Spectator Area HVAC Design for Gymnasiums

Overview

Spectator seating areas in gymnasiums require distinct HVAC design approaches from activity floors due to fundamentally different metabolic rates, occupancy patterns, and thermal comfort expectations. While athletes generate 300-400 Btu/hr per person during activity, sedentary spectators produce only 100-130 Btu/hr, creating different temperature and ventilation requirements within the same space. Intermittent high-occupancy events further complicate load calculations and system sizing decisions.

Thermal Load Characteristics

Sensible Heat Gain from Spectators

Spectator metabolic heat generation depends on activity level and clothing insulation. ASHRAE Fundamentals provides base values modified for actual conditions.

Sensible heat per person:

$$q_s = q_{total} \times SHF$$

Where:

• $q_s$ = sensible heat gain (Btu/hr per person)
• $q_{total}$ = total metabolic heat generation (Btu/hr per person)
• $SHF$ = sensible heat fraction

For seated spectators:

$$q_{total} = 100 \text{ to } 130 \text{ Btu/hr (typical 115 Btu/hr)}$$

$$SHF = 0.65 \text{ to } 0.75 \text{ (typical 0.70)}$$

$$q_s = 115 \times 0.70 = 80.5 \text{ Btu/hr per person}$$

Total Spectator Area Cooling Load

The complete cooling load calculation includes occupant sensible heat, lighting, and envelope gains:

$$Q_{total} = (n \times q_s) + Q_{lights} + Q_{envelope} + Q_{solar}$$

Where:

• $n$ = number of spectators
• $q_s$ = sensible heat per spectator (Btu/hr)
• $Q_{lights}$ = lighting heat gain (Btu/hr)
• $Q_{envelope}$ = transmission and infiltration (Btu/hr)
• $Q_{solar}$ = solar heat gain through windows (Btu/hr)

Example calculation for 500-seat bleacher section:

• Occupant load: $500 \times 80.5 = 40,250$ Btu/hr
• Lighting (1.0 W/ft², 5000 ft²): $5,000 \times 3.41 = 17,050$ Btu/hr
• Envelope and infiltration: $8,500$ Btu/hr (calculated separately)
• Total sensible load: $65,800$ Btu/hr (5.5 tons)

This calculation assumes peak occupancy. Design decisions must address the economics of sizing for infrequent peak events versus accepting temporary discomfort during maximum occupancy.

Ventilation Requirements per ASHRAE 62.1

ASHRAE Standard 62.1 classifies spectator seating under “Places of Assembly” with specific occupancy-based requirements.

Assembly Space Ventilation Rates

For spectator seating (gymnasium/arena):

Total outdoor air requirement:

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

Where:

• $V_{oz}$ = outdoor air flow rate for zone (cfm)
• $R_p$ = outdoor air rate per person (7.5 cfm/person)
• $P_z$ = zone population (number of people)
• $R_a$ = outdoor air rate per area (0.06 cfm/ft²)
• $A_z$ = zone floor area (ft²)

Example for 5,000 ft² bleacher section with 750 seats:

$$V_{oz} = (7.5 \times 750) + (0.06 \times 5,000)$$

$$V_{oz} = 5,625 + 300 = 5,925 \text{ cfm}$$

This represents the minimum outdoor air requirement at full occupancy. The system must be capable of delivering this ventilation rate while maintaining appropriate pressurization and temperature control.

Intermittent High-Occupancy Event Design

Gymnasium spectator areas experience extreme occupancy variations, from zero during practice sessions to full capacity during tournaments, creating conflicting design objectives between peak performance and energy efficiency.

Occupancy Pattern Analysis

The annual occupied spectator-hours represents a small fraction of total building operation, typically 5-10% of annual hours.

