Zehnder ComfoAir Q + KNX: вентиляция с управлением по CO2 через Modbus TCP

Heat recovery ventilation (HRV) units such as the Zehnder ComfoAir Q350 and Q450 are the backbone of airtight residential and office ventilation systems. While the ComfoAir Q series offers the ComfoBus proprietary protocol for native Zehnder peripherals, integrators working with KNX building automation systems require a Modbus path. The ComfoAir Q provides RS-485 Modbus RTU on the ComfoBus port, which — via a Moxa NPort 5110 serial device server — becomes accessible as Modbus TCP on the LAN. This article details the complete integration from register-level Modbus mapping through to CO2-driven demand control via KNX.

ComfoAir Q RS-485 Modbus RTU interface

The ComfoAir Q350/Q450 exposes a Modbus RTU slave interface on its ComfoBus RJ45 port (pins: A+, B−, GND). Serial parameters: 38400 baud, 8 data bits, no parity, 1 stop bit (8N1). Modbus unit ID: 1 (fixed, not configurable without service firmware access). The Moxa NPort 5110 RS-485 to TCP converter connects via its TB-5 terminal block (D+, D−, GND) to the ComfoAir Q ComfoBus port. NPort configuration: Operation Mode = TCP Server, port 4001; RS-485 mode (2-wire); baud 38400 8N1. On the KNX Modbus gateway side, connection parameters: IP = NPort IP, TCP port 4001, Modbus RTU framing over TCP (not Modbus TCP framing — an important distinction when configuring Intesis or Ewon gateways).

Key ComfoAir Q Modbus register map: Register 209 — Fan speed percentage (input register, 0–100%). Register 235 — Ventilation mode (holding register: 1 = Away/Mode 1, 2 = Low/Mode 2, 3 = Normal/Mode 3, 4 = Boost/Mode 4). Register 274 — Supply air temperature (input register, unit 0.1°C). Register 275 — Exhaust air temperature (input register, unit 0.1°C). Register 308 — Filter operating hours (input register, hours since last reset — trigger maintenance at 3500h). Register 227 — Bypass valve position (input register, 0 = closed, 100 = fully open).

KNX Modbus gateway mapping and group address scheme

Use an Intesis IN701KNX300O000 or WattNode Modbus-to-KNX gateway. Map the following: Register 235 (ventilation mode) → KNX GA 5/0/0 DPT 5.010 (unsigned byte) — bidirectional (write mode from KNX, read status back). Register 209 (fan speed %) → GA 5/0/1 DPT 5.001 (percentage) — read-only. Register 274 (supply air temp) → GA 5/0/2 DPT 9.001 (°C) — read-only. Register 275 (exhaust temp) → GA 5/0/3 DPT 9.001 (°C) — read-only. Register 227 (bypass position) → GA 5/0/4 DPT 5.001 (%) — read-only. Register 308 (filter hours) → GA 5/0/8 DPT 7.001 (unsigned 2-byte) — read-only; compare with threshold 3500 to trigger filter alarm GA 5/0/9 DPT 1.005.

CO2 sensor integration for demand-controlled ventilation

A KNX CO2 sensor (e.g., Elsner CO2-T-UP KNX, Thermokon SR65-CO2) sends CO2 concentration to GA 5/1/0 DPT 9.008 (ppm). A KNX logic module (ABB CL/KNX or IP-Symcon rule) implements 3-level ventilation demand: CO2 ≤ 800 ppm → write value 1 (Away/Mode 1) to GA 5/0/0; CO2 800–1200 ppm → write value 3 (Normal/Mode 3); CO2 > 1200 ppm → write value 4 (Boost/Mode 4). Apply hysteresis of 50 ppm to avoid rapid mode switching at threshold boundaries. The logic module also adds a 10-minute minimum dwell time — once boost mode activates, it holds for at least 10 minutes even if CO2 drops below 1200 ppm. This prevents hunting and reduces motor wear on the ComfoAir Q fan.

Scheduling, filter alarm and commissioning

KNX time controller (JUNG 2138 REG or ETS6 internal scheduler): Night ventilation 22:00–06:00 → force Mode 1 (Away) unless CO2 override. Occupied hours 06:00–22:00 → CO2 demand control active. Weekly override: Monday morning 06:00 → write Mode 3 (Normal) for 30 minutes to purge any accumulated CO2 from unoccupied weekend. The filter maintenance alarm (GA 5/0/9 DPT 1.005 = 1) triggers a push notification via ARISTO BewO or KNX-to-email gateway. Commissioning steps: (1) Verify Modbus RTU connection with Modbus Poll software via NPort — read register 235, confirm response. (2) In ETS6 Group Monitor, write value 4 to GA 5/0/0 and confirm register 235 changes to 4 in Modbus Poll (fan ramps to boost audibly). (3) Inject 1400 ppm CO2 test gas at sensor → verify logic triggers Mode 4 within 2 minutes. (4) Check filter hours register 308 — document baseline value for next maintenance interval.