Type B Gas Vent (C) Daniel Friedman Type B-Vents for Gas Fired Appliances
Specifications for use of Type B gas vents, unsafe Type B vents, B-Vent fire clearances

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Metal B-Vent flue installation requirements & inspection suggestions.

This article describes B-vent metal chimneys used for gas-fired appliances. What are Type B vents and where are they used? B-vents for gas fired appliances are explained. We include photos of B-vent markings & labels for identification.

We give chimney cap requirements for B-vents and we discuss fire safety, fire clearances, and other gas appliance venting details for metal 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.

Type B-Vents for Gas Fired Appliances

Type B Vent (C) Carson Dunlop Associates

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

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. 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.

Article Series Contents

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

As you see in our close-up photo below, a rated or UL-listed type B-vent chimney will usually be clearly labeled.

Type B Gas Vent (C) Daniel Friedman

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

In the photo just above and just below are labels and the UL listing mark on a Type B Gas Vent produced by Amerivent.

Type B gas vent installation specifications (C) Daniel Friedman

Below is a similar product produced by the Canadian manufacturer Selkirk.

Selkirk B-Vent (C) Daniel Friedman

Below 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

L-Vents vs B-Vents - What's the Difference?

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).

Type B Vent Installation Errrors to Avoid

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

Here is a close-up of the vent label: note the "up" arrow.

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?

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

Type B gas vents are not to be used with incinerators or with appliances burning solid or liquid fuels. The gas vent is correctly installed here on a gas fired heater.

Type B gas vent installation specifications (C) Daniel Friedman

Below, [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.

Type B gas vent installation specifications (C) Daniel Friedman

Stupid Home-Made Gas Vent Tricks

Besides choosing the right chimney material for the appliance it is to vent, a safe installation follows applicable building codes and standards.

Type B gas vent unsafe installation (C) Daniel FriedmanBut 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.

Well what can I say about this photo pair above and below ?

This creative "gas vent" and home made "chimney cap" had nothing right and nothing safe about it.

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.

Type B gas vent  unsafe installation (C) Daniel Friedman

Below: a damaged DIY gas vent with no cap.

Type B gas vent unsafe installation (C) Daniel Friedman

In our photo below 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

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

Type B Chimney Fire Clearance Details have moved to a separate article now found


Type B Gas Vent Rooftop Chimney Height & Clearances

This article has moved. Please see detailed B-vent chimney rooftop clearance table


Also see CHIMNEY HEIGHT & CLEARANCE CODE - separate article that compares the B-vent with other chimney types.

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.

However B-vent chimney caps 12" and smaller may terminate a distance above the roof if 8 feet or more away from a vertical surface as given in this table:


Chimney & Vent Codes & Installation Manuals


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

Or see TYPE B-VENT CHIMNEY FAQs - questions & answers posted originally at this page





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

Suggested citation for this web page

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


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Ask a Question or Search InspectApedia

Questions & answers or comments about B-vents chimneys & flues: installation specifications.

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher - Daniel Friedman