Heat Recovery Ventilation and KNX: Zehnder ComfoAir Q Modbus Integration and Bypass Control
Heat recovery ventilation (HRV) integrated with KNX via Modbus TCP enables granular fan speed control for CO2 demand-controlled ventilation, automatic bypass activation for summer free cooling and filter maintenance alerting — going beyond the limited on/off/boost control of standalone HRV thermostats.
HRV role in KNX buildings
Heat recovery ventilation provides balanced mechanical ventilation — equal supply and extract airflow — with heat recovery efficiency of 75–92% depending on the unit. Passivhaus standard requires minimum 75% heat recovery at design airflow; the Zehnder ComfoAir Q achieves 92% at nominal conditions.
Why KNX HRV integration
- Variable fan speed from CO2 DCV logic (not just 3 fixed modes)
- Bypass forced open for night cooling via KNX time schedule
- Window contact integration: reduce ventilation when windows open
- Filter dirty alarm → ARISTO maintenance alert email
- Temperature monitoring: supply/extract/outdoor via KNX GAs
Heat recovery economics
- Passivhaus 350m² house: 92% HRV saves ~3,500 kWh/year vs. extract-only
- At €0.25/kWh heat: €875/year heating cost saving
- Payback on HRV vs. extract fan: 5–8 years (well within product life)
- Combined with CO2 DCV: additional 30–40% fan energy saving
- BREEAM Hea 02 credits: combined HRV + CO2 monitoring
Zehnder ComfoAir Q hardware and ComfoConnect LAN C
The ComfoAir Q350 (350 m³/h, dual EC fans, counterflow heat exchanger with bypass damper) is the standard residential and small commercial HRV unit. The ComfoConnect LAN C is the Ethernet bridge module that provides network access to all ComfoAir Q data — including Modbus TCP, REST API and BACnet/IP.
ComfoConnect LAN C setup
Installation: plug ComfoConnect LAN C into ComfoAir Q bus connector (rear panel, right side) RJ45 Ethernet → network switch → same subnet as KNX gateway IP address: DHCP assigned by router (check router DHCP table or use Zehnder network scanner app) Assign static IP in ComfoConnect web interface for reliability Enable Modbus TCP: ComfoConnect web UI → Settings → Modbus → Enable Modbus TCP port: 502 (default) Modbus device ID: 1 (default) Firmware: update to v3.0+ for BACnet/IP support (Modbus TCP available from firmware v2.0+) Update via ComfoConnect web interface → Firmware → Update
ComfoAir Q Modbus register map
The ComfoConnect LAN C Modbus register map provides read/write access to fan speed, operating mode and bypass control, plus read-only access to temperature sensors, filter alarm and energy data.
| Register | Description | Type | Values / Notes |
|---|---|---|---|
| 337 | Supply fan speed | 0–100%, R/W | Write to set fan speed directly |
| 338 | Extract fan speed | 0–100%, R/W | Typically same as supply |
| 341 | Operating mode | uint16, R/W | 0=Away 25%, 1=Low 50%, 2=Medium 75%, 3=High 100%, 4=Boost 20min |
| 209 | Bypass damper state | 0/1, R/O | 0=closed (heat recovery active), 1=open (bypass/free cooling) |
| 211 | Filter dirty alarm | 0/1, R/O | 1=filter replacement due; reset via ComfoConnect web UI |
| 221 | Supply air temperature | °C×10, R/O | Temperature after heat exchanger (before supply grille) |
| 222 | Extract air temperature | °C×10, R/O | Room exhaust temperature |
| 225 | Outdoor temperature | °C×10, R/O | Supply air before heat exchanger — outdoor temperature proxy |
Intesis gateway MAPS signal table
Configure the Intesis IN701KNX as Modbus TCP master connected to the ComfoConnect LAN C IP address. The MAPS configuration tool maps each Modbus register to a KNX group address with the correct DPT and scaling factor.
Intesis IN701KNX MAPS signal table — ComfoAir Q
Signal Register DPT Scale GA Description 1 341 5.001 ×25 8/1/0 Ventilation mode (0=Away to 4=Boost) 2 337 5.001 ÷1 8/1/1 Supply fan speed (%) 3 209 1.001 — 8/1/2 Bypass damper open (0/1) 4 211 1.001 — 8/1/3 Filter dirty alarm (0/1) 5 221 9.001 ÷10 8/1/4 Supply air temperature (°C) 6 222 9.001 ÷10 8/1/5 Extract air temperature (°C) 7 225 9.001 ÷10 8/1/6 Outdoor temperature (°C) Mode scaling (register 341 ↔ DPT 5.001): DPT value 0 (0%) → register 0 (Away, 25%) DPT value 25 (25%) → register 1 (Low, 50%) DPT value 50 (50%) → register 2 (Medium, 75%) DPT value 75 (75%) → register 3 (High, 100%) DPT value 100(100%)→ register 4 (Boost, 20 min) Poll interval: 30 seconds (all registers) Modbus timeout: 5 seconds Retry on timeout: 3 attempts
DCV integration: CO2 to HRV mode
The CO2 DCV logic outputs a mode command to the ComfoAir Q via GA 8/1/0 — selecting Away, Low, Medium or High ventilation mode based on CO2 concentration thresholds. Presence detection adds standby mode when the building is vacant.
