The term “Induced Draught (ID) fan” is most commonly used in the boiler industry, but more generally refers to any fans that produce negative pressure (i.e. a partial vacuum) in a combustion air system such as a kiln. Together with FD fans, they are often used to effect pressures slightly below ambient air pressure (5-10 mm of water column is one guideline that is used) and remove combustion gases from the furnace to the stack. The ID fan is started before the FD fan for this purpose.

In coal-fired boilers, the ID fan is typically located between the electrostatic precipitator and the chimney in the flue gas path.

Centrifugal blowers are commonly used for this purpose. Radial-blade and backward-curved double-suction fans with inlet guide vanes and often with variable-frequency drives are used. These blowers must be able to handle large volumes of hot flue gases containing dust and ash. The temperature may reach 150˚C. All leakages in the system must also be handled by the ID fan so that air leaks in, rather than ash and fuel particles leaking out. This means that the fan must be powerful enough to overcome any pressure drops in the system. It is important to minimize leaks in the flue gas path to reduce the load on the ID fan, in order to save power and ensure adequate performance. Cold air leaking into the system can cause a sharp reduction in the effective capacity of the fan, because the density of flue gases increases considerably.

ID fans may be direct-driven, belt-driven or direct coupled. The direct-driven models may only be suitable for gas temperatures that are relatively low.