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 cross-section 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 cross-section 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.