Now this would be fine, if we were just interested in manual control, but we actually want this to be automatic, like the original, based on temperature, so we need to give the Arduino the ability to "switch" these circuits, which are nothing more than 24vac relay coils in the furnace and air conditioner. I'm not sure what the current is that those coils pull, but I believe that these SSR's will handle it.
The next step is the DS18B20 Temperature chip, as described previously in this blog, and at http://www.hacktronics.com/Tutorials/arduino-1-wire-tutorial.html
