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 rarely the only cause of a venting problem. Nevertheless, chimneys in locations such as the one above may be susceptible to wind-induced failure, partly because they were failure-prone to begin with.
The Neutral Pressure Plane
In cold weather the buoyancy of the warm air in a house causes a slight pressure difference from the highest to the lowest point. The pressure high in the house is positive relative to atmospheric pressure and it is negative low in the house. Between the high and low pressure areas is a zone of neutral pressure called the neutral pressure plane. When the air is calm, the NPP is roughly horizontal. The idea of the NPP and its likely position under various conditions can be useful in describing what happens to pressures inside a house in windy weather.
The force of wind blowing around a house produces a positive pressure zone on the windward side and a negative pressure zone on the downwind side. These pressures act on the leaks in the envelope, causing air flow through them and changing the pressures within the house. These pressure changes are best illustrated by looking at their effect on the position of the neutral pressure plane. The NPP can tilt away from the horizontal, but no illustration can properly convey the ragged, messy shape that the zone of neutral pressure can be distorted into by wind effects. Perhaps the best way to visualize the wind-induced pressure variations in a house is to compare the NPP to the surface of rough water. The plane of neutral pressure will have waves, curves, peaks and valleys responding to the aerodynamic influences around the building envelope. This understanding renders inherently inaccurate any simple attempt to define and illustrate the position of the NPP under windy conditions.
In strong winds, the pressures experienced by the building envelope can be very powerful—several times the normal pressures produced in chimneys through natural draft. In gusting winds, the pressures and position of the NPP are in constant change, further complicating the diagnostic process.
The design and setting of a house can influence the pressure environment inside during high winds. Imagine that the house on the left backs onto an attractive ravine and that the architect located most of the windows to take advantage of the view. The majority of leaks in the envelope could be on the exposed two-storey section. When a strong wind blows from the front of the house, the entire interior could be placed under negative pressure. This effect could have disastrous consequences for a hearth system installed inside.
The effects of wind acting on leaks in the building envelope can cause wild fluctuations of the pressure inside. An open window on the downwind side can cause the pressure inside to become extremely negative. Likewise, an open window on the windward side can pressurize the house. This effect can help to explain many venting failures of wood heating systems and illustrates the importance of looking at the whole house, and not just the top of the chimney, when diagnosing venting failures. That is, wind acting on the building envelope can cause smoke to be sucked out of a stove or fireplace due to negative pressure in the house.
The effect of wind on the pressures around and inside a building are complex and unpredictable. In general, however, the leakier the building, the more pronounced and immediate is the effect on pressures inside. The unpredictable effects of wind pressure is one reason why the installation of a specialized chimney cap may not cure a venting problem. The pressure changes inside the house may be either driving or adverse to the desired flow of exhaust gases up the chimney.
The illustration to the left shows why wind is more likely to depressurize than pressurize a house as it flows around it. The air flowing parallel to the sides of the house exerts a pressure lower than atmospheric pressure on the house surfaces. Combined with the negative pressure zone on the downwind side, this means that three of the four sides are likely to experience negative pressure. This is a simplified example. In reality, aerodynamic effects are more complex than this.
Building codes call for the chimney project at least three feet above the highest point at which it touches the roof and that its top must be two feet higher than any roofline or obstacle within a horizontal distance of ten feet. Like all building code provisions, these are the minimums allowable and may need to be exceeded in order to meet performance objectives.
Although the effects of wind are unpredictable, one thing is abundantly clear: wood heater and chimney systems of good design are highly resistant to wind-induced venting failure. A chimney that is installed inside the envelope, that penetrates the roof near the peak and that has a baffled cap is unlikely to be negatively affected by wind.