The 80/20 recirculation system is a hybrid make-up air system. Supply air is a variable mixture of return air and fresh outdoor air. Outdoor air volumes vary from 20% to 100% of total airflow, often configured to respond to building pressure fluctuations. The 80/20 units are excellent for facilities with mechanical exhaust systems, especially when the exhaust volume is variable. The supply fan will run continuously to respond to varying exhaust volumes, and the burner will modulate to satisfy the remote space set point. Discharge temperatures typically range from 70°F to 110°F.