CO2 in HVAC Systems and Indoor Air Quality

CO2 is the most direct indicator of human occupancy in an enclosed space, which is why it's the reference variable for demand-controlled ventilation (DCV). This note focuses on why sensor quality determines whether a DCV system works reliably for years, not on where to place transmitters — for that, see the technical note on how to measure CO2.

A sensor that drifts out of calibration silently doesn't trigger an alarm — it produces a bad ventilation decision every day, unnoticed until air quality complaints show up or an energy bill doesn't add up.

This note covers

• Why demand-controlled ventilation saves energy
• Why single-beam sensors drift out of calibration over time
• How dual-wavelength technology avoids that problem

Demand-Controlled Ventilation: Occupancy-Based Energy Savings

Demand-controlled ventilation (DCV) reduces outside air intake when a space is unoccupied and increases it when people are present, instead of ventilating at a constant rate at all times. Compared to a constant air volume system, a CO2-based DCV system can save more than 50% of HVAC energy use — the impact is greatest in small offices, retail stores, and supermarkets compared to other building types.

The principle is simple: CO2 level in a space indicates human presence, and that data can be used to modulate ventilation in real time. But that energy saving depends entirely on the CO2 reading being accurate — a sensor that gets it wrong produces the wrong ventilation decisions, either wasting energy or compromising air quality.

Why Single-Beam Sensors Drift Out of Calibration Over Time

A single-beam, single-wavelength infrared sensor suffers from substantial long-term drift: the incandescent bulb it uses as a light source degrades with use — tungsten evaporates and blackens the glass, reducing emitted light intensity. The sensor interprets that loss of intensity as a rise in CO2, producing increasingly unreliable readings.

To compensate, many manufacturers use automatic background calibration that assumes the lowest recorded reading corresponds to fresh air (400 ppm). That assumption fails in two common cases: in occupied buildings, where CO2 rarely drops below 600–800 ppm; and in new buildings, where concrete carbonation can push ambient CO2 below 400 ppm. In both cases, automatic recalibration introduces a systematic error that ends in inadequate ventilation and worse air quality.

Dual-beam sensors (two light sources) add a second lamp to try to compensate for drift, but that introduces an extra potential failure point and uneven dust accumulation between the two optical paths — in practice, they turn out to be relatively unreliable over the long term.