THRM ONE

A tiny, hackable OpenTherm Master device with Ethernet

The THRM ONE connects your OpenTherm-enabled boiler to your smart home. Powered by an ESP32-C6, it comes with ESPHome out of the box, and it seamlessly connects to Home Assistant in a variety of ways:

• Ethernet
• WiFi
• Zigbee (requires reconfiguration)
• Thread (requires reconfiguration)

Once connected to a supported boiler, the THRM ONE reads the status of the boiler, along with any faults. It can control the water temperature of both tap water and the heating system. When connected to Home Assistant, this device forms the backbone of your fully custom, fully local smart heating system.

Combine it with the CTRL ONE to get accurate readings of the temperature in each room, and integrate it fully with your smart home platform of your choice.

Place the wired water leak sensor (sold separately) under your boiler to instantly know if there's a water issue your boiler didn't catch. The onboard temperature sensor can provide a backup reading if your network goes down. Connect it via Ethernet for the ultimate in reliability, as boilers are often placed in a corner of the home that isn't well served by WiFi.

What's in it for you

There's a variety of home heating systems on the market that come with an "it just works" app, and they work great, once you create an account and log into their cloud. While they make some effort to integrate with other solutions such as Home Assistant, they often don't give you the full power outside of their app, and your heating system should never rely on your internet connection being up.

elvinhome is all about giving that power entirely to you. There's no app, and no cloud. You buy the hardware, you own it, and the implementation is in your hands. The THRM ONE gives you total control over your boiler. Build your own smart heating system from scratch, or rely on the plethora of ready-made solutions built by the community. That's the power of open source.

The THRM ONE is one part of that solution. Combine it with CTRL ONEs to read the temperature in each room, and connect it all via Home Assistant so your boiler isn't just controlled by the temperature in one room, but all of them. Then, choose a powerful solution from the Home Assistant Marketplace such as SAT & Versatile Thermostat to bring it all together.

Fully open source, right down to the hardware

This is a device you truly own, with the schematics and files to prove it. The THRM ONE's PCB is fully open source under the CERN-OHL-S license. This means that not only are the schematics available, but the original KiCad design files too. The THRM ONE is on the internet forever.

We benefit from the hard work of many others to implement the OpenTherm protocol and make it truly open for all, which the THRM ONE builds upon. As such, it just made sense to contribute back so that others may benefit too.

The source will be made available once the first units go on sale.

Easily connect to your boiler

The THRM ONE's case has slots to hang to screws on the wall, or it can simply be placed on the metal casing of the boiler through the included magnets in the base. All the terminals are easily accessible through a lid that clips on, no tools required.

The OpenTherm specification only provides a bare amount of energy to power the most basic of thermostats, so the THRM ONE must be externally powered. This can be done through the USB-C port that is also used to flash the device, or by wiring in any nearby 5-28V DC power source.

OpenTherm-enabled boilers only support one connected device at a time. The THRM ONE is an OpenTherm master, replacing your existing thermostat or gateway to become the sole device controlling the boiler. It then exposes full control to platforms such as Home Assistant, where you can combine multiple temperature sensors or smart thermostats such as the CTRL ONE to control your boiler for every room in the house!

Want to 3D print the case yourself? Purchase the THRM ONE without the case, and grab the 3D model files from Printables. You'll lose out on the magnets and laser-engraved lid, but it functions just the same.

Your choice of firmware

ESPHome is what the THRM ONE ships with out of the box, as it provides the most flexibility and a base for you to customise it further, while integrating deeply with Home Assistant.

But at the end of the day, this is a standard ESP32-C6, and it's possible to flash the firmware of your choice via the USB-C port! Ethernet connectivity on a device like this is a new innovation, so we contributed to the alternative open-source firmware OTGateway to support it. It will be in the next release.

Detect water leaks with the optional wired sensor

The water leak port on the side of the THRM ONE can be used to connect a wired water leak sensor, sold separately. Place the sensor under your boiler to give you peace of mind, and build automations to alert you if an issue is detected.

Any water pooling around the two contacts is detected in seconds, and reported alongside all other boiler status indicators.

Is my boiler supported?

First, check the manual of your boiler to see if it has terminals for OpenTherm. Many boilers support simple on/off thermostats and OpenTherm through the same terminals. If it does, you're in luck! However, while OpenTherm is a well defined standard, not every boiler manufacturer has implemented it perfectly.

It's worth doing a quick web search of your boiler model plus "ESPHome" to see if others have had success getting theirs to work. Some boilers might require some specific messages to be sent on start to work correctly, and this can be configured in ESPHome. Furthermore, support for each of the different settings depends on what's implemented by your boiler.

The Wiki for the OTGateway firmware has a great list of boilers that have been tested by the community. If your boiler's on this list, it will work with the THRM ONE. Don't worry if you don't see yours on here - it's just a list of ones that have been tested, and it's likely yours will work too. If it does work, share the love and add it! It's a wiki page that anyone can edit.

And in the unfortunate case where you can't get it to work, you can return the THRM ONE within the 14 day return period for your money back, no questions asked.

Tech Specs

*Cables are not included.

**The temperature sensor is a backup only, as its reading is affected by the heat from the THRM ONE and the boiler it's next to. It's intended to prevent your home from going below a safe temperature, not as an accurate measure of the temperature in the room.

***There is no support for PoE (Power over Ethernet). Perhaps in a future "Pro" version, if there's demand for it.

Do not operate the THRM ONE without a case. It will be placed near pipes and other metal objects that could short circuit the exposed contacts of the board. When purchasing the THRM ONE without a case, you must 3D print the case from the provided model files.

We are not responsible for damage caused due to the board making contact with conductive surfaces.

OpenTherm® is a registered trade mark of The Open Therm Association. THRM ONE is an independent product. It is not affiliated with, endorsed by, certified by or licensed by The Open Therm Association. The name is used only to describe interoperability with boilers that implement the OpenTherm® protocol.