Most DIY heating control starts with a generic dev board, a hand-wired OpenTherm interface and a pile of custom YAML to glue it all together. The THRM ONE takes a more finished route: a dedicated, ESPHome certified board designed to connect to an OpenTherm boiler and expose full control in Home Assistant. It is fully open source hardware, and the maker built it to broaden the range of heating systems supported beyond CTRL ONE, the thermostat with a touchscreen ePaper display released about half a year earlier.
Under the hood sits an ESP32-C6, configurable for WiFi, Thread or Zigbee, whichever works, alongside 100Mbit Ethernet on an RJ45 jack. Ethernet was chosen for reliability, as boilers often sit in corners with poor reception. The OpenTherm interface is opto-isolated, keeping the boiler terminals apart from the rest of the board, and power comes in over USB-C or 5-28V DC through screw terminals.
Construction looks tidy. There are terminal screw blocks for the OpenTherm connection and a leak sensor, plus a SOICbite programming header, all inside a compact white case with screw mounts and magnets in the base for fixing to the boiler's metal casing. A wired leak sensor can be placed under the boiler and expands the diagnostics, while a simple onboard NTC acts as a backup temperature sensor that keeps things from freezing if Home Assistant goes down. Testing covered a simulated boiler and a real one, with insulation resistance measured at over 540 MΩ.
One thing is still open: the maker needs more data on how the device behaves with a wider range of real boilers. The current revision is v0.2, which sorted out the excessive ripple voltage that crashed the first prototype, and the giveaway units are final prototypes carrying minor issues like slightly off NTC temperatures, none of which affect OpenTherm functionality. KiCad files under the CERN-OHL-S license will follow once the first units go on sale.
Project by elvinhome. Watch it on their YouTube channel: