OTThing

OTthing

A compact WiFi <-> OpenTherm (master & slave) interface

Project homepage

https://www.seegel-systeme.de/2025/01/05/ot-thing-das-universelle-wifi-opentherm-interface/

OTthing board

OTthing wiring

OTthing web interface

Home Assistant integration

Overview

OTthing is a versatile OpenTherm gateway that connects your heating system to the Internet via WiFi. It acts as an OpenTherm master (controlling your boiler) and/or slave (integrating with room thermostats), providing real-time monitoring and control through a modern web interface and Home Assistant integration.

Key Features

• Dual OpenTherm Modes: Operate as both an OpenTherm master and a slave simultaneously
• Single-Board Compact Design: Cost-efficient SMD-based single-board hardware with a small enclosure footprint
• OpenTherm Compatibility: Works with OpenTherm-compatible equipment that implements the functions supported by OTthing; available features vary by device
• USB-C Powered: Simple and reliable power supply through a standard USB-C connector
• Web Dashboard: Real-time status monitoring with system metrics
• Home Assistant Integration: Native MQTT discovery and seamless HA integration
• Multi-Zone Support: Control up to 2 heating circuits with independent parameters
• Time Program: Built-in scheduling for automatic heating setpoint changes
• Heating Curve Control: Outdoor-compensated flow temperature control with configurable curve parameters
• Flow & Setpoint Management: Fine-grained control for CH temperature targets, modes, and operating limits
• Raw OpenTherm Tools: Direct read/write request endpoint for diagnostics and advanced integrations
• Real-Time Telemetry: Live updates via WebSocket for logs and status changes
• REST API: Full HTTP interface for status, configuration, control commands, and topic discovery
• WiFi Provisioning: Built-in network scan and credential setup through the web interface
• OTA Firmware Update: In-browser firmware upload endpoint with automatic reboot on success
• Local Data Export: Device data and diagnostics export from the web UI for troubleshooting
• External Inputs & Sensors: Optional DS18B20, pulse input, or photointerrupter support (for example, condensate monitoring)
• Selectable Operating Strategies: Run OTthing as a full heating controller or in monitoring-first mode for telemetry-only operation
• Automatic Bypass Behavior: Allows you to restore conventional room-thermostat operation without rewiring
• Failsafe Runtime Status: Device health and communication state tracking for robust operation
• Local Control: Responsive web UI with no cloud dependency
• Modular Design: Extensible architecture supporting multiple integrations
• Open-Source Platform: Open hardware and firmware for custom extensions and modifications
• Advanced Configuration: Room modes (off/heat/auto), flow control, heating curves, and more
• Detailed Logging: Real-time log streaming to monitor system behavior
• Turbo Mode: Temporarily raises the flow target by a configurable amount and duration
• Automatic Suspend: Optional suspension based on outside temperature, room conditions, and minimum flow thresholds
• Customizable dashboard: add metrics to dashboard

Architecture

The firmware consists of several functional modules:

• OpenTherm Control (otcontrol.cpp): Manages master/slave communication with the boiler
• Master Requests (masterrequests.cpp): Builds and schedules OpenTherm master requests and polling cycles
• OpenTherm Values (otvalues.cpp): Stores, normalizes, and exposes decoded OpenTherm data points
• Heating Curve (heatingcurve.cpp): Calculates the target flow temperature from heating-curve parameters
• MQTT Integration (mqtt.cpp): Publishes device state and subscribes to control topics
• Device Status (devstatus.cpp): Tracks runtime health, connectivity, and status flags
• Auxiliary Input (auxInput.cpp): Handles external input signals for additional control logic
• Web Portal (portal.cpp): Serves the responsive dashboard and API endpoints
• Heating Control (CHcontrol.cpp): Manages heating circuits, setpoints, and modes
• Sensor Integration (sensors.cpp): Collects data from external sensors
• Configuration Management (devconfig.cpp): Handles persistent device settings

Hardware

• MCU: ESP32-C3 (or a compatible MCU with UART pins)
• Communication: OpenTherm interface
• Power: via USB
• Optional: DS18B20 1-Wire temperature sensors and auxiliary inputs

OpenTherm support and available controls depend on the boiler or other equipment. Check the device documentation and OTthing schematics before installation. Disconnect equipment from mains power before wiring; have boiler wiring performed by a qualified installer where required. Connect OpenTherm through the board's designated terminals, never directly to MCU GPIO pins.

Installation & Setup

1. Hardware Setup:

   • Connect the OpenTherm slave side (boiler / ventilation / solar storage) to the Boiler screw terminal on OTthing
   • Optionally connect the room unit to the Roomunit screw terminals on OTthing
   • Connect OTthing to a USB-C power supply

2. Access the Web Interface:

   • The device creates a WiFi access point for provisioning or connects to its configured network
   • The default access point password is 12345678; change it before deployment
   • Access the web UI at http://<otthing-ip>/ (default: 4.3.2.1)

3. Configure:

   • Set your boiler type and heating circuit parameters
   • Configure the MQTT broker if you are using Home Assistant
   • Adjust setpoints, heating curves, and control modes

Build and Flash (Developers)

Install PlatformIO, then run these commands from the Firmware/ directory:

platformio run --environment release
platformio run --environment release --target upload

The first command builds the firmware. The next two upload the firmware and the LittleFS web assets; perform both uploads when installing a complete build. The firmware project reads access point SSID, password, and hostname from default.ini unless overridden by the build environment. Set a unique access point password before deploying devices.

