Chimney location affects performance (C) Carson Dunlop Associates 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
  • POST a QUESTION or READ FAQs about chimney draft: requirement, combustion air code requirements, chimney repairs, factors that affect chimney draft

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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.

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Chimney Location, Draft & Performance

Chimney location affects performance (C) 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

Metal chimney too short (C) Daniel FriedmanThe 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.
  • Blocked chimney 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 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
  • Dead end chimney flues: are a chimney design highly likely to become blocked. See DEAD END CHIMNEY FLUE HAZARDS.
  • 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.

    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 BRICKS, CHIMNEY COLLAPSE

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