6" ASHT inside exiting 8" prefab pipe?

[gadgetgeek]

I'm replacing a prefab fireplace circa mid 1980's and my chimney sweep said the inside dimension of the current (air cooled?) pipe is 8". Can I install a new 6" (inside diameter) ASHT pipe inside the old pipe or will the old pipe need to be ripped out? The pipe dimensions in the installation manual show that a 6" ASHT pipe is 8" in diameter on the outside so I'm guessing it will have to be replaced. The model pipe I may be using is: Security Chimney's 6" LD Secure Temp ASHT.

 

[thechimneysweep]

Hey GG,

The OD of Security 6" Chimney is slightly over 8": you'll never get it down an 8" flue.

 

[jtp10181]

Find out the OD of the larger pipe section in your double wall pipe. It is hopefully at least 12". The inner sections should come out easily from the roof leaving just the larger outer section in place. Then you could drop the new pipe right down it. The tricky part would be keeping your 2" clearance all the way down. The clearance should be an air space, so if the OD of the old pipe does not allow that 2" air space then it should come out. Ripping it out is a piece of cake once its not attached to the unit anymore.

 

[n6crv]

Hello Gadgetgeek, thats is about what I did last year. I had triple walled chimney. A 6" class A chimney pipe is just a little to big to fit inside the 8". I took the 8" out and left the 10" and the 12" outer pipe. The 6" is inside the 10" and has vermiculite around it to keep it centered and for extra insulation. It seems to be working just fine as I have way over the 2" clearance to any thing that can burn.
Don

 

[gadgetgeek]

Cool!

I thought this was a dumb question but I'm glad I asked it anyway. I never would have thought you could rip out the inner pipe. My chimney sweep thinks the outer pipe is 12" which would be just enough clearance for new 8" (outside clearance) pipe + 2 inches air. Since the new pipe is not a flex pipe, I'm assuming the 2" clearance could be maintained by whatever mounting devices are used at the top and bottom of the pipe? I don't have an exact number but I think the chase is about 20' high.

Thanks so much for the feedback. My budget is getting crushed right now so a bit of savings while improving safety goes a long way.

 

[jtp10181]

N6CRV, technically putting the vermiculite around the pipe violates the clearances (which must be an AIR SPACE) and will not allow it to cool properly and the whole system could overheat.

gadgetgeek, I would use a spacer or two someplace on the pipe just to keep it centered. Or, make sure it is centered at the top and again at the very bottom.

 

[n6crv]

Hello jtp10181, not to take over the thread but you now have me wondering. I have the vermiculite between the 6" class A chimney and the 10" pipe. Then there is 2"of air to the 12" pipe then 2" of air space to the wood roof sheeting. So I have 4" of air space or do I? I would think it would be about the same as building a chase around the class A chimney and adding rock wool or vermiculite as a filler/insulation? I can take the vermiculite out as it is centered at the top and the bottom of the triple wall chimney. Here is a picture of the install the triple wall piece is 4' also I have gone up another 3' of class A and the correct chimney cap and support braces.
Don

 

[jtp10181]

I would think it would be about the same as building a chase around the class A chimney and adding rock wool or vermiculite as a filler/insulation?

Which would also not be legal. If you have rigid double wall Class A HT pipe it is supposed to have a 2" AIR SPACE directly around it in all directions, end of story. I am not saying that what you did is going to burn your house down, but I also know that's not how they test the pipe so no one really knows what could possibly happen.

 

[n6crv]

Thanks JTP, well looks like the vermiculite will be coming out this Fall. Better safe then sorry.
Don