It’s one of the most common questions we get from plant engineers: what’s actually the difference between an FD fan and an ID fan?
An FD (Forced Draft) fan sits on the clean-air side of a boiler or furnace. Its job is simple — push fresh ambient air into the combustion chamber to support efficient burning. Because it’s handling clean air, FD fans are generally simpler to design and maintain.
An ID (Induced Draft) fan sits on the opposite end of the system, pulling hot, dust-laden flue gases out of the furnace and sending them toward the stack or pollution control equipment. This is a much harsher operating environment — higher temperatures, particulate erosion, and often corrosive gases — which is why ID fans are typically built with more wear-resistant materials and heavier-duty bearings.
Many combustion systems use both together: the FD fan pushes air in, combustion happens, and the ID fan pulls the resulting flue gas out, maintaining a slightly negative pressure inside the furnace (balanced draft). Getting the sizing right on both sides is what keeps combustion efficient and emissions within compliance limits.