Wire My ThermostatWhich wire goes where? Get your exact thermostat wiring.
Replacing your thermostat with a Nest, ecobee, Honeywell or Amazon smart thermostat? Tick the letters on your old one (or snap a photo) and see which models work with your wires, which adapter you need, and a wiring diagram you can follow.
- Covers 5 top smart thermostats
- Heat pumps, no C wire, 2-stage, dual transformer
- Based on manufacturer wiring guides
- Free, no sign-up
1Snap a photo of your old thermostat’s wires (optional)
AI reads the terminal letters and wire colors, then you double-check them. Photos are not saved.
2Tick every terminal that has a wire
Pull the faceplate off your old thermostat. Use the letter printed next to each wire, not its color. Skip short jumper wires between two terminals.
More labels (W3, X, X2, K, PEK, L, S, T, U, HUM, DEHUM)
3A few quick questions
4Your wiring
Tick at least one terminal above and your wiring shows up here instantly.
Popular wiring setups
- 2 wires, heat only1-stage heat, no C wire · 2 wires
- 3 wires, heat + fan1-stage heat, fan, no C wire · 3 wires
- 4 wires, no C wire1-stage heat, 1-stage cooling, fan, no C wire · 4 wires
- 5 wires with C wire1-stage heat, 1-stage cooling, fan, C wire present · 5 wires
- AC only, with C wire1-stage cooling, fan, C wire present · 4 wires
- 2-stage heat + 2-stage cool2-stage heat, 2-stage cooling, fan, C wire present · 7 wires
- Heat pump, no C wireHeat pump, 1-stage compressor, 1-stage aux heat, fan, no C wire · 5 wires
- Heat pump with C wireHeat pump, 1-stage compressor, 1-stage aux heat, fan, C wire present · 6 wires
- Heat pump + emergency heatHeat pump, 1-stage compressor, 1-stage aux heat, emergency heat, fan, C wire present · 7 wires
- Heat pump with B valve wireHeat pump, 1-stage compressor, 1-stage aux heat, fan, C wire present · 6 wires
- B wire used as common1-stage heat, 1-stage cooling, fan, C wire present · 5 wires
- Rh + Rc dual transformer1-stage heat, 1-stage cooling, fan, no C wire, dual transformer · 5 wires
- K wire (Honeywell wire saver)1-stage heat, wire saver on K (fan + cooling), C wire present · 4 wires
Wiring guides by thermostat
- Google Nest Learning Thermostat (4th gen)Google Nest
- ecobee Smart Thermostat Premiumecobee
- ecobee Smart Thermostat Enhancedecobee
- Honeywell Home T9 Smart ThermostatHoneywell Home
- Amazon Smart ThermostatAmazon
Decode your thermostat wire letters
- R Wire on a Thermostat
- Rh and Rc Thermostat Wires Explained
- What Is a C Wire?
- W and W1 Thermostat Wire
- W2 and AUX Thermostat Wires
- Y, Y1 and Y2 Thermostat Wires
- G Wire on a Thermostat
- O/B Thermostat Wire
- B Wire on a Thermostat
- E Terminal on a Thermostat
- X and X2 Thermostat Wires Explained
- K Terminal on a Thermostat
- L, S and T Thermostat Terminals
How it works
- Read your old wiring. Upload a photo or tick each terminal letter that has a wire.
- We identify your system: furnace, heat pump, stages, C wire, and tricky labels like B, X and K.
- Compare thermostats and get a diagram, parts list and app settings for the one you pick.
Before you start
Frequently asked questions
Does thermostat wire color matter?
Not reliably. Installers often reuse colors, so a white wire can be on Y and a blue wire on O/B. Always go by the letter of the terminal the wire was connected to. See our wire colors chart.
What if I don’t have a C wire?
Most homes can still use a smart thermostat. ecobee includes a Power Extender Kit, Honeywell uses a wire saver, Amazon sells a C-wire adapter, and Nest usually runs without one. The tool picks the right option for your wires. See the no C wire guide.
Which smart thermostat works with my wires?
Tick your terminals above. The tool grades the Google Nest Learning (4th gen), ecobee Premium, ecobee Enhanced, Honeywell Home T9 and Amazon Smart Thermostat for your exact wiring and shows the wiring diagram for each.
Is it safe to replace a thermostat myself?
Most 24 V thermostats are a common DIY job once power is off at the furnace or breaker. Stop if you see thick wires with wire nuts or 120/240 V labels (line voltage), or if a breaker trips. Read our step-by-step safety guide.
Do you store my photo?
No. The photo is sent to our AI model only to read the terminal letters and wire colors, and it is not saved. You can also skip the photo and tick the wires yourself.
What does the B wire on my thermostat mean?
It depends on the brand: B is the heat pump reversing valve on Rheem and Ruud systems, but the common (C) wire on many Trane, American Standard and Carrier systems. The tool asks about your heat pump to tell them apart. More about B.