@vali I was able to test a similar theory with a pair of 438mhz ones here.
Get any simple RF 868mHz pair of modules (4pin probably), and then record the real device triggering, using the rx module connected to something like a raspberry pi to do the recording. Then play that recording back through the transmit module and see if the boiler triggers. Repeat the trigger test with off and then see if it still works for both. If so you have basic success, now either look at decoding, or just add your own thermometer and send off/on message from the esp instead of raspberry pi. Not that simple in reality but not crazy complex either. Combine a few tutorials/videos...

I have a 110mA max (16uS, but about ~63mA average) 5V sensor, that will take +/- 10%, and I wondered if there was an onboard boost converter for the display when running on battery or does it just use battery voltage? What does the +5V pin access when powered by battery?

@moumi32 examples / factory
or examples / arduino / factory

This post has instructions with link to official repository and flashing instructions: https://community.lilygo.cc/post/295

There is also a GMT offset and DST (daylight savings) you can modify in the header file pin_config.h

Look at the timezone functions for esp32 and you'll see you can specify a timezone instead and forget about offsets, but requires changing one line.