The Chimney Sweep Company's guide to chimneys, flues and their problems...
Click on a heading to move to the section...
- Introduction
- Definitions
- 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
c... How a flue works
Before looking at the causes of chimney problems, it is important to understand how a flue works. A flue is basically a column of hot air and gases, which is lighter than an equivalent column of cold air outside. Observe a bonfire on a still day. The smoke naturally gathers in a column rising vertically above the fire, forming an invisible flue. But once it cools to the temperature of the surrounding air it rapidly disperses. This observation of a bonfire provides two important rules for the best performance of a chimney:
- Smoke wants to rise vertically, therefore any bends or sloping sections in a flue are going to slow down the flow and hence the clearance of smoke from the fireplace.
RULE - Flues should ideally be vertical. If an offset is necessary, it should be as near vertical in angle as possible. 30 degrees from the vertical is the recommended maximum. 45 degrees bends are really too steep, although permitted in some circumstances.
- The smoke only rises as long as it is warmer than the surrounding air. The greater the temperature difference, the faster it will rise.
RULE - Flues should be insulated to keep the gases warmer therefore improving the 'draw' and the clearance of smoke from the fireplace or appliance.