Make-up air is outside air purposefully brought back into your home to replace the air hoods exhaust outside. Providing for make-up air protects against carbon monoxide poisoning and ensures hoods perform at their best.
We won’t bore you with too many details, but high-end range hoods with faster exhaust rates inhibit normal air pressure from pulling in clean, outside air through chimneys, window and door seals, and so on. Without make-up air, “backdrafting” occurs which pulls poisonous carbon monoxide back into the home when it should be venting outward. This same phenomenon can occur in a dryer vent and push highly flammable lint into a gas-burning motor, hence why cleaning out your lint trap is so important.
The need for make-up air is stressed even more so in today’s energy efficient homes because they’re practically air-tight. Section M1503.4 of the 2009 International Residential Code notes that you don’t need a make-up air system for range hoods rated at 400 CFM or less.
Implementing a make-up air system is the only “right way” to meet code and protect against carbon monoxide poisoning. The concept shouldn’t be new to any licensed contractor and there are many acceptable methods (like the example below).
However, if the “right way” isn’t an option, Zephyr Hoods introduced a new feature called ACT (Airflow Control Technology), which allows the blower strength to be reduced down to 390 CFM. Many popular hood styles are only available on units with 600 - 1,200 CFM and the ACT feature allows for those styles to be used safely without the need for make-up air. The downside? It’s not available on all models.