Concrete block chimney clearance

[sagent]

So I came across an old woodstove and decided to put in the garage. I have an existing concrete block chimney I hooked it up to. I have cellulose in the attic up against the chimney. Is this a fire hazard and if so what type of material to use to insulate against the chimney ?

 

[LEVE]

Concrete block with no liner? Yep, it's a fire hazard. The mortar can fail and let flame breach the joint and you may have a attic fire.

Install a liner, clay, steel, etc..

 

[bmxdad]

I had a concrete block chimney, but it was made to have a liner installed in the middle of the stacked blocks ... if no liner, its not safe IMO.

 

[rlitman]

Even with a liner, a dam should be built around the chimney to have I believe at least a 1" air gap without insulation.

 

[warren57]

You can buy a stainless steel liner (looks like a piece of flex duct). In various lengths and diameters. You just go up on the roof, drop it down the chimney and install the recommended vent cap on top. Connect to the flange on the heater.
Code here requires we install these liners on every old masonry chimney whe we replace furnaces.
You can buy them from and commercial HVAC supplier.
Think the last one I bought as about $150-200 for a 20' long, 4" diameter.
Google "SS chimney liner" to see what they look like.

 

[Jackfre]

Your masonry chimney is regulated by NfPA211. A liner is required. An insulated liner I's best for an exterior chimney. You can use SS flex as noted or, if it is a straight shot you can use a 316l Ridgid SS pipe. You need a clean-out tee at the bottom. Is there an ash door at the bottom of your block chimney. A liner system I've had good results with is Dura-vents Dura-liner.

 

[sagent]

There is a 7 inch clay flue inside the concrete. does this make a difference ?

How do the liners work ? Is it just a matter of sliding the pipe down the chimney to where the the stove pipe terminates into the the concrete / clay liner or do you have to secure it into the stove pipe ?

 

[Highbeam]

There is a 7 inch clay flue inside the concrete. does this make a difference ?

How do the liners work ? Is it just a matter of sliding the pipe down the chimney to where the the stove pipe terminates into the the concrete / clay liner or do you have to secure it into the stove pipe ?

If you already have a clay liner then what you have is just a typical masonry chimney. No SS flex liner is required. You can use it as-is and check NFPA for clearance requirements. If your masonry chimney was somehow compromised then not only must you use a SS liner but that liner must be insulated. The only time you can use an uninsulated SS liner is when the masonry chimney is safe and servicable.

This is from the woodstove world. Venting HVAC may have different rules.

 

[warren57]

Highbeam is correct, unless the claytile liner is damaged, you don't need a new liner!! You can save a few bucks. Wish my shop had an existing masonry chimney existing in it.....

 

[rlitman]

If the clay liner is in good shape, you may have a functional masonry chimney (it is absolutely not safe without it). A stainless liner may not be required, but that doesn't mean it isn't still a good idea.

Regardless of the liner, a masonry chimney requires clearances to combustibles. It must not be in contact with insulation or framing, and I just looked it up and there needs to be a 2" gap around the masonry. This gap can only be filled by flashing or firestop.