Dave,
Nice project. One of these days I will get off my butt and write up mine and post it somewhere. I took a somewhat different approach, using the x10 universal modules to directly control the HVAC calls. (My controlling program reads the temperature from one of several selectable thermometers in various locations around the house.) So, although the commercial thermostat is not absolutely needed, I connect it in series with my homebrew circuitry as a safety mechanism: for example, if I intend to control the house temperature between, for example, 64°F and 70°F, I set the commercial thermostat to 73 degrees Fahrenheit. If my system is working correctly, the commercial thermostat is always trying to call heat. However, if the x10 unit that calls for heat fails closed, when the temperature reaches 73°F the commercial thermostat heater-call will open.
I wanted to include an image file with this message but I couldn't figure out how to upload it!?
Regards,
Joe