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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

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

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

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

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.

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.

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.

Type B Gas Vent (C) Daniel Friedman

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.

B-Vent Chimney Outdoor Temperature Limits

Question: B-Vent outdoor temperature limits

Can B-Vent be used as an exterior chimney in New York where temps reach 10 below? - Edward J Foley 2022/04/28

This Q&A were posted originally at TYPE B-VENT ROOFTOP CLEARANCE

Moderator reply: recommendation: insulated chimney chase for tall outdoor B-Vent chimneys in very cold climates

NO, that is, probably not without an enclosing and insulated chimney chase. Also check with your local building department about what's permitted and safe where you live.

New York 2020 Fuel Gas Code (based on the 2018 International Fuel Gas Code) states the following:

503.7.2 Cold Climate

Uninsulated single-wall metal pipe shall not be used outdoors for venting appliances in regions where the 99-percent winter design temperature is below 32°F (0°C).

504.2.9 Chimney and vent locations.

Tables 504.2(1), 504.2(2), 504.2(3), 504.2(4) and 504.2(5) shall be used only for chimneys and vents not exposed to the outdoors below the roof line. A Type B vent or listed chimney lining system passing through an unused masonry chimney flue shall not be considered to be exposed to the outdoors.

Where vents extend outdoors above the roof more than 5 feet (1524 mm) higher than required by Figure 503.6.5, and where vents terminate in accordance with Section 503.6.5, Item 2, the outdoor portion of the vent shall be enclosed as required by this section for vents not considered to be exposed to the outdoors or such venting system shall be engineered.

A Type B vent shall not be considered to be exposed to the outdoors where it passes through an unventilated enclosure or chase insulated to a value of not less than R8.

Details:

While it's often permissible to extend the B-vent chimney from the building interior to its exterior, and while we see that illustrated in typical installation instructions, that's not the end of the question about exposure of a B-vent to low outdoor temperatures.

Watch out: B-vent chimney manufacturers may provide different instructions for their products, some permitting the use of the B-vent outside with no chimney chase and others requiring that the chimney be enclosed in a chaseway until it reaches the rooftop height.

The concern - which I am guessing you suspect as well - is that at very low outdoor temperatures the draft inside the B-vent chimney flue could be insufficient.

The result risks production and release of fatal carbon monoxide gas in the building served by the heating equipment thus vented.

There could also be a problem of corrosion or equipment damage from high levels of condensate forming in the very cold flue, or even frost formation near the chimney top, blocking or obstructing the venting of flue gases in extremely cold climates.

In reading literature about type B Vent chimneys we often find manufacturers or literature noting that a chimney chase may be needed in cold climates to assure adequate and safe draft.

A Type B-vent chimney

Receives incoming flue gases in the 400 - 480°F (249°C) - that's the temperature of combustion gases entering the chimney

The B-vent's exterior surface is designed to not exceed 90°F - thus meeting fire clearance and safety code requirements, but it does so with an air space between an inner and outer metal flue.

Contrast that with other more-insulated chimneys such as Class-A Chimneys whose construction involves an insulator

see CLASS A CHIMNEYS, MetalBestos™

Building Code Requirements for Outdoor Use of B-Vent Chimneys

Excerpt from the New York State Code

[The underscore emphasis is ours - Ed.] - Note that these are excerpts, not the entire code.

503.7.2 [Single-Wall Metal Chimneys in ] Cold Climate

Uninsulated single-wall metal pipe shall not be used outdoors for venting appliances in regions where the 99-percent winter design temperature is below 32°F (0°C).

503.10.2.2 Vent Connectors Located in Unconditioned Areas

Where the vent connector used for an appliance having a draft hood or a Category I appliance is located in or passes through attics, crawl spaces or other unconditioned spaces, that portion of the vent connector shall be listed Type B, Type L or listed vent material having equivalent insulation properties.

Exception:

Single-wall metal pipe located within the exterior walls of the building in areas having a local 99-percent winter design temperature of 5°F (-15°C) or higher shall be permitted to be used in unconditioned spaces other than attics and crawl spaces.

504.2.9 [B-Vent ] Chimney and vent locations.

Tables 504.2(1), 504.2(2), 504.2(3), 504.2(4) and 504.2(5) shall be used only for chimneys and vents not exposed to the outdoors below the roof line.

