Positive-pressure ventilation: making a perfect cone

By Mark van der Feyst

The importance of having one entrance and one exit opening when using the positive-pressure fan is to ensure that the structure is pressurized to force the smoke to travel from the high-pressure zone to the low-pressure zone. To obtain this pressurization, the air current created by the fan needs to surround the opening of the structure. An air seal is created around the door or window when the cone of air from the fan completely covers the entire opening.

When there is an incomplete seal around the opening, complete pressurization will not take place. To ensure that the opening is completely covered place your hand at the upper corners of the opening to feel for any air current coming from the fan. If you feel the air then you have a good cone of air surrounding the opening. If you don't feel anything, you will need to adjust the fan to obtain the cone. Remember to shut the fan off before moving it forward or backward.