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Flue Vent Connectors / Stackpipes for Heating Boilers, Furnaces, Water Heaters & Chimney Flues
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
Flue vent connector specifications & fire clearances: this article describes flue vent connectors used to connect heating appliances to a chimney in order to safely deliver combustion gases to a chimney for venting outside the building. Flue vent connectors are not the same thing as a chimney and they have their own safety and installation requirements including slope, materials, diameter, and clearance from combustibles.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Flue Vent Connectors - Heating Boilers, Furnaces, Water Heaters & Chimney Flues
Readers should also see the fire safety distances required for flue vent connectors at FIRE CLEARANCES, Single-Wall Metal Flues. Our photo (page top) shows three heating appliance, each connected by its own flue vent connector to a masonry block chimney. Chimney inspection methods and chimney repair methods are also discussed.
Flue vent connectors for "B" or "L" Vents: A vent connector connects gas equipment to a flue or chimney.
Appliances having draft hoods and installed in an attic or concealed space must
use Type B or L connectors. Appliances installed in basements can use Type B
or Type L or metal pipe (.0304" thick) or aluminum pipe (.012" thick).
Gas fired equipment should not be connected to any flue serving solid fuel
appliances.
Multiple gas vents into a single flue:
Where two or more vent connectors enter a common gas vent, chimney
flue or single wall metal pipe, the smaller connector shall enter at the highest
level consistent with available headroom and clearance to combustibles. Avoid
unnecessary bends and secure all joints with sheet metal screws or other
approved means.
Chimney Blocked at the Flue Vent Connector
Debris falling down a "dead end flue" easily blocks the flue vent connection and chimney (photo, below right).
Carson Dunlop's sketch (below left) shows fallen bricks blocking the bottom of the flue so that even where a "dead end" flue was not constructed - that is, where a chimney cleanout was installed - falling debris can so fill up the chamber at the bottom of the chimney flue that the effect is the same: a blocked chimney flue and the production of carbon monoxide.
A blocked flue case study is reported at UNLINED FLUE INSPECTIONS.
Flue Vent Connector Too Far into Chimney
Carson Dunlop's sketch shows what happens if the flue vent connector is jammed too far into the chimney flue.
Draft may be blocked, resulting in improper and potentially dangerously unsafe heating equipment operation.
For example, inadequate draft on a gas fired appliance makes the production of potentially fatal carbon monoxide gas likely.
The symptoms of this mistake are about the same as a blocked flue, and like a blocked flue, the diagnosis requires some disassembly and inspection of the flue vent connector and chimney by an expert. |
Proper Joint Connections for Single Wall Metal Flue Vent Pipes
Metal flue vent connectors such as shown in Carson Dunlop's sketch (above left) should also be joined with the female (wider) ends facing "up" so that condensate and creosote that may form inside of the flue connector remain inside of the flue pipe. Our photo (above right) shows that foil tape was used to join two flue vent connector pipes of different diameters from a gas fired heating appliance (not shown). Foil tape is not a suitable substitute for the necessary adapter to join the two different diameter flue vent connectors.
It's easy to spot an improperly-joined flue vent connector pair of sections: you will often see stains from leaks oozing out at the section joints.
Flue vent connector horizontal length limits
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 Horizontal vent length limits:
The horizontal length of a vent connector to a natural draft chimney or
vent serving a single draft hood appliance shall not be more that 75% of the
height of the vertical portion of the chimney above the connector. [NFPA 1992
(7.10.10)]. We show some long flue vent connector runs at
The maximum horizontal length of vent connectors per the GAMA tables
is limited to 1.5 feet per inch of diameter with a provision of a 10% reduction in
capacity for each multiple of the length permitted. The venting tables must be
consulted to size an adequate venting system.
Watch out: allowable flue vent connector length calculation can be a bit more complex than that rule of thumb, as the Q&A below illustrates.
Our photo shows a very long dead flue vent connector joining a heating appliance to a dead end chimney.
