Dangerous fire clearance distance on a gas flue vent connector (C) Daniel Friedman Fire Clearance from Combustibles
Single-Wall Metal Flue Vent Connectors

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Combustible clearance specifications for single wall metal flues:

This article describes the fire safety clearance distances required between oil and gas fired heating equipment and the nearest combustible surfaces.

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Fire Clearance for Single-Wall Metal Flue Vent Connectors

Rusted metal heating flue (C) Daniel Friedman

Fire clearance distances for gas-fired heating appliance flues. Indoor fire clearances for flues, vent connectors, and vents.

Our page top photo shows a gas fired heating appliance flue vent connector routed under and touching wood stairs. Our photo above / left shows a rusted-through flue vent connector also too close to building framing. [Click to enlarge any image]

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.

Article Contents

Pyrolysis, the process and temperatures under which wood deteriorates and becomes more readily combustible explains why fire clearances between flue vent connectors and nearby wood framing or other combustibles is very important.

Details about this process are at PYROLYSIS EXPLAINED.

Reduction in Fire Clearance from Flues by Using Heat Shields

Combustible fire clearance can often be reduced by proper installation of an approved heat shield, as we discuss below in an exchange with fire expert NHFireBear.

Good heat shield design includes use of noncombustible shield material, a space for air to circulate behind the heat shield, and mounting using connectors that do not transmit heat to the surface being protected.

Common fire clearance requirements indoors (C) Daniel FriedmanBe sure that any reduction in fire clearance and any heat shield that you employ has been approved by your local building department and/or fire inspector. A mistake can result in a fatal fire.

[Click to enlarge any image]

Watch out: at left the sketch titled "Flue Connections and Clearances" shows 18" from the drywall-covered partition to the "smoke pipe" - which is correct for oil fired heating equipment, but the drawing also shows a reduction to 13 1/2" if the wall has a one-hour fire rating. Your local fire inspector might approve this clearance reduction but a fire expert has warned that the drawing is obsolete on this point.

18 Sept 2015 NHfirebear said:

The diagram in the "reduction of clearance" section seems to show an allowance based upon the fire-resistance of the adjacent surfaces. US fire codes and standards, however, generally refer to clearance from "combustible materials", not just the fire rating.

If the wall CONTAINS combustible materials, it probably doesn't matter that the surface is non-combustible. Clearance is measured from "combustible material" inside the fire-rated wall or ceiling.

Even if the surface is completely fire-proof (e.g., metal plate screwed to the wall), the heat will still transfer to the wall and its combustible structure (i.e., wooden studs), resulting in pyrolysis.

A proper heat shield on a wall having combustible construction would have a one-inch ventilated airspace (or with mineral wool batts or fiberglass insulation) between the wall and the shield, even if the wall has a non-combustible or fire-resistant surface.

NFPA 211 (2010): 9.5 Clearance from Chimney or Vent Connectors. Similar rules apply to the reduction of clearances from wood stoves. NFPA 211: 12.6.2 Clearance Reduction, Solid Fuel-burning Appliances.

Editor's reply:

Thanks for the important clarification FireBear. I agree completely and will be sure it appears in the article.

The exception that can reduce fire clearance distances usually involves an approved heat shield that incorporates not just an air space but a design that has air circulating through that space. In that case a heat shield may be approved by a fire inspector and may reduce clearance. We see this in some wood stove installation instructions.

Watch out: Our photo below shows tremolite asbestos fireproof panels placed on a ceiling, in this instance as a fire barrier not a heat shield.

This material is an environmental hazard. See FIREPROOFING ASBESTOS SPRAY-ON for details.

Tremolite asbestos ceiling heat shield (C) Daniel Friedman

Reader comment: example of adding a heat shield at a heating flue vent connector to improve fire safety

Inadequate fire clearance for oil fired heater flue vent connector (C)

Above: Our first photo shows the flue vent connector for an oil fired heater. The flue passes too close to wood framing.

Below: heat shielding has been installed to reduce fire hazard and improve fire clearance above a flue vent connector for a basement or crawl space furnace.

Heat shielding installed over a single thickness metal flue vent connector (C) InspectApedia NH FireBear

[Click to enlarge any image]

I have an update on last year's "oil flue clearance" fiasco. The contractor eventually fixed it (the day before he was otherwise going to be arrested for a criminal violation of fire code).

