This script can be used on a Shelly 2PM (gen3) to nicely control a french heater that's using the "fil pilote" protocol.

shelly-french-heater.js

const CONFIG = {
  // ID of the virtual component to select the mode
  componentId: 200,
  // ID of the relay with the diode providing a negative wave
  negativeWaveSwitch: 0,
  // ID of the relay with the diode providing a positive wave
  positiveWaveSwitch: 1,
};

const simpleModes = {
  comfort: [false, false],
  eco: [true, true],
  antifrost: [true, false],
  off: [false, true],
};
const timerDurations = {
  comfort_minus1: 3,
  comfort_minus2: 7,
};

CONFIG.componentName = 'enum:' + CONFIG.componentId;
let currentMode = null;
let timer = null;

Shelly.call('Enum.GetStatus', {id: CONFIG.componentId}, function (res) {
  print('Initial mode', res.value);
  currentMode = res.value;
  handleModeChange();
});

Shelly.addStatusHandler(function (evt) {
  if (evt.component !== CONFIG.componentName) { return; }
  print('New mode', evt.delta.value);
  currentMode = evt.delta.value;
  handleModeChange();
});

function setOutput(a, b) {
  Shelly.call('Switch.set', {id: CONFIG.negativeWaveSwitch, on: a});
  Shelly.call('Switch.set', {id: CONFIG.positiveWaveSwitch, on: b});
}

function handleModeChange() {
  print('Mode changed to', currentMode);
  if (timer !== null) {
    print('Clearing timer');
    Timer.clear(timer);
    timer = null;
  }
  const simple = simpleModes[currentMode];
  const duration = timerDurations[currentMode];
  if (simple) {
    print('Simple mode -> updating relays:', simple);
    setOutput.apply(null, simple);
  } else if (duration) {
    print('Time-based mode -> starting timer:', duration);
    timedCycle(duration);
  } else {
    print('Invalid mode', currentMode);
  }
}

function timedCycle(duration) {
  const onDuration = duration * 1000;
  const offDuration = 300000 - onDuration;
  print('Starting ON cycle', duration);
  setOutput(true, true);
  timer = Timer.set(onDuration, false, function () {
    print('Starting OFF cycle', offDuration / 1000);
    setOutput(false, false);
    timer = Timer.set(offDuration, false, timedCycle, duration);
  });
}