In natural ventilation systems, the drafts in the flues or ducts are caused by the difference in density between the air in the ducts and the outer atmosphere. The higher the temperature in the ducts, the more rapid will the draft become.

In the following table 50 per cent, (a fair average in good work) has been deducted from the theoretical flow, to offset all ordinary resistances in the flues, such as friction, change in direction, etc. The difference in temperature given in the table is that existing between the outer atmosphere and the average of the air in the flue. Knowing the velocity per minute, and the cubic feet of air per minute to be removed, the area of the flue in square feet is found by dividing the volume by the velocity.

Flow Of Air In Flues Per Square Foot Of Section. By Natural Draft, In Cubic Feet Per Minute