markpeace:
Thanks, that's useful - so to paraphrase, and check I understand, anything that controls an external solenoid, element, etc should be on a circuit that uses a pull-down resistor, rather than the internal pull-up resistor.
Yes.
That way, if the arduino dies - and the voltage reverts to low - it will close/turn off.
Yes. "Fail safe". For some control points it may not matter. For others (like a heater) it is very important.
Can you see anything I've done wrong?
For the Boiler / Fill LED, where is pin 37 connected?