How do CO₂ sensors work in demand-controlled ventilation?

NDIR (Non-Dispersive Infrared) CO₂ sensors measure the absorption of infrared light at 4.26 μm wavelength by CO₂ molecules in the air sample, and are now the standard sensor type for building automation due to their long-term stability and accuracy (±30–50 ppm). The sensor outputs a 0–10V or 4–20mA signal proportional to 0–2,000 or 0–5,000 ppm and the BMS controller calculates the required outdoor air based on the differential between the zone CO₂ concentration and the outdoor ambient concentration (typically ~400–450 ppm). Per ASHRAE 62.1, demand-controlled ventilation using CO₂ is permitted in spaces with variable occupancy where the population-area outdoor air rate (Rp) component dominates, and the setpoint is typically 1,000–1,100 ppm above outdoor ambient. For accuracy, CO₂ sensors should be wall-mounted at breathing height (4–6 feet AFF), away from doors, supply diffusers, and direct sunlight, with ABC (Automatic Baseline Calibration) logic enabled to periodically reset the sensor to the lowest reading over a 7–14 day period.