HVAC for Wide Capacity Variations 10-1000 Persons

Engineering Challenge: 100:1 Load Variability

Ballrooms and banquet facilities present one of HVAC design’s most demanding challenges: maintaining comfort across occupancy ranges from 10 people during setup to 1000 during peak events. This 100:1 variation creates extreme swings in sensible and latent cooling loads, ventilation requirements, and zone pressurization demands.

The fundamental challenge lies in the non-linear relationship between occupancy and load. While sensible heat from occupants scales linearly at approximately 250 BTU/hr per person, latent loads from respiration and perspiration increase disproportionately during high-density events. System design must prevent overcooling at low occupancy while maintaining capacity for peak loads.

Ventilation Load Dynamics

The outdoor air requirement dominates system sizing for variable-occupancy spaces. ASHRAE Standard 62.1 prescribes ventilation as:

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

Where:

• $V_{oz}$ = zone outdoor airflow (CFM)
• $R_p$ = people outdoor air rate (5-7.5 CFM/person for assembly spaces)
• $P_z$ = zone population
• $R_a$ = area outdoor air rate (0.06 CFM/ft²)
• $A_z$ = zone floor area (ft²)

For a 10,000 ft² ballroom with 1000-person peak capacity:

Peak occupancy: $V_{oz} = 7.5 \times 1000 + 0.06 \times 10000 = 8100$ CFM

Minimum occupancy (10 people): $V_{oz} = 7.5 \times 10 + 0.06 \times 10000 = 675$ CFM

This 12:1 ratio in ventilation demand drives the need for CO2-based demand-controlled ventilation rather than fixed outdoor air delivery.

CO2-Based Demand Ventilation Strategy

Demand-controlled ventilation (DCV) modulates outdoor air intake based on measured CO2 concentration, which serves as a proxy for occupancy. The control strategy exploits the mass balance equation:

$$\frac{dC}{dt} = \frac{G - Q(C - C_o)}{V}$$

Where:

• $C$ = space CO2 concentration (ppm)
• $G$ = CO2 generation rate from occupants (0.005 CFM per person)
• $Q$ = ventilation rate (CFM)
• $C_o$ = outdoor CO2 concentration (~400 ppm)
• $V$ = space volume (ft³)

DCV System Requirements:

The control algorithm adjusts outdoor air dampers to maintain setpoint, with the outdoor airflow requirement calculated as:

$$Q_{required} = \frac{N \times G \times 10^6}{C_{setpoint} - C_o}$$

Where $N$ = estimated occupant count based on CO2 rise rate.

VAV System Sizing for Extreme Turndown

Traditional VAV systems struggle with the 10:1 to 20:1 turndown ratios required for ballroom applications. Terminal unit minimum airflow settings, typically 30% for cooling-only boxes, would deliver excessive air during low occupancy.

Optimized VAV Architecture:

graph TD
    A[Central AHU with VFD] --> B[Primary VAV Boxes: 4-6 zones]
    B --> C[Zone 1: 20% min turndown]
    B --> D[Zone 2: 20% min turndown]
    B --> E[Zone 3: 20% min turndown]
    B --> F[Zone 4: 20% min turndown]

    G[Independent Small AHU] --> H[Low-occupancy perimeter zones]

    I[CO2 Sensors] --> J[DDC Controller]
    J --> K[Modulate OA dampers]
    J --> L[Reset discharge temp]
    J --> M[Stage AHU capacity]

    style A fill:#e1f5ff
    style G fill:#ffe1e1
    style J fill:#f0f0f0

Design Strategy:

1. Multiple air handlers: Deploy a base-load unit (30% capacity) for low occupancy and larger units for peak events
2. Series fan-powered boxes: Enable lower minimum flows (15-20%) while maintaining air circulation
3. Pressure-independent controls: Ensure accurate flow delivery across the operating range
4. Zone subdivision: Divide large ballrooms into 4-6 control zones rather than single-zone treatment

