An idea for anyone building such a system in a region where temperatures go low enough to freeze pipes:
Make it fail-safe. What if your Ardu-stat turns the heat completely off, by malfunction?
Easy to do:
Keep your old thermostat in the circuit. Set it to turn the heading on at, say, 40 deg F.
Wire your Arduino circuits in parallel across your existing thermostat (assuming that your system turns the heat ON when a switch in the thermostat CLOSES).
Now your Arduino can ADD heat to your house, but even if the Arduino is never calling for heat, the old thermostat can step in, save the day, should the temperature fall to 39.
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