Demand-Controlled Ventilation CO2 Setpoint Calibration
Why this matters
Demand-controlled ventilation (DCV) modulates outside-air based on CO2 readings in densely occupied zones (conference rooms, classrooms, auditoriums) to capture energy savings during partial occupancy. ASHRAE 62.1-2022 Section 6.2.7 permits DCV in zones with average occupant density above 25 people per 1000 sq ft. The setpoint is not a magic 1000 ppm; it is a calculation tied to design population, area, and outdoor CO2 baseline. A miscalibrated DCV setpoint either under-ventilates a full meeting room or runs minimum OA continuously and captures zero savings.
The math you owe the zone
The CO2 setpoint for DCV equals outdoor CO2 plus the steady-state differential that corresponds to the design per-person ventilation rate at design occupancy:
CO2 setpoint = CO2 outdoor + (Cper-person x 1,000,000) / Rp
where Cper-person is the CO2 generation rate per occupant (0.0084 cfm CO2 per person at low metabolic activity, MET 1.2 to 1.4), and Rp is the design people-component ventilation rate from ASHRAE 62.1-2022 Table 6-1 in cfm per person. For an office at Rp 5 cfm per person and outdoor CO2 of 420 ppm, the calculation yields 1680 ppm + 420 ppm = roughly 2100 ppm. That feels high but is what the math returns; in practice, the area-component (Ra) also delivers OA, so the actual indoor steady-state lands well below.
The simpler engineering shortcut used by most controls vendors: target indoor CO2 of 700 to 800 ppm above outdoor at the design rate, which for typical 400 to 450 ppm outdoor gives a control setpoint of 1100 to 1200 ppm. ASHRAE itself notes the 1000 ppm "common" setpoint is a heuristic, not a code requirement.
Sensor selection and placement
Use NDIR (non-dispersive infrared) sensors with documented accuracy of +/- 50 ppm or better. Resistance-based and electrochemical sensors drift too quickly for DCV. Place the sensor 4 to 6 feet above the finished floor in the breathing zone, on an interior wall away from exhaust grilles, supply diffusers, doorways, and HVAC return paths. A sensor in the supply-air return path reads diluted air and the zone over-ventilates. A sensor near a door reads outdoor-corridor air during occupancy transitions and the zone under-ventilates.
Outdoor CO2 reference strategy
The setpoint above outdoor is more accurate than an absolute setpoint because outdoor concentrations have risen and continue to rise; the 400 ppm assumption from a decade ago is now closer to 420 to 430 ppm depending on site. Two options: install a dedicated outdoor CO2 sensor and program a differential setpoint, or use a fixed offset of 450 ppm in the program with an annual review. The differential approach is more code-defensible; ASHRAE 62.1 Section 6.2.7.1.2 explicitly accepts it.
Sequence for VAV with DCV
The DCV signal modulates the AHU's OA damper minimum position (or in single-zone VAV, the box-level OA fraction at the unit). Below the setpoint, OA holds at the zone's Ra (area-only) minimum. As CO2 rises, OA ramps proportionally toward the full Voz (area + people). The ramp band is typically 200 ppm; below setpoint minus 100 the zone is at Ra-only minimum, above setpoint plus 100 the zone is at full Voz. Hysteresis prevents damper hunting on doors-opening transients.
Annual calibration procedure
NDIR sensors drift; ASHRAE Guideline 36-2021 recommends annual recalibration. Procedure:
- Block the sensor's intake and expose it to a known calibration gas (typically 400 ppm and 1500 ppm bottle gas via diffusion cap).
- Record the displayed value at both points and adjust the sensor span and offset per OEM manual.
- If the sensor drifts more than +/- 100 ppm at either point, replace it; calibration of a worn IR source is not durable.
- Document the calibration in the BAS records and tag the sensor with the calibration date.
Compliance documentation
Title 24 Part 6 Section 120.1(c)4 requires DCV in California in densely occupied spaces of certain types; the acceptance test (NA7.5.6) must be filed. Federal code-cycle adopters of 90.1-2022 require DCV per Section 6.5.7.1; documentation of design occupancy, setpoint calculation, and sensor calibration record is the basis for inspector sign-off and ongoing M+V.
References
- ASHRAE Standard 62.1-2022, Section 6.2.7 Dynamic Reset of Outdoor Air Intake
- ASHRAE Guideline 36-2021, Section 5.6 Zone Group Control Sequences
- ASHRAE Standard 90.1-2022, Section 6.5.7 Ventilation Controls
- 2022 California Title 24 Part 6 NA7.5.6 Demand Control Ventilation Acceptance Test
- Vaisala GMW90 Series CO2 Transmitter Application Note (NDIR calibration)