Dangerous fire clearance distance on a gas flue vent connector (C) Daniel Friedman

Fire safety Clearance Required from Combustibles for Single-Wall Metal Flue Vent Connectors
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What are the required distances between flue vent connectors and combustible surfaces?
Fire clearance distance for oil fired heating system flues
Fire clearance distances for gas-fired heating appliance flues
Indoor fire clearances for flues, vent connectors, and vents
Questions & answers about chimney fire clearance specifications & requirements for metal flues

Combustible clearance specifications for single wall metal flues: this article describes the fire safety clearance distances required between oil and gas fired heating equipment and the nearest combustible surfaces.

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Fire Clearance for Single-Wall Metal Flue Vent Connectors

These articles on chimneys and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects. Chimney inspection methods and chimney repair methods are also discussed. Pyrolysis, the process and temperatures under which wood deteriorates and becomes more readily combustible is discussed here and also at SOLAR COLLECTOR WOOD HOUSINGS. Our page top photo shows a gas fired heating appliance flue vent connector routed under and touching wood stairs.

Reduction in Fire Clearance from Flues by Using Heat Shields

Combustible fire clearance can often be reduced by proper installation of an approved heat shield.

Good heat shield design includes use of noncombustible shield material, a space for air to circulate behind the heat shield, and mounting using connectors that do not transmit heat to the surface being protected.

Watch out: Our photo (below right) shows tremolite asbestos fireproof panels placed on a ceiling, in this instance as a fire barrier not a heat shield. This material is an environmental hazard. See Fireproofing containing Asbestos for details.

Common fire clearance requirements indoors (C) Daniel Friedman

Tremolite asbestos ceiling heat shield (C) Daniel Friedman

Metal Flue Fire Clearance Requirements - Single Wall Metal Pipe Flues & Oil Fired Equipment

Flue vent connectors, also called smoke pipe, stack pipe, or flue pipe by some people, are typically single-walled metal pipes connecting a heating appliance to a chimney, vent, or flue.

Metal flue pipe clearance requirements (C) Carson Dunlop

Inadequate fire clearance flue to wood (C) Daniel Friedman

Oil-fired heating equipment: Unless we have different explicit guidance from the manufacturer of an oil-fired heating appliance being vented, we want to see at least 18" of clearances between the flue vent connector and the nearest combustible surface. Sketch (above left) courtesy Carson Dunlop. Photo (above right) shows a 4 1/2" distance between an oil-fired heating flue vent connector and wood framing. Also notice the leak stains on the flue exterior?

See Flue Vent Connectors - Boilers, Furnaces for details about flue vent connectors.

Metal Flue Fire Clearance Requirements - Single Wall Metal Pipe Flues & Gas Fired Equipment:

Subtle fire clearance hazard (C) Daniel Friedman

Gas-fired heating equipment: fire clearances required range from 6" to 36" depending on the equipment. 9" is a typical clearance between a gas-fired boiler or furnace flue connector and combustibles.

Inadequate fire clearance from combustibles may not be obvious until you open a door such as our client is pointing out in this photograph.

If someone simply leaves the door open so that it touches the heating flue, there is a fire risk. we have found charred door edges in just this installation.

For a more complete listing of clearances by gas-fired appliance type see the table of Listed Flue Vent Connectors for Gas Fired Heating Appliances.

Frequently Asked Questions (FAQs) about metal flue vent connector fire safety & fire clearance distances

Question: Is there a reason why the open 10" area around the furnace flue, at the ceiling level, is surrounded by a mesh screen and open to the attic space?

I have a 5 year old combination forced air, natural gas fired furnace/air conditioner unit located in my hall closet. The metal furnace flue extends vertically through the closet ceiling, through the attic and roof. The vertical flue at the ceiling level is surrounded by and attached to a horizontal piece of open 1/2" X 1/2" metal mesh screen (about 10" square). The mesh design allows an open, back and forth air flow from the interior heating/air unit closet into and from the attic area.

The original louvers on the hall door have recently been sealed/boarded shut, no air coming through the louvers. Normal return air to the unit is directly below the unit adjacent to the floor.

Most of the furnace flue extensions I've seen are constructed through solid sheet rock at the ceiling level, (with no surrounding open area into the attic space) then through the roof. No ceiling mesh.

Questions: Is there a reason why the open 10" area around the furnace flue, at the ceiling level, is surrounded by a mesh screen and open to the attic space? This opening in the ceiling allows a hot and cold air draft flow into and out of the interior of the closet area along with considerable amount of attic debris. Could it have been designed that way to allow combustion air to be introduced to the unit? Can the screen mesh area around the flue at the ceiling level be sealed/closed off and not affect the performance of the heat/air unit? - Ray Haines

Reply: Do not close off cooling air venting around a metal flue/vent without also checking the flue material and its required fire safety clearances - you may need to replace the flue

Ray,

About fire clearances and heating flues including the one you describe, I suspect that your flue is an older installation that relied on air circulation around the flue as it passed through the ceiling for cooling and to meet fire clearance regulations. It's possible, not having seen your system, that either that opening or the louvered doors you described, were also providing combustion air, just as you suggest.

I would be very concerned about closing off the louvered door opening as you describe, because you may be reducing the amount of combustion air, resulting in dangerous, even fatal levels of carbon monoxide produced by your gas fired heater.

I would also not close off the openings around the flue as you may also increase temperatures and interfere with its fire -rating.

Take a look at both FIRE CLEARANCES, Single-Wall Metal Flues [the article above] and at COMBUSTION AIR DEFECTS

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CHIMNEY INSPECTION DIAGNOSIS REPAIR

Metal Chimneys & Flues

Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
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
Chimney Inspection Checklist, Carson Dunlop, Associates, Toronto, Ontario
"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.
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.
Fireplace & Chimney Design, Repair Books - Fireplaces, Chimneys: design, repair
Fireplace & Chimney Inspection Books - Inspecting and diagnosing chimney problems, fireplace problems, chimney & fireplace standards
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 Inspection Checklist, Carson Dunlop, Associates, Toronto, Ontario
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.
Fireplaces, a Practical Design Guide, Jane Gitlin
Fireplaces, Friend or Foe, Robert D. Mayo
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)
Principles of Home Inspection: Chimneys & Wood Heating (Principles of Home Inspection), Carson Dunlop
NFPA 211 - 3-1.10 - Relining guide for chimneys
NFPA 211 - 3-2 - Construction of Masonry Chimneys
NFPA 211 - 3-3 - Termination Height for chimneys
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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Fire Clearance for Single-Wall Metal Flue Vent Connectors

Reduction in Fire Clearance from Flues by Using Heat Shields

Metal Flue Fire Clearance Requirements - Single Wall Metal Pipe Flues & Oil Fired Equipment

Metal Flue Fire Clearance Requirements - Single Wall Metal Pipe Flues & Gas Fired Equipment:

Frequently Asked Questions (FAQs) about metal flue vent connector fire safety & fire clearance distances

Question: Is there a reason why the open 10" area around the furnace flue, at the ceiling level, is surrounded by a mesh screen and open to the attic space?

Reply: Do not close off cooling air venting around a metal flue/vent without also checking the flue material and its required fire safety clearances - you may need to replace the flue

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair