Wiring guide
Amazon Smart Thermostat wiring for 3 wires, heat + fan
The Amazon Smart Thermostat is not compatible with this setup. See the alternatives below.
Three wires on R, W and G control a furnace’s heat and blower but not AC, and there is no C wire. Some smart thermostats cope; others need extra wire. Read the full 3 wires, heat + fan guide for how to confirm your setup and mistakes to avoid.
Wiring diagram
Detected system: 1-stage heat, fan, no C wire · 3 wires
Amazon Smart Thermostat
Not compatibleNo C wire and too few wires for an adapter. You’ll need a new 18/5 thermostat cable from the equipment to the wall.
| Wire | Color | Job | Connect to | Note |
|---|---|---|---|---|
| R | red | 24 V power | R | |
| W | white | Heat, stage 1 | W | |
| G | green | Fan | G |
Install steps
- Turn off power to the furnace/air handler (switch or breaker) and the AC condenser. Confirm the old thermostat is dead.
- Photograph the old wiring, then label each wire with the letter it came off. Ignore short jumpers.
- Connect each wire to the Amazon Smart Thermostat terminal shown above. Cap unused wires one at a time.
- Restore power and finish setup in the app.
- Test heat, cool and fan. If a breaker trips or equipment runs nonstop, power off and call a pro.
What you need
- Amazon Smart ThermostatThe thermostat itselfCheck price
- 18/5 thermostat wireAdds the missing C wireCheck price
As an Amazon Associate we earn from qualifying purchases, at no cost to you.
Confirm against the official Amazon install guide. The app also checks your wiring during setup.
Frequently asked questions
Where does each wire go on the Amazon Smart Thermostat?
R → R; W → W; G → G.
Do I need a C wire for the Amazon Smart Thermostat?
No C wire and too few wires for an adapter. You’ll need a new 18/5 thermostat cable from the equipment to the wall.