Induced draft motors are designed to keep the burner box in negative pressure. This will assure that the flame stays in the burner box and doesn't roll-out. They are usually paired up with a pressure switch, to make sure that the correct pressure is in the burner box. They allow the hot flue gases to pass right through them, so they are designed to be able to withstand a lot of heat. In an 80% efficient furnace, the housings are usally made of metal. In 90% furnaces, they can be made of plastic because of the lower flue gas temperatures.

metal housing induced draft motor	plastic housing induced draft motor

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