Installation & Inspection of Type B-Vents for Gas Fired Appliances
InspectAPedia® -
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
Fire clearances for B-vent chimneys
Chimney cap requirements for B-vents
What's the difference between a B-vent and an L-vent chimney?
Questions & answers about B-vents chimneys & flues
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.
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.
Type B Vent Chimney Labels & Identification
Our photos below show the common embossed and printed labels that identify Type B Gas vents.
Chimney Height Requirements for Type B Vents
Heights for these both B-Vent and L-Vent types of metal chimney vents are shown in Carson Dunlop's sketch.
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 chimney surface as well.
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:
ROOF PITCH - rise/run in inches
MINIMUM HEIGHT*
Flat to 7/12 - 1.0 foot above the roof surface
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
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.
L-Vents vs B-Vents - What's the Difference Between These Two Vents
As we stated at Type L Vents, 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.
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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Additional technical contributors & reference sources for this article are listed below.
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Thanks to Luke Barnes for suggesting that we add text regarding the hazards of shared chimney flues. USMA - Sept. 2008.
Arlene Puentes, an ASHI member and a licensed home inspector in Kingston, NY, and has served on ASHI national committees as well as HVASHI Chapter President. Ms. Puentes can be contacted at ap@octoberhome.com
Roger Hankey is principal of Hankey and Brown home inspectors, Eden Prairie, MN, technical review by Roger Hankey, prior chairman, Standards Committee, American Society of Home Inspectors - ASHI. 952 829-0044 - hankeyandbrown.com
NFPA 211 - Standards for Chimneys & Fireplaces, NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, 2006 Edition (older editions and standards are found at the same bookstore)
NFPA #211-3.1 1988 -
Specific to chimneys, fireplaces, vents and solid fuel burning appliances.
NFPA # 54-7.1 1992 -
Specific to venting of equipment with fan-assisted combustion systems.
GAMA -
Gas Appliance Manufacturers' Association has prepared venting tables for
Category I draft hood equipped central furnaces as well as fan-assisted
combustion system central furnaces.
National Fuel Gas Code, an American National Standard, 4th ed. 1988 (newer edition is available) Secretariats, American Gas Association (AGA), 1515 Wilson Blvd., Arlington VA22209, and National Fire Protection Association (NFPA), Batterymarch Park, Quincy MA 02269. ANSI Z223.1-1988 - NFPA 54-1988. WARNING: be sure to check clearances and other safety guidelines in the latest edition of these standards.
Fire Inspector Guidebook, A Correlation of Fire Safety Requirements Contained in the 1987 BOCA National Codes, (newer edition available), Building Officials and Code Administrators International, Inc. (BOCA), Country Club HIlls, IL 60478 312-799-2300 4th ed. Note: this document is reissued every four years. Be sure to obtain the latest edition.
Uniform Mechanical Code - UMC 1991, Sec 913 (a.) Masonry Chimneys,
refers to Chapters 23, 29, and 37 of the Building Code.
New York 1984 Uniform Fire
Prevention and Building Code, Article 10, Heating, Ventilating, and Air Conditioning Requirements
New York 1979 Uniform Fire Prevention & Building Code, The "requirement" for 8" of solid masonry OR for use of a
flue liner was listed in the One and Two Family Dwelling Code for New
York, in 1979, in Chapter 9, Chimneys and Fireplaces, New York 1979
Building and Fire Prevention Code:
"Top Ten Chimney (and related) Problems Encountered by One Chimney Sweep," Hudson Valley ASHI education seminar, 3 January 2000, contributed by Bob Hansen, ASHI
"Rooftop View Turns to Darkness," Martine Costello, Josh Kovner, New Haven Register, 12 May 1992 p. 11: Catherine Murphy was sunning on a building roof when a chimney collapsed; she fell into and was trapped inside the chimney until rescued by emergency workers.
"Chimneys and Vents," Mark J. Reinmiller, P.E., ASHI Technical Journal, Vol. 1 No. 2 July 1991 p. 34-38.
"Chimney Inspection Procedures & Codes," Donald V. Cohen was to be published in the first volume of the 1994 ASHI Technical Journal by D. Friedman, then editor/publisher of that publication. The production of the ASHI Technical Journal and future editions was cancelled by ASHI President Patrick Porzio. Some of the content of Mr. Cohen's original submission has been included in this more complete chimney inspection article: InspectAPedia.com/chimneys/Chimney_Inspection.htm. Copies of earlier editions of the ASHI Technical Journal are available from ASHI, the American Society of Home Inspectors.
