Chimney Draft vs Diameter vs Height Questions

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leeave96

Minister of Fire
Apr 22, 2010
1,113
Western VA
I may be looking at a chimney that is as high as 35ft worse case. Based on some of the stuff I have searched/read, it sounds like if I have a 6" dia chimney, the thing is likely to draft like crazy - unless I put-in a stove pipe damper, which I don't really want to do.

I also have read that some of the stoves that open to the front and have a screen for a open fireplace type option can have an 8 inch stove pipe.

I gather that the taller the chimney, the stronger the draft and the larger the diameter chimney, the slower the draft, hence the 8 inch stove pipe on the front/screened type stoves to keep them from burning out of control due to strong draft.

So if one builds a really tall chimney, what diameter should you consider? Are there rules of thumb with regard to chimney draft vs diameter vs height?

Thanks!
Bill
 
One would need to know what you're venting. A fire place? Stove?

I went with a 12" x 12" tile lined chimney as it wasn't much more cost and will fit any type of liner and insulation.
 
I am venting a Woodstock Keystone woodstove.

Thanks!
Bill
 
leeave96 said:
I gather that the taller the chimney, the stronger the draft and the larger the diameter chimney, the slower the draft, hence the 8 inch stove pipe on the front/screened type stoves to keep them from burning out of control due to strong draft.

This is not quite correct. Draft strength is a function of chimney height and temperature differential. At the same flue and outside temps, a 20' tall 6" diameter chimney will measure the same draft as an 8" one of the same height. The reason that larger diameter chimneys don't pull as hard in some installations is because the flue gases expand and cool inside the flue, and that causes a lower temperature differential. But the 8" flue has almost twice the flow capacity as a 6" flue. An open fireplace or stove front lets in a lot of air, so it needs a larger pipe to accommodate the increased air flow.

On a stove designed for an 8" flue, installing a 6" pipe will cause the stove to smoke when the doors are open, but it will not make it burn out of control at all. In fact, it will be a sluggish performer with a small pipe and will never be able to live up to its BTU potential. A taller chimney will help to a degree because the higher velocity inside the pipe will boost the flow rate some, but the best way to increase capacity is with larger pipe, not a taller chimney.

This doesn't answer your question, but it should help you figure out what you need based on your stove. A stove pipe damper may be what you will need with a 35' chimney. Using 8" pipe may cool the flue gases down too much and cause creosote to condense on your flue walls.
 
Call the folks at Woodstock with the specifics of your install. Believe me, they will advise you well and they know your stove best. The have nothing to gain from misleading you and everything to gain from making sure your stove burns well and you have a positive experience.

If it is a toss-up between the two I would go with the 6" simply due to the fact that 8" tends to be much more expensive. Long term it seems the majority of stoves are being designed for 6" pipe. There are of course a few noteworthy exceptions - the BKK is the first that comes to mind. Only reason I bring this up is in case you are thinking along the lines of "what if I ever replace this stove with another one."
 
Like others have stated call Woodstock and see what they say but I think these Woodstock stoves are very controllable and you may be just fine without a pipe damper and a tall 6" chimney. Remember this is a cat stove so it already has a built in damper, (the catalyst) unlike non cats which have that non controllable secondary air supply.
 
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