Guide to Diagnosing & Correcting Chimney Draft & Performance Problems
CHIMNEY DRAFT & Performance - CONTENTS: Chimney draft effects & problems: 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
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Chimney draft troubleshooting: 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. These articles on chimney construction, design, troubleshooting, cleaning & repair include description of how to perform a thorough visual inspection of chimneys for chimney safety, draft, chimney fire hazards, chimney collapse hazards and other defects. Our sketch of types of chimney placement on a building is courtesy of Carson Dunlop Associates.
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.
[Click to enlarge any image]
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 1000°F continuous flue gas temperature although most gas appliances operate with a flue gas temperature of about
300°F and oil burners with a flue gas temperature of about 500°F.
19 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.
Adjacent chimney flues: Metal chimneys too close to one another can interfere with proper draft. See Adjacent Metal Chimney Separation and of course leaks between flues or between a flue liner and the chimney structure (and air gaps) can also cause draft problems, fire hazards, carbon monoxide hazards, other operation and safety concerns.
Ash pit doors & ash pit design defects: masonry or other fireplaces that include an ash pit door are intended to permit the disposal of ashes from the fireplace floor into a fire-safe ash pit below the fireplace. Shared ash pits among fireplaces can be a source of air leaks and draft defects; ash pit doors that are stuck open may interfere with both fire safety and proper combustion air flow; ash pits that are not properly constructed, for example including exposure of the ash pit area to combustible framing, are unsafe and can result in a building fire.
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 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 requirements for fireplaces: 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.
Current model building codes such as Chapter 10 of the IRC require provision of a source of outdoor combustion air for masonry fireplaces and some other fireplace designs. 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 CHIMNEYS, MASONRY BLOCK
Draft Inducers on chimneys: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 on chimneys:DRAFT REGULATOR, DAMPER, BOOSTER and DRAFT HOOD, GAS HEATER 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 defects: 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.
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 BRICKS, CHIMNEY COLLAPSE
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Question: poor chimney draft, smoke from first floor fireplace comes out of basement fireplace.
Hi, I am a builder in Georgia. Last year we completely demolished a house except for the basement walls and the existing fireplace. We rebuilt the first floor, added a second floor and extended the existing fireplace. The house is sprayed foam and it is cooled and heated by a geo-thermal system.
The chimney is shared, two separate flu stacks, one coming from basement and the second from the first floor.
When a fire was started on the first floor fireplace, smoke was coming through the basement fireplace. I am guessing we have a negative air pressure causing this.
My fireplace contractor has recommended installing a fan on top of the chimney, my question is
1. Is this a good solution?
2. Do we need to install some kind of system to bring in fresh air to balance the air being taken out? - H.K., Georgia, 1/16/2014
Reply: serious red flags on chimney safety are raised by draft and smoke observations
I'm unsure what you meant by "... extended the existing fireplace" and I'm worried that the "extension" means someone added a fireplace without giving it its own flue.
And I am more confused by "... The chimney is shared, two separate flu stacks," given that you observed that "...When a fire was started on the first floor fireplace, smoke was coming through the basement fireplace"
A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone, but from your description and observation some serious safety questions and possibly building code violation questions are raised that I'm sure you'll want to get clarified.
Regarding your use of the phrase "shared chimney" - A chimney structure may contain one or more individual flues or passages to vent combustion products. Each of those flues must be properly constructed and intact throughout its passage and cannot communicate with other flues, as such communications or inter-flue leakage is unsafe and also risks draft and fireplace or heating appliance performance and other safety problems.
Heating appliances between floors can never share a chimney flue.
Some building codes in some jurisdictions permit two or more individual heating appliances to be vented into the same flue if those appliances are on the same floor, and some codes/jurisdictions permit oil and gas fired appliances on the same floor to vent into the same flue provided the connections are properly located and installed with respect to one another.
Watch out: When you saw smoke coming out of the basement fireplace in response to starting a fire in the upper floor fireplace this is a significant red flag - as you doubtless recognized. But the problem is far more serious than just a draft defect for the upper fireplace chimney flue.