API Reference

OTthing exposes the following REST endpoints and WebSocket connection:

When web authentication is configured, protected endpoints require a valid login session. Requests without a valid session return 401; log in through the web UI before using these endpoints.

Core Status Endpoints

• GET / - Web dashboard (HTML)
• GET /status - Current device and boiler status (JSON)
• GET /config - Device configuration and settings (JSON)
• POST /config - Update device configuration (JSON body)
• GET /otitems - Raw OpenTherm items and values from master and slave (JSON)
• GET /topics - List all available MQTT control topics (text/plain)

Control Endpoints

• GET /set?key=value - Update settings via query parameters

  • Examples: /set?chSetTemp1=50, /set?chMode1=heat
  • Use /topics to discover supported control keys. URL-encode values when they contain reserved characters.
  • Returns 200 on success, 503 if the value cannot be set

• GET /slaverequest?id=X&rw=Y&data=HEX - Send a raw OpenTherm slave request

  • id: OpenTherm message ID (integer)
  • rw: 1 for READ_DATA, 0 for WRITE_DATA
  • data: Hex-encoded data value
  • Returns JSON with response type, ID, and data

System Endpoints

• GET /scan - Scan for available WiFi networks

  • Returns JSON with the SSID, RSSI, and channel for each network
  • Status field: -2 (scanning in progress), -1 (failed), 0+ (number of networks found)

• POST /setwifi - Configure WiFi credentials

  • Parameters: ssid and pass (URL-encoded)
  • Triggers a device reboot and a WiFi connection attempt

• GET /reboot - Trigger a device reboot

  • Returns 200 and schedules a reboot

• POST /update - Firmware update (binary upload)

  • Returns 200 on success, 500/503 on error
  • Triggers an automatic reboot if the update succeeds

Real-time Communication

• WebSocket /ws - Real-time updates and log streaming
  • Receives log messages and status updates
  • Can be used to monitor device behavior in real time

Testing & Development

A Python mock server is included for local UI and API testing. Install its dependencies and start it from the Firmware/ directory:

python -m pip install -r requirements.txt
python tools/mock_otthing.py

The server listens on http://127.0.0.1:8081/. It is useful for UI development and API integration, but it does not replace testing against physical OpenTherm hardware.

MQTT Notes

The device subscribes to control topics under otthing/<device-id>/<key>/set; /topics lists the supported keys. Home Assistant discovery is provided through MQTT when a broker is configured.

Changes sent through the web API are also published to the corresponding MQTT /set topic as retained messages. A broker can replay retained commands when the device reconnects, including after a reboot. Keep this in mind for temporary controls such as Turbo mode, and clear any stale retained command from the broker when needed.

Troubleshooting

• Cannot reach the web UI: Confirm the phone or computer is connected to the OTthing access point or the same network as the device, then check the device IP.
• No OpenTherm communication: Check the board terminals, wiring polarity where applicable, boiler compatibility, and the device's status and logs.
• Home Assistant entities are missing: Confirm broker connectivity and MQTT discovery settings, then review device logs and the broker's topics.
• A setting returns 401: Log in through the web UI and retry the request with the browser session.
• A temporary action returns after reboot: Check whether a retained MQTT /set command is replaying it.

Schematics

Discussion

https://community.home-assistant.io/t/ot-thing-an-opentherm-wifi-gateway-with-integrated-ot-master-slave/824667

Reporting issues

When reporting issues, please supply:

• Brand and model of the boiler and room unit
• OTthing firmware version and the steps that reproduce the issue
• expected behaviour
• Log
• Status JSON
• Configuration JSON
• OT items JSON
• Data history JSON

All of this data can be exported from the OTthing using the export function at the bottom of the web UI. Only one concern per issue!

Contributing

• Make changes in a new branch based on the develop branch.
• Address only one issue per PR.
• Create the PR against the develop branch.

Known Working Devices

Below is an excerpt of hardware devices reported to work successfully with OTthing. If you have verified additional devices, contributions and reports are very welcome!

Boilers & Heating Systems

• De Dietrich Inidens 20/24 MI
• De Dietrich MCR 24 Home
• De Dietrich MPX 24/29 MI Compact
• Domusa BioClass HM 16 (Pellet)
• Hoval TopGas
• Intergas HRE 36/30A
• Junkers Cerasmart ZSB 3-16 (with OpenTherm module OTM 1)
• Remeha Tzerra 28c
• Remeha Tzerra Ace
• Termet Gold Plus 2
• Viessmann Vitodens 100-W (B1KG)
• Westen Boyler Digit+
• Windhager BioWin (Pellet)

Room Units & Thermostats

• Baxi RC 06.20
• De Dietrich AD304
• De Dietrich iSense (AD289)
• Honeywell DT4M
• Hoval RS-OT
• Junkers TF 30
• Siemens QAA73
• Theben / Windhager RAM 786

Feel free to open a Pull Request or issue to report additional compatible devices!