Laboratory ventilation systems are designed to isolate and protect occupants from hazardous fumes and to provide outside air at a comfortable temperature. Program-matic needs—such as exhaust devices, inter-nal loads, and safety-mandated air changes— usually call for a large volume of conditioned make-up air. Where a high volume of air must be exhausted, a once-through ventilation system is usually required. Once-through sys-tems are often mandated by codes that pro-hibit the recirculation of air from a laboratory space to adjacent spaces. The operational costs associated with high-airflow systems are magnified when laboratory ventilation sys-tems are operated continuously—24 hours per day, 7 days per week.