A 200-person conference room sits empty 90% of the day. Designed-to-code ventilation pumps 2,000 cfm of pre-conditioned outdoor air through it constantly. Half-occupied, the room over-ventilates by 50%; empty, it over-ventilates by 100%. Demand controlled ventilation modulates outdoor air to match actual occupancy, recovering 25-40% of OA-related conditioning energy in highly variable spaces.
This guide explains the two DCV strategies (CO₂ versus occupancy counting), the sensor placement that makes or breaks accuracy, and the dynamic VRP correction in ASHRAE 62.1.
The two DCV strategies
CO₂-based DCV
Measure CO₂ in the return air (or zone). Modulate outdoor-air damper or ERV speed to maintain a setpoint. Implicitly assumes one occupant generates one ASHRAE-defined CO₂ rate.
Occupancy-based DCV
Count people directly via occupancy sensors (PIR, time-of-flight, badge-in/out, ML-based optical counters) and modulate to deliver per-person ventilation rate per ASHRAE 62.1 Table 6-1.
Each strategy has trade-offs:
| Aspect | CO₂-based | Occupancy-based |
|---|---|---|
| Sensor cost | ₹15-25k per zone | ₹40-80k per zone (HD optical) |
| Lag | 15-30 min until CO₂ rises | Real-time |
| Accuracy in mixed zones | Poor (drinkers/smokers/coffee bias readings) | Good |
| Compliance with ASHRAE 62.1 §6.2.7.2 | Standard | Acceptable but needs documentation |
| Maintenance | NDIR sensors drift; calibrate every 2 years | Generally maintenance-free (no calibration) |
For office and hospitality, CO₂-based is industry standard. For high-density variable spaces (auditoriums, classrooms, conference centres), occupancy-based catches up on accuracy.
CO₂ setpoint logic
ASHRAE 62.1 §6.2.7.2 requires the system to deliver at least the area-component of the VRP (Ra × Az) at all times. The people-component (Rp × Pz) is what scales with occupancy.
The practical setpoint logic:
- Setpoint: 700-1100 ppm above outdoor (typically outdoor ~410 ppm, so setpoint 1100-1500 ppm).
- Below setpoint: outdoor air at minimum (= area-component only).
- Above setpoint: outdoor air ramps up to maintain setpoint; max OA = full design VRP.
- Hysteresis: 100-200 ppm dead-band to avoid hunting.
A CO₂ setpoint of 1,000 ppm corresponds approximately to ventilation of 7-8 L/s per person — close to the ASHRAE 62.1 default for an office (Rp = 5 L/s/person + Ra component for floor area).
Sensor placement: the make-or-break detail
CO₂ sensor location dictates DCV accuracy. Three valid placements:
1. Return-air duct (or grille immediately upstream of return mixing). Best for single-zone systems. Reading represents the average occupancy of all served spaces.
2. Wall-mounted in zone, 1.5-1.8 m above floor, away from windows/doors and HVAC supply diffusers. For multi-zone systems where each zone is metered separately.
3. Plenum-return ceiling location, when ductwork is plenum-return type. Less ideal — stratification can bias.
Avoid: sensor at supply diffuser (reading is conditioned air, not zone air); sensor at desk level (CO₂ pools higher near floor); sensor near a coffee maker or kitchenette.
ASHRAE 62.1 dynamic VRP correction
For multi-zone systems, ASHRAE 62.1 §6.2.7.1 requires the dynamic ventilation efficiency calculation to be re-run with current occupancy. The Vot (outdoor air at the system) is the maximum of:
- Vot at design occupancy (minimum guaranteed)
- Vot at current measured occupancy
Static VRP gives one number for design occupancy; dynamic VRP recalculates per the current Pz across all zones. In a mixed-occupancy multi-zone system, this matters: a single critical zone (auditorium fully occupied) can drive system-wide outdoor-air requirement up by 30%.
Modern BAS controllers (Trane Tracer SC+, JCI Metasys, Honeywell WEBs) include a VRP module that runs this calculation continuously.
Sequence of operation
A typical DCV-enabled VAV terminal box sequence:
1. CO2 monitored in return; reset OA damper position
2. If CO2 < setpoint - 100 ppm: OA = minimum (area component only)
3. If CO2 > setpoint - 100 ppm: ramp OA toward design (max 100% of design VRP)
4. If CO2 > setpoint + 200 ppm for > 15 min: send maintenance alert
5. Once daily (e.g. 4 AM): force full OA for 30 min flush
6. Lockouts: do NOT reduce OA below 30% of design (NBC 2016 minimum) regardless of CO2
That last lockout is important — Indian code specifies a hard minimum of 5 ACH or 30% of design OA, whichever is higher. DCV cannot drop below it.
Worked example: 200-person conference room
Conference room: 200 m² floor, design occupancy 200 people, ASHRAE 62.1 default Rp + Ra.
Vbz = Rp × Pz + Ra × Az = 0.0025 × 200 + 0.0006 × 200 = 0.62 m³/s = ~1,310 cfm
At Ez = 0.8 (typical fan-coil):
Voz = Vbz / Ez = 0.78 m³/s = ~1,650 cfm at 100% occupancy
Without DCV: 1,650 cfm continuously, even when room is empty.
With DCV CO₂-based setpoint 1000 ppm:
| Occupancy | OA delivered |
|---|---|
| 0 (empty) | Ra component only = 150 cfm (minimum 300 cfm by NBC lockout) |
| 50 (25%) | ~600 cfm |
| 100 (50%) | ~1,100 cfm |
| 200 (100%) | 1,650 cfm (full VRP) |
If average daily occupancy is 30%, average OA delivered is ~700 cfm vs 1,650 baseline → 58% reduction in OA-related conditioning. At Delhi summer conditions (~67 kW total OA load at 1,650 cfm), savings = 67 × 0.58 = 39 kW peak. Annual savings: roughly ₹400,000-500,000 at typical office occupancy patterns.
Five mistakes that ruin DCV performance
1. Sensor calibration ignored. NDIR drift can be 100-200 ppm/year. Annual calibration plan or sensor replacement at 5 years.
2. Setpoint too aggressive (> 1500 ppm). Occupants report stuffy / foggy / sluggish. Stick to 1000-1100 ppm.
3. Hard lockout below 30% of design OA forgotten. NBC 2016 requires it. AHJ rejection if not implemented.
4. DCV applied to single-zone systems with a CO2 sensor in supply. Reading conditioned air, not zone air. Sensor location matters.
5. No daily flush for IAQ purge. A morning flush at full OA reduces overnight VOC accumulation; without it, occupants notice.
Quick checklist
- [ ] Strategy chosen: CO₂ (industry standard) or occupancy-based (high-density)
- [ ] Sensor placement: return-duct (single-zone) or zone-wall (multi-zone)
- [ ] Setpoint 1000-1100 ppm with 100-200 ppm dead-band
- [ ] Hard lockout at 30% of design OA (NBC compliance)
- [ ] Daily flush sequence at low-occupancy hour
- [ ] Sensor calibration plan (annual, replace at 5 years)
- [ ] BAS supports dynamic VRP for multi-zone systems
References: ASHRAE 62.1-2022 §6.2.7 (Demand Controlled Ventilation); ECBC 2017 §5.2.6; LEED v4.1 EQc1 IAQ; ASHRAE Handbook HVAC Apps 2023 Ch 47.
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