"Chimneys and Vents", 789 CMR 68.00 Massachusetts Building Code, web search 10/15/2010, original source:
http://www.mass.gov/Eeops/docs/dps/780%20CMR/780068.pdf 03/23/2007
780 CMR: STATE BOARD OF BUILDING REGULATIONS AND STANDARDS
THE MASSACHUSETTS STATE BUILDING CODE - quoting: 6801.11 Multiple-Appliance Venting Systems.
Two or more listed and labeled fossil fuel-fired
appliances shall not be connected to a common
natural draft venting system unless permitted per
applicable requirements of 248 CMR or 527 CMR.
For solid fuel-burning appliances, see 780 CMR
6801.12.
Chimneys and Vents, Chapter 18, M1801,model building code - [on file as Code_Chim_Res_C_18.pdf] - widely used by U.S. & Canadian Municipalities,
"Model DFS L-Vent / Type B Gas Vent Installation Instructions", Selkirk Corporation, 5030 Corporate Exchange Blvd., Grand Rapids MI 49512, Tel: 800-433-6341 & Selkirk Canada Corporation, PO Box 526, Depot 1, Hamilton ON L8L 7X6, 888-735-5475, web search 10/15/2010, original source: www.mass.gov/Eeops/docs/dps/780%20CMR/780068.pdf
"Type L Temperature Venting Systems [on file as L_Vent_Metal_Fab.pdf] - ", Installation and Maintenance Instructions, Metal-Fab Inc., PO Box 1138, Sichita KS 67201, 316-943-2351, Email: info@mtl-fabinc.com website: www.mtl-fabinc.com
Natural Gas Weekly Update: http://tonto.eia.doe.gov/oog/info/ngw/ngupdate.asp Official Energy Statistics from the U.S. Government
US Energy Administration: Electrical Energy Costs http://www.eia.doe.gov/fuelelectric.html
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Ceramic Roofware, Hans Van Lemmen, Shire Library, 2008, ISBN-13: 978-0747805694 - Brick chimneys, chimney-pots and roof and ridge tiles have been a feature of the roofs of a wide range of buildings since the late Middle Ages. In the first instance this ceramic roofware was functional - to make the roof weatherproof and to provide an outlet for smoke - but it could also be very decorative.
The practical and ornamental aspects of ceramic roofware can still be seen throughout Britain, particularly on buildings of the Victorian and Edwardian periods. Not only do these often have ornate chimneys and roof tiles but they may also feature ornamental sculptures or highly decorative gable ends. This book charts the history of ceramic roofware from the Middle Ages to the present day, highlighting both practical and decorative applications, and giving information about manufacturers and on the styles and techniques of production and decoration.
Hans van Lemmen is an established author on the history of tiles and has lectured on the subject in Britain and elsewhere. He is founder member and presently publications editor of the British Tiles and Architectural Ceramics Society. Available at the InspectAPedia Bookstore.
Chimney & Stack Inspection Guidelines, American Society of Civil Engineers, 2003 - These guidelines address the inspection of chimneys and stacks. Each guideline assists owners in determining what level of inspection is appropriate to a particular chimney and provides common criteria so that all parties involved have a clear understanding of the scope of the inspection and the end product required. Each chimney or stack is a unique structure, subject to both aggressive operating and natural environments, and degradation over time. Such degradation may be managed via a prudent inspection program followed by maintenance work on any equipment or structure determined to be in need of attention. Sample inspection report specifications, sample field inspection data forms, and an example of a developed plan of a concrete chimney are included in the guidelines. This book provides a valuable guidance tool for chimney and stack inspections and also offers a set of references for these particular inspections.
NFPA 211 - Standards for Chimneys & Fireplaces, NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid Fuel-Burning Appliances, 2006 Edition (older editions and standards are found at the same bookstore)
NFPA 211 - 3-4 - Clearance from Combustible Material
NFPA 54 - 7-1 - Venting of Equipment into chimneys
Brick Institute of America - Flashing Chimneys
Brick Institute of America - Proper Chimney Crowns
Brick Institute of America - Moisture Resistance of Brick
American Gas Association - New Vent Sizing Tables
Chimney Safety Institute of America - Chimney Fires: Causes, Effects, Evaluation
National Chimney Sweep Guild - Yellow Pages of Suppliers
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