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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.
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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.
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.
Our photos below show the common embossed and printed labels that identify Type B Gas vents.
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.
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.
At below left you can see the Type B gas vent installed "right way up". Why is this important?
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.
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.
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.
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.
[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:
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.
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:
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