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Flue Vent Connector Loose, Leaky, Not Sealed at Chimney
Carson Dunlop's sketch (above left) shows a loose flue vent connector at the chimney. This is one of the most common chimney and venting defects we observe in buildings.
This defect can often be seen by careful visual inspection of the flue vent connector at the chimney. Our photo (above right) shows our client pointing to a flue vent connector that was not sealed at the chimney thimble.
The results of a loose or leaky vent connection at the chimney include inadequate chimney draft (unsafe heater operation) and leakage of potentially dangerous combustion gases into the building.
Unsafe Rusted Hole in Metal Flue Vent Connectors
We suspect that the root cause of this unsafe metal heating flue is that it was routed out of the building at or below ground level - into a dead-end chimney.
Water from roof spillage or surface runoff have rusted out the flue vent connector.
This is an unsafe installation even before we think about the added hazards of fire clearances and adequate draft. |
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This flue vent connector (photo at left) has rusted through from water leaking down the chimney into the vent connector elbow.
On an oil-fired system we expect soot to blow out of this opening at system startup and during the system run cycle the opening may interfere with proper system draft. The opening leaves a fire risk should a spark blow out of this opening.
On a gas-fired system a flue vent connector with a rust hole increases the chance of inadequate draft and dangerous carbon monoxide release in the building.
This flue vent connector elbow,rusted through with a large hole, needs to be replaced and more, we need to identify and repair the source of water leakage into the chimney that caused this damage. |
Proper Flue Vent Connector Slope
Metal flue vent connectors such as shown in Carson Dunlop's sketch should:
- slope upwards from the heating appliance to the chimney connection at no less than 1/4" per foot of horizontal run
- be kept as short as possible - long runs get cool, produce condensate, soot and debris and rust and clogging
- have flue pipe sections joined by three screws (two screw joints easily separate to vent dangerous flue gases into the building.
- Also see DRAFT REGULATORS, DAMPERS, BOOSTERS
All draft hood vent connectors
must slope upward toward the flue connection at 1/4" per foot of length. |
Readers should also see the fire safety distances required for flue vent connectors at FIRE CLEARANCES, Single-Wall Metal Flues.
Frequently Asked Questions (FAQs) about flue vent connectors & stackpipes
Question: is there a Simple rule that relates allowable horizontal flue length and flue vent diameter?
What is the maximux allowable horizontal length of a category 1 appliance vent connector in feet for each inch of its diameter. - S.W.
Reply: Not quite so simple, here are the factors that determien allowable horizontal flue length:
From your question, a simple, straight answer on allowable flue length per inch of flue diameter is not quite possible.
Take a look at the National Fuel Gas COde NFPA 54, or ANSI Z223.1, for example, Tables G, and you'll see that the tables consider the following factors that set the allowable horizontal flue length or run:
- the height or flue vent connector rise and/or chimney height
- the lateral distance (0, 2', 6', 12', up to 100 feet), flue diameter
- the number of appliances served
- the maximum appliance BTUH input rate of the appliance(s)
An example (from a 1988 copy of the National Fuel Gas Code):
a 6" diameter Type B double wall connector with a 15' lateral run and a 15' high chimney can vent 198,000 BTUh of capacity.
So a simplistic equation that gives allowable horizontal distance of a flue vent connector or vent expressed in feet per inch of diameter simply would be wrong and potentially unsafe, risking a carbon monoxide or equipment operating problem.
You will want to consult the proper tables either in one of the codes or in the heating equipment manufacturer's equipment installation procedures (the manufacturer would be expected to have the final say).
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
 Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.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:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
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 Home Reference Book, a reference & inspection report product for building owners & inspectors.
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.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- 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.
- 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.
- 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.
- Gas: National Fuel Gas Code, ANSI Z223.1, 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 (at Amazon.com), 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.
- International and Uniform Plumbing Codes Handbook, International Plumbing/Mechanical Assoc (2006), ASIN: B0012QKNZI
- 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/chimneys.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
 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.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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- Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment
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.
- 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.
- 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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