Thanks for the replies! I checked the monovalve by pulling, cleaning and reconnecting the wire, then tapped on the top of the unit to see if it was simply stuck. The heat hasn't been coming on since then, so I'm thinking y'all were right. I also noticed that the center vents don't seem to have air blowing out of them, regardless of climate or fan settings, so think the blower might be gone as well. I want to go ahead and rebuild the monovalve when I get into that blower motor, but haven't found a repair kit as described. Any leads on where that can be found?

Thanks again for the help, y'all rock!