CO₂ sensor: driving mechanical ventilation by air quality in 2026

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 properly renewed, it drops back toward the outdoor value. It's a simple indicator, useful on site to check whether the mechanical ventilation and air inlets are bringing in enough fresh air, without getting lost in calculations.

CO₂ values to know on site: simple benchmarks and limits

Practical benchmarks. Outdoors, you're usually around 400 to 450 ppm. In an occupied home, ideally aim for under 800 to 1,000 ppm. Above 1,500 ppm, the air is often perceived as stuffy. Beyond 2,000 ppm, you're in fast-alert mode and need to find the cause (flow rates, vents, poorly sized rooms).

CO₂ and humidity: what the sensor doesn't tell you, and what it helps you anticipate

A CO₂ sensor doesn't measure humidity, VOCs, or particulates. But a sustained rise in CO₂ often signals heavy occupancy, and therefore more water vapour. If CO₂ stays high after a shower or cooking, you can anticipate a risk of condensation, mould or odours. Adjust the flow rates, and check the actual negative pressure room by room.

Choosing the right CO₂ sensor to drive mechanical ventilation

NDIR, accuracy and drift: concrete criteria to avoid bad surprises

To drive mechanical ventilation on air quality, favour an NDIR sensor (infrared). Look for a stated accuracy of around ±(30 to 50 ppm + 3% of reading) and check how drift is handled. A model with auto-calibration (ABC) or manual calibration prevents the CO₂ reading from "creeping up" on its own after a few months. Also check the response time (T90) and the measurement range, often 400 to 2,000 or 5,000 ppm.

Location and installation: where to place the sensor for a reliable reading

Install the sensor in the breathing zone. Generally 1.1 to 1.7 m above the floor, away from extract vents, 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 system, you'll 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 to avoid under-ventilation. 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-based control, the unit switches between low and high speed when CO₂ crosses a threshold — suited to simple homes and 2-speed units. With modulation, the flow rate varies continuously, more comfortable in a well-sealed house and with heat-recovery ventilation.

Parameters to set: CO₂ thresholds, delays, minimum and maximum flow rates

Set a trigger threshold and a return threshold. Aim for a level that avoids staying above 1,000 ppm for long during occupancy. Add a delay (5 to 20 min) to avoid yo-yoing. Set a minimum hygienic flow rate, then a maximum that stays acoustically acceptable.

Practical cases: humidity-controlled and self-regulating single-flow, heat-recovery ventilation

With humidity-controlled systems, CO₂ mainly serves as an occasional "boost" to high speed. With self-regulating systems, it's the same — you don't get fine variation. With heat-recovery ventilation, modulation based on CO₂ works well in the living room and bedrooms, with the 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 (continuously high flow, noise, air too dry, draughts). Conversely, under-ventilation lets CO₂ climb despite occupancy. The result: lingering odours, condensation, a feeling of stuffy air, and sometimes yo-yoing flow rates.

On-site checks: measurement, calibration, wiring, air inlets, ductwork leaks

Before touching the settings, check the measurement. A CO₂ sensor that's poorly placed or poorly calibrated is enough to throw off the whole control loop.

• Compare the reading with a second device, then measure in outdoor air.
• Check the location. Away from the kitchen, away from supply air, at breathing height.
• Check the power supply and wiring (0-10 V, dry contact, Modbus) and the correct input assignment.
• Inspect air inlets, vents, 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. Then set a minimum ventilation level, a realistic maximum, and add a delay or measurement smoothing to avoid oscillations. A gradual, room-by-room adjustment stabilises comfort without running the fan flat out. For more on how ventilation affects wellbeing, see also comfort and sleep quality.

Client pitch and compliance in 2026: selling CO₂-driven ventilation

Easy benefits to explain: healthier air, comfort, controlled consumption

You're selling ventilation that adapts to occupancy. When the CO₂ sensor detects a rise, the flow rate increases. The result: healthier air, fewer odours, and less of that "stuffy home" feeling. And when the house is empty, you avoid ventilating for nothing.

Traceability and evidence: how to present CO₂ readings to the client

Give simple proof. A screenshot or a 7-day export, before and after adjustments. Explain an understandable benchmark. In normal use, the aim is to stay mostly under 1,000 ppm during occupancy. Keep these readings in the job file. It's useful if there's ever a question or an inspection.

Points of caution: maintenance, filters, responsibilities, limits of the control

Compliance rests on a baseline of permanent ventilation. Control adjusts around it — it doesn't replace the minimum flow rates. Plan for regular maintenance: filters, vents, and sensor checks. Clarify who does what between you and the occupant. And remember the limit. CO₂ speaks to ventilation, not to humidity, VOCs or radon.