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A forced-air furnace is one in which a fan forces air through the ducts . The problem with forced-air heating is that as it gives ( heat ) , it also takes away ( humidity ) . The target for forced-air incubators is usually 99.5 F. Forced-air ventilation is common in the conventional industry , using fans , air intakes , and exhausts , often combined with evaporative cooling pads . The ability to retrofit existing cold rooms for use as forced-air coolers is also an economic advantage .