That part I do know about, but thanks anyway. I have percolating in my head ideas about how to, for example, switch between manual and automatic control, and how complex I want to get (likely, not very). But the auto mode will function only from noon to 8pm, or something like that, and will just turn it on and latch, and let the unit's thermostat function. And I'll put in some code to use a switch to go between manual / auto, or off / on / auto. Simplest thing is to just hit the off button on the unit at night, and then remember to turn it on in the morning, so the Arduino can then turn it on if need be.