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CHIMNEY INSPECTION DIAGNOSIS REPAIR
BACKDRAFTING HEATING EQUIPMENT
CARBON MONOXIDE - CO
CHIMNEY COMPONENT DEFINITIONS
CHIMNEY FIRE ACTION / PREVENTION
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
FLAME COLOR, BLUE vs YELLOW COMBUSTION
HEATING SYSTEM INSPECTION
HOME HEATING SAFETY
ODORS GASES SMELLS, DIAGNOSIS & CURE
SAFETY RECALLS CHIMNEYS VENTS HEATERS
STAIN DIAGNOSIS on BUILDING EXTERIORS
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
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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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.
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.
Our page top photo shows a gas fired heating appliance flue vent connector routed under and touching wood stairs. Our photo at left shows a rusted-through flue vent connector also too close to building framing.
Combustible fire clearance can often be reduced by proper installation of an approved heat shield.
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.
Watch out: Our photo (below right) 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.
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.
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" distance between an oil-fired heating flue vent connector and wood framing. Also notice the leak stains on the flue exterior?
See FLUE VENT CONNECTORS, HEATING EQUIPMENT for details about flue vent connectors.
.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.
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
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