University Lecture Hall Renovation in Austin: High-Density Occupancy Challenge

Scenario

Renovation of a 300-seat tiered lecture hall at the University of Texas at Austin (ASHRAE Climate Zone 2A). The space had chronic stuffiness complaints and elevated CO₂ (>1,200 ppm) during back-to-back classes. Constraints included fixed ceiling height (no duct depth > 18”), historic façade preservation limiting wall penetrations, and a hard cap on electrical service upgrade (max +15 kW).

Given Data

• Number of People: 300 (full capacity, per seating plan)
• Outdoor Airflow Rate per Person: 17 cfm/person (increased from baseline 15 to account for extended occupancy duration and teaching activity per Appendix A guidance)
• Area of the Space: 5,800 ft² (including stage, aisles, and fixed seating footprint)
• Outdoor Airflow Rate per Unit Area: 0.12 cfm/ft² (per Table 6-1 for “classrooms, lecture halls” — higher than general offices due to density and activity)

Calculation

• People-based ventilation = 300 people × 17 cfm/person = 5,100 cfm
• Area-based ventilation = 5,800 ft² × 0.12 cfm/ft² = 696 cfm

Total required ventilation rate = max(5,100, 696) = 5,100 cfm

This exceeds the original AHU’s OA capacity (2,400 cfm), confirming the root cause of poor IAQ.

Result and Decision

Rather than replacing the entire AHU (prohibited by electrical cap), engineers implemented a hybrid solution: (1) upgraded the existing OA damper actuator and control sequence for full-range modulation, (2) added two dedicated, wall-mounted, in-line ERV units (each rated 2,600 cfm) with minimal duct runs — exploiting existing exterior masonry openings — and (3) tied both ERVs to the BMS with occupancy-scheduled staging. Total installed OA capacity became 5,200 cfm, meeting the 5,100 cfm requirement with 2% safety margin.

Lesson

In high-occupancy spaces where area-based ventilation is negligible compared to occupant-driven demand, always verify that mechanical systems can physically deliver the people-based rate — and when infrastructure limits exist, modular, distributed solutions (e.g., multiple smaller ERVs) often outperform single-point upgrades.