Indoor air quality (IAQ) directly affects occupant cognitive performance, comfort, and health. EN 16798-1 defines IAQ categories: Category I (hospitals, schools, sensitive occupants) requires CO2 ≤ 800 ppm above outdoor; Category II (offices, general use) targets CO2 ≤ 1220 ppm. CO2-driven demand-controlled ventilation (DCV) is one of the highest-ROI applications of KNX building automation — studies consistently show 30–50% fan energy savings compared to fixed-schedule ventilation, while maintaining better air quality during actual occupancy peaks. This guide covers Elsner CO2 sensor integration, 3-level KNX ventilation logic, DALI colour temperature indicators, and Zehnder HRV Modbus fan speed control.

Elsner CO2-T-UP KNX sensor configuration

The Elsner CO2-T-UP KNX sensor (flush-mounted KNX device) measures CO2 concentration via NDIR (non-dispersive infrared) principle with automatic baseline calibration (ABC logic, 14-day cycle). Group address assignments in ETS6: GA 5/2/0 — CO2 concentration DPT 9.008 (ppm, sent every 60 seconds and on change > 50 ppm). GA 5/2/1 — Room temperature DPT 9.001 (°C). GA 5/2/2 — Relative humidity DPT 9.007 (%). The sensor also supports a local alarm output (configurable threshold, DPT 1.005). For open-plan offices with multiple CO2 sources, deploy multiple sensors and average their readings. KNX logic module averaging: sum of all CO2 GA values ÷ number of sensors. Example for 4-sensor open plan: (GA 5/2/0 + GA 5/3/0 + GA 5/4/0 + GA 5/5/0) ÷ 4 → averaged value to GA 5/6/0 (master CO2 input for ventilation logic). Calibration: mount sensors at breathing height (1.1–1.5m), away from supply air diffusers (minimum 1m) and windows. Factory calibration accuracy: ±50 ppm or ±3% of reading, whichever is greater. Annual field calibration check recommended using certified reference gas (1000 ppm CO2 in nitrogen cylinder).

3-level ventilation control logic with hysteresis

KNX logic module (ABB CL/KNX or IP-Symcon rule) implements 3-level DCV based on master CO2 GA 5/6/0: Level 1 (Basic): CO2 ≤ 800 ppm → write value 1 to ventilation mode GA 5/0/0 (minimum airflow, approximately 30% nominal). Level 2 (Normal): CO2 800–1200 ppm → write value 3 to GA 5/0/0 (nominal design airflow, 100%). Level 3 (Boost): CO2 > 1200 ppm → write value 4 to GA 5/0/0 (maximum airflow, 130% with EC motor). Hysteresis to prevent rapid switching: upward transitions use the threshold as defined. Downward transitions apply a 100 ppm offset: Level 3 → Level 2 only when CO2 drops below 1100 ppm (not 1200). Level 2 → Level 1 only when CO2 drops below 700 ppm (not 800). Minimum dwell timer: once boost (Level 3) activates, hold for minimum 15 minutes before reverting to Level 2, regardless of CO2 reading. This protects HRV fan motor from rapid cycling and ensures the elevated CO2 is genuinely cleared before reducing airflow.

DALI colour temperature indicator integration

DALI DT8 (Tunable White, Device Type 8) luminaires with Tc (colour temperature) control serve as a real-time CO2 indicator visible to occupants without requiring a dashboard or app. Configuration: a KNX-DALI gateway (e.g., GIRA DALI gateway, ABB DA/S) drives DT8 luminaires on a dedicated DALI group (Group 1 = IAQ indicators, typically a perimeter strip or ceiling accent). KNX logic maps CO2 level to colour temperature: CO2 ≤ 800 ppm → DALI Tc command 4000K (neutral white — "good air quality"). CO2 800–1200 ppm → Tc 3000K (warm amber — "ventilation active"). CO2 > 1200 ppm → Tc 2700K (warm red/amber — "boost active, please open window if boost insufficient"). The KNX group address for DALI colour temperature is GA 4/5/0 DPT 9.001 (Kelvin value, sent to KNX-DALI gateway). The DALI gateway translates Kelvin value to DALI DT8 Tc command (DALI range 0–65535 maps to physical Tc range). Transition time: configure DALI fade time = 10 seconds for smooth colour transition that is visible but not distracting.

Alarm, energy saving quantification and commissioning

CO2 alarm GA: when CO2 > 1500 ppm (override threshold for crowded meeting rooms or school classrooms), GA 5/7/0 DPT 1.005 = 1. This triggers: KNX push notification (ARISTO BewO → SMS to FM team), DALI luminaire GA 4/5/1 DPT 1.001 = 1 (alarm flash mode at 1 Hz for 30 seconds to alert occupants). Energy saving quantification: a fixed-schedule ventilation system running Level 3 (maximum) 24/7 consumes fan energy E_fixed = P_max × 8760 hours. With DCV, average fan power is approximately 45% of maximum (mix of Level 1 and Level 2 during typical office occupancy). Annual savings: (P_max − 0.45 × P_max) × 8760 = 55% fan energy reduction. For a 10,000 m² office with 18.5 kW total HRV fan power: annual saving ≈ 55% × 18.5 kW × 8760h = 89,000 kWh/year (approximately €13,500/year at €0.15/kWh). Commissioning and calibration procedure: (1) Power up CO2 sensors minimum 24 hours before commissioning for ABC calibration initialisation. (2) In a ventilated space with known CO2 (outdoor: ~415 ppm), verify sensor reading is within ±100 ppm. (3) Simulate CO2 buildup: seal room with 4 people for 30 minutes → verify logic transitions through Levels 1, 2, 3 sequentially as CO2 rises → observe DALI colour shift from 4000K to 2700K. (4) Verify Zehnder HRV Modbus register 235 changes with each level command. (5) Check alarm GA 5/7/0 activates at 1500 ppm and push notification is received within 60 seconds.