ComAir HRUC-Plus Modbus Integration for Home Assistant

Custom Home Assistant integration for ComAir HRUC-Plus 3 / Vent-Axia Sentinel Kinetic Advance MVHR ventilation units via Modbus RTU over TCP.

Features

40 Entities: Comprehensive sensor and control coverage
Config Flow UI: Add via Settings → Integrations (no YAML editing)
BMS Settings: Configuration matches Vent-Axia Connect app
Climate Control: HVAC-like entity with preset modes
Energy Tracking: Integrated energy sensor for HA Energy dashboard
Heat Recovery: Calculated efficiency sensor
Translations: English, Slovak, Czech, Polish — terminology taken from the manufacturer's own manuals (Comair HRUC-Plus NL and PL editions)

Entities

Platform	Count	Entities
sensor	20	Temperatures (4), Humidity (2), CO2 (2), Fan RPM (2), Fan Speed % (2), Power, Energy, Heat Recovery, Timers (3), Diagnostics (3)
binary_sensor	6	Attention LED, Cooling Enable, Preheater Enable, Controlled Cooling/Heating, Summer Bypass
switch	10	Virtual Inputs 1-10 (BMS control mapping)
button	1	Sync Clock (write HA time to MVHR)
select	1	Ventilation Mode (Auto/Low/Medium/High/Boost)
number	1	Mode Duration (15-240 min, step 15)
climate	1	Ventilation with preset modes

About the Summer Bypass sensor

The unit publishes no documented summer-bypass flag, so Summer Bypass is inferred from the air temperatures: when the bypass is open, supply tracks intake and exhaust tracks extract. When intake and extract are within 3 °C of each other there is not enough signal to judge, and the sensor reports unknown rather than guessing.

That inference has now been checked against a unit's own bypass status by a user on a different model: when the bypass was genuinely open the sensor read on, and when it was closed the sensor read off or unknown. So it is sound, with unknown as the honest answer in the narrow-spread case rather than a wrong one.

The manual's own rule ("Zomer bypassmodus") is that the bypass engages only when the indoor and outdoor thresholds are both exceeded and the outdoor temperature is below the indoor temperature, and that it disengages as soon as either threshold is crossed back. The sensor uses the necessary half of that: whenever intake is at or above extract the damper cannot be open, so it reports a confident off instead of unknown. The thresholds themselves are user-configurable on the unit and not exposed over Modbus, so they are not assumed. unknown is now limited to the genuinely ambiguous band — outdoor cooler than indoor, but by less than 3 °C.

The manual also describes bypass modes the bus cannot report: Off, Normal, Evening cooling (runs 5 hours, then reverts) and Night cooling (runs until the outdoor temperature rises again).

There is no register that reports the bypass directly. Register 30025, listed in the official Gen V map only as "Other output sources… TODO/TBC", was tested as a candidate and ruled out: it sits permanently at 1 on both an HRUC-Plus 3 VR and a Sentinel Econiq SC while the real bypass opens and closes. The relay outputs 30021–30024 (cooling enable, preheater enable, controlled cooling/heating) are not the bypass either — they appear to drive external heating and cooling equipment and follow temperature.

Required Hardware

1. ComAir HRUC-Plus Ventilation Unit

The ComAir HRUC-Plus 3 (also sold as Vent-Axia Sentinel Kinetic Advance) is a whole-house heat recovery ventilation unit (MVHR) with built-in Modbus RS485 support via the BMS connector.

Supported models:

Device	Variants	Tested
ComAir HRUC-Plus 3	250, 350	350 tested
Vent-Axia Sentinel Kinetic Advance	250S/SX, 350S/SX (LH/RH)	350SX RH tested
Vent-Axia Sentinel Econiq	SC	Tested — confirmed working by a user
Vent-Axia Sentinel Kinetic Apex	Gen V	Should work (same Modbus map)

All Gen V units share the same Modbus register map, which is why models beyond the tested ones generally work. If you get another model running, please open an issue so it can be added here.

2. Modbus RTU to TCP Gateway

You need a WiFi or Ethernet RS485 gateway to bridge the unit's RS485 bus to your network. The gateway converts Modbus RTU (serial) to Modbus TCP (network).