He installed a double-walled pipe (listed for 2-inch clearance) and a non-combustible heat shield for clearance reduction on the remaining single-wall pipe located less than 18 inches from combustible ceiling. - NH Fire Bear by private email 2016/07/26


Thanks FireBear. I added a pair of red arrows pointing to the mechanical connections of the heat shield to the wood-framed floor above the flue vent connector in your photo. Some fire experts warn that nails or screws through a heat shield may conduct heat through to combustible materials, thus reducing the fire-safety-effectiveness of the heat shield system. My OPINION is that ... it depends. It probably depends on

Readers wanting to understand how heat as low as 200 degF. can over time lower the combustion point of nearby combustible materials such as wood framing will want to see PYROLYSIS EXPLAINED

Also see WOOD STOVE OPERATION & SAFETY for more discussion of fire safety and heat shielding.

Metal Flue Fire Clearance Requirements - Single Wall Metal Pipe Flues & Oil Fired Equipment

Flue vent connectors, also called smoke pipe, stack pipe, or flue pipe by some people, are typically single-walled metal pipes connecting a heating appliance to a chimney, vent, or flue.

Metal flue pipe clearance requirements (C) Carson Dunlop Associates Inadequate fire clearance flue to wood (C) Daniel Friedman

Oil-fired heating equipment: Unless we have different explicit guidance from the manufacturer of an oil-fired heating appliance being vented, we want to see at least 18" of clearances between the flue vent connector and the nearest combustible surface. Sketch (above left) courtesy Carson Dunlop.

Photo (above right) shows a 4 1/2" (unsafe) distance between an oil-fired heating flue vent connector and wood framing. Also notice the leak stains on the flue exterior?

Other photos of flues with inadequate fire clearance are on file - Ed.

Watch out: This clearance is not reduced simply by covering the wall with a non-combustible material, as heat transmitted through the non-combustible covering can still set combustible elements in the wall (such as a wall stud) on fire.

See FLUE VENT CONNECTORS, HEATING EQUIPMENT for details about flue vent connectors.

Metal Flue Fire Clearance Requirements - Single Wall Metal Pipe Flues & Gas Fired Equipment:

Subtle fire clearance hazard (C) Daniel Friedman

Gas-fired heating equipment: fire clearances required range from 6" to 36" depending on the equipment. 9" is a typical clearance between a gas-fired boiler or furnace flue connector and combustibles.

Inadequate fire clearance from combustibles may not be obvious until you open a door such as our client is pointing out in this photograph.

If someone simply leaves the door open so that it touches the heating flue, there is a fire risk. we have found charred door edges in just this installation.

For a more complete listing of clearances by gas-fired appliance type see the table of Listed Flue Vent Connectors for Gas Fired Heating Appliances.

Reader Question: Is there a reason why the open 10" area around the furnace flue, at the ceiling level, is surrounded by a mesh screen and open to the attic space?

I have a 5 year old combination forced air, natural gas fired furnace/air conditioner unit located in my hall closet. The metal furnace flue extends vertically through the closet ceiling, through the attic and roof.

The vertical flue at the ceiling level is surrounded by and attached to a horizontal piece of open 1/2" X 1/2" metal mesh screen (about 10" square). The mesh design allows an open, back and forth air flow from the interior heating/air unit closet into and from the attic area.

The original louvers on the hall door have recently been sealed/boarded shut, no air coming through the louvers. Normal return air to the unit is directly below the unit adjacent to the floor.

Most of the furnace flue extensions I've seen are constructed through solid sheet rock at the ceiling level, (with no surrounding open area into the attic space) then through the roof. No ceiling mesh.

Questions: Is there a reason why the open 10" area around the furnace flue, at the ceiling level, is surrounded by a mesh screen and open to the attic space? This opening in the ceiling allows a hot and cold air draft flow into and out of the interior of the closet area along with considerable amount of attic debris.

Could it have been designed that way to allow combustion air to be introduced to the unit? Can the screen mesh area around the flue at the ceiling level be sealed/closed off and not affect the performance of the heat/air unit? - Ray Haines

Reply: Do not close off cooling air venting around a metal flue/vent without also checking the flue material and its required fire safety clearances - you may need to replace the flue


About fire clearances and heating flues including the one you describe, I suspect that your flue is an older installation that relied on air circulation around the flue as it passed through the ceiling for cooling and to meet fire clearance regulations. It's possible, not having seen your system, that either that opening or the louvered doors you described, were also providing combustion air, just as you suggest.

I would be very concerned about closing off the louvered door opening as you describe, because you may be reducing the amount of combustion air, resulting in dangerous, even fatal levels of carbon monoxide produced by your gas fired heater.

I would also not close off the openings around the flue as you may also increase temperatures and interfere with its fire -rating.


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