A simple but high-impact demand-controlled ventilation (DCV) strategy we used on a medium-sized office renovation was CO2-based modulation of outside air in shared conference rooms. Historically these spaces were served by a constant-volume system delivering the design ventilation rate regardless of actual use. By installing stand-alone NDIR CO2 sensors tied into the building automation system, we were able to reduce the supply air volume when CO2 levels were below 600 ppm and ramp up to full design flow only when the space was fully occupied. We also added occupancy sensors to ensure ventilation remained on during use even if CO2 lagged, and we kept a minimum ventilation baseline to maintain good IAQ. Over a 12-month measurement period the HVAC energy consumption for the conference room group dropped by roughly 25% compared to the previous year, contributing to an overall EUI reduction of about 5 kBtu/ft2*yr for the building. Occupant comfort surveys showed no increase in stuffiness, and in fact many appreciated the system's ability to ramp up quickly during high-density meetings. One commissioning lesson we learned was the importance of sensor placement and calibration. In our first iteration we mounted the CO2 sensor on a wall directly beneath a supply diffuser, which caused readings to consistently underreport concentrations and delayed ventilation ramp-up. During commissioning we relocated the sensor to the breathing zone away from drafts and trained facility staff to calibrate it annually. We also discovered that too low of a setpoint could cause the system to hunt, so we selected a 800 ppm upper threshold with hysteresis rather than trying to maintain a lower number. Finally, we integrated the DCV logic into the BAS trend logs and monitored the CO2 and airflow curves during different seasons to verify that the economiser and heat recovery sequences didn't conflict with the DCV algorithm. The key takeaway is that a straightforward CO2-based DCV can deliver meaningful energy savings, but success depends on thoughtful sensor placement, realistic setpoints and commissioning data review.
Based on my own experiences, a CO2-based demand-controlled ventilation system produced excellent results for a high-end art gallery client that had to deal with a large amount of variable occupancy. This resulted in a dramatic reduction in their Energy Use Intensity (EUI). On the other hand, as someone who has some experience, I would say that proper sensor placement is key. If you put sensors where there are lots of occupants coming and going, it will improve your ability to capture all of the changes in occupancy levels, thus improving your overall energy efficiency.
One demand controlled ventilation strategy that actually reduced EUI was tying outside air rates to CO2 trends with an occupancy floor, not full on off control. A commissioning week sticks with me. The system looked perfect on paper, but it felt odd seeing fans run hard in lightly used zones because one sensor was reading "busy" all day. The fix was sensor placement. We moved CO2 sensors away from supply diffusers and avoided corners, then calibrated them during normal occupancy. One small tweak mattered. Smoothing. We added a short averaging window so the VAVs didn't chase spikes from doors opening or brief gatherings. After that, EUI dropped about 8 percent and comfort complaints fell. The lesson is simple. Bad sensors create bad control. Commissioning is not just checking boxes, it's watching real behavior in the space, abit patiently.