SAFETY: Turn off power at the breaker or the furnace switch before touching any thermostat wiring. Confirm the thermostat display is dark before pulling it off the wall. The control side is only 24V AC, but a shorted wire can blow the board fuse or kill the transformer.
Thermostat wiring comes down to one rule: match the terminal letters, not the wire colors. The letters on the terminal block (R, C, W, Y, G, O/B) mean the same thing on every 24V thermostat made in the last 40 years. The wire colors are whatever the installer had on the truck.
Everything else on this page hangs off that rule. Photograph your old wiring, note which color lands on which letter, and you can wire any thermostat: Nest, ecobee, Honeywell, or a $20 basic model.
What each terminal does
| Wire color (typical) | Terminal | What it does |
|---|---|---|
| Red | R (or Rc / Rh) | 24 V AC power from the transformer. Rc = cooling power, Rh = heating power; one system uses a jumper between them. |
| White | W (W1, W2) | Heat call. W2 is second-stage or auxiliary heat. |
| Yellow | Y (Y1, Y2) | Cooling call — energizes the compressor contactor. On heat pumps, Y runs the compressor for heating too. |
| Green | G | Indoor fan (blower) relay. |
| Blue | C | Common — completes the 24 V circuit and gives smart thermostats constant power. Sometimes black. |
| Orange | O (or O/B) | Heat-pump reversing valve, energized in cooling (most brands). |
| Dark blue | B | Heat-pump reversing valve, energized in heating (Rheem/Ruud). Occasionally used as C — verify. |
| Brown | E | Emergency heat — manual backup that locks out the compressor. |
| Varies | AUX | Auxiliary heat — automatic electric backup on heat pumps. Often shares a wire with W2. |
Three terminals do the real work in most homes:
- R (red): 24V AC power. Everything the thermostat "switches on" is this wire being connected to another terminal. On thermostats with Rc and Rh, cooling and heating power come in separately. See the Rc/Rh section before touching anything.
- W (white): connect R to W and the heat runs. That's all a heat call is.
- Y (yellow): connect R to Y and the compressor runs. On a heat pump, Y runs the compressor for both heating and cooling; the O/B wire decides which.
The C wire (usually blue) doesn't switch anything. It completes the 24V circuit so the thermostat itself has constant power, which is why smart thermostats care about it so much (no C wire? your options).
Count your wires to identify your system
Pull the thermostat off its wall plate and count the wires attached to terminals (ignore spares tucked into the wall, but note them: a spare wire is a free C wire).
| Wires | Almost always | Diagram + guide |
|---|---|---|
| 2 | Heat only (furnace, boiler, wall heater) | 2-wire wiring |
| 3 | Heat + common, or heat + fan | 3-wire wiring |
| 4 | Heat + cool + fan, no C | 4-wire wiring |
| 5 | Heat + cool + fan + C, the modern standard | 5-wire wiring |
| 6 | Two-stage heat or a basic heat pump | 6-wire wiring |
| 7–8 | Heat pump with aux/emergency heat | heat-pump wiring |
Wiring by wire count
2-wire: heat only
R and W. The thermostat is a switch: close R→W, the furnace fires. Gas furnaces, boilers, and millivolt wall heaters use this. Batteries power the thermostat since there's no C wire, so a dead display here is almost always dead batteries.
Full guide: 2-wire thermostat wiring.
3-wire: heat plus common (or fan)
Usually R, W, C on boilers: same as 2-wire but the thermostat gets constant power. Some older furnaces run R, W, G instead (separate fan control). Check the old sub-base labels: if the third wire lands on G, it's fan, not common.
Full guide: 3-wire thermostat wiring.
4-wire: heat, cool, fan, no C
R, W, Y, G: the classic setup in homes wired before smart thermostats. Everything works except constant thermostat power. Installing a Nest or ecobee here means an add-a-wire kit, the Power Extender Kit (ecobee), or one of the other C-wire workarounds.
Full guide: 4-wire thermostat wiring.
5-wire: the modern standard
R, C, W, Y, G. Every smart thermostat installs cleanly on this with no adapters. If you're running new cable, don't pull 5-conductor. Pull 18/8 (eight-conductor) so the next upgrade is free. See thermostat wire gauge.
6-wire: second stage
The sixth wire is usually W2 (second-stage heat on two-stage furnaces), or O/B if you have a heat pump. If your outdoor unit runs in winter, it's a heat pump; read the heat-pump section.
Full guide: 6-wire thermostat wiring diagram.
7–8 wire: heat pumps with aux heat
Heat pumps add three ideas on top of standard wiring:
- O/B, the reversing valve. Y runs the compressor; O/B picks the direction. O (most brands: Carrier, Trane, Goodman, Lennox) energizes in cooling. B (Rheem, Ruud) energizes in heating. You set O or B in the thermostat's installer menu, never by rewiring. Wrong setting = heat when you asked for cool.
- AUX/W2, automatic backup. Electric strips that kick in when the heat pump can't keep up.
- E, emergency heat. The manual switch that runs strips instead of the compressor (use it when the heat pump itself is broken, not just because it's cold).
Full guides: heat pump thermostat wiring, 8-wire two-stage, plus brand-specific: Honeywell heat pump wiring, Nest heat pump wiring, Amazon Smart Thermostat heat pump wiring.