System Sizing Strategy

Full-capacity design approach:

Size HVAC systems for maximum occupancy load to maintain comfort during all events. This results in:

• Higher first cost (oversized equipment)
• Lower operating efficiency during typical loads
• Guaranteed comfort during peak events
• Simplified control sequences

Reduced-capacity design approach:

Size systems for typical high-occupancy events (80-90% of maximum), accepting brief discomfort during rare peak events:

• Lower first cost (appropriately sized equipment)
• Better part-load efficiency
• Potential comfort complaints during tournaments
• May require pre-cooling strategies

Recommended approach:

$$Q_{design} = Q_{typical} + 0.5 \times (Q_{peak} - Q_{typical})$$

This mid-point strategy sizes equipment between typical and peak loads, providing acceptable comfort for most events while maintaining reasonable first costs and operating efficiency.

Bleacher and Seating Area Air Distribution

Effective air distribution to spectator areas addresses the unique geometry of tiered seating while maintaining comfort without excessive noise or drafts.

Air Distribution Methods

1. Under-Seat Supply Diffusers

Low-velocity air delivered through diffusers integrated into bleacher construction provides direct conditioning to the occupied zone.

Advantages:

• Delivers air directly to breathing zone
• Minimizes mixing with warmer upper-level air
• Quiet operation at low velocities (100-200 fpm)
• Effective use of temperature stratification

Design parameters:

• Supply temperature: 55-60°F (avoid cold drafts)
• Discharge velocity: 100-200 fpm at outlet
• Spacing: 4-6 feet on center along bleacher rows
• Airflow: 15-25 cfm per seat

2. Sidewall or Rear-Wall Supply

High-capacity diffusers mounted behind or above bleacher sections project conditioned air across seating areas.

Advantages:

• Lower installation cost than under-seat systems
• Simpler retrofit application
• Easier maintenance access

Design parameters:

• Throw distance: 40-60 feet horizontal
• Supply temperature: 55-65°F
• Discharge velocity: 400-800 fpm at diffuser
• Terminal velocity: <150 fpm in occupied zone

3. Overhead Displacement Systems

Low-velocity supply from overhead with thermal buoyancy driving circulation relies on heat generation from occupants.

Advantages:

• Minimal draft risk
• Quiet operation
• Effective ventilation efficiency

Limitations:

• Less effective for cooling dense occupancy
• Requires higher ceilings (>15 feet clear)
• Supply temperature limited to 63-68°F

Return Air Configuration

Return air location significantly affects circulation patterns and energy use:

• High-level return: Captures stratified warm air, reduces cooling load, recommended for heating-dominated climates
• Low-level return: Forces complete mixing, increases cooling load, better for moisture control
• Mixed return: Combination of high and low returns with damper control for seasonal optimization

Separate Zone Control Requirements

Spectator areas require independent temperature control from activity floors due to different metabolic rates and comfort expectations.

Temperature Setpoint Strategy

This temperature differential maintains equivalent thermal comfort between zones despite different activity levels. ASHRAE Standard 55 predicted mean vote (PMV) calculations confirm that sedentary occupants at 72°F experience similar comfort to active occupants at 68°F.

Zoning Implementation Methods

1. Separate Air Handling Units

Dedicated AHU serves spectator zones with independent control:

• Complete independence from activity floor systems
• Optimal equipment sizing for spectator loads
• Separate ventilation control based on spectator occupancy
• Higher first cost, simpler operational control

2. VAV Terminal Unit Zoning

Central air handler serves multiple zones through VAV boxes with reheat:

• Single outdoor air system simplifies ventilation management
• Zone-level temperature control via airflow modulation
• Lower first cost than separate AHUs
• Requires careful minimum airflow settings to maintain ventilation

3. Split System with Zone Dampers

Packaged rooftop unit with zone dampers and optional reheat:

• Lowest first cost option
• Suitable for smaller gymnasiums (<10,000 ft²)
• Limited zone independence
• Ventilation control challenges during low-load conditions

Supplemental Radiant Heating

Radiant heating systems address the unique challenge of providing thermal comfort to sedentary spectators without excessive air temperature or airflow.

