Demand-Controlled Ventilation with KNX: CO2 Logic, VFD Fan Control and EN 16798-1
Demand-controlled ventilation reduces HVAC fan energy 30–50% by varying airflow based on actual occupancy and CO2 concentration rather than the design-maximum occupancy rate that applies for only a fraction of operating hours. KNX provides the integration layer between CO2 sensors, zone VAV dampers, AHU VFDs and the EN 16798-1 Category II indoor air quality target.
DCV principle and EN 16798-1 categories
EN 16798-1 (which replaced EN 15251 in 2019) defines four indoor air quality categories based on CO2 concentration above outdoor level. DCV maintains Category II (the standard design target for offices and schools) by continuously adjusting ventilation rate to match actual CO2 concentration — not design-maximum occupancy.
Constant ventilation baseline
Design-maximum occupancy ventilation runs at full fan speed regardless of actual occupancy. A 30-person meeting room ventilated for 30 people runs at 100% even when only 3 people are present — wasting 90% of fan energy during typical partial-occupancy periods.
DCV improvement
DCV runs at 20–30% minimum airflow when the room is empty or at low occupancy. Fan power follows the cube law: at 40% speed, fan uses only (0.4)³ = 6.4% of full-speed power. Measured energy savings in office buildings: 35–50% of HVAC fan energy over a heating season.
KNX DCV control chain
The KNX DCV control chain links CO2 sensor group addresses through ETS6 logic to AHU VFD speed control via Modbus TCP. Each component in the chain must be correctly configured for the system to maintain EN 16798-1 Category II.
DCV control chain — signal flow
[CO2 sensor NDIR] → KNX TP bus → GA 9/1/room_no (DPT 9.008, ppm) → MDT Logic Module SCN-LOGM.01 (Max aggregation) → GA 9/1/10 (Maximum Zone CO2, DPT 9.008) → MDT Logic Module PI controller → GA 8/0/0 (AHU Fan Speed %, DPT 5.001) → Intesis IN701KNX (KNX/Modbus TCP gateway) → AHU VFD Modbus register 1 (0–20000 = 0–100% speed) → AHU supply fan variable speed Zone VAV parallel path: → GA 9/1/zone (zone CO2) → MDT Logic (zone threshold) → GA 8/2/zone (VAV damper %, DPT 5.001) → KNX 0-10V analogue output → VAV box actuator (Belimo GK230A-SR 0-10V) → Zone supply airflow proportional to CO2
ETS6 PI control logic for DCV
The MDT Logic Module SCN-LOGM.01 PI function block converts CO2 concentration to AHU fan speed percentage. The PI controller maintains CO2 at the setpoint (900 ppm — providing margin below the 1,220 ppm Category II limit) by continuously adjusting fan speed.
MDT Logic Module PI parameters — DCV
Process variable: GA 9/1/10 (max zone CO2, ppm) Setpoint: 900 ppm (target — margin below 1220 ppm) Output GA: 8/0/0 (AHU fan speed %, DPT 5.001) PI parameters: Proportional gain Kp: 0.1 (fan speed % per ppm deviation) Integral time Ti: 300 seconds Output minimum: 20% (minimum outdoor air for CO2 dilution) Output maximum: 100% AHU Modbus scaling (Intesis gateway MAPS): GA 8/0/0 (0–100% DPT 5.001) → Modbus register 1 Scale: 0% → 0, 100% → 20000 (AHU VFD 16-bit register) At CO2 = 900 ppm (setpoint): fan at 20% (minimum) At CO2 = 1100 ppm (+200 ppm): PI output ≈ 40% At CO2 = 1300 ppm (+400 ppm): PI output ≈ 60% At CO2 > 1500 ppm: PI output → 100%
Multi-zone DCV with VAV dampers
In open-plan offices with multiple independently occupied zones, each zone requires its own VAV (Variable Air Volume) box to deliver the correct airflow per zone. The central AHU fan maintains duct static pressure while zone VAV boxes modulate based on their local CO2 reading.
5-zone VAV DCV — ETS6 configuration
Zone VAV boxes: Belimo GK230A-SR (50–200mm duct, 0-10V) → KNX analogue output MDT AMS-0816V.01 (0-10V per zone) Per zone GA mapping: GA 9/1/zone (CO2 ppm) → MDT Logic threshold → GA 8/2/zone (VAV %) CO2 < 700 ppm: VAV at 20% (minimum fresh air) CO2 700–900: VAV 20–50% (linear proportional) CO2 900–1100: VAV 50–80% CO2 > 1100: VAV 100% (maximum zone airflow) Central AHU static pressure control: Duct static pressure sensor (0-10V) → KNX analogue input → GA 8/3/0 (duct static pressure, DPT 9.006, Pa) → MDT Logic PI → GA 8/0/0 (AHU fan speed %) Setpoint: 50Pa duct static pressure AHU fan adjusts speed to maintain 50Pa as VAV dampers open/close All 5 zones satisfied at low CO2: → all VAVs at 20% → duct pressure rises above 50Pa → AHU fan reduces speed → energy saving achieved
Occupancy-CO2 combined logic
Combining PIR presence detection with CO2 control improves DCV response time — CO2 rises 10–15 minutes after people arrive, but presence detection is immediate. The combined logic provides a 5-minute pre-dilution boost at room entry before handing control to CO2-based DCV.
