CO2 Sensor in BMS

The Founder's Explanation

"CO2 — if CO2 level increases, fresh air intake happens."

Human Analogy

You are sitting in a meeting room. 20 people. Windows closed. Air conditioning running but no fresh air. After 45 minutes — you feel sleepy. You cannot concentrate. Someone suggests "take a 5-minute break." Everyone steps outside. Immediately you feel alert again.

What happened inside? Everyone was exhaling CO2. The concentration built up slowly. Above 1000 ppm — human performance degrades. Above 2000 ppm — headaches, significant loss of concentration. You felt it without knowing the number.

The CO2 sensor prevents this automatically.

How CO2 Sensor Works in BMS

CO2 sensor in AHU return air duct
(return air carries the air from all rooms on the floor)
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Measures CO2 concentration in ppm (parts per million)
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Below 800 ppm: fresh air damper at minimum position (save energy)
800–1000 ppm: fresh air damper opens more
Above 1000 ppm: fresh air damper opens significantly
Above 1200 ppm: alarm raised, maximum fresh air
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Fresh outside air dilutes CO2
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CO2 drops back to acceptable level
        ↓
Damper returns to minimum (energy saved again)

This is Demand Control Ventilation (DCV) — the building breathes exactly as much fresh air as the occupants need. Not less (people suffer), not more (energy wasted).

CO2 Levels Reference:

Outside air:        400 ppm (clean outdoor air)
Good indoor air:    600–800 ppm
Acceptable:         800–1000 ppm
Noticeable effect:  1000–1200 ppm (slightly drowsy)
Poor air quality:   1200–2000 ppm (significant impact on focus)
Unacceptable:       Above 2000 ppm (health concern)

CO2 Sensor Technology — NDIR:

NDIR = Non-Dispersive Infrared. CO2 molecules absorb infrared light at a specific wavelength. The sensor shines infrared light through the air sample and measures how much is absorbed — more absorption means more CO2.

No chemical reaction. No consumables. Long life — typically 10–15 years before replacement.

Common Mistake:
Placing CO2 sensor near the supply air diffuser — reads fresh diluted air not the actual room air. Must be in return air duct or in the occupied zone away from supply air.

Memory Hook:

CO2 rises → people exhale more than fresh air replaces → drowsy people
CO2 sensor detects → BMS opens fresh air damper → alert people