In this post I describe how I integrated my Proxon ventilation heating and the T300 hot water tank into my IoT system. The goal is to make good use of the surplus power from my PV system:

• Cooling with the air conditioning function of the Proxon, which I had retrofitted by the company Zimmermann.
• Heating water with the boost function of the T300, which heats the water up to 55 °C.

The project requires a few hardware and software components. They do not necessarily have to be the same ones I use – what matters is that they offer the same functions.

Components used

Hardware

• Raspberry Pi 4B with 8 GB RAM
• USB-to-RS485 adapter for reading and writing via Modbus (any suitable adapter will do)
• Shelly Plus 1 with a potential-free switching output for the boost function of the T300

And of course …

• PV system with battery whose values (including the state of charge/SOC) can be read via MQTT – in my case an E3/DC home power station
• Proxon FWT 2.0 ventilation heating with air conditioning function
• Proxon T300 hot water tank, matching the Proxon

Software

• Node-RED
• Mosquitto MQTT broker
• Node-RED palettes:
  • node-red-contrib-modbus
  • node-red-contrib-pushover
  • @flowfuse/node-red-dashboard
• Optional: a Pushover account for push notifications

Setup

The entire project runs on the Raspberry Pi 4B. It runs the current Node-RED version 5.0.8 as well as the Mosquitto MQTT broker for data exchange via MQTT.

• Modbus connection to the Proxon: The USB-to-RS485 adapter is connected to the Raspberry Pi via USB and to terminal X6 on the Proxon control board.
• Boost function of the T300: The switching output of the Shelly Plus 1 is connected to terminal X13 on the T300 board. This allows the boost to be switched on and off as needed.

You can find more about controlling the T300 in my post Node-RED Control Proxon T300, E3DC, Shelly.

Using free solar power for cooling and hot water: My Node-RED flow automatically switches the cooling function of my Proxon ventilation heating and the Boiler Boost of the T300 hot water tank – but only when the PV system delivers enough power, the home battery is sufficiently charged and (for cooling) it is actually warm outside.

The values come from the E3/DC home power station via MQTT and directly from the Proxon via Modbus RTU (RS485). Every switching action is reported via Pushover, and in the Node-RED Dashboard 2.0 I can see all values and conveniently adjust the thresholds with sliders.

Important for the Proxon: The Modbus interface uses 19200 baud, 8 data bits, parity even, 1 stop bit. With parity none you will not get any response.

Features

PV-controlled cooling (air conditioning)

• Switches on only if outside temperature > threshold AND battery level ≥ threshold AND PV power > threshold.
• The operating mode (holding register 16) and the cooling release (holding register 62) are written to the Proxon via Modbus.
• The outside temperature comes from the Proxon's own T3 fresh air sensor (input register 198).
• As long as no temperature value is available, cooling stays off for safety.
• After cooling, the unit is intentionally switched off completely (operating mode 0).

PV-controlled Boiler Boost (T300)

• The Shelly Plus 1 is switched via MQTT when battery level ≥ threshold AND PV power ≥ threshold – so hot water is produced with free electricity only.

Hysteresis – protecting the compressor

• Cooling: at least 15 minutes on, at least 10 minutes off.
• Boiler Boost: at least 10 minutes on, at least 10 minutes off.

Notifications and dashboard

• Pushover message on every automatic switch-on and switch-off, including the current values and thresholds.
• Dashboard 2.0 with gauges for outside temperature, PV power, house consumption and battery level.
• Status tiles for the heat pump operating mode (Off / Summer / Winter / ECO), the cooling function and the Boiler Boost.
• Sliders for all thresholds. The settings are restored automatically when the dashboard is reopened.
• A generic Modbus decoder with the complete register list of the Proxon (unit 41) and the T300 (unit 20) – easy to extend with your own values.

Data sources

MQTT topics (E3/DC and Shelly)

Proxon Modbus registers (unit 41)

The function node Register decodieren contains the complete register list with name, scaling, offset, unit and sign for unit 41 and unit 20. Polling is done in three blocks (holding 16–22, holding 62–70, input 195–223) every 5 minutes.

