My Rinnai tankless water heater can be connected to a wifi module that controls the recirculating pump and other functions.

The only connection between the PCB of the water heater and the external wifi modules consists of two wires, not polarized.

This powers the wifi module (which does NOT seem to have any battery inside) and allows the controller to communicate with the heater; I believe comm is half duplex only, but I am not 100% sure.

Te only interface I know that uses onmly teo wires for power and comms is 1-wire from Dallas, but I doubt it can deliver enough power for wifi and it is polarized.

What interface/protocol can it be? I am quite suprised

Water heater: Rinnai RUR199i Wifi Controller: Rinnai Control-R