I built a “dipping bird” boiler reset (metaphorically). Our old boiler had a fault where it would lock out every few months until manually reset. So, I wired a normally closed relay to it and a 10 line Arduino sketch to open the relay 1 minute out of every 120.
That ran for about 7 years; when I was researching replacing the boiler with air-to-water heat pump, I had to prove the house could be heated with lower temp water, so I changed to an ESP8266, added a platinum temperature sensor, a webserver, data logging, and ran a bunch of experiments with lower supply temps to see how the house would react.
https://imgur.io/a/VM7nD74 (that chart is entirely SVG, the whole content screenshotted was generated on the ESP)