Type B gas vent installation specifications (C) Daniel FriedmanType B-Vents Ceiling Floor Wall Clearances
B-Vent clearances to combustibles

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Metal B-Vent clearance codes & specifications:

Clearance distances for a B-vent to combustible materials in floors, walls, ceilings, attics, roofs.

This article series describes B-vent metal chimneys used for gas-fired appliances. We discuss fire safety, fire clearances, and other gas appliance venting details for b-vent chimneys.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Ceiling, Wall & Wall Clearances for B-Vent "flue pipes"

Type B Vent (C) Carson Dunlop Associates

Type B Chimney Fire Clearance Details

Sketch of a Type "B" vent provided courtesy of Carson Dunlop Associates [at REVIEWERS], a Toronto home inspection, education & report-writing firm. [Click to enlarge any image]

Type B-vents are double-walled metal chimneys/flues and can be used only with listed, draft-hood equipped gas appliances.

A Type B vent is intended for relatively low-heat applications.

B-vents are not permitted for use with incinerators and are not intended for use with appliances burning anything other than LP or natural gas.

Interior Installations of Gas Vent Connectors in all Buildings

Where the chimney extends through any zone of a building (outside that in which the heating appliance connected to it is located), it shall be provided with an enclosure having a fire resistance rating equal to or greater than that of the floor, ceiling, wall or roof assemblies through which it passes. (Selkirk 2015)

Vertical (Floor, Ceiling and Roof) Penetrations

All vertical penetrations where the vent passes through a combustible floor, ceiling or roof, require a Fire Stop (p/n 5x18CI) or Roof Jack be installed.

See Table 1 for proper framing dimension and refer to the Fire Stop or Roof Jack Section for proper installation. Non-combustible Floor, Ceiling & Roof Penetrations do not require a Fire Stop or Roof Jack. (Selkirk 2015)

Horizontal (Wall) Penetrations

Horizontal systems passing through a combustible wall require the use of a Wall Penetration, for relative temperatures with clearances. See Table 1 for proper framing dimensions and refer to Wall Penetration section for installation instructions.

Non-combustible wall penetrations do not require a Wall Penetration.  - (HeatFab 2015)

1) If a ceiling or wall has a fire resistance rating and is penetrated by a Models G, PS, or IPS Grease Duct, then the duct shall be enclosed with a continuous enclosure extending from the penetration, through any concealed spaces, to or through the roof so as to maintain the integrity of the fire separations required by the applicable building code.

NOTE: If penetrated by Z3 or Z4 no additional enclosure is required however the appropriate Through Penetration Firestop (TPF) must be used. See Through Penetration Section.

2) If a ceiling or wall does not have a fire resistance rating and is penetrated by a Model G, PS or IPS Grease Duct installed at the correct minimum clearance for unenclosed duct, then no enclosure is required.

3) Where the Model G, PS and IPS ducting extends through any story of a commercial building above that in which the connected appliances are located, it must be enclosed in the upper stories with walls having a fire resistance rating of not less than one hour for buildings of two or three stories in height.

If the commercial building is four stories or more in height, the enclosure wall shall have a fire resistance rating of not less than two hours.  (Selkirk 2015)

Question: For a propane furnace flue pipe how much clearance is needed through ceiling area?

Brenda Grant said:

For a flue pipe from a propane furnace how much clearance is needed as it goes through ceiling area?

Can one put in metal mesh to prevent insects or critters from coming down from the attic?

This question was posted originally at FIRE CLEARANCES, SINGLE WALL METAL FLUES & VENTS

Reply: use a ceiling shield, wall shield, collar, support that is Listed, Approved as a fire block and for fire safety


Where a metal chimney passes through building floors, ceilings, walls,

Most-common gas vent

If you are using a B-vent (typically a double-walled air-insulated vent for gas appliances, typically you'll see a fire clearance stamped right into the metal of the chimney, stating 1-inch MINIMUM clearance to combustibles - as you'll see in this photo

Type B gas vent installation specifications (C) Daniel Friedman

[Click to enlarge any image] You'll see that the manufacturer, Amerivent has stamped this Type B gas vent as requiring 1" clearance to combustibles.

If you cannot meet the 1-inch B-vent clearance you may be able to use a zero-clearance chimney material. More information: CLASS A CHIMNEYS, MetalBestos™

Possible gas vent

If you are using a "zero-clearance" insulated chimney such as CLASS A CHIMNEYS, MetalBestos™ the product label might it for zero clearance - or rated for in-contact with combustibles.

However a review of contemporary insulated chimneys such as the SuperVent and SuperPro by Selkirk frequrenly require a 2" fire clearance around most penetrations through building walls, floors, ceilings, roofs, unless the wall or floor is made of noncombustible material (like concrete).

Unlikely: single wall propane gas appliance vent

As you've read above, a B-vent is the proper venting connection between a conventional LP gas or natural gas heating appliance and the outdoors.

Occasionally we find an older installation of a gas fired heater that uses a single-wall metal flue between the appliance and a masonry chimney: this might be acceptable depending on local codes and restrictions where you live, if proper fire clearance distances are kept.

The proper term for this connector is a "flue vent connector" but plenty of normal people use the word "flue" or the phrase "stack pipe" loosely for any sort of chimney or chimney connector.

Watch out: a single wall metal flue vent is not safe if it passes through combustible floors, walls, ceilings, roofs.


How to Cover the Open Fire-Clearance Gap at B-vent Passage through Walls or Ceilings

Watch out: You do NOT and should NOT use a home-made insect screen to cover the fire clearance opening.

Rather you should buy a UL-Listed (approved for fire safety) Type B Gas Vent collar and firestop such as the MetalBest 104460 Galvanized RV 4" Type B gas vent collar and firstop or such as the MetalBest RV-GC shown in the photo below. The products you use for this application, if you're in North America, should be Tested and Listed to UL1738 & ULC S636.

Ceiling or wall collar for B-vent chimney flue (C)

and widely available from your heating and plumbing supplier, building supply stores, and even online at Walmart.

Watch out: even using light-weight B-vent chimney sections, the total weight of the chimney extending up through the roof will require support at one or more locations, typically using a ceiling support, attic support, or roof support system provided by the manufacturer. Additional fire blocking and shields are required at each of those passages.

Watch out: Keep in mind that a building permit and inspection is usually required for installing a heating appliance and its vent or chimney - something that you might think you won't enjoy but something should ask-for, since the inspection will improve the safety of your home.

Chimney & Vent Codes & Installation Manuals


Continue reading at TYPE B-VENT ROOFTOP CLEARANCE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see ADJACENT METAL CHIMNEY SEPARATION - separation between metal chimneys and their rain caps



Or see CHIMNEY WET TIME & CORROSION for a discussion of causes of corrosion in B-vents and other metal chimneys

Or see this

Article Series Contents

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TYPE B-VENT CEILING FLOOR WALL CLEARANCES at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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