Yes, safety and robustness were my primary concerns while designing this project. This house is more like a vacation home, and we don’t live there permanently. So my main goal is being able to control remotely as much of my heating system as possible, making sure not to burn the house down at the same time :) Here is what I took into consideration:
1. My device interfaces with the boiler through optocouplers, so I don’t touch boiler circuits with my crappy engineering skills.
2. The boiler itself, as well as OpenTherm protocol, seem to be able to recover automatically from many failure modes. It doesn’t matter much if some frames are dropped or mangled, since full set of control signals is repeated every couple of seconds.
3. I rewrote critical parts of OpenTherm component for ESPHome to make code more resilient to failure. The whole firmware is designed in a way that incorrect values or broken frames don’t bring anything down catastrophically. I also made sure that boiler has its own maximum temperatures set to sensible values, so that incorrect signals from the thermostat will be discarded.
4. The device is designed to be completely self-contained and doesn’t rely on Home Assistant or Wi-Fi being up.
There are a couple of things that I plan on adding to this setup, like a Shelly relay on boiler power input to be able to do a hard restart remotely.
Ideally, I would also prefer not to McGyver a custom device, but unfortunately, there are no such dumb smart thermostats for my boiler that would allow remote control.