When range hoods exhaust air out of your home, negative air pressure is created unless the exhausted air is replaced by fresh outdoor air. Zephyr's Universal Make-Up Air Damper is designed to balance the air pressure in your home by automatically opening to allow fresh air in while the range hood is operating. For use with any Zephyr hood operating over 300 CFM. 8" Damper for 1,000 CFM Blowers.