Thread: Type B vent pipe clearance

Is it proper for Type B vent pipes to be in contact with AC duct board?

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No - minimum clearance to combustible material and insulation is one inch for Type B gas vents.

Originally Posted by Jerry Peck

No - minimum clearance to combustible material and insulation is one inch for Type B gas vents.

Is that requirement in the IRC or from the manufacturer. I've got a contractor that says duct board is non combustible and that type B pipe just has to have 1" clearance from combustibles.

First, duct board is not noncombustible: (bold and underlining are mine)
- http://www.certainteed.com/resources...%202010_v2.pdf
- Fire Resistance: - - Surface Burning Characteristics:
- - - UL 723 and ASTM E84
- - - Max. Flame Spread Index: 25
- - - Max. Smoke Developed Index: 50
- Limited Combustible:
- - NFPA 259
- - - < 3,500 BTU/lb

Second, the code requires all material to be installed in accordance with the code and ... and ... the manufacturer's installation instructions:
- (Mechanical section)
- M1301.3 Installation of materials.
- - All materials used shall be installed in strict accordance with the standards under which the materials are accepted and approved. In the absence of such installation procedures, the manufacturer’s instructions shall be followed. Where the requirements of referenced standards or manufacturer’s instructions do not conform to minimum provisions of this code, the provisions of this code shall apply.
- (Fuel Gas section)
- G2426.1 (502.1) General. - - All vents, except as provided in Section G2427.7, shall be listed and labeled. Type B and BW vents shall be tested in accordance with UL 441. Type L vents shall be tested in accordance with UL 641. Vents for Category II and III appliances shall be tested in accordance with UL 1738. Plastic vents for Category IV appliances shall not be required to be listed and labeled where such vents are as specified by the appliance manufacturer and are installed in accordance with the appliance manufacturer’s installation instructions.
- (And manufacturer's installation instructions specify that an insulation shield be installed to keep insulation away from the vent.)

And last but no least:
- (Fuel Gas section) (bold and underlining are mine)
- G2426.4 (502.4) Insulation shield.
- - Where vents pass through insulated assemblies, an insulation shield constructed of steel having a minimum thickness of 0.0187 inch (0.4712 mm) (26 gage) shall be installed to provide clearance between the vent and the insulation material. The clearance shall not be less than the clearance to combustibles specified by the vent manufacturer’s installation instructions. Where vents pass through attic space, the shield shall terminate not less than 2 inches (51 mm) above the insulation materials and shall be secured in place to prevent displacement. Insulation shields provided as part of a listed vent system shall be installed in accordance with the manufacturer’s installation instructions.

The duct board duct is insulation and is therefore an insulated assembly (the code does not state 'attic insulation', 'wall insulation', or 'floor insulation', although that is what first comes to mind when insulation is referenced.

If Bob H sees this, he can provide additional information, and can help explain why it is needed (don't want anything touching the vent as it could cause a hot spot or a cold spot, neither is good or permissible).

Originally Posted by Gary Mohel

Is that requirement in the IRC or from the manufacturer. I've got a contractor that says duct board is non combustible and that type B pipe just has to have 1" clearance from combustibles.

The vent pipe needs the clearance around it so that it can be cooled by the surrounding air. Also, heat from the vent pipe can be channeled to combustible material if insulation is touching it.

Don't get into a argument with the builder, just report what it is supposed to be.

Originally Posted by Scott Patterson

Don't get into a argument with the builder, just report what it is supposed to be.

Easier said than done, but when one are not sure of the details of what is required, one asks.

Then one has the backup documentation to report "what it is supposed to be" ... after which the contractor can only fume.

I suspect that is why he was asking - not so much because he was in an argument with the contractor, but so that he would have the supporting documentation necessary to say how "it was supposed to be".

Thank you very much! This post was very helpful. My furnace B pipe was awaiting attention for no cover plates and a little cellulose insulation surrounding it in the attic. Some of the insulation had fallen on the bathroom sub-floor behind two bathtub bottoms in the open chase to the basement. Eyeballs says I better check the clearance too. I can imagine some cold tales in wintertime when those tubs were precooled by the air leakage from the attic clearance gap.

Originally Posted by james hiatt

My furnace B pipe was awaiting attention for no cover plates and a little cellulose insulation surrounding it in the attic. Some of the insulation had fallen on the bathroom sub-floor behind two bathtub bottoms in the open chase to the basement.

Not sure if you are describing what I am envisioning, but ... a firestop piece needs to be installed at the floor and at the ceiling (each of which also needs an insulation shield if insulation is present) where the Type B gas vent passes through.

Two of the common pieces necessary are shown in the two attachments, these are from DuraVent - http://www.duravent.com/docs/product/L820_W.pdf

Each manufacturer has their own pieces for these uses, and each are likely to have their own design differences and will be tested and listed for use with their Type B gas vents (to my knowledge, the parts are not interchangeable with parts from other brands as that would not be in conformance with its listing and labeling, and not only invalidate the listing but also be a code violation.

Ya'll pretty much covered it guys. The vent must be installed to its listing and that is installing it to code. However, the listing does leave out a few items such as fireblocking, not to be confused with firestopping. A listed firestop is a tested, listed component provided by the mfr. It is more appropriately called a "firestop/spacer" since it serves the major function of maintaining the stated clearance to the combustible subfloor as the vent penetrates it. You can't just use site fabricated sheet metal because it does not force this clearance (usually 1" with B-vent). Now, you can NOT put dual firestops at both the top and bottom of a joist as this would compartmentalize the heat and it wasn't tested this way. It must have exposure to free air to dissipate the heat. By packing insulation around hot things, the things around them get even hotter as does the vent pipe. In combustion venting, the surfaces at stated clearance cannot rise more than 90°F above ambient. Packing a firestop will grossly exceed this temp. and is a fire hazard. B-vent is tested at 540°F. However, many FVIR water heaters can exceed 600°F at the flue gas outlet.

When you have gaps in the structure greater than the footprint of the listed firestop, you must install "fireblocking" as prescribed by code. This blocks off the surrounding spaces from movement of air, smoke and gases to an extent. It is not intended to be perfect and is not fire rated. If you find insulation from the attic down below then you know either one or both firestop(s) or fireblocking are missing. A firestop and fireblock are required at each floor level or every 10 ft. vertical.
HTH