Many homeowners presume that the utility company or their fuel supplier is monitoring them, and don't realise this isn't necessarily true until something goes wrong. As a result, bird's nests and other debris sometimes cause soot and gases to drift back into the house, causing a potentially dangerous situation. All types of chimneys should have regular, professional inspection and maintenance. Most chimney sweeps service all kinds of flues, not just masonry chimneys or those attached to wood-burning appliances. Corrosive elements in these gases can affect both metal and masonry chimneys.
Moisture can enter uncapped chimneys, or through cracked concrete tops and even through the bricks.
Excessive condensation from gases within can also cause problems. In the past, some furnaces have been vented into unlined masonry chimneys, which can cause inefficient drafting and deterioration from water damage. In the interest preservation and safe, efficient operation, venting selection and installation is extremely important. The pipe connecting the furnace to the chimney should be as short as possible, with as few elbows as possible. It is important that metal chimneys erected on the exterior of houses be insulated, and of an approved type for outdoor use.