Demand-Controlled Ventilation with KNX CO2 Sensors: IAQ and Energy Savings

Why DCV: the case for CO2-based ventilation control

Traditional ventilation systems are sized for maximum design occupancy and run at fixed speed regardless of actual occupancy. A meeting room designed for 20 people runs the same ventilation flow at 06:30 with zero occupants as at peak occupancy at 10:00. Fan power scales with the cube of air velocity — halving the flow reduces fan power to 12.5% of maximum. Even modest flow reductions deliver large energy savings.

CO2 is the primary indicator of human occupancy. Outdoor air is approximately 400-420 ppm CO2. Each person generates approximately 20 l/min of CO2 at rest (office work). As occupancy increases, CO2 rises above outdoor background. Maintaining CO2 below 1000 ppm above outdoor levels ensures adequate fresh air per person per EN 13779. KNX CO2 sensors provide continuous ppm measurement that directly drives ventilation modulation.

KNX CO2 sensors: product comparison

Several KNX-native CO2 sensors are available for flush-mount, DIN rail, and panel installation. All transmit CO2 concentration as DPT 9.008 (ppm) on KNX TP. Calibration method, measurement range, and additional parameters differ.

DCV logic: four-threshold ventilation control

The DCV logic maps CO2 concentration to ventilation flow rate using four thresholds aligned with EN 13779 indoor air quality categories. Hysteresis between thresholds (a lower switch-down point than the switch-up point) prevents rapid oscillation of fan speed when CO2 hovers near a threshold.

Input: KNX CO2 sensor GA, DPT 9.008 (ppm)
Output: ventilation speed command GA, DPT 5.001 (0-100%)

Threshold table (with 50 ppm hysteresis):
  CO2 < 700 ppm (empty/outdoor level):
    → Ventilation: 20% speed (minimum, code-minimum airflow)

  CO2 700-1000 ppm (low occupancy):
    → Ventilation: 50% speed (moderate flow)
    → Switch down at 650 ppm (hysteresis to 20% level)

  CO2 1000-1400 ppm (medium-high occupancy):
    → Ventilation: 80% speed
    → Switch down at 950 ppm

  CO2 > 1400 ppm (overcrowded / alarm):
    → Ventilation: 100% speed (maximum boost)
    → KNX alarm telegram GA = ON → building management alert
    → Switch down at 1350 ppm

ETS6 logic block (MDT SCN-LCRM.01):
  Comparator 1: input A (ppm) < 700 → output = 20
  Comparator 2: input A >= 700 AND < 1000 → output = 50
  Comparator 3: input A >= 1000 AND < 1400 → output = 80
  Comparator 4: input A >= 1400 → output = 100
  Output written to GA (DPT 5.001) → Systemair SAVE or TROX TVZ

EN 13779 CO2 categories and compliance targets

EN 13779:2007 (Ventilation for non-residential buildings) defines Indoor Air Quality categories IDA 1 through IDA 4 based on CO2 concentration above outdoor reference (typically 400 ppm). The DCV system should target IDA 2 during occupied hours as a minimum — this equates to maintaining CO2 below 800-1000 ppm above outdoor level, or approximately 1200-1400 ppm absolute in most European climates.

Night purge sequence and morning pre-conditioning

During unoccupied hours, CO2 gradually decreases to outdoor levels and the DCV logic reduces ventilation to minimum (20% speed). However, VOCs (volatile organic compounds) from cleaning products, furniture off-gassing, and building materials accumulate overnight even without occupants. A night purge sequence runs the ventilation system at 100% for 30 minutes before the scheduled occupancy start, flushing overnight VOC and CO2 accumulation before occupants arrive.

KNX time switch (MDT SCN-LCRM.01 weekly timer):

Night minimum (23:00 → occupancy start - 30 min):
  Write ventilation speed GA = 20 (20% minimum)
  DCV CO2 logic disabled — override by time switch
  Exception: if CO2 rises above 1000 ppm (unexpected night use)
    → DCV logic resumes and overrides night minimum

Night purge (06:00, 30 min before 06:30 occupancy start):
  Write ventilation speed GA = 100 (100% boost)
  Duration: 30 minutes (timer resets at 06:30)
  At 06:30: release override → DCV CO2 logic resumes control

Occupancy period (06:30-22:00):
  DCV logic active: CO2 thresholds control fan speed
  KNX presence detector (MDT PIR) reinforces DCV:
    No presence detected for 30 min → force minimum ventilation
    regardless of CO2 (room may have CO2 from previous occupants)

Weekend / holiday mode:
  KNX calendar block → extend night minimum to full day
  Night purge still runs at 06:00 Saturday/Sunday (HVAC system keep-alive)

BREEAM Hea 02 compliance and energy payback

BREEAM New Construction issue Hea 02 (Indoor Air Quality) awards credits for CO2 monitoring with automated ventilation response and occupant-visible CO2 display. A KNX DCV system with Elsner CO2 sensors, ETS6 logic control, and a KNX touchpanel (Gira X1 or MDT Glastaster II) displaying live CO2 concentration satisfies the monitoring and automated response criteria. Data logging (ARISTO BewO) provides the performance evidence required during BREEAM assessment.

Energy payback for DCV is typically 1-2 years in meeting rooms and conference spaces. A meeting room with maximum design ventilation of 500 m3/h running at an average of 40% actual occupancy saves approximately 60% of maximum fan energy. At 0.15 EUR/kWh and a 1.5 kW fan motor, this equates to approximately 900 kWh/year saved, or 135 EUR/year per room. CO2 sensor cost (Elsner KNX CO2-T-UP) is approximately 180 EUR — payback in 16 months. At scale across 20 meeting rooms, the system pays back in under 12 months excluding installation efficiencies.