouldn't have been moving that fast in the first place. Sounds like a non sequitur to me.

Needless to say I could be very very wrong here. Someone please educate me.

 

[slipknot91]

I'm doing the same thing as you. I dont think mine is quite that big though. I think mine is 9x9. I too have an 8'' hole in the chimney and all I did was run the 6'' from my Clayton up to the closest spot to the chimney inlet and put what they call an increaser on there. You can get them at lowes. I haven't had any issues yet and I ran my Papa bear like that for aver a year with only 1 cleaning. Even then I had maybe a coffee can full of black soot and small flakes of creosote. That soot was probably from the oil furnace.

 

[laynes69]

Its the cooling that happens when it goes into a larger flue that effects draft. If that chimney doesn't stay hot enough it won't draft correctly. 8x12 is a large flue for something with a 6" outlet. Older stoves like the papa bear and even the older wood furnaces put alot of heat up the chimney which keeps the drafts high. The whole reason that lining is important is you match the proper size to the stove. Having a warmer flue creates a better draft for the unit. Also cuts down on condensation which will cause creasote. You can run it and see how it does, it may be fine. But lining it would be a benefit as far as performance and also safety.

 

[lobsta1]

Kong,
The following quote is from: http://www.chimneysweeponline.com/library.htm
They are a very good source of info. Look at in particular over sized wood stove chimneys & Installing wood stove chimney liners.

Remember, the new stoves MUST have really seasoned wood & the air flow & temp of that air flow is much less than the old smoke dragons.
Al

3) The existing chimney is lined, but the liner is too large. Oversized flues cause numerous problems, like smoke spillage into the house when they're cold, stove overfiring when they finally heat up, and the kind of excessive creosote formation that leads to chimney fires. Code prohibits venting any woodburner into a chimney flue with a cross-sectional area (CSA) that is more than three times the CSA of the flue collar on the stove. Thus, if your stove has a 6" flue collar (28 sq.in. CSA), the chimney flue must have a CSA less than 84" (3 x 28). In other words, if the chimney flue is larger than 10" round (78 sq.in. CSA) or 9" x 9" (81 sq.in. CSA), you must install a 6" stainless liner. This rule gets tougher if one or more of the chimney surfaces is on an outside wall, so it is exposed to outdoor temperatures for its entire length: in those cases, the CSA of the flue can't be larger than two times the CSA of the flue collar, so you must reline if the existing flue is larger than 8" round or 7" x 7".