Venting gas fireplace through back of chimney

[ruggles531]

We recently pulled out an old gas insert and pulled off the brick mantle as part of a remodelling project. I am in the process of building and framing out a new linear design unit and have a question about venting. The old fireplace vented directly up the chimney and had a cool air draw coming in underneath it. When I bought the new unit the seller recommended venting out directly through the back of the chimney, so that is the plan.

My question is this. The drawings recommend building a wood frame on the exit wall for the outside vent to mount in. I would assume this is what needs to be done when venting through a traditional wall that has wood studs. As I am venting directly out the back wall of the chimney it is solid brick. Can I just cut out a round hole which is big enough for the pipe to pass through and leave it at that?

I'm a newbie to this and would appreciate your input. I can also take some pictures if it would explain things better. Thanks in advance...

 

[DAKSY]

Yes, you can go thru the brick without using a thimble, but with the thimble in place, you can seal any gaps in the hole around it with hydraulic cement.
The finished install will look better with the thimble, as well. If you ever have to remove anything to do work on or replace something, the thimble has a
slip section that allows for easy disassembly. You may have to struggle to disengage the venting if you simply seal around the vent pipe.

 

[ruggles531]

Thanks for the information. It's interesting because the guy who sold me the fireplace also sold me a flex hose with the external termination plate already attached. The box is labelled SFKIT36S (it's a Lennox fireplace). It doesn't seem to have a thimble which attaches to both sides of the wall. I'll have to take a look and see what options I have.

 

[ruggles531]

I have another question in relation to this install. We removed the old steel firebox from the chimney and found they had packed pink insulation between the steel firebox and the brick of the chimney. As the new fireplace is quite a bit wider than the old one it won't fit all the way into the chimney area and juts out about 10 inches. I therefore have a 10" gap between the back of the new gas fireplace and the back wall of the brick chimney. I am planning on capping the chimney and sealing it off from the bottom (as I will vent the new fireplace out the back wall of the chimney.

What options do I have for insulating this area? Would this be a good application for spray foam insulation? Does it need to be fire rated?

 

[DAKSY]

It shouldn't need to be fire rated. Not sure on the composition of the spray foam, but unfaced fibreglass is commonly used to insulate DV co-linear venting & the exhaust side on that system gets as hot as almost anywhere else in the firebox. If you could frame, insulate & sheetrock a false wall back there, you'd be good to go...

 

[ruggles531]

Thanks again! I was thinking of framing out the space and putting regular old insulation back there. Thanks for confirming this would be a good approach.