Wiring guide

Thermostat wiring by setup

Find the setup that matches your wall, then see if each smart thermostat works and exactly how to wire it.

Compatibility matrix

Setup	Nest Learning (4th gen)	ecobee Premium	ecobee Enhanced	Honeywell T9	Amazon Smart Thermostat
2 wires, heat only	Works with an adapter	Works with an adapter	Works with an adapter	Works with an adapter	Not compatible
3 wires, heat + fan	Works with an adapter	Works with an adapter	Works with an adapter	Works with an adapter	Not compatible
4 wires, no C wire	Works: direct swap	Works with an adapter	Works with an adapter	Works with an adapter	Works with an adapter
5 wires with C wire	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap
AC only, with C wire	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap
2-stage heat + 2-stage cool	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap	Not compatible
Heat pump, no C wire	Works with an adapter	Works with an adapter	Works with an adapter	Works with an adapter	Works with an adapter
Heat pump with C wire	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap
Heat pump + emergency heat	Works: direct swap	Probably works: verify first	Probably works: verify first	Works: direct swap	Probably works: verify first
Heat pump with B valve wire	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap
B wire used as common	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap	Works: direct swap
Rh + Rc dual transformer	Pro install recommended	Pro install recommended	Pro install recommended	Pro install recommended	Not compatible
K wire (Honeywell wire saver)	Pro install recommended	Pro install recommended	Pro install recommended	Works: direct swap	Works: direct swap

All setups

Guide2 wires, heat only1-stage heat, no C wire · 2 wires
Guide3 wires, heat + fan1-stage heat, fan, no C wire · 3 wires
Guide4 wires, no C wire1-stage heat, 1-stage cooling, fan, no C wire · 4 wires
Guide5 wires with C wire1-stage heat, 1-stage cooling, fan, C wire present · 5 wires
GuideAC only, with C wire1-stage cooling, fan, C wire present · 4 wires
Guide2-stage heat + 2-stage cool2-stage heat, 2-stage cooling, fan, C wire present · 7 wires
GuideHeat pump, no C wireHeat pump, 1-stage compressor, 1-stage aux heat, fan, no C wire · 5 wires
GuideHeat pump with C wireHeat pump, 1-stage compressor, 1-stage aux heat, fan, C wire present · 6 wires
GuideHeat pump + emergency heatHeat pump, 1-stage compressor, 1-stage aux heat, emergency heat, fan, C wire present · 7 wires
GuideHeat pump with B valve wireHeat pump, 1-stage compressor, 1-stage aux heat, fan, C wire present · 6 wires
GuideB wire used as common1-stage heat, 1-stage cooling, fan, C wire present · 5 wires
GuideRh + Rc dual transformer1-stage heat, 1-stage cooling, fan, no C wire, dual transformer · 5 wires
GuideK wire (Honeywell wire saver)1-stage heat, wire saver on K (fan + cooling), C wire present · 4 wires