Class A chimney height in relation to existing masonry chimney

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swiftsoles

New Member
Aug 3, 2022
3
Boone, NC
I have an existing masonry chimney, but want to install a new class A chimney pipe. The new pipe will be about 2 feet away from the existing masonry chimney. This is because instead of having to go into the masonry chimney through a thimble by way of a right angle coming up off the stove, the new pipe will just go straight up from the stove, and through the roof. My reason for installing the new class A, is because the existing chimney is not lined, and a new pipe will be easier to clean with being able to go straight up, instead of through the thimble. I also don't know if a liner will fit down the existing chimney.

I understand the 3-2-10 rule, as it relates to the roof ridge line. How about the existing chimney though? Does the new chimney need to be 2 feet taller than the existing one? Further, the existing masonry chimney actually has 2 flues, as there's another woodstove in the basement. Although I don't use it now, I might want to in the future as backup. Could I still use the masonry chimney, or would I need to discontinue it completely? Obviously both chimney's can't be 2 feet taller than the other, so how would this work if I wanted to use both (if possible)?

To complicate matters, it's a 12/12 roof, and the ridge line is 10ft away. Which means the class A height is a minimum of 12 feet. It's hard to find info about the maximum height of class A chimney from the manufactures. Would that height be an issue? Thanks for any help.
 

stoveliker

Minister of Fire
Nov 17, 2019
6,483
Long Island NY
You need to have bracing every 5 ft above the roof penetration. And I'd make the chimney 1 ft taller than the others. That's a good number for adjacent flues (to avoid smoke from one getting sucked down in the other.) I'm not sure how this'll work for flues 2 ft apart. But it's good to get above the ridge to have less chances of downdrafts due to wind curling over the ridge and down into the chimney.
 

bholler

Chimney sweep
Staff member
Jan 14, 2014
31,159
central pa
I have an existing masonry chimney, but want to install a new class A chimney pipe. The new pipe will be about 2 feet away from the existing masonry chimney. This is because instead of having to go into the masonry chimney through a thimble by way of a right angle coming up off the stove, the new pipe will just go straight up from the stove, and through the roof. My reason for installing the new class A, is because the existing chimney is not lined, and a new pipe will be easier to clean with being able to go straight up, instead of through the thimble. I also don't know if a liner will fit down the existing chimney.

I understand the 3-2-10 rule, as it relates to the roof ridge line. How about the existing chimney though? Does the new chimney need to be 2 feet taller than the existing one? Further, the existing masonry chimney actually has 2 flues, as there's another woodstove in the basement. Although I don't use it now, I might want to in the future as backup. Could I still use the masonry chimney, or would I need to discontinue it completely? Obviously both chimney's can't be 2 feet taller than the other, so how would this work if I wanted to use both (if possible)?

To complicate matters, it's a 12/12 roof, and the ridge line is 10ft away. Which means the class A height is a minimum of 12 feet. It's hard to find info about the maximum height of class A chimney from the manufactures. Would that height be an issue? Thanks for any help.
Honestly, I would consider relining the masonry chimney. But no, it doesn't have to be 2' higher than the other chimmey
 

swiftsoles

New Member
Aug 3, 2022
3
Boone, NC
Honestly, I would consider relining the masonry chimney. But no, it doesn't have to be 2' higher than the other chimmey
I agree that might just end up being easiest. I was trying to avoid having multiple angles in the inside portion of the stove pipe, and then a 90 to get into the wall thimble, which makes it difficult to clean. I can't really clean top-down due to the chimney height outside, so I need to clean bottom-up.
 

EbS-P

Minister of Fire
Jan 19, 2019
4,082
SE North Carolina
If you can fit a 6” insulated liner, price the difference between new class A and a liner. I’m just guessing but new class A will be much more expensive.

Two 45s is better than a 90.
At the thimble height I think you can install a T then short piece of liner down to a clean out cap. Soot eaters can make the bends and clean bottom up.
 

begreen

Mooderator
Staff member
Nov 18, 2005
98,102
South Puget Sound, WA
I agree that might just end up being easiest. I was trying to avoid having multiple angles in the inside portion of the stove pipe, and then a 90 to get into the wall thimble, which makes it difficult to clean. I can't really clean top-down due to the chimney height outside, so I need to clean bottom-up.
Is there a cleanout door at the bottom of the current masonry chimney?
 

swiftsoles

New Member
Aug 3, 2022
3
Boone, NC
Is there a cleanout door at the bottom of the current masonry chimney?
No, I have to go in through the thimble to clean it. Anything that falls, just falls down to the bottom of the flue, whatever is down there. I can dangle my shop-vac hose down there to suck up that stuff when needed.
 

begreen

Mooderator
Staff member
Nov 18, 2005
98,102
South Puget Sound, WA
Understood. I was thinking that adding a cleanout door, and then a capped extension to the tee would make cleaning easier. Additionally, to make bottom-up cleaning consider using 45s with an offset instead of a 90 to the thimble. This will also assist draft.

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