What is a "shared flue" in a chimney and why are they dangerous
Multiple heating appliances, fireplaces, woodstoves all using the same chimney flue
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This article describes shared chimney flues, multiple heating appliances, fireplaces, woodstoves all using the same chimney venting path through a building and to outside. Shared flues can create fire and smoke spread hazards and carbon monoxide hazards in buildings.
Here we outline how to detect shared chimney flues, how they are repaired, and we list the exceptions to the rule, showing when, in some jurisdictions, it is permitted to vent more than one heating appliance into a single chimney flue and when it is permitted to vent both oil and gas-fired appliances into the same flue.
Our photo (page top) shows three heating appliance, each connected by its own flue vent connector to a masonry block chimney. Is this OK? the answer is, it depends. On the chimney flue size and other installation details. Notice too that we are sharing the flue between oil and wood fired appliances. See Exceptions: Shared Flues Permitted?
A "shared chimney flue" means that the flue or pathway for venting combustion gases out of a building is shared among two or more things that need to be vented, such as a fireplace and a heating boiler. A "flue" refers to a sealed pathway out of the building to vent combustion gases, such as a metal flue pipe or clay flue tiles stacked up inside of a chimney.
A single chimney can safely have multiple flues inside the chimney, each flue venting an appliance, fireplace, or other device, provided that the chimney was properly constructed, including the necessary spacing or sealant between the individual flues.
Our photo at the very top of this page and repeated here shows an unlined chimney flue which, by luck of
illumination from our flash camera, also showed us that a common flue was being used to
vent two fireplaces - a fire code violation in many jurisdictions. You can see that this chimney has two brick flues entering its common upper section. There is no flue divider in this chimney above the two flues entering in its bottom - shown in the center of our photo.
Dedicated chimney flues are required: Each device that produces combustion gases needs to have its own dedicated flue to vent combustion gases safely out of the building.
Fire spread and combustion gas or carbon monoxide hazards: gases may be conducted from a heating appliance to another area in the building when the flue divider is missing.
For example, dangerous carbon monoxide from a basement gas fired heater or a coal stove may leak into an upper building floor through a fireplace or woodstove connected to the same flue.
As Carson Dunlop's sketch points out (above left), the missing divider from the chimney shown in our photo above and in the sketch at left may permit dangerous flue gases or carbon monoxide to enter the living area through a fireplace. Proper construction of chimney walls in a modern masonry flue is shown at above right.
Our photo (above/left) shows that the clay flue tiles are just 1.5" apart (left flue pair) and about 2" apart (right flue pair) - too close according to the sketch above.
This is especially the case if elsewhere in the building a gas (or oil) fired heater is using the same chimney.
In the unfortunate fireplace at left, we see that someone has "fixed" an unsafe shared chimney flue problem.
The fireplace flue was shared with the heating boiler in the basement.
To end this sharing the repairman installed clay flue tiles between the bottom of the fireplace and the upper fireplace flue.
This is no longer a working fireplace. Is it a safe flue?
Examples of Unsafe Shared Chimney Flues
It was common for chimney flues to be shared in older buildings in the following ways (and probably others you can suggest to us). All of the examples in the following list are unsafe and should be corrected immediately.
A single large chimney with multiple fireplaces on the same floor - for example back to back fireplaces on the first floor of a colonial home may share the same flue
A single chimney serving fireplaces on two or more floors may be unsafe and may not be permitted in your jurisdiction. See Two Fireplaces below.
A single chimney flue serving a basement heating appliance: boiler, furnace, or water heater that is also used by a fireplace on the same level in the building or on an upper floor
A single chimney flue serving a fireplace on one floor and a woodstove on an upper floor may be unsafe and may not be permitted in your jurisdiction. See Two Fireplaces below.
A single chimney flue serving a basement heating boiler and a coal stove or woodstove on an upper floor
Single-wythe thick brick masonry flues and some other un-lined chimney flues: see Unlined Chimney Flues: Inspection Procedures, Advice Safety Requirements for old chimneys - photos, dangerous carbon monoxide and blocked flue case report, chimney flue standards, chimney inspection suggestions
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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.
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 Chimney 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 - 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)
More Information on Building Diagnostic Inspections and Repairs
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
InspectAPedia® Home & Site Map - Building & Environmental Inspection, Testing, Diagnosis, Repair, & Problem Prevention Advice: In-depth research & advice on diagnosing, testing, correcting, & preventing building defects & indoor environmental hazards. Unbiased information, no conflicts of interest.
The Mold Information Center: What to Do About Mold in Buildings, When and How to Inspect for Mold, Clean Up Mold, or Avoid Mold Problems
Environmental Inspection, Testing, & Diagnosis On-Site IAQ, Gas, Air Testing, Mold Investigation, Sick Building Diagnosis, Lab Services, & Remediation Plan Preparation - indoor air quality testing, problem source determination, supporting lab work, written remediation plan addressing removal of environmental and other hazards and prevention of their recurrence.
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Fire spread and combustion gas or carbon monoxide hazards: gases may be conducted from a heating appliance to another area in the building when the flue divider is missing. 
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