Flue Sizing

Buckley Rumford Fireplaces
The Mystery of Flue Sizing
Fully Explained

by Jim Buckley

Published as a "Toolbag Tip" in the Magazine of Masonry Construction, September, 1995, pg 434-5.

This is boring and, I'm sure, more than most of you ever wanted to know about flues. But, if you design, sell or build chimneys, you can save 20% on the in-the-wall cost of the chimney. That could be several hundred dollars in your pocket or the competative edge you need to take back some of the market from those tin cans. So, pay attention.
Square and rectangular clay flues are nominally designated by their maximum exterior dimensions. For example a 13"x13" flue is really about 12 3/4 inches square outside and about 11 inches square inside - with rounded corners. Round flues are always designated by their actual inside diameter.
This is a relatively new way of designating flues and is detailed in ASTM C-315, the clay flue lining standard. Just a few years ago flues that were designed to work with standard brick were called "standard flues" and flues that were designed to fit modular brick were called "modular flues." A "standard 12"x12" flue" was what we now call a 13"x13" flue. A "modular 12"x12" flue" is what we now just call a 12"x12" flue.
As I said, standard flues were designed to fit with standard brick. For example, a 13"x13" flue can be wrapped with eight standard brick per course to build a chimney that is 21"x21".

>Problem is most brick these days and all block are modular - that is the units are less than 4" or 8" or 16" but lay up, with a mortar joint, to these dimensions.
If you try to wrap a standard 13" flue with modular brick or block you either end up with one inch head joints, cut little slivers of brick or block or, more likely, build the enclosure another 4" bigger.

Look how awkward all of these solutions are with block.
So, flue manufacturers made modular flues - 12"x12, "12"x16" and 16"x20" - as well as standard flues - 13'x13," 13"x17" and 17"x21".
Comparing the cost of a modular flue in a block chimney (below) to a standard flue in a block chimney (above), we save one whole block per course - four instead of five - plus the labor to cut and set it. The savings is 20% in the wall.

I know many of us want to sell more brick and block, but from the builder's perspective, using modular flues with modular brick or block will save 20% in the wall for the whole chimney. A chimney that might cost a builder $1,000 to build with 13"x13" standard flues will only cost him $800 with 12"x12" modular flues.
Look how well a 12"x12" clay flue and Rumford fireplace work with a 6" residential masonry wall system.

If you use 13"x13" flues the whole shell would have to be made an awkward one inch bigger - which really means four inches bigger - in both directions.
It's by thinking through and integrating our systems we will be able to save our builder customers money and show them how to build better masonry fireplaces competitively. We'll get that one brick or block per course back many times over when we convince builders to use masonry chimneys instead of wooden chimneys.

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