Just to clarify to anyone who may care what I am planning to do and why I
asked the original question...

I wanted to know which two wires control the fan On/Auto function. I have
since learned from this thread it's green and red. I *will* open the
thermostat to double check that before I do anything else.

I plan to connect a normally-open relay in parallel with the On/Auto
switch. The relay will be controlled by my home automation system. This
will enable me to turn the fan On/Auto from my automation system. I am
not trying to control the heating/cooling functions of my HVAC; just the
fan On/Auto.

Assuming I pick the correct pair of wires (which I think I can now do even
though I am not a "professional") I don't see how this will damage my HVAC
system. If anyone thinks I'm wrong about that I'd certainly like to hear
what you have to say.

Peter
--
**Remove Xs to reply by e-mail**