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story · 11-11-2009, 06:36 PM

I'm venting a unit heater horizontally out through a wood wall (the inside wall is some type of wall board, then the insulation, then the outside wood wall). And then the vent goes up, above the roof line. The code says there must be a one-inch gap around the b-vent (which is 8"), and is used to go through the wall. The rest of the venting in 26-gauge single-wall.

Anyway, what should I use to fill in the 1" gap around the 8" b-vent that goes through the wall? Hopefully something not too expensive. I saw an 8" thimble at Menards, but it was $40+ dollars.

I think I know what tocover* the gap with, once I fill-in the gap. But, as mentioned, I'm not sure what would be economical and safe to fill in the gap.

THe unit heater says it must be fire rated to 360 degrees. But that seemed odd to me, since even the special "fire resistant" caulk was only rated to 240 degrees.

Obviously, I'm no HVAC specialist. But I did follow everything in the manual and had the building inspector out here to check it out once I had it hung and the gas line connected--and he gave it the thumbs up so far.

Thanks in advance,

~john

Dave Carney · 11-11-2009, 07:11 PM

You need a thimble.

**beenthere** · 11-11-2009, 07:18 PM

Thimble.

dac122 · 11-12-2009, 08:00 AM

For safest and best looking spend the money on the thimble. If you're sure you have at least a 1" gap all around, and don't care what it looks like, you could try a can of fire rated expansion foam.

story · 11-12-2009, 09:26 AM

Quote:

Originally Posted by dac122

For safest and best looking spend the money on the thimble. If you're sure you have at least a 1" gap all around, and don't care what it looks like, you could try a can of fire rated expansion foam.

I looked at the fire-rated specialty foams at Menard's, and none of them said they were good for temps above 240 degrees.

Is that a high enough rating to spray around the B-vent? Or am I looking for some special type of fire-rated foam, that's only available at a specialty store?

I mentioned earlier that I have a very elegant/attractive way to *cover* both sides of the b-vent hole--much better looking than a huge thimble.

So, yes, I'm *really* looking for something behind the covers I have, to hold the b-vent in place with the one-inch gap.

Any ideas? Is the 240-degree foam good enough?

John

Dave Carney · 11-12-2009, 12:01 PM

thimble

**beenthere** · 11-12-2009, 01:22 PM

No, it will become brittle and crack from the higher heat of the flue.

use a thimble.

yuri · 11-12-2009, 03:26 PM

Metal is ALWAYS best. If you use something else and a few yrs later it deteriorates and there is a problem you could be liable or have insurance company problems.

danstheman · 11-12-2009, 03:58 PM

There are few terms as misused as "fire-rated". There is no foam on the market which is rated for high temperatures. There are fireblock foams, which are not designed exposed to temperatures any higher than standard foam. Fireblock foams contain a fire retardant, and in the event of a house fire will resist burning a little longer than standard foam.

The same thing goes for fireblock caulk, and fire stop caulk as well, both of which are commonly referred to as "fire-rated". Fireblock, Fire Stop, and fire break products are only designed to resist burning in the event of a building fire. They are not designed to withstand high temperatures on a regular basis.

If you want a product for high-temperature applications, look for something that says "high temperature" or "high heat", for example high-heat mortar(~2000-3000ºF) or high-temperature silicone caulk(600ºF).

Most contractors, HVAC 'professionals', and even inspectors I have discussed this with have no idea what the difference between fireblock, firestop, and high-temperature products is. They just assume that anything red is temperature resistant. I think eventually someone is going to get hurt or at least have some big problems as a result of this ignorance.

Quote:

Originally Posted by dac122

For safest and best looking spend the money on the thimble. If you're sure you have at least a 1" gap all around, and don't care what it looks like, you could try a can of fire rated expansion foam.

danstheman · 11-12-2009, 04:17 PM

I recently sealed the opening where the stovepipe for my wood stove entered the wall. There was about a 1" gap, which was covered with a thimble but was not air tight. I make no claims that this method is officially recognized or code-approved, but it worked well and seems pretty safe:

I pulled back the thimble, and stuffed the hole as densely as I could with fiberglass. Then I covered the fiberglass with a generous layer of stove cement(rated to 3000ºF), covering all the fiberglass and meeting the stovepipe and the wall. Before applying the stove cement I sprayed the fiberglass with water to make it damp, as per the cement instructions. I let it dry a few days before pushing the thimble back and firing the stove. It worked great and there is only one tiny crack, despite how thickly I applied it. I think the flexibility of the fiberglass helps reinforce the cement and prevent it from cracking.

Note: Do not try to seal two pieces of metal together with cement. Last year I tried sealing the thimble to the stovepipe using high-heat mortar, and the expansion of the metal caused the mortar to crack and pop and eventually fall off.

**beenthere** · 11-12-2009, 06:15 PM

Despite common thought that it doesn't.
Fiberglass burns. And is a combustable material. So your install is a code violation.

danstheman · 11-12-2009, 09:42 PM

Fiberglass does not "burn", but it does support combustion, if there are flammable materials nearby. I actually tried burning fiberglass with an open flame, and it changed color and gave off a tiny bit of smoke, but the glass did not burn. Everything it is in contact with is non-combustable, and by reducing the stack effect there is less of a draft to feed a fire if one did occur, so I think it actually reduced the risk of fire.

But like I said, I make no claims about this method meeting codes.

Quote:

Originally Posted by beenthere

Despite common thought that it doesn't.
Fiberglass burns. And is a combustable material. So your install is a code violation.