How chimney location affects chimney draft and performance
How to diagnose poor chimney draft for a smoky fireplace, woodstove, or heating boiler/furnace
18 things to check if your chimney has bad draft or your fireplace is smoky
How to correct inadequate chimney draft
Questions & answes about chimney draft: requirement, repairs, factors that affect draft
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This article describes how the location of the chimney on an exterior wall, imbedded in the wall, or located inside of the building affects chimney draft and performance. Our sketch of types of chimney placement on a building is courtesy of Carson Dunlop.
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.
As Carson Dunlop's sketch shows, a warm chimney works best at developing good draft which in turn helps assure that the appliances or fireplaces being vented by the chimney will perform properly.
Construction of the chimney running through the interior of a home was originally done to get the most heat out of the chimney in cold weather.
Even though it is easier to build the chimney on the outside wall of a building, a central chimney provided heat through its masonry to the building interior on all floors.
Draft: Thermal Performance of Chimneys
A chimney's thermal performance provides the "draft" by maintaining a warm interior lining. The draft is the pressure difference between ambient air
and the less dense flue gases within the chimney. The lighter gases are buoyant and rise to be displaced by heavier ambient air.
The chimney must contain the hot gases and protect the surrounding materials against combustion. Residential masonry chimneys must protect the
building while under exposure to 1000oF continuous flue gas temperature although most gas appliances operate with a flue gas temperature of about
300oF and oil burners with a flue gas temperature of about 500oF.
18 Causes of Poor or Inadequate Chimney Draft, Draw, or Causes of Smoking Fireplaces & Heaters
The vertical distance from the top of this chimney to the top of the oil fired heating boiler it serves is less than six feet. The oil fired boiler has blown soot into the utility room and garage throughout its' life, a constant source of annoyance that probably stems from inadequate total draft even when the oil burner, boiler, and chimney flue are up to full operating temperature.
We could address this short chimney with a draft inducer fan, but a taller flue would be smart anyway, to get the chimney top higher than the roof surface. We discuss draft inducer or "draft boosting" fans for heating systems (and maybe for some fireplaces) in detail at Draft Inducer Fans
The articles listed below assist in diagnosing other causes of poor chimney performance.
Blocked Flues: A chimney will not draw properly if it has been blocked for any reason, such as a birds nest, collapsing flue liner, creosote, or a flue vent connector pushed too far into the chimney at the thimble. See Blocked Chimney at the Flue Vent Connector and see Extended Too Far into Chimney.
Chimney flue size & location: a chimney which has a flue diameter too small will not vent combustion products properly and may be unsafe. But a chimney flue that is too big may also not vent properly. For example, venting a gas fired water heater into a large diameter masonry flue (perhaps one previously used for a fireplace) may mean that the heater never produces enough heat to establish a good up-draft in the chimney. One of our clients suffered headaches every October when cold weather approached and her gas fired appliances vented only into the building rather than up through the chimney. A chimney located on the outside of a building is colder and may have a harder time establishing a good draft to vent small appliances.
Chimney location: an outside flue may be too cold to develop adequate draft for the appliance vented into it. See Chimney Location & Draft Performance
Chimney too short: a chimney that is too short in total height may not develop adequate draft and also may be a fire or flue gas release hazard. See our photo above and see Chimney Too Short and Chimney Height Extensions
Chimney too low on roof: such chimneys are not only a fire hazard, they are more likely to be affected by roof shape and may have inadequate draft. See Masonry Chimney Roof Clearance
Chimney cleanout doors: that are missing or left ajar let air into the bottom of the chimney, interfering with development of adequate draft - and are unsafe. See Missing Chimney Cleanout Door
Chimney rain cap: a rain cap not only avoids water damage to the chimney and flue, a properly-designed and installed chimney cap actually improves chimney draft by avoiding downdrafts from local winds. See Missing Chimney Rain Cap
Combustion air: if a fireplace or heating appliance lacks combustion air it will not operate properly and may be very unsafe, risking production of fatal carbon monoxide gas in some cases. See Wood Burning Heaters Fireplaces Stoves for more discussion of combustion air. See COMBUSTION AIR DEFECTS for a discussion of inadequate combustion air for heating boilers and furnaces.
Cracked brick chimneys: may have holes that let outside air into the flue, preventing establishment of proper draft - such chimneys are unsafe. See Cracked Brick Chimney Sides
Cracked masonry block chimneys may let outside air into the flue, preventing establishment of proper draft - such chimneys are unsafe. See Cracked Concrete Block Chimneys
Draft Inducers:Draft Inducer Fans can be installed on heating and fireplace flues as a last resort - but first make sure none of the other causes of bad draft are present - safety hazards may be present.
Draft Regulators:DRAFT REGULATORS, DAMPERS, BOOSTERS and DRAFT HOODS - gas fired must be properly installed and working for heating boilers, furnaces, and water heaters to work properly. See these linked-to articles for details about draft, how it works, how it is measured, why it is important for safe appliance operation.
Fireplace design: if a fireplace is improperly designed, for example with an improper ratio of fireplace opening to throat size or chimney diameter, the fireplace will not draw properly and will be smoky. See Wood Burning Heaters Fireplaces Stoves and FIREPLACES & HEARTHS. Similarly, if the placement of the fire in the fire box puts the burning logs too close to the fireplace front it may not draw well; finally, if the fireplace lacks adequate combustion air it will be a smoky installation.
Flue vent connector length: a metal flue vent connector ("stackpipe") that is too long, especially long horizontal runs, is prone to clogging with debris, rust-through, and other draft problems. See Length Limits for a Flue Vent Connector
Shared chimney flues: venting too many appliances into a single flue may exceed its capacity and may be unsafe. Conversely, sometimes we find that a small gas-fired appliance venting into a shared flue vents properly only when the chimney draft is boosted by an oil fired appliance vented into the same opening. Since it is unlikely that both appliances will always run at once, this is a bad design. See Shared Chimney Flue Examples
Split chimneys are not only very dangerous, releasing flue gases, sparks, presenting a fire risk, and risking collapse, they also fail to develop a proper draft. Watch for this dangerous condition when diagnosing poor chimney draft. Split Openings in Brick & Chimney Collapse
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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 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.
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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