Gateway	Input Voltage	Interface	Tested
Elfin EW11A	5-36V DC	WiFi	Yes
Waveshare RS485 to ETH	5-36V DC	Ethernet	Should work
USR-W610	5V DC	WiFi	Should work

The Elfin EW11A is recommended — it can be powered directly from the BMS connector (5V or 24V), requires no external power supply, and fits neatly inside the ventilation unit.

3. RJ12 Cable (6P6C)

A standard RJ12 6-pin cable to connect the gateway to the BMS connector on the HRUC unit. You can use:

RJ12 breakout adapter (recommended — clean, no soldering)
Cut and strip an RJ12 cable

BMS Connector Pinout

The HRUC-Plus has a 6P6C RJ12 BMS connector (J20) for Modbus RS485 communication.

RJ12 Pin Numbering

Looking at RJ12 jack (clip facing down):

        ┌─────────────────────────┐
        │  1   2   3   4   5   6  │
        │ 24V GND  A   B  GND +5V │
        └──────────┬──────────────┘
                   │
                 clip

Complete Pinout

Pin	Signal	Description	Connect to Gateway
1	P24VF9	+24V DC (fused 500mA)	VCC (if gateway supports 24V)
2	GND	Ground	(alternative GND)
3	MOD A	RS485 Data+	A
4	MOD B	RS485 Data-	B
5	GND	Ground	GND
6	P5VF10	+5V DC (fused 500mA)	VCC (if gateway needs 5V)

Wiring Diagram

HRUC BMS RJ12 (J20)          Gateway Terminal
─────────────────────        ────────────────────
Pin 1 or 6 (power) ──────── VCC
Pin 3 (MOD A)  ───────────── A  (Data+)
Pin 4 (MOD B)  ───────────── B  (Data-)
Pin 5 (GND)    ───────────── GND

Power Pin Selection

Pin	Voltage	Use for
Pin 1	24V	Gateways rated 5-36V (e.g. Elfin EW11A)
Pin 6	5V	Gateways rated 5V only

Both pins are fused at 500mA via F9 (24V) and F10 (5V).

RS485 Termination

The BMS board has a 120Ω terminator (R51) enabled via jumper J4:

Enable if gateway is at end of RS485 bus
Enable if cable length > 10 meters
Enable if communication errors occur

Gateway Configuration

This integration speaks Modbus RTU over TCP: Home Assistant builds the complete RTU frame (including the CRC) and the gateway must pass the bytes through unchanged. The gateway must therefore be a transparent serial bridge — it must not do Modbus protocol conversion.

Serial settings

Setting	Value
Protocol	None (transparent) — see warning below
CLI	Disable — see warning below
Baud Rate	115200
Data Bits	8
Parity	None
Stop Bits	1
Flow Control	`Disable` or `Half-Duplex` — both confirmed working
Buffer Size	512
Gap Time	anywhere in 10–50 — both ends of that range confirmed working

RS485 is a two-wire bus and therefore always half duplex; that is a property of the wiring, not a setting you need to hunt for. The EW11A exposes a single Flow Control dropdown in which Half-Duplex is one of the values, and either it or Disable works.

Frame Length and Frame Time are not on the Serial Port Settings page. They exist only in the gateway's configuration export, and the defaults are fine — if you cannot find them in the web interface, nothing is wrong.

Network settings

Setting	Value
Protocol	TCP Server
Local Port	502
Route	Uart
Security	None

Two settings must be right, and both fail the same silent way.

Protocol = None. The Elfin EW11A offers Modbus in the Protocol dropdown. That mode makes the gateway expect Modbus TCP (MBAP-framed) requests from the network and silently discard the RTU frames this integration sends.

CLI = Disable. With CLI set to Serial String the gateway watches the serial stream for its escape sequence (+++) so it can drop into command mode — meaning it inspects, and can absorb, bytes in transit. Modbus RTU is raw binary in which any byte pattern may occur, so a gateway hunting for a trigger string is not a clean pipe.

Either mistake produces No response received after 3 retries with perfect wiring. See Troubleshooting.

The values above are read from a working EW11A, not from a datasheet.