[Note: for Type B-Vent chimneys see TABLE 504.3(1) TYPE B DOUBLE-WALL VENT - Ed]

A Type B vent or listed chimney lining system passing through an unused masonry chimney flue shall not be considered to be exposed to the outdoors.

Where vents extend outdoors above the roof more than 5 feet (1524 mm) higher than required by Figure 503.6.5, and where vents terminate in accordance with Section 503.6.5, Item 2, the outdoor portion of the vent shall be enclosed as required by this section for vents not considered to be exposed to the outdoors or such venting system shall be engineered.

A Type B vent shall not be considered to be exposed to the outdoors where it passes through an unventilated enclosure or chase insulated to a value of not less than R8.

Table 504.2(3) in combination with Table 504.2(6) shall be used for clay-tile-lined exterior masonry chimneys, provided that all of the following are met:

1. Vent connector is a Type B double wall.

2. Vent connector length is limited to 11/2 feet for each inch (18 mm per mm) of vent connector diameter.

3. The appliance is draft hood equipped.

4. The input rating is less than the maximum capacity given by Table 504.2(3).

5. For a water heater, the outdoor design temperature is not less than 5°F (-15°C).

6. For a space-heating appliance, the input rating is greater than the minimum capacity given by Table 504.2(6).

Source: 2020 Fuel Gas Code of new York State, based on the 2018 Inernational Fuel Gas Code, retrieved 2022/04/28 original source: https://codes.iccsafe.org/s/NYSFGC2020P1/chapter-5-chimneys-and-vents/NYSFGC2020P1-Ch05-Sec504.2.9

Note: The Fuel Gas Code Table 504.2 Application of Single-Appliance Vent Tables 504.2(1) Through 504.2(6)

provides required chimney heights for vents of various diameters given various appliance input BTUH rating (in thousands of BTUs/Hour) .

Type B Vent Installation Errors 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 Only On 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

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 applications 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).

Note: some specific B-vent manufacturers may set a different maximum flue gas temperature. For example see Hart and Cooley's documentation given below and in which the company states

"Type B Gas Vent is designed for negative pressure applications and flue gas temperatures not exceeding 400°F above ambient for Category I appliances.".

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).

 

Chimney & Vent Codes & Installation Manuals

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

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

at TYPE B-VENT CEILING FLOOR WALL CLEARANCES

Type B Gas Vent Rooftop Chimney Height & Clearances

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

at TYPE B-VENT ROOFTOP CLEARANCE

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:

TYPE B-VENT ROOFTOP CLEARANCE

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-12-04 by (mod) - do I need to build a chase to enclose a B-vent chimney routed outside of my building?

Jim

Short answer: yes, you will want to enclose your B-vent in a chimney chase, especially as your building is in a cool or cold climate.

From the ARTICLE INDEX above we list information about B-vent chimneys starting at this page:

TYPE B-VENT CHIMNEYS - https://inspectapedia.com/chimneys/B-Vent-Chimneys.php

that describes B-vent installation procedures and mistakes to avoid, and that provides links to PDF copies of B-vent installation instructions from several manufacturers.

Of course you'll want to check the installation instructions for your specific water heater and the B-vent that you purchase to use with it.

Using Selkirk's IO manual (from that web page cited above) as an example, you'll see that the company expects the chimney to be exposed outdoors and has both safety and durability suggestions of which I include examples below:

Vent Location: enclose exterior portions of B-vents

Selkirk Type B Gas Vents are recommended to be installed within the heated portion of the structure whenever possible.

If an exterior location is necessary it is highlyrecommended to enclose exposed portions (especially in cold climates) to reduce heat loss which may in turn lead to poor draft and/ or condensation/icing problems.

Safety (under "Vent Location"): provide adequate room air supply

Appliances served by an exterior gas vent must have an air supply to the appliance room adequate to balance indoor and outdoor pressures.

Otherwise, “stack action” of the heated building can cause reverse venting action when the appliance is off, or operating on its pilot.

To prolong the life and appearance of the galvanized steel outer casing

and other parts of Selkirk Gas Vents located outdoors, use proper painting procedure at time of installation. Remove oil and dirt with a solvent.

Paint first with a good quality zinc primer or other primer recommended for use on galvanized steel.

Next apply an appropriate finish coat.

Ordinary house paints applied directly to galvanized steel may not adhere well and do not prevent underfilm corrosion which leads to paint loosening and peeling.

