12. Don't bend the rules when you bend the chimney.

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.
XXThe 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.
XXThe 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.
XXThere 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!
XXUnavoidable 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 apart.
 

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