Demand-controlled ventilation (DCV) adjusts fresh air supply to match actual occupancy by measuring CO2 concentration as a proxy for human presence and metabolic activity. EN 15251 (superseded by EN 16798-1 in 2019) defines indoor air quality categories: Category I (IEQ Level 1) = 550ppm above outdoor (~950ppm total); Category II (standard offices, schools) = 800ppm above outdoor (~1150ppm total); Category III (acceptable, industrial) = 1350ppm above outdoor. KNX DCV connects CO2 sensors to variable-speed fans via ETS6 logic and Modbus gateways, delivering up to 40% fan energy savings versus constant-volume systems while maintaining EN 16798-1 compliance.

KNX CO2 sensors — hardware options

Elsner CO2-AD1 KNX: flush wall-mount (55×55mm frame), NDIR (Non-Dispersive Infrared) CO2 measurement (0-5000ppm, ±50ppm accuracy), temperature sensor (±0.5°C), relative humidity. Native KNX TP connection (BCU integrated). ETS6 objects: CO2 ppm (DPT 9.008), temperature (DPT 9.001), humidity (DPT 9.007), CO2 alarm thresholds (DPT 1.005). MDT SCN-CO2W12.01: similar specification, 0-2000ppm or 0-5000ppm selectable, compatible with MDT Room Thermostat for combined CO2/temp display. ABB KNX CO2 sensor (SAM/A 5.10.1): panel-mount version for mechanical ventilation rooms where wall aesthetics are secondary. NDIR vs electrochemical sensors: NDIR (all KNX CO2 sensors above) is the correct technology for building ventilation — stable measurement, no oxygen dependency, 10-year lifetime without sensor replacement. Electrochemical CO2 sensors (used in some cheap IoT devices) drift over months and are unsuitable for HVAC control.

ETS6 CO2 sensor configuration

Elsner CO2-AD1 ETS6 parameters: Measurement range: 0-5000ppm (for fresh air building automation; 0-2000ppm if range not exceeded). Alarm threshold 1: 1000ppm (EN 15251 Category II standard office threshold). Alarm threshold 2: 1500ppm (elevated ventilation urgency). Telegram send: on change of 50ppm OR cyclic every 300 seconds (heartbeat). CO2 DPT: 9.008 (ppm as 2-byte float). Group address: GA 4/0/1 = living room CO2, GA 4/0/2 = bedroom CO2, GA 4/0/3 = office CO2. Calibration: initial factory calibration. Field recalibration: ABC (Automatic Baseline Calibration) assumes sensor periodically reaches fresh outdoor air CO2 levels (~420ppm) and calibrates minimum accordingly — recommended for residential. Manual calibration: inject known-concentration gas (optional for laboratory accuracy; not required for HVAC control).

ETS6 logic for fan speed modulation

ETS6 PI logic block (available in ABB and MDT ETS6 applications): receives CO2 ppm input, outputs 0-100% fan speed setpoint. Configuration: input = GA 4/0/3 CO2 value DPT 9.008; setpoint = 800ppm (Category II target); output = DPT 5.001 (0-255 = 0-100%) on GA 4/1/0 fan speed command. PI parameters: P-gain = 5, I-time = 120 seconds (slow response appropriate for slow-changing CO2 concentration). Output limits: minimum speed = 30% (minimum ventilation for building pressurisation and moisture control even at CO2 = 400ppm); maximum = 100%. At CO2 = 800ppm → output = 30% (minimum). At CO2 = 1200ppm → output = 80%. At CO2 > 1500ppm → output = 100% (alarm + maximum ventilation). Alternative: simple stepped logic (no PI block needed for smaller systems): CO2 < 700ppm → fan at 20%; 700-900ppm → 50%; 900-1200ppm → 80%; > 1200ppm → 100% (ETS6 threshold function objects).

Multiple zone aggregation logic

For commercial office floor with 8 CO2 zones: maximum aggregation strategy — KNX ETS6 Maximum function (MDT or ABB ETS6 logic object) takes 8 × DPT 9.008 CO2 inputs, outputs single maximum value → PI controller → single VFD fan speed command. Alternative: per-zone VAV damper control — each zone has independent KNX VAV damper actuator (Belimo NM24A-KNX, 0-100% position DPT 5.001) driven by zone CO2 PI; shared VFD maintains duct static pressure at setpoint (250Pa) measured by pressure differential sensor (0-1000Pa, 0-10V → KNX 0-10V input module). Duct pressure KNX input: Elsner FT-100 pressure sensor (0-10V) → KNX 0-10V input module (MDT SCN-ISAP.01) → DPT 5.001 pressure % → ETS6 PI loop → VFD speed via Modbus.

VFD integration — Modbus gateway

Intesis IN70xKNX Modbus RTU/TCP to KNX gateway: MAPS configuration: Signal "Fan Speed Command", KNX GA 4/1/0 (DPT 5.001), Modbus holding register 40001 (ABB ACH580 process reference register, 0-20000), scaling 0-255 → 0-20000 (multiply by 78.4). KNX writes DPT 5.001 value 200 (=78.4%) → Intesis writes Modbus register 40001 = 15680 → ACH580 runs at 78.4% speed → supply air fan at ~78Hz. VFD speed feedback: Modbus register 40003 (ACH580 process output, 0-20000) → scale to DPT 5.001 → KNX GA 4/1/1 actual fan speed status. WAGO 750-893 CODESYS alternative: Modbus master function block writes process reference register in SFC cycle; reads speed feedback every 2 seconds; KNX TP object reads/writes via KNX object library.

Energy savings and compliance

EN 16798-1 (replacing EN 15251) requirement for DCV: CO2 sensor-based control is explicitly recognised as a valid method for verifying occupancy-related ventilation rates. BREEAM HEA 02 (Indoor air quality): CO2 monitoring required for credits; DCV with CO2 sensors earns 1-2 BREEAM credits. LEED v4 EQ Prerequisite: minimum air quality with CO2 monitoring for densely occupied spaces (>25 people per 1000 sq ft). Fan energy savings: VAV fan follows cubic affinity law (power ∝ speed³). Average office DCV scenario: 8h occupied at average 65% speed (CO2 averages 900ppm) + 16h unoccupied at 20% (minimum ventilation). Annual saving vs CAV at 100%: (1 - 0.65³) × 8h + (1 - 0.20³) × 16h = (1 - 0.274) × 8h + (1 - 0.008) × 16h. Fan energy saving ≈ 68% vs 24h CAV at 100%. For 11kW supply fan: annual saving = €3,900/year at €0.20/kWh. CO2 sensor payback: each Elsner CO2-AD1 KNX ≈ €180, VFD ≈ €800 — payback from fan energy saving alone: under 6 months.