Look up into the duct if possible. There should be a damper in the duct called a "backdraft damper" that closes and prevents cold air from entering the duct when the fan shuts off. The damper could also be located near the outlet of the fan which could be a wall or near the roof level (maybe in the attic if it goes through there).
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Look up into the duct if possible. There should be a damper in the duct called a "backdraft damper" that closes and prevents cold air from entering the duct when the fan shuts off. The damper could also be located near the outlet of the fan which could be a wall or near the roof level (maybe in the attic if it goes through there).