3. Chimney diameter (b)  The rule
of 1:8
How do we determine correct chimney
diameter in any given situation?
XXA
rule of thumb that has been used for years is to use the 1:8
formula. A square 10" x 10" chimney has a crosssection
of 100 square inches and will serve an aperture 800 square inches
at the bottom.
XXFor
those whose school days are in the dim & distant past you
calculate the crosssection of a round chimney using the formula
3.14 x r². In the example at right, the radius of an 11"
chimney is 5.5". So 3.14 x (5.5" x 5.5") = 95"²
or near enough 100"²
XXThe
1:8 rule of thumb is not foolproof but it's often used as a basis
for designing new chimneys – and it's a useful starting
point for diagnosis when things go wrong.
XXA
chimney that is very short, or which is runs cold owing to extreme
exposure, may need to be closer to 1:7
XXA
warm dry chimney with plenty of height may allow you to get away
with 1:9
XXOne
trap to avoid is to think your chimney should be the same diameter
as the flue collar on your stove. This is not always the case.
Always consult your appliance manufacturer's recommendation first
(if it is available) before falling back on the 1:8 formula.
To view a table showing the size
of apertures that can be served by different sizes of square
and round chimney click here.