For smoke to come out of the lower floor fireplace when a fire was ignited on the first floor there must be some flue gas and smoke communication between the two fireplaces - which is a prohibited condition that is unsafe as well as not functional. Such leaks mean that draft is uncontrolled as well as defective, they invite fire spread between building floors, and depending on what other chimney flues exist and appliances are connected to them there is also risk of dangerous flue gas or even carbon monoxide poisoning of building occupants.
So your first order of business is to have an expert, certified chimney inspector examine the chimney and flues to find the defects and hazards.
Now with respect to poor chimney draft and down-flow of smoke between floors, beyond the unacceptable cross-flue leakage I've already cited, I add that cold air falling down a chimney can cause downdrafts but normally as the fire is ignited and chimney is warmed this condition quickly switches to updraft and proper drafting for the fireplace. But a leak between flues, such as an opening between a basement fireplace and first floor fireplace flues that are supposed to be isolated from one another can also cause inadequate draft for both fireplaces.
Once you have found and repaired the unsafe and cross-leaking chimney flues, if draft is still inadequate, a last resort is a chimney top draft inducer fan. I am afraid of fans in fireplace chimneys because of a concern that in event of a chimney fire or other unsafe condition the fan may add to the hazard. (We do see draft inducers on oil fired heating equipment connected to inadequate chimneys on occasion.)
A better solution is to provide outside combustion air - a design that is required for wood burning (and possibly other) fireplaces in modern construction codes.
Finally, with the flues properly intact and separated and isolated from one another, with an assurance that no other chimney or fireplace design or installation safety hazards remain, and with outside combustion air provided for the fireplaces, and with a check of chimney height and clearances, you will want to assure that a properly designed chimney cap is installed both to protect the flue from weather damage and to reduce site-induced downdrafts.
Fireplace ash pit door safety & draft check
Don't forget to include a check on ash pit doors in fireplaces: often I find these doors open to a common ash pit for fireplaces between floors - a possible source of communication between fireplaces if the fireplace ash pit opening doors are not properly constructed, installed, located, and normally closed.
Example Building Code Specification for Fireplace Combustion Air
Chapter 10 of the 2009 IRC Section R1006 defines combustion air requirements for masonry fireplaces.
R1006.1.2 permits installation of listed combustion air ducts in accordance with the manufacturer's listing.
R1006.2 requires combustion air to be taken directly from the exterior of the building or from non-mechanically-ventilated parts of the building (such as ventilated crawlspaces or attics)
The exterior air intake shall also be covered with a corrosion resistant screen of 1/4" mesh - presumably to avoid rodent infestation and nesting that can both block combustion air and form a fire hazard.
This section also states that the combustion air intake shall not be located in a garage, a basement, and shall not be located higher than the firebox.
R1006.3 requires a minimum 1" clearance to combustible material for any combustion air duct for the duct within 5ft of the duct outlet.
R1006.4 requires the duct passageway to be a minimum of 6 sq inches in area and a maximum of 55 sq inches except when listed combustion air systems are used for listed fireplaces.
R1006.5 permits the outlet opening for the combustion air duct to be located in the back or sides of the firebox chamber or within 24" of the firebox opening on or near the floor.
This outlet shall be closeable and designed to prevent burning material from dropping into concealed combustible spaces. Pay particular attention to this rule, as I have on occasion found fireplaces built such that both combustion air passages and ash pit passages included combustible framing - a rather frightening fire hazard in my view.
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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 firstname.lastname@example.org
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 #211-3.1 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
IRC Chapter 10 Chimneys and Fireplaces. Chapter 10 contains requirements for the safe con-
struction of masonry chimneys and fireplaces and establishes the standards for the use and installa-
tion of factory-built chimneys, fireplaces and masonry heaters. Chimneys and fireplaces constructed
of masonry rely on prescriptive requirements for the details of their construction; the factory-built
type relies on the listing and labeling method of approval. Chapter 10 provides the requirements for
seismic reinforcing and anchorage of masonry fireplaces and chimneys. This chapter includes the following sections:
R1001 Masonry Fireplaces 453
R1002 Masonry Heaters 456
R1003 Masonry Chimneys 457
R1004 Factory -built Fireplaces 461
R1005 Factory-built Chimneys 462
R1006 Exterior Air Supply 462
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.
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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 & 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.