Part-Load Efficiency Optimization

System efficiency degrades at part load due to component performance curves, fixed losses, and control compromises. The relationship between cooling delivered and energy consumed follows:

$$EER_{part} = EER_{rated} \times PLF \times \frac{1}{1 + CD \times (1-PLR)}$$

Where:

• $PLF$ = part-load factor from manufacturer data
• $PLR$ = part-load ratio (actual load / rated load)
• $CD$ = degradation coefficient (0.1-0.25 for modern equipment)

Efficiency Enhancement Measures:

The fan energy relationship is particularly powerful:

$$P_{fan} = \frac{Q \times \Delta P}{\eta_{fan} \times \eta_{motor}} \propto Q^3$$

Reducing airflow to 50% of design (500 CFM vs 1000 CFM per person equivalent) cuts fan power to 12.5% of full load.

Avoiding Low-Occupancy Overcooling

Overcooling during setup periods or small events creates comfort complaints and wastes energy. The phenomenon occurs when:

1. Minimum airflow exceeds cooling requirement: VAV boxes at minimum stops deliver more cooling capacity than the space needs
2. Discharge air too cold: Supply temperature sized for peak latent loads overcools at low sensible-only conditions
3. Thermostat location errors: Sensors near doors or in dead zones misrepresent space conditions

Control Solutions:

flowchart LR
    A[Space Temperature Input] --> B{Occupancy Level}
    B -->|High >500 people| C[Supply Air: 55°F<br/>Max Flow Rate]
    B -->|Medium 100-500| D[Supply Air: 58°F<br/>Reduced Flow]
    B -->|Low <100 people| E[Supply Air: 60-62°F<br/>Minimum Flow]

    F[CO2 Measurement] --> G{ppm Level}
    G -->|>1000 ppm| H[Increase OA damper<br/>Boost airflow]
    G -->|800-1000 ppm| I[Maintain current OA]
    G -->|<800 ppm| J[Reduce OA to minimum]

    C --> K[Zone Discharge]
    D --> K
    E --> K

    style B fill:#ffffcc
    style G fill:#ccffcc

Implementation sequence:

1. Occupancy detection: Deploy CO2 sensors + scheduling system to determine operating mode
2. Supply temp reset: Increase discharge temperature from 55°F to 60-62°F when CO2 < 800 ppm
3. Reduced minimum flows: Lower VAV minimums to 15-20% or use fan-powered boxes with zero primary air capability
4. Zone temperature averaging: Use multiple sensors (3-5 per large ballroom) to prevent single-point control errors
5. Time-of-day scheduling: Pre-condition spaces 2 hours before events, setback during setup

The energy impact is substantial. A 10,000 ft² ballroom operating at low occupancy 60% of annual hours can reduce energy consumption by 40-50% with proper controls versus fixed-volume, fixed-temperature operation.

Quick Response Control Requirements

Event spaces transition rapidly between occupancy states. A ballroom can fill from 50 to 800 people within 30 minutes as guests arrive for dinner service. The thermal mass of air is minimal:

$$\tau = \frac{\rho \times V \times c_p}{Q \times \rho \times c_p} = \frac{V}{Q}$$

For a 10,000 ft² ballroom with 20-foot ceilings and 40,000 CFM airflow: $\tau = \frac{200,000}{40,000} = 5$ minutes.

This five-minute time constant means space conditions respond quickly to occupancy changes, but control system response lags create problems:

Required Response Characteristics:

• Sensor update rate: 1-minute intervals for CO2, 30-second for temperature
• Damper/valve actuators: 60-90 second stroke time maximum
• Controller PID tuning: Aggressive with minimal integral action to prevent overshoot
• Anticipatory control: Use building management system integration to pre-condition based on event schedule

Components

• Small Ballroom 100 To 300
• Medium Ballroom 300 To 700
• Large Ballroom 700 To 1500
• Grand Ballroom Over 1500
• Divisible Space Design
• Partition Wall Integration