This page has some really good information on heating system wiring in America. One thing I can't tell from it though, is what exactly the 24VAC call-for-heat and call-for-cool signals drive.
Presumably they don't supply the actual power for heating or cooling (24VAC wouldn't get you far), so I guess it is some kind of control signal, but what is the load? Does it drive a relay? If so what is the coil resistance, etc.?
In other words, what exactly is in the question marks here?