Though it’s a complex HVAC issue, simply put, a hood with a high 600 to 900 CMFs used with high BTU ranges, for example, will need what’s called a makeup air system. Makeup air literally makes up for the indoor air that’s being pulled out of your home through the hood. An appropriately located motorized damper kicks on to pull fresh air from the outside into your house when your range hood is pulling large amounts of air out of the house while you’re cooking.