The effects of wind

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 (Bernoulli's) 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.

Wind flowing over a chimney assists natural draft by sucking out the gases.

Despite the fact that wind flowing over a chimney produces a driving pressure, it cannot be depended upon for appliance performance because it is highly variable and unpredictable. The only dependable driving pressure in a chimney operating on natural draft is produced by temperature difference.

Winds can flow in a direction adverse to draft in an open chimney after flowing over a taller obstacle.

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 flows from either above or below the chimney top can be adverse to upward flow.

A couple of useful definitions:

Driving pressure: A pressure that produces flow in the desired direction.
Adverse pressure: A pressure that inhibits flow in the desired direction.

The effects of wind on chimney performance

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.

Wind flowing from above or below an open chimney top can have an adverse effect on upward flow by creating turbulence and/or positive pressure at the opening.

Chimney caps help to resist adverse pressures caused by wind. A cap with a vertical baffle performs well in adverse wind conditions.

A simple cap like this one is much better than no cap.

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.

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, a 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 that the building envelope as in the illustration below.

The velocity pressure of wind flowing against a raised part of a building behind the chimney can produce an adverse effect on draft. Note that this system would already have a problem because the top of the chimney is lower than the upper part of the building envelope. This is a good example of how adverse characteristics can combine to create serious venting problems.

Often, wind gusts simply cause a vulnerable system that verges on failure to spill the distinctive puffs of smoke that imply 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 effects of wind on the house envelope
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 (left), 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.

Wind can cause dramatic pressure fluctuations inside the building and can cause the neutral pressure plane to tilt, and cause the position and strength of positive and negative zones to vary wildly.

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.

This whole house could be depressurized by wind because of the location of the majority of leaks .

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 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.

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: hearth 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.

Summary

wind flowing over the top of a chimney can produce a driving pressure, increasing draft
wind may also create adverse pressures at the top of a chimney because of its direction of flow or turbulence created as it flows over nearby obstacles, or when the top of the chimney is in a positive pressure zone
wind pressure induces air flow through leaks in the building envelope
wind causes pressure changes inside a building which in turn changes the position of the neutral pressure plane
wind is an unreliable source of draft because it is highly variable and may be driving or adverse to chimney venting
every chimney should have a cap, preferably one that is baffled, to minimize the potentially adverse effects of wind
specialized chimney caps may reduce the effects of adverse winds, but are sometimes used in error as a cure-all for troubled venting systems
the best defence against wind-induced venting failure is good system design

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