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.
Count the wires attached to terminals on your old thermostat, then jump to that diagram below: 2-wire, 3-wire, 4-wire, 5-wire, 6-wire, 7-wire, 8-wire, heat pump, or two R wires (Rc/Rh). Every diagram on this page prints cleanly: hit Ctrl+P (Cmd+P on Mac) and take the page to the furnace with you.
This page is the diagram library. For the full explanation of what each terminal does and how to replace a thermostat step by step, use the thermostat wiring guide. The one rule that makes every diagram below work: match the terminal letters, not the wire colors. Red usually means R and white usually means W, but nothing enforces that, so photograph your old wiring and go by the letters.
Which diagram do you need?
Pull the thermostat off its wall plate and count only the wires landed on terminals. Ignore spare conductors tucked into the wall (but note them: a spare is a free C wire). Then follow the chart.
Wire color reference
Before any diagram, the color-to-terminal chart. Colors are conventions; verify every wire at both ends.
| 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. |
Full per-color breakdown, including the blue-wire trap: thermostat wiring color code guide.
2-wire thermostat wiring diagram
This is your system if: you have heat only, no air conditioning. Gas furnaces, boilers, and millivolt wall heaters run on two wires: R (24V AC power) and W (heat call). The thermostat is a simple switch that connects them. No C wire means the thermostat runs on batteries, so a blank display here is almost always dead batteries. Full guide: 2-wire thermostat wiring.
3-wire thermostat wiring diagram
This is your system if: you have heat only, plus one extra conductor. On boilers the third wire is usually C (common), which gives the thermostat constant 24V AC power. On some older furnaces it’s G (fan) instead. Check the terminal label on your old thermostat before assuming: a wire landed on G is fan control, not common. Full guide: 3-wire thermostat wiring.
4-wire thermostat wiring diagram
This is your system if: you have heat and AC in a home wired before smart thermostats. R, W, Y, G covers power, heat, cooling, and fan, but there’s no C wire for constant thermostat power. A Nest or ecobee here needs an add-a-wire kit, ecobee’s Power Extender Kit, or another C-wire workaround. Full guide: 4-wire thermostat wiring.
5-wire thermostat wiring diagram
This is your system if: you have heat, AC, and a C wire. R, C, W, Y, G is the modern standard, and every smart thermostat installs on it with no adapters. If your five colors don’t match the chart above, trust the terminal letters on the old thermostat. This is the diagram most single-stage gas furnace + AC homes need. Full guide: 5-wire thermostat wiring.
6-wire thermostat wiring diagram
This is your system if: you have two-stage heat (the sixth wire lands on W2) or a basic heat pump (the sixth wire lands on O/B). The distinction matters: if your outdoor unit runs in winter, you have a heat pump and should use the heat pump diagram instead. Full guide: 6-wire thermostat wiring diagram.
7-wire thermostat wiring diagram
This is your system if: you have a heat pump with auxiliary or emergency heat. The seven conductors typically cover R, C, Y, G, O/B, AUX/W2, and E. This is the most common heat-pump configuration in cold climates, where electric strips back up the compressor. Full guide: 7-wire Honeywell thermostat wiring diagram.
8-wire thermostat wiring diagram
This is your system if: you have a two-stage heat pump or a heat pump with separate AUX and E wiring. The eighth conductor is usually Y2 (second-stage compressor) or a dedicated E wire. If you’re pulling new cable for any system, pull 18/8 so future upgrades are free; see thermostat wire gauge.
Heat pump thermostat wiring diagram
This is your system if: one outdoor unit both heats and cools your home. The wire that makes a heat pump different is O/B: Y runs the compressor in both modes, and O/B picks the direction by energizing the reversing valve. O energizes in cooling on most brands (Carrier, Trane, Goodman, Lennox); B energizes in heating on Rheem and Ruud. You set O or B in the thermostat’s installer menu, never by rewiring. Full guide: heat pump thermostat wiring.
Rc/Rh dual-transformer wiring diagram
This is your system if: two separate R wires come out of your wall, one from the furnace transformer and one from the AC transformer. Land each on its own terminal (Rh and Rc) and never jumper them: out-of-phase transformers can put roughly 48V across the circuit and blow the board fuse or cook a transformer. One R wire? Then Rc and Rh get connected instead, which modern thermostats do internally. Full guide: Rc/Rh jumper wiring.
Printing these diagrams
Every diagram on this page prints cleanly. Print the whole page (Ctrl+P, or Cmd+P on Mac) or right-click any single diagram and save the image. Two tips for the furnace-side visit:
- Print the color-code chart along with your diagram. You’ll want both when you’re matching conductors at the control board.
- Write your own color-to-terminal mapping on the printout as you disconnect each wire. Your photo plus the marked-up printout is your recovery plan if anything gets confusing.
⚠️ 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
Which wiring diagram do I need for my thermostat?
Count the wires attached to terminals on your old thermostat and use the matching diagram above. Two wires is heat-only, four is heat + AC without a C wire, five is the modern standard, and six or more usually means two-stage heat or a heat pump. If your outdoor unit runs in winter, use the heat pump diagram regardless of wire count.
Are these thermostat wiring diagrams printable?
Yes. Print the page with Ctrl+P (Cmd+P on Mac), or right-click any diagram to save it as an image. Each diagram is a clean vector graphic, so it stays sharp at any print size.
Where are the Honeywell and Nest wiring diagrams?
The diagrams on this page apply to any 24V thermostat, including Honeywell and Nest, because terminal letters are universal. For brand-specific terminal layouts, jumper behavior, and installer menus, use the Honeywell thermostat wiring guide and the Nest thermostat wiring guide.
My wire colors don’t match the diagram. What do I do?
Ignore the colors and go by the terminal letters on your old thermostat. Wire colors are installer conventions, not rules, and Honeywell’s own documentation says so. Photograph the old wiring, label each wire with the terminal letter it came from, and land it on the same letter on the new thermostat. The color code guide covers how to verify each wire with a multimeter.
What voltage is thermostat wiring?
24V AC on the control side, stepped down by a transformer in the furnace or air handler. A healthy reading across R and C is roughly 24 to 28V AC. If your thermostat has thick wires joined with wire nuts, that’s a line-voltage (120/240V) system for baseboard or wall heaters: stop and call an electrician, these diagrams do not apply.
Last reviewed: July 2026
