Rusted metal flue lining an old masonry chimney in Two Harbors MN (C) Daniel FriedmanMetal Chimney Flue Separation Requirements Inside a Common Chimney or Chase

  • METAL CHIMNEY FLUE SEPARATION CODES - CONTENTS: Separation requirements for metal chimnney flues in a wood frame chimney chase or inside existing masonry chimneys during chimney repair or relining jobs.
  • 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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Metal chimney flue separation or spacing requirements inside a common chase or inside a masonry chimney:

Separation requirements for metal chimnney flues in a wood frame chimney chase or inside brick, concrete block, or stone chimneys during chimney repair or relining jobs. Code & Design Requirements for metal chimney flue Separation widths, fire blocking, and other parameters. How much space is required between metal chimney flues in a common chaseway?

This article series 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. Page top photo of a rusted metal liner inside a brick chimney of a Minnesota home illustrates several unsafe conditions.

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

Metal flue clearance requirements (C) Carson Dunlop AssociatesMetal Chimney Flue Clearances Within Common Chimney Chase or Masonry Chimney Relining

Sketch of common metal chimney fire clearances provided courtesy of Carson Dunlop Associates, a Toronto home inspection & education firm.

[Click to enlarge any image]

Reader Question: what separation is needed between multiple stainless steel chimney re-liners in a single masonry flue?

2016/06/17 Carole said:

Have a 33 year old house with ONE chimney for three flues (one for oil heat, one for woodburning stove and one for living room fireplace. Had to repair flues, all terra corra tiles had shifted.

Was told that putting three stainless cylinders dropped in chimney would suffice AND that wythes(separators would not be needed.
My understanding from site is that no matter whether cast in place or stainless tubes are used that wythes separating the flues is code and need to be done.
Thank you

Reply: what you need to know for safe multiple chimney flue re-lining with metal inside a masonry chimney

Metal re-lining of a masonry flue in New York (C) Daniel Friedman

Above: single metal flue re-lining in a masonry chimney.

Interesting question, Carole. I'd start with the local fire inspector and ask an opinion; let me know what you're told. You will also need to know

Be sure you've also seen our discussion of metal chimney flue separation - a separate article at


That page includes cleareances between individual flues and flue types - note the "not permitted" in some entries of the table.

Watch out: improper chimney lining type, size, location, spacing, or any other details risks fire and potentially fatal carbon monoxide hazards. Be sure that your final chimney safety inspection is by a qualified expert who has no financial or other relationship with the contractor performing the chimney work.

A metal chimney inspection checklist (not exactly for your situation but helpful) is at TRIPLE-WALL METAL FIREPLACE CHIMNEYS

Reader follow-up:

Carole said:

We went to Building inspector and there seems to be very little help. Most building inspectors do not even deal with fireplaces.
I live in MA and who is the local fire inspector? I have been laughed at for this question.
Is it someone at the fire dept?

Basically I was told by repair place no separators needed. But that means that stainless walls have nothing to support them and just float around.
Really would like your help in researching further.

Our coverage for insurance has the add on for building to code. This should cover the wythes.
I do not feel good about free floating stainless tubes.

Moderator reply:

See Massachusetts Chimneys and Vents Code 780 CMR 68.00 (2007) [PDF] excerpted below,

in particular

6801.3.4 Clearances.

Chimneys and vents shall have airspace clearance to combustibles in accordance with 780 CMR 51.00 through 99.00 and the chimney or vent manufacturer’s installation instructions.

Exception: Masonry chimneys equipped with a chimney lining system tested and listed for installation in chimneys in contact with combustibles in accordance with UL 1777, and installed in accordance with the manufacturer’s instruction, shall not be required to have a clearance between combustible materials and exterior surfaces of the masonry chimney. Noncombustible firestopping shall be provided in accordance with 780 CMR 51.00 through 99.00. 

6801.4 Space around Lining.

The space surrounding a flue lining system or other vent installed within a masonry chimney shall not be used to vent any other appliance. This shall not prevent the installation of a separate flue lining in accordance with the manufacturer’s installation instructions and 780 CMR 51.00 through 99.00. - Source: Massachusetts Chimneys and Vents Code 780 CMR 68.00 (2007) [PDF] retrieved 2016/06/16, original source:

More metal chimney flue separation requirements from FLUE SEPARATION REQUIREMENTS

Exception: When venting only one appliance, two flues may adjoin each other in the same chimney with only the flue lining separation between them. The joints of the adjacent flue linings shall be staggered at least 4 inches (102 mm). - Chapter 4101:8-10 Chimneys and Fireplaces, Ohio, retrieved 11/9/2015, original source:, retrieved 2015/11/09


Continue reading at CHIMNEY HEIGHT & CLEARANCE CODE where we describe the importance of varying the height among adjacent or nearby chimney flues of all types, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Or see FIRE CLEARANCES, SINGLE WALL METAL FLUES & VENTS - indoor flue vent connector clearance requirements

Or see FLUE SEPARATION REQUIREMENTS - for masonry chimneys



Or see METAL CHIMNEYS & FLUES - home for an index to metal chimney installation and repair guides

Suggested citation for this web page

METAL CHIMNEY FLUE SEPARATION CODES at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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