CO2 → HRV mode logic — MDT Logic Module
Input: GA 9/1/10 (max zone CO2, ppm)
Output: GA 8/1/0 (ventilation mode, DPT 5.001 × 25)
Thresholds:
CO2 < 700 ppm: → send value 0 (Away mode, 25%)
CO2 700–900 ppm: → send value 25 (Low mode, 50%)
CO2 900–1100 ppm: → send value 50 (Medium mode, 75%)
CO2 > 1100 ppm: → send value 75 (High mode, 100%)
Presence interlock:
All rooms unoccupied 30 min → send value 0 (Away)
First room presence = 1:
→ send value 25 (Low mode, 50%) immediately
CO2 takes over after 5 minutes
Residential simplified (3 modes only):
Unoccupied → Away (25%)
Occupied, CO2 < 900 ppm → Low (50%)
Occupied, CO2 > 900 ppm → High (100%)Summer bypass control — free cooling
The ComfoAir Q bypass damper opens automatically when outdoor temperature is below indoor temperature — allowing cool outdoor air to bypass the heat exchanger and flush the building. KNX enhances this with forced bypass logic for night cooling during summer.
Night free cooling logic — KNX time schedule
Conditions for KNX-forced night cooling:
GA 8/1/6 (outdoor temperature) < 16°C
AND GA 8/2/0 (indoor temperature) > 24°C
AND time: 22:00–06:00
Action: send value 75 (High mode) to GA 8/1/0
→ ComfoAir Q runs at 100% with bypass open
→ Cool outdoor air flushes building overnight
→ Indoor temperature falls 2–4°C before 06:00
Bypass damper read-back:
GA 8/1/2 (bypass state, 0=closed, 1=open)
If mode = High AND outdoor < indoor AND GA 8/1/2 = 0:
→ Bypass not opening — check ComfoAir Q parameters
(ComfoConnect: bypass enabled and outdoor < indoor
condition may need 2-hour delay to trigger)
Daytime bypass (automatic by ComfoAir Q):
No KNX action required — ComfoConnect handles
automatic bypass based on its own sensorsWindow contact interlock: if a window is open (KNX window contact GA = 1), send mode 0 (Away) to GA 8/1/0 — no point ventilating mechanically when a window provides natural ventilation. This also prevents the HRV from drawing unconditioned air through an open window and reducing the other zone temperatures.
Filter maintenance integration
ComfoAir Q filter replacement is due every 12 months (G4 supply filter and F7 extract filter). The Modbus filter alarm register triggers a KNX group address change that fires an ARISTO maintenance alert.
ARISTO BewO filter alert rule
Trigger: GA 8/1/3 (Filter Dirty Alarm) changes to 1
(Modbus register 211 read = 1)
ARISTO BewO rule:
IF GA("8/1/3") = 1
THEN ALARM "ComfoAir Q filter replacement due — " + DATE
TO EMAIL maintenance@building.com
CC facilities@building.com
After filter replacement:
Reset alarm via ComfoConnect web interface:
Settings → Maintenance → Reset filter timer
Modbus register 211 returns to 0 within 5 minutes
GA 8/1/3 returns to 0 → alarm auto-clears in ARISTO
O&M manual (BREEAM Man 01 — sustainable procurement):
Include filter part numbers (G4: Zehnder 523050270,
F7: Zehnder 523050280) and 12-month replacement schedulePluggit aerocon HRV alternative
The Pluggit aerocon Plus (280/400/550 m³/h variants) is a German-made HRV unit comparable in performance to the Zehnder ComfoAir Q series. For projects where Zehnder premium pricing is not justified by specification requirements, Pluggit offers 10–15% lower cost at the same airflow capacity.
Pluggit integration
- Modbus TCP: same ComfoConnect protocol subset as Zehnder
- Register map: compatible with Zehnder (minor differences in register offsets)
- Home Assistant: HACS Pluggit integration (REST API)
- KNX: same Intesis IN701KNX approach — adjust MAPS register numbers
- Preferred for German residential projects under €500K budget
When to specify Zehnder
- Passivhaus projects (Zehnder PHI certified at 92% efficiency)
- Projects requiring BACnet/IP (Zehnder firmware v3.0+)
- Zehnder already specified by M&E consultant or architect
- ComfoConnect REST API required for home automation integration
- Service contract with Zehnder service network required
Commissioning and airflow balancing
Commission the ComfoAir Q KNX integration in eight steps. Complete Modbus connectivity verification before ETS6 download to catch network or configuration issues early.
| Step | Action | Pass criterion |
|---|---|---|
| 1 | Install ComfoConnect LAN C in ComfoAir Q bus connector | ComfoConnect web UI accessible at assigned IP — firmware version shown |
| 2 | Enable Modbus TCP in ComfoConnect web interface | Modbus port 502 responds — verify with Modbus polling tool (ModScan or similar) |
| 3 | Load MAPS signal table in Intesis IN701KNX | MAPS Test: read register 221 (supply temp) — shows current supply temperature |
| 4 | Download ETS6 project: GA assignments to Intesis device | Intesis programming complete — no ETS6 download errors |
| 5 | Verify GA 8/1/4 (supply temperature) in ETS6 Group Monitor | Value matches ComfoAir Q display supply temperature ±1°C |
| 6 | Send mode 3 (High) to GA 8/1/0 | ComfoAir Q display shows High (100%) mode — fans at full speed (audible) |
| 7 | Measure supply and extract airflow at diffusers (anemometer) | Supply and extract within ±5% of each other — adjust ComfoAir Q balance setting if needed |
| 8 | Trigger high CO2 (simulate via ETS6) → verify mode increases | DCV logic correctly drives ventilation mode from Away to High as CO2 threshold crossed |
Need HRV Modbus KNX integration commissioned?
We configure ComfoConnect LAN C Modbus TCP, load Intesis MAPS signal tables and commission DCV and bypass logic in ETS6 — delivering HRV KNX integration with airflow balance records and filter maintenance schedules.
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