Free thermostat wiring tool
Which 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
Step 1 of 4
How do you want to enter your wires?
Pull the faceplate off your old thermostat first so you can see the wires.
Step 2 of 4
Tick every terminal that has a wire
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)
Tick at least one terminal to continue.
Step 3 of 4
A few quick questions
Step 4 of 4
Your wiring
Tick at least one terminal above and your wiring shows up here instantly.
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Smart thermostats we cover
The tool above grades each of these for your exact wiring. Each one has its own wiring guide.
- 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).Wiring guide →
- 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.Wiring guide →
- ecobee Smart Thermostat Enhancedecobee’s mid-range model with the same core wiring as the Premium (no accessory terminals). Power Extender Kit included.Wiring guide →
- 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.Wiring guide →
- 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.Wiring guide →
The tool’s result links to Amazon. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Our compatibility verdicts are the same for every brand.
Guides
Thermostat wiring guides
Common wiring setups
- 2 wires, heat only
- 3 wires, heat + fan
- 4 wires, no C wire
- 5 wires with C wire
- AC only, with C wire
- 2-stage heat + 2-stage cool
- Heat pump, no C wire
- Heat pump with C wire
- Heat pump + emergency heat
- Heat pump with B valve wire
- B wire used as common
- Rh + Rc dual transformer
- K wire (Honeywell wire saver)
What your wire letters mean
- R Wire
- Rh and Rc
- What Is a C Wire?
- W and W1
- W2 and AUX
- Y, Y1 and Y2
- G Wire
- O/B
- B Wire
- E Terminal
- X and X2
- K Terminal
- L, S and T Thermostat Terminals
How-to guides
- Thermostat Wire Colors Chart
- No C Wire? Your Options for Every Smart Thermostat Brand
- Heat Pump Thermostat Wiring
- Line Voltage vs Low Voltage Thermostats
- How to Replace a Thermostat Safely
- Smart Thermostat Not Working After Install? Troubleshooting
- Thermostat Clicking but AC Not Turning On
- How to Test Thermostat Wires With a Multimeter
- Smart Thermostat Settings to Check After Installing
- Heat Pump Using Aux Heat Too Much? Causes and Fixes
- Thermostat Wire Gauge, Conductors and Running New Cable
- Thermostat Wire Not Detected? Fixing Y, G and Rh Wire Errors
- Thermostat Not Working After a Power Outage or Battery Change
- Aux Heat vs Emergency Heat
- Thermostat With 6 Wires
- ecobee PEK Wiring
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.