Demand-Controlled Ventilation (DCV)

Demand-controlled ventilation (DCV) adjusts outdoor air supply based on actual occupancy, providing energy savings while maintaining indoor air quality in educational facilities with highly variable occupancy patterns.

DCV Fundamentals

Operating Principle

DCV modulates ventilation based on occupancy indicators:

$$\dot{V}{OA} = \dot{V}{min} + K \times (Indicator - Baseline)$$

Where:

• $\dot{V}_{OA}$ = outdoor air flow rate
• $\dot{V}_{min}$ = minimum (unoccupied) ventilation
• K = proportional gain
• Indicator = CO₂, occupancy count, or other

Occupancy Detection Methods

CO₂ as Proxy

CO₂ concentration indicates occupancy:

$$CO_2 = CO_{2,outdoor} + \frac{N \times G}{Q}$$

Where:

• N = number of occupants
• G = CO₂ generation rate (~0.31 L/min for adults)
• Q = ventilation rate

ASHRAE 62.1 Basis:

• 1,000 ppm indoor = approximately 15 CFM/person
• 800 ppm differential above outdoor

Educational Facility Applications

Classroom Implementation

Classrooms experience predictable but variable occupancy:

Typical Pattern:

• Empty: Before school, after hours
• Partial: Study halls, small groups
• Full: Standard class periods
• Peak: Special events

DCV Benefits:

• 30-50% ventilation energy savings
• Automatic response to actual occupancy
• Maintained IAQ regardless of schedule changes

Assembly Spaces

Gyms, auditoriums, cafeterias with highly variable loads:

Characteristics:

• Very low to maximum occupancy
• Short notice events
• Extended vacancy periods

DCV Essential: Fixed ventilation would be grossly oversized or undersized.

Laboratory Spaces

Science labs with fume hood requirements:

• DCV for general area when hoods satisfy minimum OA
• Coordinate with exhaust requirements
• Maintain proper pressure relationships

System Design

CO₂ Sensor Placement

Proper sensor location critical:

Recommended Locations:

• 3-6 ft above floor (breathing zone)
• Away from supply diffusers
• Away from doors and windows
• Minimum 3 ft from occupants
• Representative of space conditions

Avoid:

• Direct sunlight
• Near HVAC equipment
• Stagnant areas
• High traffic paths

Control Strategies

Single-Zone DCV:

• One sensor controls one zone
• Simplest implementation
• Common for classrooms

Multi-Zone DCV:

• Multiple zones share AHU
• Critical zone reset
• System-level optimization

$$\dot{V}{OA,system} = \frac{\sum \dot{V}{OA,zone}}{E_v}$$

Where $E_v$ = system ventilation effectiveness

Setpoint Selection

Equipment Requirements

CO₂ Sensors

Sensor Specifications:

• Technology: NDIR (non-dispersive infrared)
• Range: 0-2,000 ppm minimum
• Accuracy: ±50 ppm or ±3%
• Output: 4-20 mA or BACnet

Calibration:

• Factory calibration
• Annual verification
• Automatic baseline correction (ABC)
• Fresh air calibration option

Damper Actuators

Outdoor air dampers for DCV:

• Modulating capability (0-100%)
• Characterized for linear flow
• Fast response (<60 seconds)
• Reliable positioning feedback

Control Integration

BAS programming requirements:

• PID control loops
• Minimum/maximum limits
• Alarm generation
• Trend logging
• Override capability

Energy Savings

Savings Calculation

$$Savings = \sum_{hours} (\dot{V}{OA,design} - \dot{V}{OA,actual}) \times \Delta h \times Cost$$

Where:

• $\Delta h$ = enthalpy difference
• Cost = energy cost per unit

Typical Savings

Payback Analysis

Simple payback typically 1-3 years:

$$Payback = \frac{Sensor + Installation + Commissioning}{Annual\ Energy\ Savings}$$

Code Compliance

ASHRAE 62.1

DCV explicitly permitted:

• Section 6.2.7 Dynamic Reset
• Allows reduced OA when occupancy reduced
• Must maintain minimum rates
• Requires OA monitoring

Building Energy Codes

ASHRAE 90.1 and IECC require DCV for:

• High occupancy spaces (>25 people/1,000 ft²)
• Systems >3,000 CFM
• Specific climate zones

Educational Codes

State education department requirements:

• Minimum ventilation rates
• CO₂ monitoring often required
• Documentation and reporting

Commissioning and Maintenance

Commissioning Requirements

Verify DCV performance:

• Sensor calibration verification
• Control sequence testing
• Response time verification
• Minimum OA verification
• Alarm testing

Ongoing Maintenance

Performance Monitoring

Track key indicators:

• CO₂ levels during occupancy
• Outdoor air delivery
• Energy consumption
• Occupant complaints

Demand-controlled ventilation provides educational facilities with optimized indoor air quality while significantly reducing energy consumption, particularly valuable in spaces with variable occupancy patterns.