The system operates by sending current signals from the boiler unit to the room unit and voltage signals inthe reverse direction.
So, the system communicates by varying the voltage and the current levels:
The most on/off room thermostats are not using mechanical relays, because the lack of space and I have never seen noisy room thermostats. I think they uses something like octocouplers as well, at least something small.
Don't think so. It's either voltage or current that's controlled, not both. Can't be both as they depend on each other.