Nice to see I'm not the only one who grabbed the schematic.
Looks good. I like how you've used the control/data lines, should make the interface program easier. Let not forget the vital green LED!
If the time constant for your system is large enough, you could get away with just the simple on/off thermostat fan control you have shown, though I wouldn't mind seeing a PWM controller attached to your newly tasked error amplifier.
What sort of plans have you for the digitals ins and outs?