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

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.

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

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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

Brenda:

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

Most-common gas vent

If you are using a B-vent (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.

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 frequently 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.

See FIRE CLEARANCES, SINGLE WALL METAL FLUES & VENTS

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) InspectApedia.com

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.

B-Vent Chimney Codes & Installation Manuals

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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 2022-02-11 by Inspectapedia Com Moderator - Can you drill a test hole in the B-vent to measure draft?

@Nate,

Continuing: you can show the photo below to your local inspector - it's from COMBUSTION APPLIANCE SAFETY & EFFICIENCY TESTING [PDF] U.S. Department of Energy

On 2022-02-11 by Inspectapedia Com Moderator

@Nate,

Continuing:

Your code inspector may be familiar with some metal chimney installations such as DuraVent that include chimney section instructions stating:

"DRILL 1/8 INCH DIAMETER HOLE. DO NOT PENETRATE INNER LINER"

But that is pertaining to chimney assembly and should not prevent making the necessary opening to perform needed combustion and draft tests.

On 2022-02-11 by Inspectapedia Com Moderator

@Nate,

Perhaps the inspector worries that you've damaged the B-vent, though I'm doubtful that the effect of a drilled 1/4" hole for a test probe could be measured or detected.

Usually on gas fired heating equipment there is a single-wall galvanized steel flue vent connector between the boiler or furnace exhaust outlet and the entry to the B-vent chimney itself.

It'd be better to make a test opening there so as to avoid an argument about messing with a muti-wall B-vent.

Typically we measure draft in that space - the breech - maybe 2-6" above the top of the heater itself. Measuring draft much higher in the flue might not give the best heater draft or combustion air adjustment results.

We've been doing that since 1976 without an issue with anybody.

Details are at DRAFT MEASUREMENT, CHIMNEYS & FLUES

I'm not clear what your local inspector's worry is about. The exhaust vent on gas fired heater flues and vents is at negative pressure - so if there were any effect of the 1/4" diameter test probe hole ( as instructed by every draft gauge and heating test instrument manufacturer) it would be the drawing-IN of a very small amount of air into the exhaust flue: not enough to measurably affect the draft.

Bottom line: the authorities that I've found (eg. cited below) state that it is perfectly acceptable to drill a test hole in the B-vent to measure draft.

"“B” Vent is a double wall pipe with an outer layer of galvanized steel and an inner layer made of aluminum. It is commonly used to vent gas fired appliances (with negative pressure draft) and it is acceptable practice to drill a test hole in this type of flue pipe to perform combustion and/or draft testing. " - source: "Appendix A: Combustion Appliance Protocols Clean, Tune & Evaluation Requirements" [PDF] - State of Vermont, retrieved 2022/02/11 original source: https://dcf.vermont.gov/sites/dcf/files/OEO/WAP/WAP-Manual-AppendixA.pdf

Copy here at

COMBUSTION APPLIANCE PROTOCOLS CLEAN, TUNE & EVALUATION REQUIREMENTS [PDF]

If at a installation there is no length of single wall flue vent connector (a B-vent is connected directly to the heater) have you considered just cutting a neat square of aluminum stick-on tape to cover the test opening after making a measurement.

On 2022-02-11 by Nate

do you approve a hole being made in the B vent near a furnace, boiler etc for a combustion analyzer to be inserted? or are there any manufacturer forms saying as much?

local inspector giving us a bunch of BS on it but ive been doing it for 18 years and has always been common practice. looking for supporting info that it is approved by b vent manufacturers

On 2021-11-27 by Inspectapedia Com Moderator

It's correct that you don't modify listed chimney parts or the result could be a fire or carbon monoxide hazards.

Also it's generally the case that too many on bends interferes with the draft so that the equipment is not vented safely or properly.

You can box around a metal chimney if you respect the required higher clearances but you want to be sure to design your enclosure so that there is access for inspection , cleaning,, or repair.

On 2021-11-27 by Lisa

We have a decent size dead space in our 1st floor hallway that contains our 4" b-vent ductwork from our basement hot water heater. There is a firestop where the b-vent comes through the 1st floor.

In all my research, it appears that the b-vent cannot be cut and multiple 45 or 90 angle pieces cannot be inserted to move the b-vent into a better location. Am I correct? We really wanted to be able to reclaim this space for a hall closet.

