Combined Force of Wind Effect and Thermal Effect
It should be noted that when the forces of wind effect and thermal effect are acting together (even when both forces are acting together without interference), the resulting airflow is not equal to the sum
of the two estimated quantities. The flow through any opening is proportional to the square root of the sum of the heads acting upon the opening.
When the two heads are equal in value and the ventilating openings are operated so as to coordinate them, the total airflow through the building is about 10 percent greater than that produced by either head acting independently under conditions ideal to it. This percentage decreases rapidly as one head increases over the other, and the larger will predominate.
The wind velocity and direction, the outdoor temperature, or the indoor distribution cannot be predicted with certainty, and refinement in calculations is not justified; consequently, a simplified method can be used. This may be done by using the equations and calculating the flows produced by each force separately under conditions of openings best suited for coordination of the forces.