Combined presence + CO2 logic — MDT Logic Module
Input GAs:
GA 7/0/zone (PIR presence, DPT 1.001: 0=vacant, 1=occupied)
GA 9/1/zone (CO2 ppm, DPT 9.008)
Output GA: 8/2/zone (VAV damper %, DPT 5.001)
Logic states:
Presence = 0 AND 30 min elapsed → VAV at 20% (standby)
Presence = 1 (first entry) → VAV at 50% for 5 minutes (pre-boost)
After 5 min: CO2 PI takes over
CO2 < 700 ppm: VAV at 30% (low occupancy confirmed)
CO2 700–1000 ppm: VAV PI control 30–80%
CO2 > 1000 ppm: VAV 80–100% (high occupancy)
Presence = 0 for 30 min: → VAV ramps to 20% minimum
Benefits:
- Immediate pre-dilution prevents CO2 spike at meeting start
- Standby mode when room vacated — energy saving
- CO2 control maintains EN 16798-1 during occupationEN 16798-1 Category II verification test
Verify DCV performance with a real occupancy test before building handover. Document results for BREEAM and LEED assessors.
DCV functional test — meeting room 30m², 8 occupants
Test conditions: Windows closed, HVAC at design conditions 8 people enter meeting room simultaneously Monitor via ETS6 Group Monitor: GA 9/1/zone (CO2) Expected behaviour: T+0 min: CO2 = 420 ppm (outdoor level, room pre-diluted) T+5 min: CO2 rises to ~600 ppm (pre-boost active: VAV 50%) T+10 min: CO2 reaches ~800 ppm → DCV PI increases VAV T+20 min: CO2 stabilises at 900–1100 ppm (Category II) T+30 min: CO2 ≤ 1220 ppm confirmed → Category II maintained Pass criterion: CO2 does not exceed 1220 ppm (Category II) at any point during 30-minute occupied period with 8 occupants Document: CO2 log CSV from ARISTO, peak CO2 value, time to stabilise. Submit to BREEAM/LEED assessor.
AHU Modbus TCP integration
Central air handling units from Swegon GOLD, Robatherm, Klingenburg and similar manufacturers provide Modbus TCP or BACnet/IP interfaces for KNX integration. The Intesis IN701KNX gateway bridges KNX group addresses to AHU Modbus registers.
Intesis IN701KNX — AHU Modbus TCP signal table
Signal Register Type DPT GA Description 1 1 uint16 5.001 8/0/0 Fan speed setpoint (0–100%) 2 3 uint16 5.001 8/0/1 Fan actual speed (read-only) 3 5 uint16 9.001 8/0/2 Supply air temperature (°C) 4 10 bit 1.001 8/0/3 Filter alarm (0/1) 5 15 uint16 9.006 8/0/4 Duct static pressure (Pa) Register scaling: Register 1 write: 0% → 0, 100% → 20000 (16-bit scale) Register 3 read: same scale, divide by 200 for % BACnet/IP alternative (some AHUs): Use Intesis IN701KNX BACnet/IP variant Object: Analog Value 1 (fan speed setpoint, 0–100%) Same KNX GA mapping — only gateway firmware differs
Energy savings calculation
DCV energy savings follow the fan affinity law: fan power is proportional to the cube of fan speed. Operating at 40% of design speed uses only 6.4% of full-speed power — the savings are substantial even at moderate speed reductions.
Meeting room DCV energy savings — worked example
Baseline (constant ventilation at design rate, 10 ACH): Fan motor: 800W (3-phase, 15Pa external duct pressure) Annual operating hours: 3,000 h Annual energy: 800W × 3000h = 2,400 kWh DCV (average 40% fan speed during occupied hours): Fan power at 40% speed: 800W × (0.40)³ = 51W Annual energy: 51W × 3000h = 153 kWh Annual saving: 2,400 − 153 = 2,247 kWh At €0.20/kWh: €449/year per meeting room 10 meeting rooms: €4,490/year DCV implementation cost (sensors + logic): ~€3,000 Simple payback: 3000 / 4490 = 0.67 years (< 1 year) Include in LEED EAc1 (Energy and Atmosphere): submit DCV design airflow data and logged actual airflow data to demonstrate measured energy savings
Commissioning and airflow balancing
Commission DCV logic after CO2 sensors are installed and verified. Test the entire control chain from sensor to VFD before live occupancy testing.
| Step | Action | Pass criterion |
|---|---|---|
| 1 | Set all zone CO2 GAs to 1200 ppm in ETS6 Group Monitor (simulate high CO2) | AHU fan increases to 80–100% speed within PI response time |
| 2 | Set all CO2 GAs to 400 ppm (simulate empty building) | AHU reduces to 20% minimum speed within 5 minutes |
| 3 | Test each VAV damper: set zone CO2 to 600 ppm → check damper position | Damper at approximately 30% open; measure airflow at diffusers with anemometer |
| 4 | Airflow balance: compare measured airflow to design values | Measured flow within ±20% of design; adjust VAV calibration if needed |
| 5 | Real occupancy test (8 people, 30 min, closed windows) | CO2 does not exceed 1220 ppm (Category II); log GA values |
Need KNX DCV logic and AHU integration commissioned?
We design and commission KNX demand-controlled ventilation systems — ETS6 PI logic, Modbus TCP gateway configuration, VAV damper calibration and EN 16798-1 Category II verification with BREEAM and LEED documentation.
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