On 2020-12-04 by Jim

I want to put an atmospheric vented 40 gallon gas water heater in the utility room on the main floor of a 2 story house. It will be located against an outside wall.

Double wall Type B vent pipe will run 2 feet vertical from the draft hood into a 90 degree elbow.

Then 16 inch horizontal through a wall thimble and into another 90 degree elbow.

Then 15 feet vertical up the outside of the house through the eve terminating 2 feet above the highest part of the roof within 12 feet.

My question is:

Can I leave the B vent going up the outside of the house in the open or does it have to be enclosed in a chase?
Rural Oregon location

This Q&A were posted originally at GAS APPLIANCE / HEATER DIRECT VENT CLEARANCES

On 2020-05-09 - by (mod) - water heater draft hood requirement

Tony

If you're asking about the vent hood that's normally right at the top of the gas-fueled water heater, that device is required for safe operation and proper draft regulation.

You can see the draft hood atop the Rheem manufactured home water heater in this snapshot

Draft hood atop a Rheem gas fueled water heater or geyser at Inspectapedia.com

Watch out: on most gas fueled water heater the draft hood is required for safe operation.

On 2020-05-09 by Tony

I am replacing the Bradford White model mims40t6ln gas water heater in my mobile home with a rheem gas water heater made for manufactured homes

. My existing hot water heater doesn't have a vent draft hood connecting to the double wall exhaust vent but the new one comes with one.

Do I install it with the new one? The water heater is in a utility closet accessible on the outside of my house

On 2020-03-18 - by (mod) -

Thanks for the question about interconnecting Amerivent and Selkirk metal chimney parts, Carl.

As you can see from the example photo below, the chimney connections are different- stick with a single manufacturer for safety.

On 2020-03-18 by Carl

Can I use amerivent pipe to Selkirk ,will they lock together?

On 2020-01-03 by Sylvain

do i need to be a licence proffetionnel to install a vent liner in my chimney

On 2019-02-04 by (mod) -

There are several safety concerns in the situation you described including water damage to the heating equipment that could make it unsafe, downdrafts that could cause improper venting and risk fatal carbon monoxide poisoning, and ultimately rust damage to the equipment or flue.

I should be trivial to see which vent the rain cap belongs on by inspecting on the roof to see which one is missing. If you can't access your roof safely yourself it's time to hire mechanic with a ladder.

On 2019-02-03 by John Ulman

The wind apparantly blew my Type B Gas vent cap(0ld 3" made by Americap) off as I found it on the ground. I am not sure which vent pipe it came off. Does this present a problem as we have been having quite a bit of rain lately?

On 2018-12-09 - by (mod) -

Todd

Sorry, that's not a specification that I have seen.

I'll do some research to see if I can find any other details.

On 2018-12-09 by todd

how far away from roof vents does b vent termination need to be ? 4" diam.

On 2018-11-11 - by (mod) -

Scott

As long as you are meeting the B-vent manufacturer's fire clearance distances including from the nearest rafter, I don't know of any reason that it needs to exit the roof precisely in the middle between two rafters.

However looking at your photo I see a few items worth mentioning:

1. You will need to remove the B-vent up close to the roof underside or otherwise protect it from both damage and from sawdust and roof debris that otherwise will fall into the vent during the cuttting of the roof exit opening.

2. Your roof decking looks liike two or more generations of pine boards, possibly on an older home that originally was roofed with wood shingles. Or were those nailers overlaid with plywood? I can't quite make that out.

In any case, if the roof opening cuts through one of the original roof nailer boards, you may need to add blocking to support free end of the board.

3. The roof framing looks a bit hodge-podge to me with incomplete rafters tacked together to the left of your B-vent, abutting what may have originally been a lower ridge board with almost no visible nails - the connections could be questionable.

If I were a roofer asked to work on this roof from above, and if I had any experience with old roofs (and workers falling through them or of later being blamed for subsequent roof failures) I would want to inspect the condition of the roof framing and support from the attic side before going on the roof itself. I might then have some suggstions.

On 2018-11-11 by Scott

I have my B-Vent securely anchored to a rafter (maintaining a 1" clearance using AmeriVent Vent Pipe Hangers).

Should I relocate it directly between 2 rafters instead before I have a roofer cut the hole and install flashing, extension pipe, storm collar, and cap?
IMAGE LOST by older version of Clark Van Oyen’s Comments Box code - now fixed. Please re-post the image if you can. Sorry. Mod.


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