You’re thinking of a relay cutting power to the radiator as a whole. That is indeed not a good idea and may damage it long term. - Fil pilote / adomotique

see also

• Automating Heating with Home Assistant

Controls Fil pilote

There are 6 commands in total. Two of them require a microcontroller to generate, as they use precise signal timing. The other 4 can be generated using relays and diodes. Two basic commands can be generated without a diode.

Without diode, only a relay needed:
0V : Mode confort
230V: Mode eco

With a diode:
Negative half wave: Mode hors gel
Positive half wave: Turns off the heater

• Utilisation des diodes - diodes IN4007