Wire letter guide
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).
Measuring an unknown X wire
- Turn the power off and disconnect the X wire from the old thermostat. Keep its bare end away from everything else.
- Turn power on and measure between R and the X wire with the meter on AC volts.
- About 24 V with no heat called means X is the common side. Close to 0 V means it is a control wire (usually emergency or aux heat) that only gets power when the thermostat calls for it.
If the readings are unclear, confirm at the air handler which terminal the wire lands on before you connect it.
Look up the old thermostat’s manual
The quickest way to settle an X or X2 wire is the installation manual for the thermostat you are removing. Note the brand and model number (usually on the back of the faceplate or on the wall plate) before you take it down; manuals for most thermostats from the last few decades are available online, and their terminal table says exactly what X and X2 do on that model.
Where X2 goes on a new thermostat
When X2 is confirmed as auxiliary heat, it goes where AUX would: AUX or W2/AUX on Nest, W1 (aux) on ecobee in heat pump mode, W2/Aux on the Honeywell T9, and W on the Amazon Smart Thermostat. The tables below show this for a typical heat pump.