Demand Control Ventilation (CO2-Based)

What it is

Ventilation costs money — every cubic foot of outdoor air you bring in has to be heated or cooled to room conditions. In a space with swingy occupancy (conference rooms, classrooms, gyms, restaurants, retail), ventilating for a full room all day when it's usually half-empty wastes a ton of energy.

Demand control ventilation (DCV) fixes that by ventilating for the people actually in the room right now. It uses CO2 concentration as a stand-in for occupancy: people exhale CO2, so rising CO2 means more people, and the control opens the outdoor-air damper to bring in more fresh air. When the room empties and CO2 falls, it throttles outdoor air back down. You get the required fresh air when you need it and stop conditioning unnecessary outdoor air when you don't.

How it works

A CO2 sensor in the occupied space (or sometimes in the return) feeds a concentration reading to the controller. The controller compares it to setpoints and modulates the outdoor-air damper between a low (base) ventilation rate and a high (full) ventilation rate:

• Below the low setpoint (room lightly occupied or empty): outdoor air sits at a base minimum — enough to handle building sources (off-gassing, etc.) and any code-required base rate. CO2 is the proxy for people, but a building still needs some baseline air for the building itself.
• Rising through the band: as CO2 climbs, the controller proportionally opens the outdoor-air damper, ramping ventilation up to match the growing crowd.
• At/above the high setpoint: outdoor air is at the design maximum for a full house.

The key concept is that CO2 is a proxy, not the pollutant of concern. We don't ventilate to "get rid of CO2" per se — CO2 just happens to track the number of breathing people, who also bring the odors, moisture, and bioeffluents that ventilation is really there to dilute. So DCV adjusts the people-related portion of ventilation while keeping a base rate for the building itself.

Why it saves energy: you're not heating or cooling outdoor air for occupants who aren't there. A conference room ventilated full-tilt for 8 hours but occupied for 90 minutes is conditioning a lot of needless outdoor air — DCV cuts that to the times it's actually full. The savings are biggest in spaces with high design occupancy and low average occupancy, and in climates with big heating or cooling penalties on outdoor air.

DCV usually rides on the same outdoor-air damper an economizer uses, and the two coordinate: economizer logic decides when outdoor air is good for cooling, DCV decides how much outdoor air is needed for ventilation. The controller takes the greater demand at any moment.

In the field

• Place the CO2 sensor in the breathing zone of the occupied space, away from supply diffusers (which blow fresh, low-CO2 air and fool the sensor low) and away from doors/windows. A wall sensor at roughly occupant-breathing height in the conditioned space is typical. A sensor blasted by a diffuser will under-ventilate a full room.
• Set the low and high CO2 setpoints to a sensible band. DCV works on the difference between indoor and outdoor CO2, not the absolute number — outdoor air is itself a few hundred ppm. A common target keeps indoor CO2 within roughly several hundred ppm above outdoor.
• Keep a base ventilation rate. Don't let DCV close outdoor air to zero when the room empties — the building still needs its base rate for non-occupant sources. Set the low position accordingly.
• Coordinate with the economizer. Make sure DCV and economizer free cooling don't fight — outdoor-air position should satisfy whichever is demanding more at the moment.
• Calibrate the sensor. CO2 sensors drift. Many use an auto-calibration routine that assumes the space periodically returns to near-outdoor CO2 (e.g., overnight when empty) — that assumption fails in a 24/7 occupied space, so know whether your sensor needs manual calibration there.
• Verify the response. Breathe near the sensor (or use a controlled CO2 source) and confirm the damper opens as CO2 rises and closes as it falls. No response = sensor, wiring, or controller config.

Normal values & targets

• Outdoor (ambient) CO2: roughly 400–450 ppm — this is your baseline; DCV works on the rise above it.
• Typical indoor control band: ventilation ramps up to hold indoor CO2 within a few hundred ppm of outdoor; a commonly used comfort/ventilation reference is keeping indoor levels in the neighborhood of ~1,000 ppm or lower in occupied spaces (used as a control target, not a hard safety limit).
• CO2 as a health hazard: the levels DCV deals with (hundreds to ~1,000-ish ppm) are comfort/IAQ targets, not dangerous. CO2 doesn't become a direct health concern until far higher concentrations — DCV operates well below that.
• Base ventilation: never zero; maintain the building's base outdoor-air rate even at low occupancy.
• Sensor drift: budget periodic recalibration; many sensors auto-calibrate on the assumption of regular near-outdoor exposure.

Common faults & what they mean

• Room stuffy when full (under-ventilating): sensor reading low because it's in a supply-air stream or poorly placed, setpoints too high, or damper not opening. Relocate/verify the sensor and confirm damper response.
• Over-ventilating (high energy use, never throttles back): sensor reading high (drifted, contaminated), setpoint band too tight, or DCV stuck at full. Calibrate the sensor and check the band.
• No damper response to occupancy changes: failed sensor, wiring fault, DCV not enabled, or actuator dead. Trace the sensor signal to the controller and the controller's command to the actuator.
• CO2 sensor reads obviously wrong (e.g., below outdoor ambient or pinned high): sensor failed or badly out of calibration. Replace/recalibrate; a bad CO2 reading makes every ventilation decision wrong.
• Ventilation drops to zero in an empty room: base rate set too low/at zero — building still needs baseline air. Raise the minimum position.

Tech tips & gotchas

• CO2 is a proxy for people, not the enemy. You're not chasing a CO2 number for its own sake — you're using it to size ventilation to the crowd. Keep that framing and the setpoints make sense.
• Sensor placement is everything. In a supply stream it reads artificially low and starves a full room; in a dead pocket it reads high and over-ventilates. Breathing-zone, good air mixing, away from diffusers and doors.
• Always keep a base rate. DCV reduces the occupant portion of ventilation, not the building's baseline. Closing all the way off when the room empties is a mistake.
• Watch the auto-calibration assumption. Many CO2 sensors self-zero by assuming the room periodically hits near-outdoor CO2. In a space that's never empty, that auto-cal drifts the sensor wrong over time — calibrate manually there.
• DCV is best where occupancy swings hard. Conference rooms, auditoriums, gyms, classrooms, restaurants — high design occupancy, low and variable actual occupancy. It does little in a steadily occupied office.
• Coordinate with the economizer, don't let them fight. Same damper, two demands (ventilation vs free cooling) — the control should serve the larger one.

Safety / code notes

• Demand control ventilation must still deliver at least the minimum outdoor-air ventilation required by the applicable mechanical-code ventilation provisions for the current occupancy; DCV modulates above the required minimum, it doesn't excuse falling below it.
• Maintain a base (building-related) outdoor-air rate even at low occupancy per the ventilation code; DCV reduces the people-component, not the base component.
• CO2 sensors and damper actuators run on low-voltage Class 2 control power; the air-handling unit's line side is full voltage — de-energize and verify before servicing inside the unit.