The Chimney Sweep Company's guide to chimneys, flues and their problems...
- How a flue works
- Flue Functions
- The smokey fire
- Primary cause of smokiness
- Secondary causes of smokiness
- Damp in flues
- Diagnosing problems
- Further sources of information
h... Damp in flues
The burning of any fuel produces water vapour. If the flue is too cool, this vapour will condense, and mixing with other by-products, will produce tar and acids. This can result in either brown stains coming through the walls, both inside and outside the house, or runny tar leaking out around register plates and flue pipe joints, particularly with woodburning stoves. These problems can occur in unlined flues built prior to 1965 or where clay liners have been installed the wrong way up. The solution generally requires lining the chimney. For closed woodstoves, insulating round the liner is advisable.
Rain can also cause similar problems, either entering the flue directly at the top, or through leaking mortar joints, defective flashings or back gutters. These should be checked and repaired by a local builder. Both the slab top and Marcone chimney pot (see diagram 4) are effective at keeping direct rain out. Generally avoid using chimney pot additions, i.e. hood inserts, as these are too restrictive to the outflow of smoke and may lead to other problems.