Wire letter guide
W2 and AUX Thermostat Wires: Second-Stage and Auxiliary Heat
W2 is a second heat stage: a two-stage furnace runs on low most of the time and uses W2 on cold days. On a heat pump, W2, AUX, or X2 on older Carrier/Trane thermostats runs backup electric heat strips.
The Amazon Smart Thermostat has one heat terminal, so it can’t run a second furnace stage.
What to set in the app
Wiring W2 or AUX is only half the job. In the thermostat’s equipment settings, tell it you have a second heat stage or auxiliary heat and what type it is (electric strips, gas or oil). On a heat pump, most thermostats also let you choose an outdoor temperature above which aux heat is locked out. Setting that lockout too low makes the home slow to warm up on cold mornings; setting it too high wastes electricity on strip heat.
Common problems
- Aux heat light on all the time: the heat pump may be low on refrigerant or iced up, or the O/B setting is reversed. That needs a technician, not a wiring change.
- Two-stage furnace never goes to high fire: check that W2 is connected at both the thermostat and the furnace board, and that the furnace’s own staging switch isn’t set to "auto" mode that ignores W2.