CO2 as a Control Parameter

Demand-controlled ventilation adjusts the airflow based on actual occupancy rather than a schedule. Under partial load—which is most of the time—this saves fan power and heating energy without compromising indoor air quality.

For this to work, the measurement must come from the occupied area, not from the exhaust air. In the exhaust air, the room air mixes with everything that enters along the way; the reading arrives too late and is too smoothed out.

CO₂ is a slow-responding parameter. After a window is opened, it takes minutes for the CO₂ level to drop. A two-point control system will therefore oscillate; a controller with a long response time that accounts for the dead time in the control loop is appropriate.

And finally: Where users are expected to ventilate the space themselves, the reading must be visible. A traffic-light system or a display in the room changes behavior; a reading in the building automation system does not.

Is this suitable for a school? Especially there: In a classroom, more people are seated in a smaller space than in an office, so CO2 levels rise more quickly, and ventilation usually comes from windows rather than a mechanical system. In that case, the display in the room isn’t just a gimmick—it’s the actual tool: It tells the class when to ventilate and lets the person in charge know afterward whether it worked. The same applies to daycare centers, lecture halls, and conference rooms.

For existing buildings, another advantage is that wireless devices don’t require new wiring—installation can be completed within a single summer break. And when classrooms are located on the ground floor or in the basement, radon must be taken into consideration; our reference project in this area is a university with 70 monitored areas.

The number of CO2 sensors required for a given area can be determined rather than estimated: Two international standards specify a density of measuring devices, and both result in figures of the same order of magnitude. For an open-plan office measuring 200 square meters, this translates to one device —as a minimum requirement.

WELL v2 requires, in Feature A08, at least one sensor per 3,500 square feet of usable floor area, or approximately 325 square meters. RESET Air for Commercial Interiors takes a different approach: one sensor per regularly used room type, with an additional limit of 500 square meters of coverage per sensor. A meeting room therefore counts as a separate unit, even if it is small.

The lower limit is therefore not the answer for every area. Where the use of a space varies—such as a meeting area, break room, or reception area—each constitutes a separate zone and should be measured individually. Therefore, plan for one device per usage zone and check the area limit as a second condition.

Regarding accuracy: WELL requires 10 percent CO2 at 750 ppm. The EnoSense® CO2 also meets this requirement, with a maximum value of 9.0 percent and a typical value of 7.0 percent. It also meets the measurement interval requirement of no more than ten minutes, with a measurement interval of five minutes. You can find the complete line-by-line verification against both standards here: WELL and RESET in Detail →