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ThermTerm: an open source heat pump controller and home automation terminal, built on ESP32


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ThermTerm: an open source heat pump controller and home automation terminal, built on ESP32


Control heat pumps manually or remotely via MQTT/Home Assistant.

I love my heat pumps. They're energy efficient and the fastest way to heat or cool any room.

But I've always hated the remote controls that come with heat pumps. They're clunky and hard to read, especially in low light. In theory, you can program schedules for your heat pumps, but in practice the remotes are too frustrating to use for that.

ThermTerm solves all the problems I've had with these physical controls, while integrating the heat pumps into Home Assistant via MQTT. Additional sensors pipe temperature, humidity and light data, for statistics and automation. Out of the box, ThermTerm supports Mitsubishi's infrared protocol, since that's the system I have. Thanks to integration with IRRemoteESP8266, you can make ThermTerm work with your own heat pumps by writing a few lines of adaptor code. No signal analysis or reverse-engineering required. See Customization for more details.

Note: you will need PlatformIO to get started.



Press the dial to toggle power.

Turn the dial to set temperature.

Change to heat mode with the bottom button, cool mode with the top. Set fan speed by pressing the center button, then turn the dial.

Through a TFT screen, readouts are visible any time of day. The chunky dial is easy to press without looking. Program automations using Home Assitant, not miserable rubber buttons like a VCR. Toggle your heat pumps from any room in the house, or even while you're away.


  • Make a heat pump controller I don't hate using
  • Use easy-to-repurpose IR, instead of a manufacturer-specific serial connection
  • Structure this project into discrete objects to make iteration, maintenance and enhancements easy
  • Provide hobbyists with a reference for Arduino code with that kind of structure (I had a hard time finding it for end projects, only libraries)
  • Build a device that expands the surface area of a home automation system, letting it gather more data, take more user input, and provide more feedback
  • Leverage as much of the existing Arduino OSS code ecosystem as possible, keeping the project tightly scoped to its problem domain

Shoutouts/core dependencies

  • Adafruit, whose thorough libraries and thoughtful breakout boards made hardware integration a breeze
  • IRRemoteESP8266, a library with bulletproof software-based PWM, a great API, and a shocking compendium of IR protocols
  • arduino-home-assistant, an MQTT library with exhaustive implementations of Home Assistant-supported devices

Bill of materials

The sensors use I2C, so with a light rewrite, you can replace the above models with alternatives if needed, or skip them entirely if you don't care about recording data back to Home Assistant.

If you want to design your own enclosure and use a different display, I suspect any ESP32-based device will work fine.

There's not a huge upside to the battery in the current implementation but I designed the case to accommodate it for future enhancements. You can skip it unless you want to tinker with power outage-related home automation tasks.


Make a copy of _Credentials.h.example, rename it to _Credentials.h. This file will be ignored by git.

Fill in these values as appropriate to your network:

#define WIFI_SSID ""       // your network SSID (name)
#define WIFI_PASSWORD "" // your network password

#define MQTT_LOGIN ""
#define MQTT_PASSWORD ""

#define MQTT_BROKER IPAddress(192, 168, 1, 100)

Home Assistant

With correct MQTT credentials, ThermTerm should be discovered automatically in the Home Assistant MQTT configuration.

To troubleshoot, uncomment this line in platfomio.ini:

build_flags = -D ARDUINOHA_DEBUG

This will log MQTT events and errors via serial.


The most urgent customization for most will be making this work with your own heat pump.

  1. Write a subclass of IRInterface. You can use MistubishiInterface as a starting point. The goal is to translate the system controller's state model into whatever features you care about for your own heat pump. Consult IRremoteESP8266 for the API that corresponds to your manufacturer.
  2. In main.cpp, replace the MitsubishiInterface instance in the SystemController constructor with your own interface:
    YourCustomInterface customInterface;
    SystemController controller(customInterface);

That should get you moving.

To tweak other details about the device, explore _Constants.h. To adapt this to alternative hardware, including other displays, you'll want to replace the implementations in src/hardware.


If you want to go all-in and asemble one of these for yourself, 3D printing files have been provided. See for detailed instructions.


There's a lot more this platform could do, but this is the minimally-useful implementation that can get your whole heat pump system integrated in with an afternoon of work. Future features:

  • Celsius support (what can I say, I think in Fahrenheit)
  • Display alerts issued from Home Assistant
  • Trigger alerts distributed to all other ThermTerm devices on the network
  • Set preferences, including WiFi and MQTT credentials, via serial terminal
  • Bulk provisioning


If you have anything useful you'd like to give back, feel free. Of particular interest:

  • A simple Celsius toggle. A preprocessor macro would be fine to start with. All the internal state is managed in Celsius, and SystemState already has partial support for it.
  • I can only test with Mitsubishi 144-bit IR receivers, since that's all I have. If you have a working interface for a different manufacturer, open a PR to add it!
  • This is the most C++ I've ever written. If there's anything goofy that could be done differently, feel free to propose it
  • Relatedly, if you have thoughts on making this project more extensible with other displays and ESP32 variants, I'd love to hear about it
  • Enhancements to documentation, or comments on code you found confusing, are great too