Oil fired forced warm air heating system with central AC. The t-stat is supposed to have an "auto" system setting, which alows heating system to come on if needed, but then automatically have the Air-Cond come on instead if, for example, I wake up to a real warm day ahead, and vice-versa. Supposed to have settings... Heat/Cool/Auto/Off. I don't have the Auto setting. Booklet I have says the auto setting may not be there depending on how it was wired. But doesn't say anything more about how to fix it. That was a nice feature I liked, being able to have it set on automatic switching back and forth from heat to cool, and they advertise it. So, I guess it must mean the installer did not use an extra wire that must have been needed? Would have been simple enough to do when it was being done, so the installer must not have known or cared about it?