Type B Gas Vent (C) Daniel Friedman Installation & Inspection of Type B-Vents for Gas Fired Appliances
Specifications for use of Type B gas vents, examples of unsafe Type B vent installations, B-Vent clearances
     

  • TYPE B-VENT CHIMNEYS - CONTENTS: Metal B-vent chimney installation specifications & inspection guide. Suggestions for the selection, installation, & inspection of B-Vents for gas fired appliances. What are Type B vents and where are they used. B-vents for gas fired appliances. Photos of B-vent markings & labels for identification. Chimney cap requirements for B-vents
  • POST a QUESTION or READ FAQs about B-vents chimneys & flues: installation specifications
  • REFERENCES

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Metal B-Vent flue installation requirements & inspection suggestions.

This article 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.

Type B gas vents such as the Ameri-Vent™ are permitted for venting gas-fired appliances and have their own set of combustible clearance and installation specifications.

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Type B-Vents for Gas Fired Appliances

Type B Gas Vent (C) Daniel Friedman Type B Gas Vent (C) Daniel Friedman

Article Contents

Type B Vent (C) Carson Dunlop Associates

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

Watch out: B-Vents are not for use with fireplaces, woodstoves, or oil-fired equipment.

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

The B-vent also requires that its own special chimney cap be installed. If the cap is lost, do not substitute something not recommended by the manufacturer or the chimney may be unsafe.

Watch out: also for a "home made" B-vent using aluminum or clothes dryer venting products. Those are not intended for venting gas appliances and would be unsafe in such use.

Type B Vent Chimney Labels & Identification

Our photos below show the common embossed and printed labels that identify Type B Gas vents.

Type B gas vent installation specifications (C) Daniel Friedman Type B gas vent installation specifications (C) Daniel Friedman

Above are labels and the UL listing mark on a Type B Gas Vent produced by Amerivent. Below is a similar product produced by the Canadian manufacturer Selkirk. At below right I'm holding a new section of Selkirk's Type B gas vent to show its double-wall construction.

Selkirk B-Vent (C) Daniel Friedman Selkirk B-Vent (C) Daniel Friedman

Get the Metal Chimney Sections Right-Way Up

Metal chimney assemblies, including Type B-Vents Includes a "right way up" which you can see Amerivent stamps or embosses right into the steel surface as well as marking it again on a printed label, giving us two chances to get it right.

Type B gas vent installation specifications (C) Daniel Friedman Type B gas vent installation specifications (C) Daniel Friedman

At below left you can see the Type B gas vent installed "right way up". Why is this important?

  • If a gas vent is installed upside down there is risk of dangerous CARBON MONOXIDE - CO gas leakage into the building - a potentially fatal mistake.
  • If a gas vent is installed upside down condensate leaks back into the heating appliance where it can cause expensive corrosion or rust damage, perforation, gas leakage, and expensive as well as dangerous conditions.

Use Type B Gas Vents Only on Gas Fired Equipment - not with other fuels

At below right [Click to enlarge] Ameri-Vent's Type B gas vent label makes plain that the vent is intended only for use with listed, draft-hood-equipped gas appliances. They are not to be used with incinerators or with appliances burning solid or liquid fuels. The gas vent is correctly installed here.

Type B gas vent installation specifications (C) Daniel Friedman Type B gas vent installation specifications (C) Daniel Friedman

Type B Vent Installation Errrors to Avoid

Besides choosing the right chimney material for the appliance it is to vent, a safe installation follows applicable building codes and standards. But the Type B vent manufacturers make a concerted effort to get their products installed correctly and safely even by fellows who prefer to use the instructions to kneel upon.

Stupid Home-Made Gas Vent Tricks

Well what can I say. This creative "gas vent" and home made "chimney cap" had nothing right and nothing safe about it. The telescoping (though creative) cap had slid down into the vent, blocking exit venting, the diameter was too small, there was no rain cap, the material used for the gas vent was not listed for that purpose, a rubber plumbing vent is being used on what was supposed to be a glue gas vent where we expect metal flashing, and so on ad nauseum.

Type B gas vent unsafe installation (C) Daniel Friedman Type B gas vent  unsafe installation (C) Daniel Friedman

It is worth taking a look at all chimneys from time to time - perhaps to notice that a cap has been lost (below left) or other damage has occurred.

