Is Flex Duct Combustible?

Do you see anything wrong with this gas furnace flue in contact with flex duct?

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combustible or not the vent requires clearances required by the type of vent (b-vent typically requires 1" clearance to combustibles)

Correct me if I’m wrong…but I think that is why he was asking if the flex pipes were combustible…to see if they needed that clearance?


That’s exactly what I’m asking. B-vents require 1" clearance to combustibles. So, I’m wondering if the flex duct is considered combustible.

That I’m not sure of…we’ll see soon enough eh?

I never tried to burn a flex-duct, but Underwriters Lab probably has, I’m sure under the right conditions most anything will burn. I know the fiberglass inside it will.

I would not want it touching a flue vent pipe in my home, and I would tell that to a client.

I believe the inner liner melts, and I always write them up.

It is my understanding the b-vent needs 1" clearace whether the material is combustible or not.

A 1" clearance from the metal straps that support it? 1" clearance from the roof flashing around it? 1" clearance from fiberglass insulation? I’m pretty sure the labels that come on them say 1" clearance from combustible material.

Thanks guys. You’re all very helpful and I appreciate it. You have all confirmed my inner thoughts that it is a concern and I’ll state it that way.

B-vent typically requires a 1 inch “air space” according to the Amerivent (brand of b-vent) installation instructions and as stamped on the pipe. In other words a foot of non-combustible rockwool insulation around the pipe would not be allowed. The pipe is designed to be air cooled.

Here’s the installation instructions from Duravent (emphasis added). I saw nothing that referenced “air space”:

“A roof flashing is placed down over the pipe, and adjusted so the flashing fits tightly against the roof, with the vent pipe held in a position maintaining the 1-inch minimum clearance from combustibles.”

And, “Insure at least 1 inch clearance to all combustibles, including the area the pipe passes through the roof.”

Yes flex duct does burn, it should never touch any thing hot in this case single wall pipe that is shown in the pic (this or not b vent or double wall pipe)

Flex is not allowed through a garage wall due to it’s fire rating. I go with the 1" requirement.