Ceramic Fiber Around Factory Chimney

[Thumper76]

I normally lurk on these forums as most of the info I'm looking for has already been discussed by other members. That being said, info on this topic isn't as easy to find- so I signed up to pick your collective brains.

I have recently installed a zero clearance fireplace (Napoleon NZ 3000). To vent this I installed a 7" id factory stainless chimney. This fireplace is centred on a gable wall of a great room that has a column in the centre of the wall that supports the ridge beam. This made going straight out and up with wall and T kit impossible. There is a chase on the exterior that houses 2 chimneys: 1 for the zero clearance and one for a wood stove that will be located on a covered deck on the exterior.

To route the chimney around the centre column within the wall, I used 2 - 30deg offset kits. The chimney passes through the wall at 30deg off of vertical. I cut an oblong hole in the sheathing and framed a chase that gives 2+ inches of clearance to combustibles and closed the gap on the exterior with 2 metal sheets lapped to shed water and gaps caulked with high temp silicone- now is the crux of my issue...

I wanted to insulate the framed opening to avoid heat loss (fireplace may not run all the time) and avoid condensation from warm house air contacting the metal sheets. I also needed to maintain 2" to combustible materials. So I installed 2" of ceramic Fiber against the chimney and then Roxul (mineral wool in the rest of framed opening. On few test fires I had this winter I did notice that this section of chimney felt quite a bit hotter to the touch which is presumably from the cf wrapped around it.

My question is: Is the 2" gap to combustibles meant to keep the exterior surface of the chimney cool or simply to ensure that nothing combustible is in contact with the chimney?

The chase inside the house is now closed in but the chase on the exterior is still open. If the gap is required for air flow, I could remove the cf from the outside. I want my house to be energy efficient but I also don't want to burn it down. I've attached some pics and I'd appreciate any input you have to offer.

Thanks

 

[double-d]

I'm impressed with your chase and the detail that you put into it.
Class A pipes use fiberglass or ceramic insulation to stay cooler on the outside and have a 2 inch clearance to combustibles. This doesn't mean you can't have more. The clearance is to keep combustibles from catching fire, per Code NFPA 211. If you have Vinyl Siding on the chase, you should probable have more than the 2" clearance, I've seen some siding, that start to "change shape" from the heat.
The angle kits do not allow angles greater than 30 degrees by code, I'm not sure if there is a limit on the number of angles that you could have, others here may know that information

 

[Thumper76]

Thanks Double D. There is a minimum 2" clearance to all wood framing, sheet rock etc- I guess my concern is where I put the 2" of ceramic blanket around the chimney in order to keep the mineral wool away from it. Just trying to figure out if even putting ceramic fibre which is non combustible in this space is a no-no- just not sure if an an air space is required to dissipate heat from the outer surface of the chimney.

I did use 30 deg offsets but for some reason I think in Canada you're permitted to use up to a maximum of 45deg - they call this out in the installation manual.

I believe 2 offset kits are max according to manual and they show as an example using 2 kits to "dogleg" out and around a joist and then back again to the original plumb-line. I thought I was ok as I was just using the 2nd kit to go out the wall.

Thanks very much for your reply.

 

[georgepds]

Not on expert on code... but.. my understanding is the code requires a 1" air clearance all around any chimneys that penetrates a roof or wall.

I'd check with the building inspector on this one

What I think is allowed is a tight metal plate, exposed to the air so it can cool off, on the sheetrock side of the chimney. You can silicone the edges of this this plate to minimize air leaks.. but there will still be conductive losses through the air gap