Power line comms X-10

[dhouston]

... I will get inspiration from the circuits particularily the mains interfaces, and just use some simple PPL tone switching at 2 different frequencies, the didtances are pretty short

I think there's an even easier way that doesn't require power-line comms. You can use a current transformer or hall-effect current sensor from Allegro to sense when the geyser is on or off and use RF links to communicate.