A single French text puts numbers on indoor CO2: the order of 27 December 2022, issued for the annual assessment of ventilation provision in the premises open to the public covered by decree no. 2022-1690. It keeps three classes, at most 800 ppm for satisfactory air renewal in occupied premises, 800 to 1,500 ppm, and above 1,500 ppm for insufficient renewal. It also sets what the instrument must be worth: non-dispersive infrared spectrometry or an equivalent technology, a range of at least 0 to 5,000 ppm, and a maximum uncertainty of ± (50 ppm + 5% of the reading). In a dwelling none of those thresholds applies: the obligation remains the extract flow rate of the order of 24 March 1982, and CO2 is only a commissioning tool.
Understanding CO₂ to fine-tune ventilation
Why the CO₂ level reflects occupancy and air renewal
CO2 is mainly produced by breathing. When people enter a room, the level rises. When the air is renewed, it drops back towards the outdoor value. It is an occupancy indicator, useful on site to check whether the ventilation and air inlets are bringing in enough fresh air, but it measures neither a flow rate nor compliance.
What the order of 27 December 2022 quantifies, and for whom
The text is short and entirely numerical. The right-hand column is the one that matters to you, because it separates what binds a housing job from what does not.
| What the order sets | Value | What it governs |
|---|---|---|
| Instrument technology | non-dispersive infrared absorption spectrometry or equivalent | hardware |
| Minimum measuring range | 0 to 5,000 ppm | hardware |
| Measurement uncertainty | ± (50 ppm + 5% of the reading) | hardware |
| Duration of display monitoring | at least two hours | protocol |
| Measurement conditions | heating season, occupancy between 0.5 and 1.5 times the design figure | protocol |
| Instrument position | 1 to 2 m above floor level, away from air inlets and outlets | protocol |
| Satisfactory air renewal | at most 800 ppm | classification |
| Intermediate class | 800 to 1,500 ppm inclusive | classification |
| Insufficient air renewal | above 1,500 ppm | classification |
This table does not apply to housing, and that is exactly what makes it useful in a client conversation: it gives you a public reference to point at, without ever letting the client believe a regulatory threshold applies at home. The protocol is as instructive as the thresholds, since it makes clear that a reading taken outside the heating season, in an empty room, or over ten minutes, means nothing.
CO₂ and humidity: what the sensor doesn't tell you, and what it helps you anticipate
A CO2 sensor measures neither humidity, nor VOCs, nor particulates. But a sustained rise usually signals heavy occupancy, and therefore more water vapour. If CO2 stays high after a shower or cooking, anticipate a risk of condensation or lingering odours. Adjust the flow rates and check the actual negative pressure room by room, instead of raising the thresholds.
Choosing the right CO₂ sensor to drive mechanical ventilation
NDIR, accuracy and drift: criteria the regulation already quantifies
To drive ventilation on air quality, take an NDIR sensor (non-dispersive infrared), the technology the order retains for regulatory measurement. Use that text's requirements as a purchasing grid even outside public premises: a range of at least 0 to 5,000 ppm, a stated uncertainty no worse than ± (50 ppm + 5% of the reading), and a declared measurement method. Also check the response time (T90) and drift handling, with a disable-able ABC function.
Location and installation: where to place the sensor for a reliable reading
For regulatory measurement the order requires the instrument to sit between 1 and 2 metres above floor level, away from air inlets and outlets. The same logic applies in a dwelling. Place the sensor in the breathing zone, away from extract points, windows and any heat source. Avoid corners and airflow paths. In a house, the living room and a bedroom are the most useful spots.
Power, communication and ventilation compatibility: points to check before buying
Check the power supply (often 230 V or 24 V) and the control output. Depending on the unit you will need a 0-10 V output, a relay, or a bus (Modbus, KNX). Confirm compatibility with your unit, the cable length, and the ability to set thresholds and a minimum flow rate so the system never drops below the regulatory floor. For more on on-site checks, see also measuring ventilation flow rate.
