High-Occupancy HVAC Design for Lecture Halls

Overview

High-occupancy educational spaces such as lecture halls and auditoriums present unique HVAC challenges due to concentrated human heat gains, elevated CO2 generation, and intermittent usage patterns. Proper system design requires accurate load calculations, appropriate ventilation rates, and strategies to manage peak demands while maintaining acceptable indoor air quality.

Occupant Density and Load Implications

Typical Occupancy Densities:

The sensible heat gain from occupants in sedentary activities is approximately 250 BTU/hr per person, while latent heat gain contributes an additional 200 BTU/hr per person at 75°F and 50% RH.

Total Occupant Load Calculation:

$$Q_{occupants} = N \times (q_s + q_l)$$

Where:

• $Q_{occupants}$ = total heat gain from occupants (BTU/hr)
• $N$ = number of occupants
• $q_s$ = sensible heat gain per person (250 BTU/hr)
• $q_l$ = latent heat gain per person (200 BTU/hr)

For a 300-seat lecture hall:

$$Q_{occupants} = 300 \times (250 + 200) = 135,000 \text{ BTU/hr}$$

This represents a cooling load of approximately 11.25 tons solely from occupants.

Ventilation Requirements per ASHRAE 62.1

ASHRAE Standard 62.1 specifies minimum ventilation rates for assembly occupancies based on occupant density and floor area. For lecture halls and auditoriums, the standard categorizes these as “Assembly Spaces.”

ASHRAE 62.1 Ventilation Rate Procedure:

$$V_{bz} = R_p \times P_z + R_a \times A_z$$

Where:

• $V_{bz}$ = breathing zone outdoor airflow rate (CFM)
• $R_p$ = people outdoor air rate (CFM/person)
• $P_z$ = zone population (number of people)
• $R_a$ = area outdoor air rate (CFM/ft²)
• $A_z$ = zone floor area (ft²)

Standard Values for Assembly Spaces:

• $R_p$ = 5 CFM/person (lecture classroom)
• $R_p$ = 5 CFM/person (auditorium seating area)
• $R_a$ = 0.06 CFM/ft²

For a 5,000 ft² lecture hall with 300 occupants:

$$V_{bz} = (5 \times 300) + (0.06 \times 5000) = 1,500 + 300 = 1,800 \text{ CFM}$$

CO2 Generation and Control

Human respiration generates approximately 0.3-0.4 CFH (cubic feet per hour) of CO2 per person during sedentary activities. In high-occupancy spaces, this can rapidly elevate indoor CO2 concentrations above acceptable levels.

Steady-State CO2 Concentration:

$$C_s = C_o + \frac{N \times G}{Q_{oa}}$$

Where:

• $C_s$ = steady-state indoor CO2 concentration (ppm)
• $C_o$ = outdoor CO2 concentration (typically 400-450 ppm)
• $N$ = number of occupants
• $G$ = CO2 generation rate per person (0.35 CFH or 583 mL/min)
• $Q_{oa}$ = outdoor air ventilation rate (CFH)

To maintain indoor CO2 below 1,000 ppm with outdoor air at 400 ppm:

$$Q_{oa} = \frac{N \times G}{(C_s - C_o)} \times 10^6$$

For 300 occupants:

$$Q_{oa} = \frac{300 \times 0.35}{(1000 - 400)} \times 10^6 = 175,000 \text{ CFH} = 2,917 \text{ CFM}$$

This calculation demonstrates that CO2 control often drives ventilation requirements above ASHRAE 62.1 minimums in densely occupied spaces.

Peak Load Management

High-occupancy spaces experience significant load variations between occupied and unoccupied periods. Effective design incorporates diversity factors and control strategies to optimize energy consumption.

Diversity Factor Application:

$$Q_{design} = Q_{peak} \times DF$$

Where:

• $Q_{design}$ = design cooling load
• $Q_{peak}$ = calculated peak load assuming 100% occupancy
• $DF$ = diversity factor (typically 0.85-0.95 for scheduled classes)

Load Components:

1. Occupant loads (highly variable, 0-100% swing)
2. Lighting loads (predictable, controlled by schedule)
3. Envelope loads (relatively constant)
4. Equipment loads (AV systems, typically constant when in use)

System Design Strategies

Underfloor Air Distribution (UFAD):

UFAD systems provide effective ventilation in lecture halls by delivering outdoor air directly to the breathing zone. Supply air temperatures of 63-65°F create stratification, reducing cooling load for the upper unoccupied zone.

Supply Airflow Calculation:

$$Q_{sa} = \frac{Q_{sensible}}{1.08 \times \Delta T}$$

Where:

• $Q_{sa}$ = supply airflow (CFM)
• $Q_{sensible}$ = sensible cooling load (BTU/hr)
• $\Delta T$ = temperature difference between space and supply air (°F)

Demand-Controlled Ventilation (DCV):

DCV systems modulate outdoor air intake based on measured CO2 concentrations or occupancy sensors. This approach significantly reduces energy consumption during partial occupancy periods.

$$Q_{oa,DCV} = Q_{oa,min} + \frac{(C_{measured} - C_{setpoint,low})}{(C_{setpoint,high} - C_{setpoint,low})} \times (Q_{oa,max} - Q_{oa,min})$$

Typical setpoints:

• $C_{setpoint,low}$ = 800 ppm (minimum ventilation)
• $C_{setpoint,high}$ = 1,000 ppm (maximum ventilation)

Acoustic Considerations

High-occupancy spaces require low background noise levels for speech intelligibility. Design targets include:

• NC 25-30 for lecture halls
• NC 20-25 for auditoriums with amplified sound
• Maximum air velocity at diffusers: 400-500 FPM
• Duct velocity limits: 1,500-2,000 FPM in occupied spaces

Low-velocity air distribution (0.3-0.5 air changes per minute) reduces noise while maintaining adequate ventilation.

Rapid Purge Ventilation

Post-occupancy purge cycles remove accumulated CO2 and contaminants. A purge rate of 3-4 air changes per hour for 30-45 minutes after class dismissal restores baseline conditions.

$$t_{purge} = \frac{\ln(\frac{C_{initial} - C_{outdoor}}{C_{final} - C_{outdoor}})}{-ACH}$$

Where:

• $t_{purge}$ = purge time (hours)
• $ACH$ = air changes per hour during purge
• $C_{initial}$ = CO2 concentration at occupancy end
• $C_{final}$ = target CO2 concentration

Thermal Stratification Management

Ceiling heights of 12-20 feet in lecture halls create temperature stratification. Destratification fans or properly designed return air systems prevent excessive vertical temperature gradients exceeding 3-5°F per 10 feet of height.

Conclusion

High-occupancy HVAC design requires integration of ventilation rate calculations, CO2 generation analysis, peak load management, and diversity factors. Systems must deliver adequate outdoor air per ASHRAE 62.1 while managing intermittent loads efficiently. Demand-controlled ventilation, appropriate diversity factors, and rapid purge strategies optimize both indoor air quality and energy performance in lecture halls and auditoriums.