In our photo at below right that aluminum, horizontal pipe near the building corner is venting a gas fired water heater. Or is it? At the stone wall we see a second larger B-type gas vent for the heating system.

Type B gas vent unsafe installation (C) Daniel Friedman Type B gas vent unsafe installation (C) Daniel Friedman

Chimney Height & Clearance Requirements for Type B Gas Vents

Heights for these both B-Vent and L-Vent types of metal chimney vents are shown in Carson Dunlop's sketch below.

The top of the chimney should be at least two feet above anything within a ten-foot radius of the chimney.

B-Vents should be at least two feet above the roof surface as well. See the detailed table below and also see TYPE B-VENT CHIMNEY CLEARANCES - separate article compares with other chimney types.

L vent and B vent height requirements (C) Carson Dunlop Associates

[Click to enlarge any image]

Table of Type B-Vent Rooftop Clearance Requirements

The 1992 Vent Sizing Tables require that all Type B gas vents terminate above the roof with a listed cap or listed roof assembly in accordance with the manufacturer's instructions.

Vent caps 12" and smaller may terminate a distance above the roof if 8 feet or more away from a vertical surface as follows:

Table of Rooftop Chimney Clearances for Metal B-Vent Chimneys & Flues
ROOF PITCH - rise/run in inches Minimum Height Above the Roof Surface (1)
Flat to 7/12 1.0 foot above the roof surface (1)
7/12 to 8/12 1.5 feet above the roof surface
8/12 to 9/12 2.0 feet above the roof surface
9/12 to 10/12 2.5 feet above the roof surface
10/12 to 11/12 3.25 feet above the roof surface
11/12 to 12/12 4.0 feet above the roof surface
* Continues to 21/12 pitch at 8.0 feet  

Notes

(1) measure on the up-roof or "shorter" height side of the chimney

(2) Watch out: while one foot above the roof surface may meet the recommendations for fire clearances, in one-storey buildings or in buildings with heating equipment on the uppermost floor, the total chimney height may be inadquate to establish sufficient and safe draft unless you extend the chimney height by more than these low minimums.

(3) Chimney Vent caps larger than 12" must be located at least 2 feet above the highest point and 2 feet higher than any portion of the building within a horizontal distance of 10 feet.

More complete details about chimney height requirements on buildings is
at CHIMNEY HEIGHT & CLEARANCE CODE

Type B Chimney Fire Clearance Details

More complete details about Type B metal chimney fire clearances indoors can be read
at FIRE CLEARANCES, METAL CHIMNEYS.

L-Vents vs B-Vents - What's the Difference Between These Two Vents

As we state at TYPE L VENT CHIMNEYS, it can be confusing distinguishing between L-Vent chimneys and B-Vent chimneys, in part because the same components may be used in both heat venting range installations.

Type L-Vents conform to UL 641 and are intended for higher temperature appications such as venting oil fired heating equipment. A Type-L vent can also be used for venting a gas appliance as explained below.

Type B-Vents are intended for lower temperature gas fired heating equipment applications and should not be used with other fuels (such as home heating oil).

Describing their model DFS L-Vent/ Type B Gas Vent installation procedures, Selkirk Corporation, a producer of metal chimneys and vents, offers these details:

Type L-Vent is designed for venting approved oil or gas appliances producing draft hood flue gases not exceeding a temperature of 570ºF (299ºC).

Type B Gas vent is designed for venting approved gas appliances equipped with draft hoods and other Listed gas appliances specified for use with Type B Gas vent which produce flue gases not exceeding 480°F (249°C).

Minimum clearance between the vent and combustible materials is 3 inches for L-Vent and 1 inch for Type B gas vent. L-Vent or B-Vent that extends through any story above that on which the connected appliance is located is to be provided with enclosures having a fire resistance rating equal to or greater than that of the floor or roof assemblies through which they pass.

Framing dimensions of enclosures and at joist or rafter levels shall be a minimum of 6" larger than the outside of the vent for L-Vent and 2” larger for B-Vent.

Near the vent base, post a notice of the type of appliance for which the DFS installation to combustibles is installed. If installed at 1” airspace, it is limited to B-Vent (gas) only. If it is installed at 3” airspace, it it may be used with oil (as an LVent) or gas (as a B-Vent).

 

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