Clothes dryer exhaust exceeding what CFM requires makeup air?

• A. 100 CFM
• B. 150 CFM
• C. 200 CFM
• D. 250 CFM

Answer: (C)

Exhausting air from the living space creates negative pressure, so when that exhaust is large enough, makeup air is required to keep the house balanced. For a clothes dryer, the code threshold that triggers makeup air is 200 CFM. If the dryer can exhaust at least 200 CFM, you must provide a source of outside air to replace the exhausted air, preventing depressurization and potential backdrafting of combustion appliances or improper venting. In typical homes, dryer exhaust is around 100 CFM, so makeup air isn’t usually required, but as the exhaust capacity approaches or exceeds 200 CFM, adding outside makeup air becomes necessary. Makeup air can be supplied via a dedicated outside air intake or through the HVAC system with an appropriate damper, ensuring the indoor pressure stays balanced.