In North Carolina masonry supply companies typically stock "standard" flue sizes (not modular). If one is designing or building a chimney with say 13" x 13" flue, the modular brick or CMU opening would be 16" x 16" (nominal), leaving 1 1/2" clear on all sides, which is more than the wall thickness of the flue. One could grout this space solid, but not withoutloosing the air space.
How is this situation resolved in the real world?
Mark Dulaney Glen, AIA
Acanthus Architecture, P.A.
Excellent question. Two answers:
1) Masons are hard to change and don't seem to care that they can save up to 20% in the total cost of building a chimney by using a flue size designed to course with the masonry unit they are using. See http://www.rumford.com/articleFluesize.html With thinking like this we have lost most of the production fireplaces to the zero clearance fireplace and wooden chimney guys - needlessly.
2) The code change is new with the 2003 ICC codes and hasn't had much impact yet. The old eastern and southern codes allowed up to 4" air space around the flue which still probably works okay in North Carolina. The "not more than the thickness of the liner" language was insisted on by the California Structural Engineers in exchange for not grouting flues in solid because they were worried that flue liners might shift enough to spill flue gasses in a seismic event if they aren't closely surrounded by masonry.
My recommendation is to insist on modular flues. Superior Clay makes both standard and modular sizes and any dealer in North Carolina can get them just as easily for about the same price as they get standard flues. To avoid high special order shipping costs, order well in advance and ask them to put your modular flues on their next regular full truckload order. It's worth a little extra planning and shopping around since you really can save up to 20% in labor and materials.
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