Can You Put an 8" Flue Stove Into a 6" Chimney

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Scottsman
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Post Fri. Feb. 25, 2011 1:25 pm

Calling on the collective widsom of the forum...I have a big coal/wood stove with an 8" flue outlet that I want to install to a 6" diameter Duravent chimney. I was thinking of using this 8" stove pipe I bought with it and using an 8" to 6" reducer right before the wall thimble. But is that allowed? I read somewhere that NFPA says you cannot reduce, but I can't find the the NFPA rules and want to verify this. :?

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freetown fred
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Post Fri. Feb. 25, 2011 4:11 pm

I've done it w/ my big old wood burner & it's worked just fine for yrs & yrs--do you feel safe with it? Does it work OK?--PS--I put the reducer on the stove end--crimp down,always
Scottsman wrote:Calling on the collective widsom of the forum...I have a big coal/wood stove with an 8" flue outlet that I want to install to a 6" diameter Duravent chimney. I was thinking of using this 8" stove pipe I bought with it and using an 8" to 6" reducer right before the wall thimble. But is that allowed? I read somewhere that NFPA says you cannot reduce, but I can't find the the NFPA rules and want to verify this. :?

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Coalfire
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Post Fri. Feb. 25, 2011 5:53 pm

NFPA= Not For Practical Application :lol:


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Freddy
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Post Fri. Feb. 25, 2011 6:26 pm

I can't speak for every state, but here in Maine you are not allowed to decrease. Even if it's not mentioned in your state code, stop & think about it. Why would a manufacturer put an eight inch pipe on a stove rather than a six? Wouldn't a 6 be cheaper? Yes, wouldn't a 6 fit more chimneys? Yes, so why would they use an eight? Because that particular stove needs it! Either the design or the capacity of the stove is such that a 6" will not removes the exhaust in a timely fashion. So, if you reduce an eight inch to a six inch, you are asking for trouble. It might work OK for a while, or maybe even forever, but the fact of the matter is that when asked for full output you will have troubles. Those troubles can be anything from not giving the heat it should, to putting carbon monoxide into your house and killing all occupants.

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Post Fri. Feb. 25, 2011 7:50 pm

Well, it all depends on the height of your 6" chimney. The height of the chimney has a direct effect on it's capacity. A chart from the 1981 copy of "Wood Heat Safety" by Jay W. Shelton, shows the following ratings for a chimney.

Height----6" Chimney----------7" Chimney----------------8" Chimney
in Feet
6'..........71,000 Btu...........102,000 Btu..............142,000 Btu
8'..........81,000 Btu...........118,000 Btu..............162,000 Btu
10'.........89,000 Btu...........129,000 Btu..............175,000 Btu
15'........105,000 Btu...........150,000 Btu..............210,000 Btu
20'........120,000 Btu...........170,000 Btu..............240,000 Btu
30'........135,000 Btu...........195,000 Btu..............275,000 Btu

So, it is possible to do successfully and safely provided you take into account the capacity of the appliance and the chimney it is connected. If you are planning on firing at the stoves rated capacity or anywhere in between this table should help.

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Coalfire
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Post Fri. Feb. 25, 2011 8:19 pm

Freddy wrote:I can't speak for every state, but here in Maine you are not allowed to decrease. Even if it's not mentioned in your state code, stop & think about it. Why would a manufacturer put an eight inch pipe on a stove rather than a six? Wouldn't a 6 be cheaper? Yes, wouldn't a 6 fit more chimneys? Yes, so why would they use an eight? Because that particular stove needs it! Either the design or the capacity of the stove is such that a 6" will not removes the exhaust in a timely fashion. So, if you reduce an eight inch to a six inch, you are asking for trouble. It might work OK for a while, or maybe even forever, but the fact of the matter is that when asked for full output you will have troubles. Those troubles can be anything from not giving the heat it should, to putting carbon monoxide into your house and killing all occupants.
I think the larger flu is cause it is a coal/wood stove. you normally need a larger flu for the wood part. So if he only burnt coal the smaller pipe would probably work. Wether it is up to code or not is another question.


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cntbill
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Post Fri. Feb. 25, 2011 8:40 pm

Coalfire wrote:I think the larger flu is cause it is a coal/wood stove. you normally need a larger flu for the wood part. So if he only burnt coal the smaller pipe would probably work. Wether it is up to code or not is another question.
This is pretty much what I found out with my VC Vigilant, it had an 8" flue because it was a wood stove and an option of the coal kit, the newer VC II is coal only and has a 6" flue. When I asked the dealer if I could use 6" he said if I only burned coal he didn't see a problem with. So I converted to a 6" collar and the stove worked fine for me as I only used coal, except of course when I used wood just to get the coal going.

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BigBarney
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Post Mon. Feb. 28, 2011 9:23 pm

Doug:

That chart you submitted says it all.

Even for a 142,000 BTU appliance you would need more than 30'

of flue above the thimble.

Almost anything larger cannot even be considered for a 6" flue.

This is not a safe practice, the manufacturer has done testing

and determined a 8" flue is needed on his appliance.

BigBarney

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Scottsman
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Post Mon. Feb. 28, 2011 9:43 pm

Thanks, guys. I was able to go online and check NFPA and it says not to reduce. It's OK to go same size or bigger, but not reduce. Rats. Oh, well--got another stove to fix up and sell. Too bad--this one is a really good one. Will make someone with an 8" chimney very happy.

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