CO2 Sensors and KNX: Elsner, MDT and Theben Sensor Selection and ETS6 Commissioning

Why CO2 monitoring matters

ASHRAE 62.1 and EN 16798-1 define acceptable indoor CO2 as no more than 550 ppm above outdoor level. With outdoor CO2 at approximately 420 ppm, the Category II indoor target is 970 ppm (420 + 550). Harvard T.H. Chan School of Public Health research found that CO2 concentrations above 1,200 ppm reduce cognitive performance by 15–30% in tasks requiring complex decision-making.

KNX CO2 measurement feeds directly into demand-controlled ventilation logic — the sensor group address drives the DCV function block in ETS6, which adjusts AHU fan speed and VAV damper positions to maintain Category II compliance while minimising fan energy consumption.

CO2 sensor technology: NDIR only

Non-Dispersive Infrared (NDIR) is the only CO2 measurement technology suitable for building automation. NDIR measures CO2 absorption of infrared light at the CO2-specific wavelength of 4.26 micrometres — a direct, specific measurement unaffected by other gases.

Elsner S1-B KNX CO2 sensor

The Elsner S1-B KNX is a flush-mount combined CO2, temperature and relative humidity sensor powered directly from the KNX TP bus — no separate local power supply required. Suitable for all standard back-box depths.

Group object    DPT         GA              Description
CO2 ppm        9.008       9/1/room_no     CO2 concentration (ppm)
Temperature    9.001       9/2/room_no     Room temperature (°C)
RH             9.007       9/3/room_no     Relative humidity (%)
CO2 alarm      1.001       9/4/room_no     CO2 alarm (1=alarm)

ETS6 parameters:
  CO2 send interval:   60 seconds (default)
  CO2 alarm threshold: 1000 ppm (configurable 500–5000)
  Temperature offset:  0.0°C (adjust after on-site check)

Bus power consumption: 3mA (KNX TP bus powered)
No local PSU required — standard KNX power budget applies
(MDT 640mA PSU: max 200 devices including this sensor)

MDT SCN-CO2W.01 with LED display

The MDT SCN-CO2W.01 adds a built-in display and LED colour indicator not present on the Elsner S1-B — making it preferable for open-plan offices where occupants benefit from real-time air quality feedback.

Theben AMUN 716 KNX ceiling sensor

The Theben AMUN 716 mounts in a ceiling junction box — suitable for open-plan offices where wall-mount sensors are impractical due to lack of partition walls. Ceiling mounting provides an average room CO2 measurement across the entire occupied space rather than a localised wall reading.

Mounting: ceiling junction box (standard 68mm depth)
Cable:    YCYM KNX TP to ceiling junction box
          (green/yellow TP bus pair + 0.5mm earth)

ETS6 application: Theben AMUN 716 v1.0
Group objects: CO2 ppm (DPT 9.008), Temperature (DPT 9.001)
               CO2 alarm (DPT 1.001)

Coverage: one sensor per 50m² open-plan area
Ceiling height: optimal at 2.4–3.0m (CO2 stratification
negligible at standard ceiling heights)

For rooms with natural ventilation (operable windows):
  Ceiling sensor averages across room — preferred over
  wall sensor near window which reads outdoor CO2 when open

Sensor placement guidelines

Correct sensor placement is as important as sensor technology selection. An accurately measuring sensor placed in the wrong location will drive the DCV system incorrectly — the sensor must measure the CO2 concentration that occupants actually breathe.

Calibration and maintenance

Verify each sensor on installation day using a portable NDIR reference meter (Testo 535 or equivalent, NIST-traceable calibration). Measure the same location within 30 seconds — readings should agree within 100 ppm. Document the verification in the commissioning record.

Equipment required:
  Certified CO2 reference gas cylinder (1000 ppm CO2,
  NIST-traceable, certificate of analysis)
  Calibration adapter (fits over sensor diffuser)

Procedure:
  1. Connect calibration adapter to sensor
  2. Flow reference gas at 0.5 L/min for 3 minutes
  3. Read GA 9/1/room_no in ETS6 Group Monitor
  4. Compare to certified concentration (1000 ppm ±5 ppm)
  5. If deviation > 100 ppm: adjust ETS6 CO2 offset parameter
  6. Repeat gas flow and verify offset correction applied

If sensor cannot be corrected to ±100 ppm:
  Replace sensor — NDIR source has degraded
  Replacement interval: 10–15 years typical

Multiple zone CO2 aggregation

For central air handling units serving multiple zones, the AHU fan speed must respond to the most demanding zone — the zone with the highest CO2. ETS6 MDT Logic Module calculates the maximum CO2 reading across all zones.

Input GAs: 9/1/0 through 9/1/7 (8 zones, CO2 ppm, DPT 9.008)
Output GA: 9/1/10 (Maximum Zone CO2, DPT 9.008)

MDT Logic Module SCN-LOGM.01 Max function:
  Output = Max(9/1/0, 9/1/1, 9/1/2, 9/1/3, 9/1/4,
               9/1/5, 9/1/6, 9/1/7)
  → GA 9/1/10

GA 9/1/10 → DCV function block → AHU fan speed

Alternative (zone DCV with VAV):
  Each zone CO2 GA → individual VAV damper actuator (0-10V)
  Central AHU fan tracks duct static pressure (constant
  pressure reset strategy — maintains 50Pa duct pressure
  regardless of individual VAV damper positions)

BREEAM Hea 02 documentation

BREEAM UK NC 2018 credit Hea 02 requires CO2 monitoring and alarm capability in occupied rooms. Demonstrating continuous Category II compliance can also contribute to an Innovation credit. Prepare the following evidence for the BREEAM assessor.