Radiant Heat Benefits for Spectators

Radiant panels deliver heat directly to occupants through electromagnetic radiation, warming surfaces rather than air:

• Higher effective temperature: Occupants feel warmer at lower air temperatures due to radiant heat exchange
• Reduced stratification losses: Heat directed downward to occupied zone rather than rising to ceiling
• Draft elimination: Minimal air movement maintains comfort
• Fast response: Radiant systems provide immediate warmth when bleachers fill

Radiant System Configurations

Ceiling-mounted infrared panels:

• Gas-fired or electric elements at 800-1400°F surface temperature
• Mounting height: 15-25 feet above occupied zone
• Heat output: 20-40 Btu/hr per ft² of floor area
• Pattern spacing: 15-20 feet on center

Low-temperature radiant panels:

• Hydronic or electric at 120-180°F surface temperature
• Mounting height: 10-15 feet above occupied zone
• Heat output: 15-25 Btu/hr per ft² of floor area
• Closer spacing: 8-12 feet on center for uniform coverage

Radiant heat flux calculation:

$$q_r = \epsilon \times \sigma \times F_{p-s} \times (T_s^4 - T_p^4)$$

Where:

• $q_r$ = radiant heat transfer (Btu/hr·ft²)
• $\epsilon$ = emissivity of surfaces (0.85-0.95 typical)
• $\sigma$ = Stefan-Boltzmann constant ($0.1714 \times 10^{-8}$ Btu/hr·ft²·R⁴)
• $F_{p-s}$ = view factor between panel and surface
• $T_s$ = surface temperature (°R)
• $T_p$ = person surface temperature (°R)

For practical design, manufacturers provide simplified output ratings at standard mounting heights and temperatures.

Integrated Radiant and Convective Strategy

Optimal spectator comfort combines forced-air ventilation with radiant heating:

Heating season:

• Radiant panels provide 60-80% of heating requirement
• Forced air delivers ventilation at neutral temperature (65-70°F)
• Lower air temperatures acceptable with radiant warmth

Cooling season:

• Forced-air system handles entire cooling load
• Radiant panels off or used for perimeter heating only
• Higher ventilation rates during high-occupancy events

This approach minimizes air circulation during low-occupancy periods while maintaining proper ventilation and provides rapid response during event setup.

Control Sequences for Variable Occupancy

Effective spectator area control adapts system operation to actual occupancy while minimizing energy use during vacant periods.

Demand-Controlled Ventilation

CO₂-based ventilation control adjusts outdoor air rates proportional to occupancy:

Setpoint strategy:

• Unoccupied: Outdoor air dampers closed, fans off or minimum speed
• Low occupancy (<100 people): 800-1000 ppm CO₂ setpoint
• High occupancy (>100 people): 1000-1200 ppm CO₂ setpoint
• Sensor location: 4-6 feet above bleacher floor, multiple sensors for large sections

Ventilation rate modulation:

$$V_{oa,actual} = V_{oa,min} + (V_{oa,design} - V_{oa,min}) \times \frac{CO_2{actual} - CO_{2,outdoor}}{CO_{2,setpoint} - CO_{2,outdoor}}$$

Event-Based Scheduling

Integration with facility scheduling systems optimizes pre-conditioning and setback:

Pre-event startup:

• Begin conditioning 30-60 minutes before event
• Purge with high outdoor air rates if economizer conditions permit
• Ramp to full capacity 15 minutes before scheduled occupancy

Post-event setback:

• Maintain full ventilation for 15-30 minutes after event
• Gradual setback to unoccupied mode
• Return to minimal conditioning within 1 hour

Unoccupied periods:

• Temperature setback: 55°F heating, 85°F cooling
• Ventilation: Intermittent purge cycles only
• Equipment cycling to maintain readiness

This approach balances comfort, indoor air quality, and energy efficiency across the full range of occupancy conditions typical in gymnasium spectator areas.

Conclusion

Spectator area HVAC design in gymnasiums requires careful analysis of intermittent high-occupancy cooling loads, separate zone control from activity floors, and compliance with ASHRAE assembly space ventilation requirements. The fundamental conflict between sizing for rare peak events and maintaining efficiency during typical operation demands strategic compromises informed by load calculations, occupancy analysis, and operating budget constraints. Proper application of under-seat air distribution, supplemental radiant heating, and demand-responsive controls creates comfortable conditions across the full spectrum of occupancy patterns while minimizing energy waste during the majority of operating hours.