L vent and B vent height requirements (C) Carson Dunlop Associates Fire Safety Clearance Requirements between Metal Chimneys & Combustible Materials

  • FIRE CLEARANCES, METAL CHIMNEYS - CONTENTS: Minimum clearances from combustibles for manufactured metal chimneys for oil, gas, or other fuel-fired appliances using metal chimneys. Table of Clearances for Listed Flue Vent Connectors for Gas-Fired Heating Appliances Minimum Distance from Combustible Materials
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Metal chimney fire clearance codes & specifications: this article describes the basic fire clearance requirements for four types of manufactured metal chimneys: B-vents, L-vents, Class A chimneys, and 650C Chimneys.

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Fire Safety Clearance Required from Combustibles for Metal Chimneys

Adjacent metal flue heights should vary (C) Carson Dunlop AssociatesThis article series on chimneys, chimney construction, and chimney safety provide detailed suggestions describing how to perform a thorough visual inspection of chimneys for safety and other defects.

Adjacent Metal Chimney Separation Requirements

In addition to the requirement for safety fire clearance from rooftops and other building components, separate metal chimneys that are too close to one another may cause damage resulting in poor chimney performance or an unsafe chimney.

Carson Dunlop's sketch at left shows that metal chimneys should be at least 16 inches apart to avoid damage.

See CLASS A CHIMNEYS, MetalBestos™

Class A, B-Vent, & 650C Metal Chimney Fire Clearance Requirements

Metal flue clearance requirements (C) Carson Dunlop AssociatesCarson Dunlop's sketch (left) summarizes combustible clearance requirements for the three main types of metal chimneys:

Sketch courtesy Carson Dunlop.



Table of Clearances for Listed Flue Vent Connectors for Gas-Fired Heating Appliances

Clearances for Listed Flue Vent Connectors for Gas-Fired Heating Appliances
Minimum Distance from Combustible Materials
Equipment B-Vents L-Vents Single-Wall Metal Pipe Factory-Built Chimneys
B-Vents: Listed equipment w/ draft hood & equipment listed for use with B-Vents
as listedas listed6-inchesas listed
Residential gas boilers/furnaces, gas conversion burner, draft hood
6-inches6-inches9-inchesas listed
L-Vents: Residential appliances listed for use with L-Vents
not permittedas listed9-inchesas listed
Incinerators - residential
not permitted9-inches18-inchesas listed
Listed gas fired toilets
not permittedas listedas listedas listed
Unlisted residential appliances with draft hood
not permitted6-inches9-inchesas listed
Residential & low-heat equipment other than items above
not permitted9-inches18-inchesas listed
Medium heat equipment
not permittednot permitted36-inchesas listed

Other Key Articles on Chimney, Flue, & Vent Clearances for Fire Safety & Building Code Compliance

Reader Question: what's the code on distance and clearance for metalbestos chimneys & vents vs oil storage tanks?

I am installing a pellet boiler and will need a second metal chimney as I am leaving my oil burner as a backup. I live in maine. The only place I would like the metal asbestos chimney would run 40” away from my oil tank. Do you know what the code is on this? I am having a heck of a time getting any answers regarding this? - R. B. Windham ME 2/27/2014


Insulated metal chimneys typically require a one-inch clearance from combustibles.

I think the reason you're not finding an answer to your question is that we're mixing up different constraints and safety concerns:

  1. The required separeation or clearance distances for flue vent connectors (generally single-wall uninsulated metal "flue pipes" or "stack pipes" that connect the heater to the chimney) - discussed at FLUE VENT CONNECTORS, HEATING EQUIPMENT
  2. The required separation of metal or insulated chimneys themselve from varous building components or combustibles - discussed at FIRE CLEARANCES, METAL CHIMNEYS and for height above roofs, CHIMNEY HEIGHT & CLEARANCE CODE
  3. The required separation of heating appliances (like an oil burner) from heating oil storage (the oil tank) - discussed at ABOVE GROUND OIL STORAGE TANK (AST) INSPECTION

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