Setting up effective CO₂-based control on mechanical ventilation
Step-based control or modulation: which setting for which home
Two logics exist. With step control, the unit switches between low and high speed when CO2 crosses a threshold, which suits two-speed units. With modulation, the flow rate varies continuously, more interesting in a well-sealed house and with heat-recovery ventilation. In both cases the control works above the permanent flow rate, never below it.
Parameters to set: CO₂ thresholds, delays, minimum and maximum flow rates
Set a trigger threshold and a return threshold, keeping enough separation to avoid hunting. Add a delay to smooth the reading. The non-negotiable point is the minimum flow rate: it follows the table in the order of 24 March 1982, not a comfort figure. The maximum follows acoustics and what the ductwork takes without whistling.
Practical cases: humidity-controlled and self-regulating single-flow, heat-recovery ventilation
With humidity-controlled systems, CO2 acts as an occasional boost to high speed, fine modulation staying driven by humidity. With self-regulating systems it is the same pattern, without fine variation. With heat-recovery ventilation, modulation on CO2 works well in the living room and bedrooms, with bypass and balancing checked. For more on this type of setup, see also key installation points for heat-recovery ventilation.
Checking and troubleshooting: when ventilation doesn't respond to CO₂
Common symptoms: over-ventilation, under-ventilation, noise, discomfort
When CO2-based control goes wrong, the most visible sign is over-ventilation, with continuously high flow, noise and draughts, until the occupant switches the system off. Conversely, under-ventilation lets CO2 climb despite occupancy. In both cases the cause is more often a false reading than a bad setting, and that is where to start.
On-site checks: measurement, calibration, wiring, air inlets, ductwork leaks
Before touching the settings, check the measurement. A sensor that is poorly placed or poorly calibrated is enough to throw off the whole control loop.
- Compare the reading with a second instrument, then measure in outdoor air to obtain a low reference point.
- Check the location, between 1 and 2 m above floor level, away from the kitchen and away from supply air.
- Check the power supply and wiring (0-10 V, dry contact, Modbus) and the correct input assignment.
- Inspect air inlets, extract points, filters, and dampers. Nothing should be obstructed.
- Track down ductwork leaks and disconnections. A parasitic air intake changes the flow rates.
Fine-tuning tips: balancing, filtering, settings to stabilise the control
Once the measurement is reliable, balance the flow rates to nominal and record them. Then set the regulatory minimum, a realistic maximum, and add a delay or measurement smoothing to avoid oscillations. A gradual, room-by-room adjustment stabilises the result without running the fan flat out. For more on how ventilation affects rest, see also comfort and sleep quality.
Client pitch and compliance in 2026: selling CO₂-driven ventilation
What you are actually selling, and what you are not
You are selling control that adapts the flow rate to occupancy above the regulatory floor. You are not selling compliance: that comes from the flow rates in the order of 24 March 1982, measured and recorded. Setting that boundary on the quote heads off the most common dispute on this measure, from the client who understood they were buying an air quality guarantee. That boundary is drawn in black and white in the costing: Argile describes each item line by line on the quote, with the mandatory wording already drafted.
Traceability and evidence: how to present CO₂ readings to the client
Give dated proof. A seven-day export, before and after commissioning, with the sensor reference, its position, and the date of its last outdoor-air check. Use the classes from the order of 27 December 2022 as a reading grid, while stating that they govern premises open to the public. Keep those records in the job file: that is what turns a setting into evidence. That evidence does not have to go back out as an attachment: with Argile, the client finds the project documents and the follow-up of the installation in a dedicated space.
Points of caution: maintenance, filters, responsibilities, limits of the control
Compliance rests on a baseline of permanent ventilation. Control adjusts around it, it does not replace the minimum flow rates. Plan for regular maintenance, filters, extract points and sensor checks, then set out in writing who is responsible for it. And remember the limit: CO2 speaks to air renewal, not to humidity, nor to VOCs or radon.
