My chimney (Lumb as I live in Aberdeenshire) is eliptical in shape,
measuring 11" on the major axis and 9" on the minor axis. It is also
completely vertical. This is a single Lumb in an end gable Granite wall
and the house was built around 1870. I installed a Bont ESSE solid
fuel stove with a water jacket in 1983 and the Lumb has NEVER been
swept. I burn a smokeless fuel (Chinacite) and fallen Beech boughs when
available. About once a year, after heavy summer rain, some soot will
fall and restrict the 90° bend into the stove. I open the inspection
hatch and Hoover it out.

In retrospect I think I should have lined the Lumb with 5.5" pipe as
this is the size that comes out of the ESSE stove. Due to the change
in cross sectional area between the 5.5" and the eliptical Lumb there
is not a great deal of draughting and sometimes the fire will smoke
when the front doors are open.

Chris.