How Wind Affects Chimney Performance
Aerodynamic effects are complicated but being aware of them helps in diagnosing venting problems
Air is a fluid that has weight, so when it gets moving it exerts pressure on anything that gets in its way. If you have experienced a fifty mile per hour wind, you know all about it. Just like water, air flows in eddies and currents when it gets turbulent, as it does flowing around obstacles. The fact that air is invisible makes diagnosing wind-induced venting failure mostly guesswork, but there is some science that provides guidance.
The higher the velocity of a stream of air, the lower is the pressure that it exerts on the surface it is flowing over. It is this principle that gives an airplane wing its lift. For the same reason, wind flowing over the top of a chimney can increase draft by producing a driving pressure that assists in pulling exhaust gases from the chimney.
Despite the fact that wind flowing over a chimney can produce a driving pressure, it cannot be depended upon for appliance performance because it is variable and unpredictable. The only dependable driving pressure in a chimney operating on natural draft is produced by temperature difference.
For example, wind can often flow down towards the top of a chimney after passing over an obstacle like a roof, adjacent building or trees. Wind may also approach the top of a chimney from below after flowing up a roofline to a chimney penetrating the peak. Wind tunnel testing has demonstrated that wind flowing from either above or below the chimney top can be adverse to upward flow by creating positive pressure at the top of the chimney.
Note that the thick black line in this and the other house drawings on this page is the building envelope, which contains the insulation and vapor barrier that encloses the warm spaces of the house.
A chimney with no cap is the most vulnerable to the adverse effects of wind. A cap, particularly one that has baffles to prevent direct line of sight access to the opening (as opposed to a simple flat rain cap) provides significant protection from the adverse effects of wind. In fact, research has shown that caps with baffles (of the sort common on factory-built chimneys) can actually enhance draft regardless of wind direction.
The cap shown at right is the type of design common on factory-built
chimneys. Note that the baffle, in the form of a band between the cap and the skirt at the base of the cap, prevents direct access of the wind to the open top of the chimney. This simple design consistently produces a driving pressure at the top of the chimney, regardless of wind direction or speed.
Adverse pressure can also occur when the top of the chimney is in a positive pressure zone caused by the velocity pressure of the wind as it flows against a raised part of the building behind the chimney (below). This is one case in which adding to the height of the chimney may help to resolve a wind-related venting problem.
Adding height to this chimney could get its top above the positive pressure zone and also make it higher than the second floor ceiling.
Some caution is warranted when diagnosing what may appear to be wind-induced venting failure, particularly when the chimney already has a suitable cap. For example, the householder might report the intermittent puffing of smoke from the appliance that occurs only on windy days. The pulsing effect of wind gusts clearly plays a role in this type of smoke puffing, but is it the only cause? Other contributing factors could be low flue gas temperature due to fire smoldering, an outside chimney, or a chimney that is shorter than the building envelope as in the illustration above.
Often, wind gusts simply cause a vulnerable system that borders on failure to spill the distinctive puff of smoke that implies wind-induced downdraft. At one time or other, most chimney sweeps and technicians have recommended the installation of a specialized "anti-downdraft" chimney cap only to find that it did not cure the problem. Adverse pressure caused by wind acting on the chimney top is 