Draft - continued

Induced Draft Fan

In this figure, the circular mechanism in the stack indicates the presence of a fan.

The figure provided here illustrates an induced draft furnace. In this type of design, a fan is used in the stack to aid the draft created by the chimney. The fan, known as the “Induced Draft Fan” or the “ID Fan”, draws (sucks) combustion air from the furnace. Usually such fans are provided with moveable vanes on the suction side, to control air flow through the furnace.

Since the ID Fan is sucking air from the furnace, the furnace cavity operates at a negative pressure. In this context the term "negative pressure" refers to the fact that the furnace operates at a pressure that is lower than the ambient pressure.