Occupancy-Based HVAC Uses CO2 and Motion Sensors Together

Occupancy-based HVAC control uses CO2 and motion sensors together to adjust ventilation and temperature only where spaces are actually occupied, eliminating conditioning of empty zones, while daylight sensors trim lighting levels without affecting comfort.

Using both sensor types rather than one is the detail that makes this work, because each fails in a way the other covers. Motion sensing misses people who are still — a meeting room full of seated attendees can read as empty within minutes, and systems that trusted motion alone became notorious for turning ventilation off on occupied rooms. CO2 catches exactly that case, since a room full of people accumulates CO2 regardless of how little anyone moves.

CO2 has the opposite weakness: it lags. Concentration takes time to build after people arrive and time to disperse after they leave, so a CO2-only system under-ventilates at the start of a meeting and over-ventilates after it ends. Motion leads, CO2 confirms, and the combination gives a control loop that responds quickly without being fooled by stillness. It is a good illustration of a general principle in sensing — two cheap sensors with uncorrelated failure modes usually beat one expensive sensor.