TROX TVZ VAV Box BACnet/IP to KNX via LOYTEC LGATE-950

TROX TVZ-D VAV controller: BACnet objects

The TROX TVZ-D volume flow controller mounts in the supply air duct and measures airflow using a differential pressure sensor across an orifice plate. The actuator drives a damper blade to maintain the setpoint flow. BACnet MS/TP communication runs on RS485 at 9600 baud — the same physical layer as Modbus RTU but a different protocol. The BACnet device ID is set via a rotary selector on the front of the unit (range 1-127). Ensure each TVZ on the same MS/TP bus has a unique device ID.

LOYTEC LGATE-950: BACnet to KNX mapping

The LOYTEC LGATE-950 is a standalone BACnet MS/TP to KNX TP router mounted on DIN rail. It connects to the BACnet RS485 bus on port A and to the KNX bus on the TP terminal. Power supply: 24V DC. The web configuration interface is accessed at the device IP address on port 80 (default: 192.168.1.254, change before deployment). BACnet UDP port: 47808 (standard). The LGATE auto-discovers BACnet devices on the MS/TP bus using Who-Is broadcast at startup.

LGATE-950 configuration steps:

1. Auto-discovery:
   Connect → Scan BACnet MS/TP network
   All TVZ units appear as BACnet devices (by device ID)
   LGATE reads object list from each device via ReadPropertyMultiple

2. For each TVZ (example: TVZ device ID 1, Zone A Room 01):
   Create mapping entries in LGATE web UI:

   TVZ AV:3 (actual flow) → KNX GA 12/1/3
     Direction: BACnet read → KNX send
     KNX DPT: 9.002 (l/s, airflow)
     Poll/COV: COV (send on 5 l/s change)

   TVZ AO:1 (damper position) → KNX GA 12/1/4
     Direction: BACnet read → KNX send
     KNX DPT: 5.001 (percent)
     Poll/COV: poll every 30s

   TVZ AI:1 (room temperature) → KNX GA 12/1/5
     Direction: BACnet read → KNX send
     KNX DPT: 9.001 (°C)
     Poll/COV: poll every 60s

   KNX GA 12/1/1 → TVZ AV:1 (min flow setpoint write)
     Direction: KNX receive → BACnet write
     KNX DPT: 9.001 (l/s)
     Trigger: on KNX telegram receive

3. Repeat for each TVZ with unique GA sub-group per zone

Zone control sequence: KNX room controller to VAV

The KNX room controller (MDT SCN-LCRM.01 with occupancy sensor) calculates the heating or cooling demand for the zone as a percentage (0-100%) based on setpoint versus measured temperature. This demand percentage is converted to an air volume flow setpoint in l/s by a KNX logic function block that scales between minimum (dead-band, typically 20% of maximum) and maximum flow rates defined during HVAC commissioning.

Zone A: meeting room, TVZ device ID 1
  Minimum flow (unoccupied / dead-band): 30 l/s
  Maximum flow (full cooling demand): 150 l/s

KNX logic function (MDT SCN-LCRM.01):
  Input: GA 20/1/1 — heating/cooling demand, DPT 5.001 (0-100%)
  Output: GA 12/1/1 — TVZ flow setpoint, DPT 9.001 (l/s)

  Formula: flow = 30 + (demand% / 100) * (150 - 30)
  At 0% demand: 30 l/s (minimum ventilation)
  At 50% demand: 90 l/s
  At 100% demand: 150 l/s

  Occupancy detection (MDT PIR or KNX presence detector):
    Occupied → use demand-based setpoint (30-150 l/s)
    Unoccupied for 15 min → force flow setpoint = 20 l/s (standby)
    Write 20 l/s to GA 12/1/1 → LGATE writes to TVZ AV:1

  Morning warm-up (KNX time switch at 06:30):
    Write maximum flow (150 l/s) to GA 12/1/1 for 30 min
    Flush overnight air buildup before occupants arrive

Multi-zone scaling and large installations

A typical office floor with 20 rooms has 20 TVZ units. Each unit requires 4-5 BACnet object mappings in the LGATE, totalling 80-100 mappings per floor. The LGATE-950 supports up to 128 BACnet devices and 500 mapping entries — adequate for a single floor. For multi-floor buildings, deploy one LGATE-950 per floor (or per 128 devices), connecting each LGATE to the building KNX backbone and to the floor RS485 BACnet bus.

For buildings with more than 128 BACnet devices per floor (large open-plan offices or warehouses), the LOYTEC L-INX automation server supports up to 4 independent MS/TP networks plus Ethernet BACnet/IP, providing capacity for 500+ BACnet devices within a single unit. The L-INX also supports LOYTEC LTE (proprietary peer-to-peer) and direct ETS6 integration for larger KNX projects.

BACnet COV and CO2 demand-controlled ventilation

BACnet Change of Value (COV) subscriptions reduce network traffic compared to periodic polling. Instead of the LGATE requesting the actual flow value every 10 seconds, the TVZ sends an update only when the flow changes by more than the COV increment (configure 5 l/s in LGATE). In a 20-device installation this reduces BACnet MS/TP traffic from 120 request-response pairs per minute to typically 5-15 change events per minute during stable operation.

TROX AirMaster option: CO2 sensor built into TVZ room controller
  BACnet object AI:3 (ppm) → mapped to KNX GA 12/1/6, DPT 9.008

KNX DCV logic (MDT SCN-LCRM.01):
  IF GA 12/1/6 > 1000 ppm (crowded room, CO2 rising):
    → Write GA 12/1/2 = 150 (maximum flow setpoint in l/s)
    → LGATE forwards: BACnet write TVZ AV:2 = 150 l/s
    → TVZ opens damper to maximum — prioritises IAQ over demand

  IF GA 12/1/6 < 700 ppm for 300s (room emptying):
    → Write GA 12/1/2 = 30 (return to minimum flow setpoint)
    → LGATE forwards: BACnet write TVZ AV:2 = 30 l/s
    → TVZ returns to demand-based control

  CO2 alarm at 1400 ppm:
    → Write KNX GA 20/0/1 = ON (alarm telegram to visualisation)
    → Gira X1 / MDT Glastaster shows red alert for zone
    → Building manager receives notification via Home Assistant