Wood stove chimney question

Started by mpls_ham, February 01, 2011, 01:48:46 PM

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mpls_ham

Hello,
As I try my best to remain patient to get back up to my 14x24 cabin, I have been working on details for when I do have access.  Here is my questions. 
I have been offered a 7' tall 3 ply chimney stack that would work nicely on my cabin.  The catch is that the stack is 8'' diameter and my wood stove is 6''.  My stove pipe would be about 8' in length to the cathedral ceiling leaving me 15' total rise (straight run up).  I've priced out what it would cost to buy all 6'' and the savings would be at least $300.  Would the stove draft properly with the larger diameter chimney stack?

Is there any advantage to installing the chimney before I tin the roof, or does the flashing sit on top of the tin?  If it does sit atop the roof tin, how will the flashing be sealed due to the ridges on the tin?

Thanks
Northern Black Hills - South Dakota

MountainDon

6" vs 8":  It may not draw as well as it should with an 8" chimney. As the smoke hits the 8 inch section it will not move as fast. The manufacturer of the stove may have recommendations. I recall seeing info on some stove where the mfg permitted 6 or 8 inch if the chimney was straight up, but stated only 6 inch with any bends. You'd best check on that.

For a metal roof you can get boots that have a flexible ring base. It is dead soft aluminum that has a silicone boot molded around it. Dek-Tite is one. Here's one source... never bought from them; I found mine at the roofing dealer where I got the metal.
http://www.pipebootexpress.com/Default.aspx?gkw=dektite+flashing&gclid=COy8mcrS56YCFRg8gwodgzs11g




We installed the metal roof, then marked the center of the chimney by drilling a hole up through the roof from underneath. Then cut the hole with a metal cutting blade.

Just because something has been done and has not failed, doesn't mean it is good design.


Alan Gage

I agree with Don. Depending on what type of stove you're using you should go with the manufacturers recommended pipe size. The newer the stove (EPA approved) the more picky it's apt to be.

Are you in the Minneapolis area? I was recently at Home Depot in Mankato and while they don't carry woodstoves they did have some chimney kits on deep clearance. They don't include the expensive class A chimney pipe but the kits contain the chimney cap, through the ceiling adapters, and I think maybe some of the pipe that connects to the stove among other nits and bits. I picked up a through the wall kit for a future basement install for $165. They were originally $300 and the cheapest I found online for the same brand was about $230. They had a couple through the ceiling kits too.

Alan

Alan

MelFol

With Don, I think going from 6" to 8" may be too much velocity change, which allows the gasses to cool and soot to build up. I don't have quite the same situation, but here is my experience.  20+ years ago I changed stoves. I previously had 6" all the way, changing to single wall above the roof. The new stove had 5" output.  I found a reducer, installed 5" triple wall through the roof, but retained 6" single wall above the roof for about 3 to save a few dollars.  I had problems year after year. Creosote and soot would build up in the flue, I suspect due to both velocity change the single wall allowing the gasses to cool. When the reducer deteriorated I tried to buy another. The dealer advised me to go the same diameter triple wall all the way up, which our building code now called for.  I followed his advise and have not regretted the decision and additional cost. I have almost no problem with soot buildup since.

Redoverfarm

Well I think in a have-to case it would be better making the transition to a larger diameter rather than vise-versa.   When you cut the roof penetration hole don't be surprised if it does not quite fit.  Especially if you mark the hole 90 deg from either face.  The hole will be slighted elongated due to the pipe sitting plumb on a angled roof. 


mpls_ham

Thanks for the input!  I poured over the stove manual and it does recommend to use 6" pipe all of the way up.  The manual does have instructions for attaching to an existing brick stack using a 90 degree elbow to enter into the back wall.  There are no requirements for the brick stack size ;)  I wonder if another option is to run a 6" single wall sleeve into the chimney stack???   I realize purchasing the whole works would be the "ideal option" but reuse of the free stack seems like a good alternative to dropping it off at the city dump.
Northern Black Hills - South Dakota

MountainDon

Because it is sometimes difficult to ascertain the condition of the inner flue in a masonry chimney some manufacturers recommend the use of a liner. They are single wall expanding length things. That way the flue is sized right. It's the same thing as going from 6 to 8 inch, if the masonry flue has a larger cross sectional area the smoke will slow and perhaps affect the stove performance.
Just because something has been done and has not failed, doesn't mean it is good design.