How the flow works

1. Polling and decoding Modbus: The Proxon registers are read and converted using the register list, and the operating mode, cooling state and outside temperature are extracted.
2. Collecting values: PV power, battery level, outside temperature and all slider values are stored in the flow context.
3. Controlling the cooling: The conditions are checked on every new value. If necessary, registers 16 and 62 are written via Modbus and a Pushover message is sent.
4. Controlling the Boiler Boost: The battery and PV thresholds are checked, the Shelly is switched via MQTT and a Pushover message is sent.

[modbus-read U41 holding 16-22 / 62-70 / input 195-223]  (every 5 min)
    │
    ▼
[Register decodieren] ──► [Extract heat pump / cooling status]
                                   │
                                   ├──► [Dashboard: operating mode, cooling, outside temp.]
                                   └──► (outside temperature → flow context)

[MQTT E3/DC: PV power, battery SOC, house consumption]
[Dashboard sliders: thresholds] ──► (flow context + global cache)
    │
    ├──► [Cooling check & switch (hysteresis 15/10 min)]
    │         ├──► [modbus-write U41 holding 16: operating mode]
    │         ├──► [modbus-write U41 holding 62: cooling release]
    │         └──► [Pushover]
    │
    └──► [Boiler Boost check & switch (hysteresis 10/10 min)]
              ├──► [MQTT → Shelly Plus 1 (T300 Boiler Boost)]
              └──► [Pushover]

[ui-control: dashboard connected] ──► [Restore slider values]

Importing and setting up the flow

You can find the complete flow in my GitHub repository in the file Klimaanlage-BoilerBoost-PV-Flow.json. Simply copy the JSON and paste it into Node-RED via Menu → Import.

Before the first start, the following points must be adapted to your own environment:

1. Modbus client

• Open the configuration node Proxon Modbus USBtoRS485.
• Enter the serial port of your RS485 adapter (e.g. /dev/ttyUSB0) and check: 19200 baud, 8 data bits, parity even, 1 stop bit, RTU-BUFFERED.
• Unit IDs: 41 for the ventilation heating, 20 for the T300.

2. MQTT

• Enter the address of your own broker in the configuration node mqtt-broker.
• Adapt the topics in the mqtt in nodes (Verbrauch Watt, Einspeisung Watt, Akku Ladung in %, PV-Boiler Boost Status).
• In the function node Boiler-Boost pruefen & schalten (Hysterese), adapt the Shelly topic in the line msgShelly = { topic: ... }.

3. Pushover

• Open one of the pushover api nodes and create the pushover-keys configuration with your own application key and user key.

4. Thresholds in the dashboard

The default values apply until a slider is moved for the first time.

5. Optional customizing

• Hysteresis: Change MIN_EIN_MS (minimum on-time) and MIN_AUS_MS (minimum off-time) in the two function nodes.
• Polling interval: Change the rate in the three modbus-read nodes (currently 5 minutes).
• Additional values: Every register from the list is available in the output of Register decodieren as u<unit>_<holding|input>_<address>.

Troubleshooting

• No Modbus response / timeouts: Check the parity (even!), the baud rate, the A/B wiring of the RS485 bus and the name of the serial port.
• Cooling never switches on: The outside temperature must be known. Check that input register 198 is being read and look at the status text of the function node (Temp. unbekannt = temperature unknown / zu kalt = too cold).
• Switching seems delayed: That is the hysteresis. During the minimum on-time or off-time, the node shows (Hysterese aktiv).
• Slider values lost after a restart: The thresholds are stored in the global context. To keep them across a Node-RED restart, configure a persistent contextStorage (e.g. localfilesystem) in settings.js.

Outlook

As the next extension for the cold season, I would like to control the heating function of the Proxon via Modbus as well. It should only switch on when outside temperature < threshold AND battery level ≥ threshold AND PV power > threshold – and switch off again when the values drop below the thresholds.

Download

The flow, the documentation (English and German) and the pictures are available in my GitHub repository. I am always happy about feedback, ideas and exchange – especially on energy management, PV, heat pumps and Node-RED.