12. Don't bend the rules when you bend
There are compelling reasons for
keeping a chimney as straight as possible. Even slight bends
introduce friction and in marginal situations this may be enough
to upset performance. Where bends are unavoidable, the standard
way to relieve unwanted friction is to size up the entire chimney
diameter by an inch or two. But as we have seen earlier, increased
chimney diameter means increased conductive heat loss through
the chimney wall. And the whole emphasis in chimney design is
to minimise heat loss so that the flue gases remain hot right
to the point where they exit the stack.
second reason for keeping a chimney straight is maintenance.
Every bend can trap and hold soot, and if there is a major fall
of soot followed by blockage, you can be quite certain the blockage
will occur on a bend. It's also a lot easier to maintain a chimney
if you can see all the way down with a good torch. That way,
stubborn patches of creosote and tar can be readily identified
as can any damage to the chimney lining.
third reason to keep a chimney straight is cost. Whether you
offset in bricks and mortar or in prefabricated stainless-steel
sections, every bend costs money. Cutting a hole in the roof
and dropping a dead straight prefabricated chimney onto the top
of a stove can cost half as much as building up from an insulated
"Tee" on the back of the appliance and introducing
sundry offsets higher up in an attempt to avoid minor obstacles.
used to be a theory that putting a bend in the chimney directly
above the hearth shielded the fire from rain and somehow reduced
'blow back.' It turned out to be only a theory and has no place
in modern design!
offsets should always be kept as gradual as possible. 45º
degree bends are strictly taboo except when a short length of
flue pipe is being used to connect a stove into the base
of the chimney. Higher up, 30º degree bends are acceptable
but should be avoided if possible. By far the best way to offset
is to use a pair of 15º bends placed a reasonable distance