Demand Control Ventilation adjusts outdoor air based on actual occupancy rather than design maximum, reducing energy while maintaining indoor air quality.
DCV uses CO2 concentration as a proxy for occupancy to modulate ventilation:
More people → Higher CO2 → More outdoor air
Fewer people → Lower CO2 → Less outdoor air
| Application | Typical Savings |
|---|---|
| Conference rooms | 20-40% |
| Classrooms | 15-30% |
| Auditoriums | 30-50% |
| Open offices | 10-20% |
| Gyms/fitness | 25-40% |
Zone Outdoor Air = People OA + Area OA
Vbz = Rp × Pz + Ra × Az
Where:
Rp = Per-person rate (CFM/person)
Pz = Zone population (people)
Ra = Per-area rate (CFM/ft²)
Az = Zone area (ft²)
| Parameter | Value |
|---|---|
| Rp (per-person) | 5 CFM/person |
| Ra (per-area) | 0.06 CFM/ft² |
| Zone area | 1000 ft² |
| Design occupancy | 10 people |
Design OA = (5 × 10) + (0.06 × 1000) = 50 + 60 = 110 CFM
With DCV at 50% occupancy:
Actual OA = (5 × 5) + (0.06 × 1000) = 25 + 60 = 85 CFM
| CO2 Level | Interpretation |
|---|---|
| ~400 ppm | Outdoor air baseline |
| 600-800 ppm | Light occupancy |
| 800-1000 ppm | Moderate occupancy |
| 1000-1200 ppm | High occupancy |
| >1200 ppm | Ventilation insufficient |
ASHRAE 62.1 allows up to 700 ppm above outdoor:
CO2_Setpoint = Outdoor_CO2 + 700 ppm
= 400 + 700 = 1100 ppm (typical)
Guideline 36 uses: 1000 ppm as occupied setpoint
If Zone_CO2 < (Setpoint - 200 ppm):
OA_Damper = Minimum_Position
If Zone_CO2 between (Setpoint - 200) and Setpoint:
OA_Damper = Proportional (Min to Design)
If Zone_CO2 > Setpoint:
OA_Damper = Design_Maximum
Zone level:
Calculate Zone_OA_Setpoint:
If Zone_CO2 < 600 ppm:
Zone_OA = Ra × Az (area-based only)
If Zone_CO2 > 600 ppm:
Zone_OA = Rp × Estimated_People + Ra × Az
Where Estimated_People = f(CO2 level)
System level:
System_OA = Sum of all Zone_OA requirements
× Diversity factor
/ Ventilation efficiency (Ev)
Zone minimum with DCV:
Zone_Min_Flow = MAX(
Cooling_Minimum,
Heating_Minimum,
Ventilation_Minimum
)
Where Ventilation_Minimum varies with CO2:
CO2 < 600 ppm: Area-based only
CO2 > 600 ppm: Area + proportional people
| Specification | Requirement |
|---|---|
| Accuracy | ±75 ppm or 3% |
| Range | 0-2000 ppm minimum |
| Response time | <2 minutes |
| Calibration | Annual or per manufacturer |
Do:
Don't:
Required for differential control:
1. Add CO2 sensor in space
2. Wire to RTU controller
3. Configure DCV sequence:
- Set CO2 setpoint (1000 ppm)
- Configure OA damper modulation
- Set minimum position
4. Commission and verify
1. Add CO2 sensor per zone (or representative zones)
2. Configure zone minimum airflow reset
3. Calculate system OA at AHU:
Sum(Zone_OA) / Ev
4. Modulate OA damper to maintain system OA
5. Use MAT or economizer logic
1. CO2 sensors in served zones
2. Highest CO2 determines DOAS output
3. Supply airflow modulates with CO2
4. Or individual zone VAV terminals
| Test | Procedure |
|---|---|
| Sensor accuracy | Compare to calibrated reference |
| Control response | Increase occupancy, verify damper opens |
| Setpoint operation | Verify at setpoint, above, below |
| Fail-safe | Sensor failure defaults to max OA |
Monitor for 2-4 weeks:
| Metric | Target |
|---|---|
| CO2 maintained | <1000 ppm occupied |
| Response time | Stabilize within 15-30 min |
| Energy reduction | Document baseline savings |
Causes:
Solutions:
Causes:
Solutions:
Causes:
Solutions:
Per ASHRAE 90.1 and energy codes:
| Condition | DCV Required |
|---|---|
| High density (>25 people/1000 ft²) | Yes |
| System >3000 CFM design OA | Yes |
| Space >500 ft² | Yes |