Occupancy Sensors for Classroom Ventilation Control

Occupancy Sensors for Classroom Ventilation Control

Occupancy sensors provide real-time detection of room occupancy for demand-controlled ventilation (DCV) systems in educational facilities. Proper sensor selection, placement, and integration directly affect ventilation effectiveness, energy efficiency, and indoor air quality in classroom environments.

Sensor Technology Types

Passive Infrared (PIR) Sensors

PIR sensors detect infrared radiation emitted by occupants. The sensor responds to temperature differentials caused by human body heat moving across detection zones.

Operating principles:

• Detect thermal radiation in 8-12 μm wavelength range
• Require motion for detection (static occupants may not be detected)
• Multiple detection zones increase coverage reliability
• Temperature differences between occupants and background affect sensitivity

Typical specifications:

• Detection range: 20-40 feet radius
• Field of view: 90-180 degrees
• Response time: 1-2 seconds
• Operating temperature: 32-120°F

Application considerations:

• Effective in spaces with moderate occupant movement
• Less reliable for seated classrooms during testing
• Performance degrades when room temperature approaches body temperature
• Line-of-sight required to detection zones

Ultrasonic Sensors

Ultrasonic sensors emit high-frequency sound waves (typically 25-40 kHz) and detect frequency shifts caused by moving objects using the Doppler effect.

Operating principles:

• Emit ultrasonic waves that reflect off surfaces and occupants
• Detect frequency changes from moving objects
• Sound waves propagate around obstacles
• More sensitive to minor movements than PIR

Typical specifications:

• Detection range: 30-50 feet radius
• Coverage pattern: omnidirectional
• Frequency: 25-40 kHz
• Response time: 2-5 seconds

Application considerations:

• Detect occupants through partitions and obstacles
• May false-trigger from air movement, HVAC operation
• Require careful gain adjustment to prevent false positives
• Effective for spaces with minimal occupant movement

Hybrid (Dual-Technology) Sensors

Hybrid sensors combine PIR and ultrasonic technologies, requiring both technologies to confirm occupancy before triggering. This configuration significantly reduces false positives and false negatives.

Operating logic:

• Both sensors must detect presence for “occupied” status
• Either sensor can maintain “occupied” status once established
• Provides redundancy for reliable detection
• Optimizes detection while minimizing false triggers

Sensor Type Comparison

Integration with HVAC Controls

Control Strategies

Binary occupancy control:

• Occupied: Full ventilation rate per ASHRAE 62.1 or IMC Section 403
• Unoccupied: Reduced ventilation (typically 0.06 cfm/ft² or complete shutoff where permitted)
• Simple two-position control logic
• Most common implementation

Time-delay settings:

• On-delay: 30-60 seconds (prevents short-duration false triggers)
• Off-delay: 15-30 minutes (accounts for brief departures, sensor blind spots)
• Longer off-delays recommended for classrooms to prevent unnecessary cycling

Gradual setback:

• Progressive reduction in ventilation rate after occupancy loss
• Maintains minimum ventilation for contaminant dilution
• Reduces thermal shock and acoustic disturbances from equipment cycling

BAS Integration

Occupancy sensors integrate with building automation systems through multiple protocols:

Communication methods:

• Hardwired relay contacts (24 VAC common)
• 0-10 VDC analog signal
• BACnet MS/TP or BACnet IP
• Modbus RTU or Modbus TCP
• Proprietary protocols (manufacturer-specific)

Data points transmitted:

• Occupancy status (binary or analog)
• Sensor diagnostics
• Battery status (wireless sensors)
• Configuration parameters

Control Sequence Coordination

Occupancy-based ventilation control must coordinate with other HVAC functions:

1. Temperature control: Maintain setpoint during occupied periods; implement setback during unoccupied periods
2. CO₂ monitoring: Occupancy sensors provide supplementary control when CO₂ sensors are primary
3. Scheduling: Override schedule-based control when occupancy detected outside scheduled hours
4. Demand reset: Adjust outdoor air damper positions based on actual occupancy versus design occupancy

Sensor Placement Requirements

Optimal mounting locations:

• Ceiling-mounted: 8-12 feet height for standard classrooms
• Wall-mounted: 6-8 feet height when ceiling mounting not feasible
• Corner placement maximizes coverage area
• Avoid mounting near supply air diffusers (air movement causes false triggers)

Coverage patterns:

• Single sensor typically covers 400-600 ft² classroom
• Multiple sensors required for irregular room geometries
• Overlap coverage zones 10-15% to eliminate detection gaps
• Consider furniture layout and potential obstructions

Code-compliant installation:

• Sensors must detect occupancy in all normally occupied areas
• Comply with ASHRAE 62.1 Section 6.2.7 (demand-controlled ventilation)
• Meet International Mechanical Code Section 403.3.1.1 (ventilation controls)
• Coordinate with IECC Section C403.2.7 (demand control ventilation requirements)

Accuracy and Reliability Factors

Environmental conditions affecting performance:

• Extreme temperatures reduce PIR sensitivity
• High air velocities cause ultrasonic false triggers
• Lighting changes affect some optical sensors
• Electromagnetic interference from equipment

Calibration and commissioning:

• Verify detection coverage through walk tests
• Adjust sensitivity and time delays for application
• Document sensor placement and detection zones
• Test under various occupancy scenarios (standing, seated, minimal movement)

Maintenance requirements:

• Inspect lenses quarterly for dust, debris, damage
• Verify operation during normal occupancy patterns
• Recalibrate after room layout changes
• Replace batteries annually (wireless sensors)

Best Practices

• Specify hybrid sensors for critical classroom applications requiring high reliability
• Implement minimum 15-minute off-delay to account for brief absences and sensor limitations
• Coordinate occupancy-based control with CO₂-based DCV for optimal performance
• Commission sensors under actual occupancy conditions, not just empty rooms
• Provide override switches for manual control during sensor failures
• Document sensor locations and coverage patterns in building automation system
• Train facility staff on occupancy sensor operation and troubleshooting
• Review actual vs. design occupancy patterns after first year of operation

Properly selected and integrated occupancy sensors enable significant energy savings in educational facilities while maintaining code-compliant ventilation rates and acceptable indoor air quality during occupied periods.