EW11A Serial Port Settings

This is a correctly configured EW11A — note Protocol Settings → Protocol = None, which is the setting that makes the gateway a transparent bridge:

EW11A Configuration Screenshots

Protocol Settings:

Route Settings:

Vent-Axia Connect App BMS Settings

The Modbus settings can be verified in the Vent-Axia Connect app under Advanced Settings → Modbus:

Installation

Method 1: HACS (Recommended)

Open HACS in Home Assistant
Click Integrations
Click the three dots menu → Custom repositories
Add repository URL: https://github.com/Koky05/comair-modbus-homeassistant
Select category: Integration
Click Add
Search for "ComAir HRUC-Plus Modbus"
Click Download
Restart Home Assistant

Method 2: Manual Installation

Download the latest release from GitHub

Copy the comair_modbus folder to your Home Assistant custom_components directory:

config/
└── custom_components/
    └── comair_modbus/
        ├── __init__.py
        ├── climate.py
        ├── config_flow.py
        ├── const.py
        ├── coordinator.py
        ├── manifest.json
        ├── sensor.py
        ├── binary_sensor.py
        ├── select.py
        ├── number.py
        ├── strings.json
        └── translations/
            ├── en.json
            ├── sk.json
            └── cs.json

Restart Home Assistant

Configuration

Go to Settings → Devices & Services
Click + Add Integration
Search for "ComAir HRUC-Plus Modbus"
Enter your gateway configuration:

Field	Default	Description
Gateway IP Address	(required)	IP address of your Modbus gateway
Modbus TCP Port	502	TCP port for Modbus communication
Device ID (Slave Address)	2	Modbus slave address of the HRUC unit
Baud Rate	115200	Serial baud rate
Data Bits	8	Number of data bits
Parity	None	Parity setting
Stop Bits	1	Number of stop bits

Click Submit

The integration will test the connection and create all entities.

Sensor Details

Temperature Sensors

Sensor	Register	Description
Intake Temperature	30100	Outside air entering the unit
Supply Temperature	30110	Heated/cooled air to rooms
Extract Temperature	30120	Room air being extracted
Exhaust Temperature	30130	Air being expelled outside

Environmental Sensors

Sensor	Register	Description
Intake Humidity	30101	Outside air humidity (%)
Extract Humidity	30121	Room air humidity (%)
Intake CO2	30102	Outside CO2 level (ppm) — if sensor installed
Extract CO2	30122	Room CO2 level (ppm) — if sensor installed

Fan & Power Sensors

Sensor	Register	Description
Supply Fan RPM	30014	Supply fan speed (RPM × 0.1)
Extract Fan RPM	30016	Extract fan speed (RPM × 0.1)
Power	30010	Current power consumption (W)
Energy	—	Accumulated energy (kWh), calculated from Power
Heat Recovery Efficiency	—	Calculated from temperatures (%)

Status Sensors

Sensor	Register	Description
Run Time	30001	Total operating days
Service Timer	30002	Months until service required
Filter Timer	30003	Months until filter change
Faults	30004-05	Active fault codes
Warnings	30006-07	Active warning codes
Notifications	30008-09	Active notifications

Ventilation Modes

Control ventilation via the select entity or climate presets:

Mode	Default Fan Speed	Description
Auto	Automatic	Automatic control based on sensors
Low	20%	Low fan speed (PR1)
Medium	30%	Medium fan speed (PR2, Normal)
High	50%	High fan speed (PR3)
Boost	100%	Maximum ventilation (PR4, Purge)

Fan speeds are configurable per mode via the Vent-Axia Connect app.

Energy Dashboard

The integration provides an Energy sensor (sensor.comair_hruc_plus_energy) that tracks total energy consumption in kWh. This sensor has state_class: total_increasing and can be used directly in the Home Assistant Energy dashboard.

Dashboard Card

A ready-to-use picture-elements Lovelace card that mimics the physical HRUC-Plus LCD controller display is included at docs/lovelace_controller_card.yaml.