Rc vs Rh: the jumper question
- One R wire out of the wall (most homes): Rc and Rh must be connected. Modern smart thermostats bridge them internally; older ones use a small metal jumper. Land your R wire on either terminal.
- Two R wires (separate furnace and AC transformers, common in older homes and add-on AC): each goes to its own terminal and they must never be jumpered: out-of-phase transformers can put ~48V across the circuit and cook both.
Quick test before you connect anything: measure across the two R wires. More than ~12V AC between them = two live transformers = no jumper.
Full guide: Rc/Rh jumper wiring.
No C wire? Four options, best first
- Use a spare wire. Extra conductor tucked in the wall? Land it on C at both ends. Free and done in 20 minutes.
- Add-a-wire kit / Power Extender Kit. A small diode module at the furnace splits an existing conductor into two circuits. ecobee ships its PEK in the box.
- 24V plug-in adapter. A wall-wart transformer feeding Rc/C directly. Fine for rentals; ugly wire run.
- Pull new 18/8 cable. The permanent fix, and the right call if the existing cable is damaged.
Full guides: install a thermostat without a C wire · can the Nest blue wire be a C wire?
How to replace a thermostat (any brand)
- Kill the power at the breaker or the furnace switch. Confirm the thermostat display is dead.
- Photograph the wiring. Two shots: wide (whole sub-base) and close (terminal letters readable).
- Label each wire with masking tape by terminal letter, not color.
- Unscrew wires, cap the cable. Clip a clothespin or tape the bundle so it can't slide into the wall cavity.
- Mount the new base, land each wire on the terminal with the same letter. Seat firmly; a loose W wire is an intermittent "no heat" call that will drive you insane in January.
- Set the installer menu: system type, O or B orientation (heat pumps), fuel type.
- Restore power and test all modes: heat, cool, fan. For heat pumps, confirm cool actually blows cold. If it heats instead, flip O/B in settings.
Troubleshooting wiring problems
| Symptom | Most likely | Check |
|---|---|---|
| Blank display after install | No power: blown 3A/5A board fuse or tripped float switch | Fuse on the furnace control board; ~24V AC across R and C |
| Heat runs but no AC (or vice-versa) | Rc/Rh jumper missing or wire loose on Y/W | Jumper setting, then terminal screws |
| Heat pump blows cold on heat | O/B set wrong | Installer menu, not the wiring |
| Fan runs constantly | G shorted or fan set to ON | Thermostat fan setting first, then G wire insulation |
| Smart thermostat drops Wi-Fi / reboots | No C wire — power stealing | Add a C wire (options above) |
| Short cycling after new thermostat | Wrong system type / cycling rate in installer menu | Match settings to equipment before suspecting hardware |
If you find chafed or broken cable in the wall, that's replacement wire time: 18-gauge solid, conductor count per the table above.
⚠️ Know when to stop
Thermostat wiring is 24 V AC low voltage — safe to handle once you cut power at the breaker (do it anyway; a short can kill the transformer or blow the board fuse). But stop and call an HVAC pro or electrician if you hit any of these:
- Line voltage (120/240 V): thick wires with wire nuts, often on baseboard or wall-heater thermostats. Miswiring these can injure you or start a fire.
- Gas components: anything past the furnace control board — gas valves, ignitors, pilot assemblies.
- Refrigerant lines or sealed-system parts on heat pumps and AC units.
- No idea which system you have: if the wiring doesn't match any diagram here, photograph it and get a pro's eyes on it before guessing.
FAQ
What do the letters on thermostat wiring mean? R = 24V power, C = common (completes the circuit), W = heat, Y = cooling/compressor, G = fan, O/B = heat-pump reversing valve, W2/Y2 = second stages, E = emergency heat. Full reference: thermostat wiring letters and terminals.
Is thermostat wiring color-coded? By convention only (red→R, white→W, yellow→Y, green→G, blue→C), and nothing enforces it. Go by the terminal letter on the old thermostat. See the full color-code guide.
Is thermostat wiring dangerous? The control circuit is 24V AC; you'd barely feel it. The risks are indirect: shorting wires kills transformers and board fuses, and some thermostats (electric baseboard) are line-voltage 120/240V, which absolutely can hurt you. Thick wires + wire nuts = line voltage = stop.
Can I use my old wires with a new smart thermostat? Almost always, if you have 4+ wires. The question is the C wire: 4-wire homes need one of the workarounds above. 2-wire heat-only homes should check the thermostat maker's compatibility tool before buying.
What gauge is thermostat wire? 18 AWG solid, sold as 18/2 through 18/10 (the second number = conductor count). Pull 18/8 for any new run. Details: thermostat wire gauge guide.
Where can I find every wiring diagram in one place? On the thermostat wiring diagrams page: every diagram from this guide plus brand-specific ones, printable.
All wiring guides: By count: 2-wire · 3-wire · 4-wire · 6-wire · 7-wire Honeywell — By system: heat pump · AC · diagrams hub — By brand: Nest · Honeywell · Emerson · White Rodgers→Nest · Trane old→new · Coleman Mach (RV) · Carrier Infinity
