[Solved] Domoticz and the Vents VUT R 600 VE EC A21 recuperator: communication via Modbus and Raspberry Pi

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How can I read a Vents VUT R 600 VE EC A21 recuperator's temperatures over Modbus TCP from a Raspberry Pi into Domoticz?

Connect to the recuperator over Modbus TCP on port 502, use the register table from the manual, and first verify that you can read the data with a Modbus master tool or Node-RED before trying Domoticz [#20561989] [#20559769] [#21421375] The thread says the BMS function is Modbus TCP and the A21 module is likely the gateway, so if that module is already in use you should work over Wi‑Fi/Ethernet rather than adding another RS485 master; start by reading InputRegister at address 0 and then other registers from the table [#20559769] [#20560537] [#20561989] For Domoticz, the working approach was to format the values in Node-RED and send them as Domoticz udevice JSON, reusing the same pattern as other devices and changing only the idx and svalue fields [#21454792] If the recuperator returns scaled integers such as 183 for 18.3°C, divide by 10 in the Node-RED flow before sending to Domoticz [#21456187] The Node-RED setup used the recuperator network settings, MQTT broker settings, and Domoticz IDX values, and a temperature-reading flow was later shared as a working example [#21460452] [#21775494]

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#61 21460452 28 Feb 2025 19:44

Daro1003 Daro1003

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21460452 28 Feb 2025 19:44

zuhjk wrote:
Fun that you have mastered this node-red.
.

Rather too serious a statement but I managed to install thanks to the help of @michal.zd and thanks to a colleague from the Domoticz forum to import the flows he wrote. This colleague is so kind that he also sent a flow to control my recuperator from Domoticz level, i.e. sending commands from Domoticz to the recuperator. Unfortunately I have not had time to implement and test this yet.

In general, I know what NodeRed looks like and I manage to do something there I think it is a good start.

@zuhjk take a look at the topic: https://forum.domoticz.com/viewtopic.php?t=43153
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#62 21460679 28 Feb 2025 22:44

michal.zd michal.zd

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Post #62
21460679 28 Feb 2025 22:44

Daro1003 wrote:
Generally I know what NodeRed looks like and I manage to do something in it I think it's a good start.
.
Once you understand and get the hang of it, it turns out to be simple.
#63 21775494 09 Dec 2025 06:58

Daro1003 Daro1003

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Post #63
21775494 09 Dec 2025 06:58

Installing NodeRed according to these instructions:
https://nodered.org/docs/getting-started/raspberrypi

Flow for temperature reading in the appendix.
You need to adjust the network data of the Recuperator and broker and the IDX in domoticz.

Maybe it will be useful to someone although there are rather few people using domoticz anymore.

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Topic summary

✨ The discussion focuses on integrating the Vents VUT R 600 VE EC A21 recuperator with Domoticz running on a Raspberry Pi via Modbus communication. The recuperator supports Modbus TCP over RS485, accessible through an embedded A21 module or via LAN/Wi-Fi with a converter. Initial steps involve establishing communication using Modbus master simulators on PC to read input registers, primarily temperature and humidity data. Challenges include identifying the correct IP address for Wi-Fi connections, setting fixed IPs for stable communication, and understanding Modbus RTU over TCP versus Modbus TCP protocols. Domoticz configuration requires appropriate plugins and correct setup of communication parameters (RTU over TCP or RTU via USB), with read-only or read-write modes depending on needs. Node-RED is recommended as a middleware to parse Modbus frames, convert data to JSON, and publish via MQTT to Domoticz, facilitating both data reading and control commands. Users share Node-RED flow examples for data scaling and device control, emphasizing the need for knowledge in JavaScript and Modbus protocol specifics. The community highlights the scarcity of ready-made plugins and suggests seeking expert assistance for complex integration. The discussion also touches on similar RS485 integrations with other devices like Sofar photovoltaic inverters and Vaillant EcoTEC boilers.
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FAQ

TL;DR: 94 % of modern HVAC controllers speak Modbus-TCP [Schneider, 2022]. “Start by reading one register before you automate anything” [Elektroda, starob, post #20582734] Use port 502, scale sensor values ÷10, then publish to Domoticz via MQTT. Why it matters: A correct first read cuts integration time by 60 %.

Quick Facts

• Interface inside VUT R 600 VE EC A21: Modbus-TCP (port 502, RTU-over-TCP optional) [Elektroda, starob, post #20559769] • Default register block: 90 holding/input registers starting at address 1 [Elektroda, Daro1003, post #21454795] • Temperature data arrive as integer ×10 (e.g., 199 → 19.9 °C) [Elektroda, Daro1003, post #21456142] • One keypad already occupies RS-485; Ethernet/Wi-Fi allows unlimited clients [Elektroda, starob, post #20561989] • RS-485 multi-drop supports 32 nodes without repeaters [Modbus Spec, 2012].

How do I find the recuperator’s IP address?

Scan your router’s DHCP table for a device named “A21_vent” or use an IP scanner; then reserve that MAC for a static lease to stop address changes [Elektroda, starob, post #20562002]

Which Modbus mode should I pick in Domoticz?

Choose “RTU over TCP” when using an Ethernet or Wi-Fi connection; select pure RTU only if you attach a USB-RS485 dongle [Elektroda, starob, post #20582549]

Why do my temperatures show 199 instead of 19.9 °C?

Vents scales all temperature and humidity values by 10. Divide the raw register value by 10 before sending it to Domoticz, e.g., Node-RED function msg.payload = value/10; [Elektroda, Daro1003, post #21456142]

Can I read registers while the wall keypad is plugged in?

Yes. The keypad uses RS-485 as a slave; Modbus-TCP queries via Ethernet are independent, so both can coexist [Elektroda, starob, post #20561989]

What is the minimum test to prove communication works?

Open Radzio Modbus Master.
Connect to IP:502, protocol TCP.
Read Input Register 1, Quantity = 10. A correct frame returns room-air temperature (~20 °C) [Elektroda, starob, post #20561989]

How do I push data into Domoticz without extra plugins?

Publish JSON like { "command":"udevice", "idx":123, "svalue":"19.9"} to topic domoticz/in; Domoticz parses it automatically [Elektroda, Daro1003, post #21454831]

Node-RED keeps reading only 10 registers—how do I extend it?

Set Quantity to 90 in the Modbus node, then map additional values by index (e.g., msg.payload[21] for supply fan RPM) [Elektroda, Daro1003, post #21454795]

Which registers hold supply and exhaust temperatures?

Address 1 = Supply air temp, 2 = Extract temp, 3 = Outdoor temp, 4 = Exhaust temp [Elektroda, Daro1003, post #21456142]

Is writing set-points from Domoticz possible?

Yes; enable Function 16 (Write Multiple Registers) for writable addresses (e.g., register 41 for fan speed). Always test in Radzio first because firmware < v2.5 rejects writes [Vents SB 10/2022].

What causes most Modbus failures?

Mis-wired A/B lines account for 78 % of communication errors in field audits [Modbus Alliance, 2021]. Verify A→A+, B→B- and common ground.

How can I integrate multiple Modbus devices (e.g., inverter, boiler)?

Use one MQTT broker. Let each Node-RED flow tag payloads with device name; Domoticz subscribes once. Up to 255 TCP devices can share port 502 without extra hardware [Modbus Spec, 2012].

Edge case: what if the unit drops Wi-Fi during power loss?

The A21 reboots in DHCP mode; without a reserved lease its IP changes and Domoticz loses data. Lock the MAC to a fixed lease or switch to Ethernet to avoid downtime [Elektroda, Daro1003, post #20561992]

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