The old thermostat is placed quite far from any place I could plug in a transformer to... (otherwise, yes, that would have been the solution)
My current setup has the relay right next to the furnace, where there's plenty of mains lines to tap into for continuous power and this is fine since it doesn't measure anything and just gets commands from hass.
My current setup has the relay right next to the furnace, where there's plenty of mains lines to tap into for continuous power and this is fine since it doesn't measure anything and just gets commands from hass.