Smart thermostat wiring guides

Google Nest Learning Thermostat (4th gen)Learns your schedule, supports up to 3 heat and 2 cool stages, heat pumps with 2 stages of aux heat, and dual transformers (pro install recommended).
ecobee Smart Thermostat PremiumRoom sensor, air-quality monitor and built-in Alexa. Handles 2 heat / 2 cool stages, heat pumps with 2 aux stages, and humidifiers via ACC terminals. Power Extender Kit included.
ecobee Smart Thermostat Enhancedecobee’s mid-range model with the same core wiring as the Premium (no accessory terminals). Power Extender Kit included.
Honeywell Home T9 Smart ThermostatRoom-by-room sensors and the widest label support: accepts K (wire saver), L/A, Rh/Rc and E terminals. Needs a C wire or wire saver.
Amazon Smart ThermostatThe budget pick, set up through Alexa. Simple systems only: single-stage heat/cool or a single-stage heat pump. Needs a C wire or Amazon’s C-wire adapter.

Compatibility at a glance

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