Chimney flue separation (C) Carson Dunlop Associates Chimney Flue Separation Requirements

  • FLUE SEPARATION REQUIREMENTS - CONTENTS: Separation requirements for masonry flues in brick, concrete block, or stone chimneys. Code & Design Requirements for Masonry Chimney Thickness and Flue Separation Thickness. How much space is required between masonry flues? Separation Requirements for Chimney Flues At the Chimney Top. How much solid masonry is necessary between masonry flues for fire and gas safety? Chimney inspection & photo guide to chimney diagnosis, & repair
  • POST a QUESTION or READ FAQs about chimney flue design specifications & codes: what is the requirement to separate individual flues passing through a single chimney structure

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Chimney flue separation or spacing requirements: this article explains the requirement for separation between individual chimney flues in a masonry chimney for fire safety and performance. These articles describe and illustrate chimney inspection procedures and critical chimney defects which can be observed from outdoors at ground level.

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What is the Requirement for Separation Between Masonry Flues in a Multi-Flue Chimney?

Inadequate separation between flues in a masonry chimney (C) Daniel FriedmanOur photo (left) shows three masonry flues in a common chimney, each separated by just an inch or so.

The problems with this design include fire and gas safety hazards - an opening may develop between the flues causing draft interference between them and even inadvertent down-draft of dangerous flue gases from one chimney flue into another (and possibly out through a fireplace or heating appliance).

Code & Design Requirements for Masonry Chimney Thickness and Flue Separation Thickness

Good masonry chimney design will normally provide 8" of solid masonry between individual chimney flues in a multi-flue chimney. In Canada, single wythe brick flues are accepted. In many United States locales, single wythe brick flues remain in use but several standards require or recommend either re-lining (and other safety measures) or the confirmation that 8 inches of solid masonry exists - i.e. a double wythe or greater flue.

State building codes vary in how explicitly they address chimney design details. Further, there may be confusion between the general requirement for thickness of masonry chimney walls and the number of inches of masonry separating flues in a multi-flue masonry chimney.

For example in New York (in 1979) the code required by Para R-906 - Flue Lining (Material) Masonry chimneys shall be lined with fireclay flue liners not less than 5/8 of an inch in thickness or with other approved liner material that will resist, without cracking or softening, a temperature of 1800 deg.F, but the code provided an Exception: Masonry chimneys may be constructed without flue liners when walls are at least 8" in thickness. This requirement was dropped when New York changed from an explicit specification code to a [stupid because it became vague] "performance" code in January 1984.

The International Residential Code® for chimneys and fireplaces requires normal masonry wall thickness of at least 4" of solid masonry units, but requires 8" of solid masonry for chimneys without liners.

This model building code specifies at least 4" of solid masonry between adjacent flues in a common chimney and specifies that the masonry wythes shall be bonded into the walls of the chimney. (An exception which probably applies only in rare cases is provided when only one appliance is being vented: two flues may adjoin each other in the same chimney with only the flue lining separating them. the joints of the adjacent flue linings must be staggered at least 4".)

See see UNLINED FLUE INSPECTIONS for additional details about building codes and the inspection, design, and safety of older chimneys.

Separation Requirements for Chimney Flues At the Chimney Top

Chimney flue separation (C) Carson Dunlop Associates

As Carson Dunlop's sketch shows, masonry flues should be separated inside the chimney structure for fire and gas leak safety.

Our photograph just above shows two clay tile flues that are well separated (though we had other issues with that chimney top).

[Click to enlarge any image]

We include chimney flue separation details as a rooftop chimney inspection item - this may be the easiest place to spot the absence of a needed flue divider.

Examples of Unsafe Masonry Flues with No Separation or Insufficient Separation

Unlined unsafe brick chimney flue interior view (C) Daniel FriedmanOur photo at left shows an older single-wythe brick chimney serving two fireplaces in a pre-1900 home. later one of the fireplaces was abandoned and its flue converted to use by a gas fired heating boiler.

But notice these defects and concerns with this flue:

  • Only a single brick thickness separates these two flues - risking holes, and gas leaks between flues,
  • Risk of down-draft of gas fired appliance flue exhaust back into the building through the fireplace - a potentially deadly carbon monoxide hazard
  • Fire hazards - inadequate masonry thickness

At SHARED CHIMNEY & FLUE HAZARDS we illustrate in more detail an old chimney that lacked this flue divider and we discuss the hazards further.

Separated masonry flues but at same height (C) Daniel Friedman

Our photo (left) shows about 4" of separation between two clay flue tile lined flues in a masonry chimney. But errors at this chimney include

  • the flues terminating at the same height (see CHIMNEY HEIGHT & CLEARANCE CODE)
  • lack of chimney cap over these flues
  • Absence of a gasket or separation between the clay liner and the concrete top seal at the chimney (crown) leading to cracks and ultimate frost damage to the chimney and flue - a possible safety hazard.

Readers should also see CHIMNEY HEIGHT & CLEARANCE CODE where we describe the importance of varying the height among adjacent or nearby chimney flues of all types

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