Fire Clearance for Single-Wall Metal Flue Vent Connectors
Fire clearance distances for gas-fired heating appliance flues. Indoor fire clearances for flues, vent connectors, and vents.
Our page top photo shows a gas fired heating appliance flue vent connector routed under and touching wood stairs. Our photo above / left shows a rusted-through flue vent connector also too close to building framing. [Click to enlarge any image]
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 explains why fire clearances between flue vent connectors and nearby wood framing or other combustibles is very important.
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, as we discuss below in an exchange with fire expert NHFireBear.
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.
Be sure that any reduction in fire clearance and any heat shield that you employ has been approved by your local building department and/or fire inspector. A mistake can result in a fatal fire.
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Watch out: at left the sketch titled "Flue Connections and Clearances" shows 18" from the drywall-covered partition to the "smoke pipe" - which is correct for oil fired heating equipment, but the drawing also shows a reduction to 13 1/2" if the wall has a one-hour fire rating. Your local fire inspector might approve this clearance reduction but a fire expert has warned that the drawing is obsolete on this point.
18 Sept 2015 NHfirebear said:
The diagram in the "reduction of clearance" section seems to show an allowance based upon the fire-resistance of the adjacent surfaces. US fire codes and standards, however, generally refer to clearance from "combustible materials", not just the fire rating.
If the wall CONTAINS combustible materials, it probably doesn't matter that the surface is non-combustible. Clearance is measured from "combustible material" inside the fire-rated wall or ceiling.
Even if the surface is completely fire-proof (e.g., metal plate screwed to the wall), the heat will still transfer to the wall and its combustible structure (i.e., wooden studs), resulting in pyrolysis.
A proper heat shield on a wall having combustible construction would have a one-inch ventilated airspace (or with mineral wool batts or fiberglass insulation) between the wall and the shield, even if the wall has a non-combustible or fire-resistant surface.
NFPA 211 (2010): 9.5 Clearance from Chimney or Vent Connectors. Similar rules apply to the reduction of clearances from wood stoves. NFPA 211: 12.6.2 Clearance Reduction, Solid Fuel-burning Appliances.
Thanks for the important clarification FireBear. I agree completely and will be sure it appears in the article.
The exception that can reduce fire clearance distances usually involves an approved heat shield that incorporates not just an air space but a design that has air circulating through that space. In that case a heat shield may be approved by a fire inspector and may reduce clearance. We see this in some wood stove installation instructions.
Watch out: Our photo below shows tremolite asbestos fireproof panels placed on a ceiling, in this instance as a fire barrier not a heat shield.
Reader comment: example of adding a heat shield at a heating flue vent connector to improve fire safety
Above: Our first photo shows the flue vent connector for an oil fired heater. The flue passes too close to wood framing.
Below: heat shielding has been installed to reduce fire hazard and improve fire clearance above a flue vent connector for a basement or crawl space furnace.
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I have an update on last year's "oil flue clearance" fiasco. The contractor eventually fixed it (the day before he was otherwise going to be arrested for a criminal violation of fire code).
He installed a double-walled pipe (listed for 2-inch clearance) and a non-combustible heat shield for clearance reduction on the remaining single-wall pipe located less than 18 inches from combustible ceiling. - NH Fire Bear by private email 2016/07/26
Thanks FireBear. I added a pair of red arrows pointing to the mechanical connections of the heat shield to the wood-framed floor above the flue vent connector in your photo. Some fire experts warn that nails or screws through a heat shield may conduct heat through to combustible materials, thus reducing the fire-safety-effectiveness of the heat shield system. My OPINION is that ... it depends. It probably depends on
Whether or not there is an air space above the heat shield, allowing for reduction in heat transmission through the heat shield as well as for cooling of the connectors
The length of the screws or other fasteners and thus the distance between the inner-side of the heat shield and the nearest combustible material
The fastener location - close to nor not close to the actual heat source, in this case, the flue vent connector. For example a heat shield supported by metal through-fasteners into wood framing that are located at the perimeter of the heat shield material and thus sufficiently distant from the heat source may be fully effective.
Readers wanting to understand how heat as low as 200 degF. can over time lower the combustion point of nearby combustible materials such as wood framing will want to see PYROLYSIS EXPLAINED
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.
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" (unsafe) distance between an oil-fired heating flue vent connector and wood framing. Also notice the leak stains on the flue exterior?
Other photos of flues with inadequate fire clearance are on file - Ed.
Watch out: This clearance is not reduced simply by covering the wall with a non-combustible material, as heat transmitted through the non-combustible covering can still set combustible elements in the wall (such as a wall stud) on fire.
See PYROLYSIS EXPLAINED for details.
Reader 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
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.
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(Mar 7, 2014) Anonymous said: how high a chimney from the gas range
Sorry anon I don't understand the question. A gas range is not normally connected to a building chimney.
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Questions & answers or comments about chimney fire clearance specifications & requirements for metal flues.
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Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: email@example.com
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 firstname.lastname@example.org
Roger Hankeyis 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 #211-3.1 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.
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_Repair.php. 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
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 - 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
Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. Tel: (416) 964-9415 1-800-268-7070 Email: email@example.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides extensive home inspection education and report writing material.
The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones