[buffalobillpatrick]

2 Arduinos in Heating system. (I play with more)

Master powers up Slave HP Controller via R5 when HP is needed, or powers off Slave when not.

Slave tells Master that it is powered on & functioning via "HP ALIVE Signal" which is active within 10 seconds of its Power On.

This is done in Slave by energizing ALIVE_RLY_R1 via its pin D4

The Slave frequently calls its custom_delay function,
which: Delays 5 sec. & Checks Source Out water temperature & GAS Level inside HP

If GAS LIMIT is EXCEEDED, then Audible Alarm is activated & ALIVE_RLY_R1 is reset via pin D4.

or if Source out is too cold to run HP, then ALIVE_RLY_R1 is reset via pin D4.

The Master reads this "HP ALIVE Signal" on its pin D12
in its code after powering On Slave

if "HP ALIVE Signal" is found to be "OFF"
then the Master powers off Slave by de-energizing R5

and "Downshifts" from using HP to using Boiler.