Since you figured out the wiring situation, I'm going to throw this in unsolicitedly: it's probably a good thing to replace the thermostat with a smart thermostat at the same time. Energy companies typically team up with Home Depot and Lowes to offer instant rebates on smart thermostats, anywhere from 50-150$ off sale value. Check on your energy company's website for a coupon. I picked up a couple ecobee smart thermostats for 15 bucks each when I rewired my furnace.