Potential Pitfalls with Chimneys
However well designed, constructed and positioned, the satisfactory performance of a flue can be adversely affected by downdraught caused by nearby hills, adjacent tall buildings or trees. These can deflect wind to blow directly down a flue or create a region of turbulence over the terminal.
In this situation, wind blowing over another tall building, tree or hill, descends onto the chimney top, causing a puff of smoke or fumes in the room, usually intermittently. See Fig. 1.
Fig. 2 shows a chimney sited in the line of the prevailing wind, with a taller object, house, roof, tree or nearby hill behind the chimney terminal. This can cause a pressure zone and puffing or continuous fume emission when the wind is blowing.
For a high pitched roof, (>30°,) the pressure on the windward side is greater than on its leeward side. See Fig. 3. A flue outlet located in this high pressure zone (Fig.4) may cause fume emission from an appliance it serves unless the outlet is raised above the high pressure zone or placed on the leeward slope in the suction zone.
If the room in which the flue originates communicates by way of windows or other openings with the suction zone, (Fig 4) the possibility of appliance smoke emission will be increased.
Therefore, for a high pitched roof it is desirable that a chimney placed on the windward slope is located as near to the ridge as possible, where a reduced air pressure exists.
For a low pitched, (<300), or a flat roof, the positioning of the chimney is not so critical and the effect of wind may be largely ignored.
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House of Flues
An interactive guide to help you select the best flue for each appliance around the home.
