Y, Y1 and Y2 Thermostat Wires: Cooling and Compressor Stages

Y / Y1 starts the outdoor compressor: cooling on an AC, or both heating and cooling on a heat pump. Y2 means a two-stage compressor, which runs on low most of the time and steps up in extreme heat. The Amazon Smart Thermostat has a single Y terminal, so it can’t run a second stage.

Where it connects on each smart thermostat

Two-stage AC

Wires: R, C, W, Y1, Y2, G · open in the tool

ThermostatY goes toWhole setup
Nest Learning (4th gen)—Works: direct swap
ecobee Premium—Works: direct swap
ecobee Enhanced—Works: direct swap
Honeywell T9—Works: direct swap
Amazon Smart Thermostat—Not compatible

Other wire letters