Demand controlled ventilation: what it does with CO₂ data

Demand controlled ventilation uses a room's CO₂ reading as a stand-in for how many people are in it, and resets the outdoor air rate up or down to match. A packed conference room gets more outdoor air than the same room empty at 7am, without a technician changing a single setting by hand. This post covers how that reset actually works, the CO₂ numbers that trigger it, and the one part of the system most buildings never check.

Key Takeaways

• Demand controlled ventilation resets outdoor airflow based on a room's CO₂ reading, used as a proxy for occupancy, under ASHRAE Standard 62.1.
• The common reset point is roughly 700 ppm above the outdoor CO₂ reading (1,100 ppm where outdoor air itself runs high), the trigger for ramping airflow back to the full design rate.
• A Lawrence Berkeley National Laboratory study of 208 CO₂ sensors in 34 commercial buildings found roughly 4 in 10 had drifted more than 75 ppm from true, and none of the buildings had calibrated a sensor since installation.
• A CO₂ sensor that has drifted 200 ppm will quietly under-ventilate an occupied room or over-ventilate an empty one, and the building automation system has no way to flag its own error.
• Demand controlled ventilation manages outdoor air volume. It says nothing about PM2.5, VOCs or radon, and a reading in the room, kept independent of the building's own sensor, tells a facilities team whether the reset is actually happening.

What demand controlled ventilation does with a CO₂ reading

Every mechanically ventilated building has to bring in a minimum amount of outdoor air, sized for the room at full occupancy. Running that rate at 7am, with three people in a room built for 40, wastes the energy spent heating or cooling air nobody needs. Demand controlled ventilation is the fix: a CO₂ sensor in the space feeds the building automation system, and outdoor airflow resets somewhere between a low floor and the full design rate as the reading moves.

ASHRAE Standard 62.1 is what most commercial systems are built against, and its 2025 edition added demand controlled ventilation control sequences directly into the standard. People, not furniture or finishes, are what drives occupant-related CO₂, so a reading that climbs is read as a room filling up, and a reading that falls is read as a room clearing out.

The CO₂ number that actually resets the airflow

Two different CO₂ numbers get confused in this conversation, and they are not the same one. The first is the comfort threshold: past roughly 1,000 ppm of CO₂, most people start calling a room stuffy, a figure ASHRAE's own position document ties to inadequate outdoor air rather than to any health effect of CO₂ itself.

The second is the reset differential a demand controlled ventilation sequence actually watches: once the indoor reading climbs about 700 ppm above the outdoor CO₂ reference (1,100 ppm in locations where outdoor air itself runs higher), the system ramps outdoor airflow back up to the full design rate. That is a relative number, measured against the air outside that day, not a fixed ceiling. It is also well short of the limit OSHA's enforceable standard sets for industrial exposure, which is five times higher and built for a different purpose entirely.

Where the sensor sits changes what the reading means

A demand controlled ventilation system is only as good as the one reading it trusts, so placement matters. Under ASHRAE Standard 62.1, the sensor has to sit in the breathing zone, roughly 3 to 6 feet above the floor, not in a ceiling plenum or a supply duct, and a space built from several rooms needs one per room rather than one for the whole zone.

Some installations put the sensor in the return air duct instead, reading mixed air from everywhere the room exhausts to. That works until supply air bypasses the breathing zone entirely, which reads lower than what people are actually sitting in. A conference room with its own door and its own return is the case to watch: a hallway-level reading will miss it every time.

What happens when the sensor drifts

A commissioning checklist rarely catches this: the sensor the whole reset depends on drifts, and nothing in the control sequence notices. The common sensor technology in commercial systems reads CO₂ optically, and that optical path fouls and ages. A 2010 study from Lawrence Berkeley National Laboratory tested 208 of these sensors across 34 commercial buildings and found that at a reading of 1,010 ppm, 40% of them were off by more than 75 ppm, and nearly 1 in 5 were off by more than 200 ppm. Not one facilities team in the study had calibrated a sensor since it was installed.

