Burning  wood creates a unique problem, this being tar build up.  As wood burns at lower tempertures than fossil fuel/smokeless fuels the residue from the sap can condense on the side of the flue, not as a soft soot but as a `treacle'.  This is particularly the case when a burner is loaded with wood, then the air supply is shut down by the householder to burn overnight.  As subsequent fires are lit producing fresh tar the deposits on the side of the flue turn to a semi liquid tar (creosote), which you may see leaking down the flue pipe or around the register plate that seals off the chimney. This wet tar like substance will then attack the fabric of the chimney, and through capillary action, work its way out to the plaster finish on the walls weakening the mortar at the same time. This usually shows up first in bedrooms with a chimney breast, a brown, half moon shape stain appearing, and a similar one on the adjacent ceiling. Over time the residue will build up, as it goes from a wet state to dry with subsequent fires and so the flue will progressively narrow. The solid tar is highly flammable and will readily catch fire. The tempertures reached in the flue during a fire can be well over 2000 degrees. The results if you are lucky is cracked plaster pot and flue.  If you are really unlucky the house is burnt to the ground. Therefore it is essential that regularly used wood burning appliances are swept at least twice a year.