Features:

Transparent background (works with any HA theme)
Outdoor/indoor temperature, humidity, heat recovery efficiency
Mode-dependent fan icon (auto, speed-1, speed-2, speed-3, boost alert)
Supply/extract fan speed percentages
Air quality smiley (happy/neutral/sad based on faults/warnings)
Power, energy, runtime, filter and service timers

Installation:

Copy SVG icons from docs/icons/ to /config/www/comair/ on your HA
Copy docs/icons/lcd_transparent.png to /config/www/comair/
Dashboard → Edit → Add Card → Manual → paste content from docs/lovelace_controller_card.yaml
Adjust the entity IDs. Home Assistant derives an entity ID from the entity's translated name when it is first created, so they depend on the UI language you installed with. The card ships with the IDs from a Slovak install (sensor.comair_hruc_plus_3_teplota_nasavania); on English you will have sensor.comair_hruc_plus_3_intake_duct_temperature. Search-and-replace before use.

Fault, Warning and Notification Codes

The unit reports these as three 32-bit bitmasks. The Faults, Warnings and Notifications sensors show the active codes as their state (OK, W-12, or W-12, W-15), and carry the meaning of each one in attributes:

state: "W-12, W-15"
attributes:
  codes: ["W-12", "W-15"]
  descriptions:
    - "W-12: Filter cleaning or replacement overdue"
    - "W-15: BMS offline"

The state stays a compact code list so automations and history keep working; the descriptions are attributes, which is where a template or a card can pick them up. An undocumented bit is passed through as raw hex (0x40000000) rather than hidden.

Faults		Warnings		Notifications
F-1	Supply air thermistor	W-1	Supply air temperature	N-1	Filter cleaning/replacement due soon
F-2	Extract air thermistor	W-2	Exhaust air temperature	N-2	Service due soon
F-3	Supply fan	W-3	Preheated air temperature	N-3	Device offline
F-4	Extract fan	W-4	Intake air humidity	N-4	Cooling suspended
F-8	Supply air too cold	W-5	Extract air humidity	N-5	Cooling insufficient
F-32	HMI communication lost	W-6	Supply air flow
		W-7	Extract air flow
		W-8	Left filter sensor
		W-9	Right filter sensor
		W-10	System overpressure
		W-11	Preheater activated
		W-12	Filter cleaning or replacement overdue
		W-13	Service interval overdue
		W-14	Lost connection to sensors or controllers
		W-15	BMS offline
		W-18	Bypass or heat exchanger efficiency
		W-19	Preheater IO offline
		W-20	Cooling unit offline

Sourced from the manufacturer's manuals, which do not cover the same set — the Dutch edition documents F-8, W-18 to W-20 and N-3 to N-5 but omits W-8 and W-9; the Polish edition (01/2024) documents W-8 and W-9 but omits the others. Two disagreements are left as the Dutch edition has them and noted in const.py: it calls F-1 the supply thermistor where Polish calls it the inlet thermistor, and W-2 the exhaust temperature where Polish calls it the extract temperature.

How warnings clear, per the Dutch manual: W-1 to W-7, W-10, W-11 and W-20 clear once the unit recovers and is power cycled; W-12 and W-13 clear once the filter or service values are reset.

Older controllers use a different scheme. An earlier HRUC-Plus edition (manual 475340, same 10040001xx reference numbers) reports faults as additive numbers rather than F-/W-/N- codes — 01 fan left, 02 fan right, 04 temperature sensor left, 08 right, and so on, summed together, so 03 means both fans. This integration reads the Gen V bitmask registers and decodes the F-/W-/N- scheme; if your controller shows plain numbers instead, it predates what these registers expose.

Modbus Register Map

Troubleshooting

Diagnostic script

Before changing anything, run tools/gateway_test.py. It needs only Python 3 (no Home Assistant, no pymodbus) and can run from any PC on the same network, or from the Advanced SSH & Web Terminal add-on:

python3 tools/gateway_test.py 192.168.1.50

It tests the TCP connection, sends a request in both RTU-over-TCP and Modbus TCP framing, sweeps device IDs 1–16, and prints which side of the chain is broken.

`No response received after 3 retries`

This error means the TCP connection to the gateway succeeded and the request was sent, but nothing came back from the RS485 side — complete silence rather than corrupted data. The network half of the setup is therefore already correct. Two possibilities remain, in order of likelihood:

1. The gateway never puts the request on the RS485 wire.

Serial settings: Protocol = None. If it is set to Modbus, the gateway expects Modbus TCP from the network and drops the RTU frames — this is by far the most common cause.
Network settings: Protocol = TCP Server, Local Port = 502, Route = Uart.
Confirmation: in tools/gateway_test.py, if test 3 (Modbus TCP framing) gets a reply while test 2 (RTU over TCP) does not, the gateway is in conversion mode.