If we forego relocating the b-vent, can we box around the b-vent with at least the minimum of 1" clearance around the b-vent?

On 2021-08-12 by Walter Dick

@inspectapedia.com.moderator,

Thanks for your reply. Yes it is a 4-sided chase that contains a B vent. This is on the main floor of our bungalow. The gas water heater is in the basement and the flue continues through the unfinished attic space and out the roof. I haven't opened the chase yet so don't know if it has drywall on the inner side of the wood studs.

On 2021-08-11 by Inspectapedia Com Moderator

@Walter Dick,

I'm unclear what sort of "wall" and what you mean by "inside the wall" we're discussing, so I can't give you a clear "yes that's fine" - let me explain:

A "wall" to me is a structure framed with 2x4 or 2x6 lumber (or metal studs) faced on two sides, possibly an interior partition wall or an exterior wall, possibly structural.

The cavity space in a wall is just 3 1/2" or may be 5 1/2" in depth and in width it's the stud spacing, e.g. 15 1/2" - so it's not likely that a gas water heater is routed inside such a wall.

A water heater B-vent might however be routed THROUGH such a wall.

As long as you seal the wall penetration for the wire properly and the wires are clear away from the B-vent itself - respecting its clearance distances (see this page above) that's ok.

If by "wall" you actually meant a framed chaseway for a chimney - a 4-sided structure that routes the B-vent chimney up to and through the roof to the exterior,
with the same caveats as I gave above on clearance and fire-rated sealants at penetrations, it's common to run wires in such a chaseway.

Beware that because chimney chases themselves have fire blocking requirements, penetrations for the wire as it runs through a chase also have to meet those fire block requirements. Typically that means using a special fire-rated sealant at the penetrations.

In general you can run wires in a chimney chase

On 2021-08-11 by Walter Dick

Can I add electric wiring and switches/outlets inside an existing wall that encloses a b vent for a gas water heater?

On 2021-06-02 by Inspectapedia Com Moderator

@jeff levitch,

Watch out: Do not run single-wall connectors through unheated spaces, partitions, or in concealed areas. That's a fire and gas leak hazard.

Source: Hart & Cooley, TYPE B GAS VENT INSTALLATION INSTRUCTIONS [PDF] (2015) Hart & Cooley Co., 5030 Corporate Exchange Blvd. SE, Grand Rapids, MI 49512 USA, Tel: (800) 433-6341 Email: info@hartcool.com Website: https://www.hartandcooley.com, retrieved 2021/06/02 original source: https://www.hartandcooley.com/assets/files/o4/150952-typebgasvent-130116.pdf

On 2021-06-02 by jeff levitch

Is it a code violation to put a type b vent (with a joint) through a "concealed space". ( MORE THAN 6" WIDE)?

On 2020-01-13 - by (mod) -

Flue

In addition to the flue clearance data given above on this page and cited in the PDF files we provide also in this article series, take a look at the imprint, stamping, or labels right on your B vent. I bet you'll see that it specifies one inch to combustibles.

On 2020-01-13 by flue pipe clearance

What is the distance for a flue pipe to vinyl siding?

On 2019-11-16 - by (mod) -

Anon

Please use the search box just above on this page to search for

Water heater pressure relief valve

And you will see our articles that will answer just about any question you may have

On 2019-11-16 by Anonymous

Water heater pressure release vavle
Proper pressure

On 2019-06-08 - by (mod) -

Let's start by identifying the brand of your chimney. Then we'll take a look at the installation instructions from that manufacturer.

On 2019-06-07 by Stephen

A previous homeowner installed a triple wall 8" flue through the ceiling from a fireplace with no Firestop or ceiling support box, how do I enclose the ceiling whole after the chimney has already been installed?

On 2018-10-06 - by (mod) -

Will

The final legal answer to your excellent question is in the hands of your local building code inspector who in turn may ask what the chimney manufacturer said.

My opinion is that there wouldn't be a functional issue with a slope but there may be leak issues around the chimney if you use non- factory angles, boots, flashing.

On 2018-10-05 by Will

Does b vent have to penetrate through the roof vertically or can it have a slight slope to ot?

 


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