Occupancy Variation in Gymnasium HVAC Design

Gymnasiums present unique HVAC challenges due to extreme occupancy variations. A typical school gymnasium might serve 30 students during a morning PE class, then host 800 spectators for an evening basketball game. This 25:1 occupancy range creates significant design complexity that demands intelligent control strategies and proper equipment selection.

Occupancy Load Characteristics

Typical Occupancy Scenarios

Gymnasium occupancy patterns vary dramatically throughout the day and week:

Metabolic Heat Generation

Athletic activities generate substantially higher metabolic heat than sedentary spectators:

• Sedentary spectators: 400 Btu/hr total (250 sensible, 150 latent)
• Light activity (walking): 550 Btu/hr total (250 sensible, 300 latent)
• Moderate exercise (PE class): 850 Btu/hr total (315 sensible, 535 latent)
• Heavy exercise (basketball, volleyball): 1,450 Btu/hr total (580 sensible, 870 latent)

The latent heat fraction increases significantly with activity intensity, ranging from 38% for spectators to 60% for heavy exercise. This affects both ventilation requirements and dehumidification capacity.

ASHRAE 62.1 Ventilation Requirements

Per ASHRAE 62.1, gymnasiums require ventilation rates based on both area and occupancy:

Breathing Zone Outdoor Airflow (Vbz) = Ra × Az + Rp × Pz

Where:

• Ra = 0.06 cfm/ft² (area component)
• Rp = 30 cfm/person (people component for gyms with spectator seating)
• Rp = 20 cfm/person (for exercise rooms without spectator seating)
• Az = floor area (ft²)
• Pz = zone population

For a 10,000 ft² gymnasium:

• Minimum (unoccupied): 600 cfm (area component only)
• 30 students (PE class): 600 + (20 × 30) = 1,200 cfm
• 800 spectators (game): 24,600 cfm (600 + 30 × 800)

This 20:1 ventilation range makes constant-volume systems energy-inefficient.

Demand-Controlled Ventilation Strategies

CO₂-Based Control

Demand-controlled ventilation (DCV) using CO₂ sensors provides the most effective approach for managing variable occupancy:

graph TD
    A[CO₂ Sensors - Multiple Locations] --> B{Control Logic}
    B --> C{CO₂ < 800 ppm}
    C -->|Yes| D[Reduce to Minimum OA]
    C -->|No| E{CO₂ 800-1000 ppm}
    E -->|Yes| F[Modulate OA Damper]
    E -->|No| G{CO₂ > 1000 ppm}
    G -->|Yes| H[Maximum OA Position]

    D --> I[Variable Speed Supply Fan]
    F --> I
    H --> I

    I --> J[Maintain Building Pressure]
    J --> K[Exhaust Air Modulation]

    style A fill:#e1f5ff
    style I fill:#fff4e1
    style J fill:#e8f5e9

Implementation Requirements:

1. Sensor Placement: Minimum three sensors in breathing zone (4-6 ft above floor), distributed to avoid dead spots
2. Setpoint Strategy: 800-1,000 ppm range with proportional control
3. Minimum Position: Always maintain area component (0.06 cfm/ft²)
4. Response Time: 2-5 minute averaging to prevent hunting
5. Override Capability: Manual high-speed override for rapid occupancy changes

Scheduling-Based Pre-Occupancy

Integration with facility scheduling systems enables anticipatory ventilation:

gantt
    title Daily Gymnasium Ventilation Schedule
    dateFormat HH:mm
    axisFormat %H:%M

    section Ventilation Mode
    Unoccupied (Min OA)     :done, unoc1, 00:00, 07:00
    Pre-Purge (50% OA)      :active, prepurge1, 07:00, 08:00
    PE Class 1 (DCV Active) :crit, pe1, 08:00, 09:00
    Unoccupied (Min OA)     :done, unoc2, 09:00, 10:00
    PE Class 2 (DCV Active) :crit, pe2, 10:00, 11:00
    Unoccupied (Min OA)     :done, unoc3, 11:00, 15:00
    Pre-Event (75% OA)      :active, preevent, 15:00, 16:00
    Game Event (DCV Active) :crit, game, 16:00, 18:00
    Post-Purge (100% OA)    :active, postpurge, 18:00, 19:00
    Unoccupied (Min OA)     :done, unoc4, 19:00, 24:00

Pre-occupancy strategies:

• 30-60 minute pre-purge before scheduled high-occupancy events
• 100% outdoor air flush for 15-30 minutes post-event to remove accumulated contaminants
• Setback to minimum OA during verified unoccupied periods
• Temperature pre-conditioning coordinated with ventilation pre-purge