2. The request reaches the wire, but the unit does not answer.

Open the gateway's status page and watch the serial Tx/Rx counters while the script runs. Tx increasing but Rx flat means the unit is not responding.
Verify the wires are on the BMS RJ12 connector (J20). The connectors labelled + A B - are the sensor connectors and carry no Modbus.
Check GND is connected, not only A and B.
Confirm the gateway is an EW11A (RS485, terminals A/B/C/D). The plain EW11 is RS232 and cannot talk to the unit.
Try swapping A and B — but only after Protocol = None is confirmed, otherwise two variables change at once.
Verify baud rate and parity match the Vent-Axia Connect app (Advanced Settings → Modbus): 115200 / 8 / None / 1.

The J4 terminator and alternative device IDs are not likely causes: a missing terminator does not produce total silence, and while the gateway is misconfigured every device ID fails, so sweeping IDs proves nothing.

Cannot Connect to Gateway

Verify gateway IP address is correct
Check gateway is powered and on network: ping <gateway_ip>
Verify TCP port 502 is accessible
Verify the gateway is in TCP Server mode on port 502

Cannot Communicate with HRUC Unit

Check the gateway serial Protocol is None, not Modbus
Check Device ID is correct (default: 2)
Verify RS485 wiring (A→A, B→B, GND→GND) on the BMS RJ12 connector
Check gateway serial settings match (115200/8/N/1)
Try enabling RS485 termination (jumper J4) if the cable is longer than 10 m

Sensors Show "Unavailable"

Wait for first data poll (up to 30 seconds)
Check Home Assistant logs for errors
Verify Modbus communication with test script

CO2 Sensors Show "Unknown"

Your unit does not have CO2 sensors installed
This is normal — the sensors will show as "Unknown"

Test Modbus Connection

With pymodbus available (inside the Home Assistant environment, for example). If you do not have pymodbus, use tools/gateway_test.py instead — it uses only the Python standard library.

from pymodbus.client import ModbusTcpClient
from pymodbus.framer import FramerType

client = ModbusTcpClient('192.168.x.x', port=502, framer=FramerType.RTU)
client.connect()

# Read intake temperature (device_id=2)
result = client.read_input_registers(address=99, count=1, device_id=2)
if not result.isError():
    temp = result.registers[0] / 10
    print(f"Intake Temperature: {temp}°C")
else:
    print(f"Error: {result}")

client.close()

Dependencies

Home Assistant 2024.1.0 or newer
pymodbus >= 3.6.0 (installed automatically)

Documentation

File	Description
BMS_WIRING_GUIDE.md	Detailed wiring instructions
MODBUS_CONFIRMED_SETTINGS.md	Confirmed Modbus register documentation
MODBUS_ANALYSIS.md	Modbus register map analysis

Credits

Peter Koval (@Koky05) — Development
Vent-Axia / Ventilair — BMS pinout documentation and technical support

License

This project is licensed under the MIT License — see the LICENSE file for details.

Contributing

Contributions are welcome! Please open an issue or pull request on GitHub.

Disclaimer

This is an unofficial integration not affiliated with Vent-Axia or Ventilair. Use at your own risk.

Name		Name	Last commit message	Last commit date
.github/workflows		.github/workflows
custom_components/comair_modbus		custom_components/comair_modbus
docs		docs
tools		tools
.gitignore		.gitignore
BMS_PIN_mapping.png		BMS_PIN_mapping.png
BMS_WIRING_GUIDE.md		BMS_WIRING_GUIDE.md
LICENSE		LICENSE
MODBUS_ANALYSIS.md		MODBUS_ANALYSIS.md
MODBUS_CONFIRMED_SETTINGS.md		MODBUS_CONFIRMED_SETTINGS.md
MVHR-BMS_ModBus Map_page2.png		MVHR-BMS_ModBus Map_page2.png
README.md		README.md
hacs.json		hacs.json

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