X and X2 Thermostat Wires Explained

X2 appears on older Carrier, Bryant and Trane heat pump thermostats and runs auxiliary or second-stage heat. It connects where AUX or W2 would.

X alone is inconsistent: some manufacturers use it for the common wire, others for emergency heat. Don’t guess. Check which terminal that wire connects to at the air handler, or measure 24 V between R and X with the system powered (common will read about 24 V to R).