Draft - continued

Forced Draft

The figure provided here illustrates a forced draft furnace. In this type of design, a fan is used to aid the draft created by the chimney.

The fan, known as the “Forced Draft Fan” or the “FD Fan” forces combustion air into the furnace. To control the air flow, usually such fans are provided with moveable vanes on the suction side. In some cases variable frequency drive may be used to run the fan. In this case, the rotational speed of the fan is used to control the flow of combustion air.

Since the FD Fan is pushing air into the furnace, the furnace cavity gets pressurized, or, in other words, it operates at a positive pressure. In this context the term "positive pressure" refers to the fact that the furnace operates at a pressure that is higher than the ambient pressure.