Equipment Selection for Variable Loads

Variable Air Volume Systems

VAV systems with variable-speed drives provide optimal energy performance:

Fan Selection Criteria:

• Design for peak occupancy (games/events)
• Select motors for 40-50% minimum speed capability
• Use backward-curved or airfoil fans for wide operating range
• Implement static pressure reset based on damper positions

Energy Savings Calculation:

At 50% airflow, fan power drops to approximately 12.5% of full load (cube law relationship):

Power₂ / Power₁ = (CFM₂ / CFM₁)³

For a 30,000 cfm design serving a gymnasium:

• Full load (800 occupants): 30,000 cfm = 30 hp
• PE class (30 students): 1,200 cfm (4%) = 0.008 hp
• Unoccupied: 600 cfm (2%) = 0.0003 hp

Annual operating hours weighted average shows 75-85% fan energy reduction versus constant volume operation.

Modular Approach for Large Gymnasiums

For facilities exceeding 15,000 ft², consider modular equipment staging:

flowchart LR
    A[Base Unit - 6,000 cfm] --> D[Gymnasium]
    B[Stage 1 - 6,000 cfm] --> D
    C[Stage 2 - 6,000 cfm] --> D
    E[Stage 3 - 6,000 cfm] --> D

    F[Occupancy Level] --> G{Control Logic}
    G -->|0-50 people| H[Base Only]
    G -->|51-150 people| I[Base + Stage 1]
    G -->|151-300 people| J[Base + Stages 1-2]
    G -->|300+ people| K[All Stages]

    H --> A
    I --> A
    I --> B
    J --> A
    J --> B
    J --> C
    K --> A
    K --> B
    K --> C
    K --> E

    style A fill:#e1f5ff
    style D fill:#fff4e1
    style G fill:#e8f5e9

Control Sequences for Occupancy Transitions

Rapid Occupancy Increase Response

When CO₂ levels rise above setpoint:

1. Immediate: Outdoor air damper modulates to 100% open
2. 0-30 seconds: Supply fan ramps to design speed
3. 30-60 seconds: Return/exhaust dampers modulate to maintain building pressure
4. 60+ seconds: Cooling/heating adjusts to maintain setpoint

Post-Event Purge Cycle

After high-occupancy events:

1. Event end trigger: Occupancy sensor or manual input
2. Maximum ventilation: 100% outdoor air for 20-30 minutes
3. Temperature override: Allow 5°F drift during purge
4. Verification: CO₂ must drop below 600 ppm before returning to minimum OA
5. System shutdown: After purge completion and temperature recovery

Energy Efficiency Considerations

Economizer Integration

Free cooling through economizer operation significantly reduces energy consumption during shoulder seasons:

• Dry-bulb economizer: Appropriate for most climates, lockout above 70°F outdoor
• Enthalpy economizer: Preferred in humid climates, prevents latent load introduction
• Integrated control: Coordinate with DCV to maximize free cooling during high-occupancy periods

Heat Recovery

Energy recovery ventilators (ERVs) become economically viable for gymnasiums with consistent high-occupancy schedules:

Payback Analysis:

• Gymnasiums operating >2,000 hours/year at >50% design occupancy
• Minimum 50% effectiveness sensible recovery
• Total effectiveness >60% for latent recovery in humid climates
• Typical payback: 4-7 years in northern climates, 6-10 years in mild climates

Destratification

High ceilings (20-40 ft typical) create significant stratification during heating:

• Ceiling fans: 10-15°F reduction in ceiling-to-floor temperature differential
• Energy savings: 20-30% heating energy reduction
• Summer operation: Reverse to pull cool air up, reducing cooling load on spectators
• Controls integration: Activate based on space temperature differential sensors

Design Recommendations

1. Always implement DCV with CO₂ sensors for gymnasiums serving multiple functions
2. Design for peak load but select equipment optimized for part-load operation
3. Install minimum three CO₂ sensors distributed throughout breathing zone
4. Integrate scheduling system with HVAC controls for anticipatory ventilation
5. Provide manual override capability for unscheduled events
6. Include post-event purge sequences in control programming
7. Consider dedicated dehumidification separate from sensible cooling to handle variable latent loads
8. Design for 0.02-0.05 in. w.c. positive pressure to prevent infiltration during low occupancy

Proper management of occupancy variation transforms gymnasium HVAC from energy-intensive to highly efficient while maintaining superior indoor air quality across all operating conditions.