iagnostic 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.