@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...