Flue clearance from type X drywall

[rbrady]

Why would you (is it required?) need 1" of clearance between a type B vent and type X drywall if the drywall is fire resistant? I always write it up, but I am beginning to wonder why.

[Wayne B. Wilson]

I believe some of it is to stop transfer of heat to nails or screws

[Larry D. Kage]

http://www.nachi.org/forum/f2/drywal...ustible-11531/

[rbrady]

Thanks Larry, I always search the forum first before asking a question, but I searched the HVAC forum and only went back a little less than 3 years. It looks like the issue is open to some interpetation, although it was definatly leaning towards being OK.

[Wayne B. Wilson]

Also the paper is not fire proof unless it is treated , Stay with the 1 inch clearance to be safe .

[Larry D. Kage]

Page 2, first paragraph by manufacturer states "Drywall is not combustible or explosive."

http://www.lafargecorp.com/wps/wcm/r...ywallMSDS5.pdf



But NFPA classifies it as limited-combustible.

In regard to gypsum wallboard, the product's paper facing prevents it from passing ASTM E 136. However, because it does have a demonstrated ability to perform in fire rated assemblies, the NFPA has placed it in a special classification called limited-combustible. This category distinguishes gypsum wallboard from other, more highly combustible products. To qualify as limited-combustible, a material must have a noncombustible structural base or core, a surface less than 1/8 in. (0.3 cm) thickness and a flame-spread rating of 50 or less.



On top of those two definitions, one should consider whether there is framing wood within the "clearance to combustibles" distance even if there is drywall between it and the vent pipe.

[Nick Gromicko]

Good post Larry.

[Brian E. Kelly]

B- Vent manufacturers say NO, and I believe underwriters Lab has not signed off or tested B-Vent to a less than 1" clearance.

http://www.americanmetalproducts.com...vent/faq.htm#6

Can I install drywall or wall board right up against the gas vent?
No. Drywall and wallboard are considered combustible materials and should not be installed against the vent including the use of this material to close off the hole around the vent. Type B Gas Vent requires a minimum of one-inch clearance to combustibles. This one-inch is a typical clearance for all brands of Type B Vent, but each section of vent pipe is marked with the clearance required by Underwriter Laboratories, so it should be checked.