Ceiling-Mounted CO₂Sensing: Effect of Location and Stratification Temperature

Carbon dioxide is an important parameter for indoor air quality (IAQ) monitoring and demand controlled ventilation (DCV). Usually, CO2 sensors are wall-mounted at 0.9-1.8 m (3-6 ft) height as prescribed by LEED, although ASHRAE standards seemed to relax this requirement. In this work, we investigate whether positioning these sensors in the ceiling is effective and advantageous. We studied CO2-level measurements for HVAC control in configurations with mixing ventilation and found that CO2 from human exhalations experiences buoyancy from several factors. We calculated buoyancy from air properties, and we introduced the notion of "stratification temperature"for exhaled air. By simulation, we test the sensitivity to temperature, and we conducted in situ in vivo measurements to acquire more detailed insights in the feasibility of ceiling sensor positions. Buoyancy calculations show that in exhaled air, the positive buoyancy of H2O approximately compensates for the negative buoyancy of CO2, so that thermal buoyancy is the most dominant factor. Exhaled air, containing CO2 to be measured, will rise towards a ceiling that has a temperature below the stratification temperature. Computational fluid dynamics (CFD) simulations of a small office space indicate that this can also be the case in the presence of air flows induced by a mechanical ventilation system. The measurement results support that using "properly mounted"CO2 sensors in the ceiling gives lower variability in CO2 measurements and faster response than wall-mounted sensors and yields slightly higher values than wall sensors. Our results highlight the need to update the standards and regulations for sensing CO2 to include ceiling-mounted sensors.