03-12-13, 10:39 AM
As Slimwilly says, back boilers can make the situation worse by lowering the exhaust temperature. They are often also sited so near to the fuel that they reduce secondary combustion.
Perhaps your parents could use the sawdust briquettes when they want maximum heat output and switch to smokeless fuel when they have the stove turned down.
Rustyrider: open fires are much smokier, but often produce a fairly loose soot deposit. At least the lower part of the flue is easier to inspect without a stove in the way.
Get the wood is as dry as possible to help it burn more cleanly.