A 200 ppm drift is not a rounding error. It is most of the margin between a stuffy room and a comfortable one, and a system reading it wrong will either hold a full room at the low floor, or run an empty one at full design rate, burning exactly the energy demand controlled ventilation exists to save. The building automation system has no independent way to catch its own sensor failing, because it only has the one number.

What demand controlled ventilation does not do

A demand controlled ventilation sequence manages one thing: how much outdoor air comes in. It says nothing about PM2.5 from wildfire smoke or a nearby loading dock, and nothing about VOCs off-gassing from new carpet or paint. Radon needs months of readings before the number means anything, and CO₂ does not touch that either. Turning up outdoor air can dilute some of those too, but the CO₂ reading driving the reset has no idea they exist.

The honest tradeoff: a demand controlled ventilation system tells a damper what to do, not whether the strategy is keeping pace with a packed room or whether its one sensor still reads true. That answer has to come from somewhere other than the system grading its own work.

How a facilities team checks whether it is actually working

The way to verify a demand controlled ventilation sequence is doing what it is supposed to is a reading taken independent of it: a monitor in the room itself, not wired into the building automation system, reporting CO₂ against the air outside and kept as history. When the conference room fills for a 10am meeting and the line on an independent monitor climbs while the building's own sensor stays flat, that gap is the sensor drift showing up before anyone has to guess at it.

That is also the conversation a workplace air quality testing program is built for: continuous readings, kept over weeks, that turn "the air on this floor feels thick" from a complaint into a number someone can check. Measured Air's monitors for commercial buildings are sized per space, report continuously against the outdoor reference, and keep every reading as history, so a facilities team has its own answer to whether the reset is happening, independent of what the one sensor behind it says it did.

FAQ

What is demand controlled ventilation? Demand controlled ventilation is a control strategy that raises or lowers a room's outdoor airflow based on a CO₂ reading used as a proxy for occupancy, instead of running one fixed ventilation rate regardless of how many people are actually in the room.

What CO₂ level triggers demand controlled ventilation to increase airflow? Most systems built to ASHRAE Standard 62.1 reset outdoor airflow back toward the full design rate once the indoor CO₂ reading climbs about 700 ppm above the outdoor reference, or 1,100 ppm where the outdoor air itself runs higher.

Does demand controlled ventilation save energy? It can, because it stops heating or cooling a full design rate of outdoor air for a room that is only partly occupied or empty. How much it saves depends on how much the room's occupancy actually swings and whether the CO₂ sensor behind the reset still reads accurately.

Where should a CO₂ sensor go for demand controlled ventilation? In the breathing zone, roughly 3 to 6 feet above the floor, in the room it controls, not in a ceiling plenum or a shared return duct that mixes air from spaces with different occupancy.

Can demand controlled ventilation go wrong? Yes. The CO₂ sensor driving the reset can drift without anyone noticing, and a building automation system has no independent way to catch its own sensor failing. A drifted sensor will under-ventilate an occupied room or over-ventilate an empty one.

Does Measured Air control a building's ventilation system? No. Measured Air reports CO₂ and other readings room by room against the air outside. It does not operate dampers or connect into a building automation system, which makes it an independent check on what a demand controlled ventilation sequence is actually doing.

The takeaway

Demand controlled ventilation does one job well when it is working: it reads a room's CO₂ as a stand-in for occupancy and resets outdoor airflow to match, instead of running one rate all day regardless of who is in the room. The reset point is a relative number, roughly 700 to 1,100 ppm above the outdoor reference, and the whole strategy depends on one sensor that commercial buildings rarely recalibrate after installation. A reading taken independent of that sensor, kept as history room by room, turns "is the reset actually happening" from a guess into something a facilities team can check. Get in touch about monitoring for your building to see what an independent reading shows on the floors you manage.