Thank you Brian Carlton. Me, being the dummie I am, replaced parts with ones I thought were good. Turns out, my "good" monovalve's solenoid would not energize. The "new" one I have, I applied voltage to and it clicks just fine. Seems like the heat varies with temp dial now, though I didn't take it on the obligatory test drive yet.

NOTE TO SELF: Test parts before installing... Boy do I feel