SAFETY: Turn off power at the breaker or furnace switch before touching any thermostat wiring, and confirm the display is dark before pulling the old unit off the wall. Thermostat control wiring is 24V AC, but a short can still blow the control-board fuse or the transformer.
Every thermostat wire does one of about twenty jobs, and the letter next to each terminal tells you which. This page is the full reference: every terminal letter, what it switches, and which ones your system actually has. For the hookup steps and diagrams by wire count, start with the full thermostat wiring guide.
Every thermostat terminal letter
| Letter | Name | What it does | Always present? |
|---|---|---|---|
| R | Power | 24V AC power from the transformer. Feeds every other terminal. | Yes, on every system |
| Rc | Cooling power | 24V power dedicated to the cooling circuit, on systems with two transformers | Only on dual-transformer systems |
| Rh | Heating power | 24V power dedicated to the heating circuit, on systems with two transformers | Only on dual-transformer systems |
| C | Common | Return path that gives the thermostat constant power. Required by most smart thermostats. | No — many older systems have none |
| W | Heat (single-stage) | Calls the furnace or boiler for heat | On heat/cool and heat-only systems |
| W1 | Heat, stage 1 | First-stage heat on multi-stage furnaces (same job as plain W) | Multi-stage furnaces only |
| W2 | Heat, stage 2 / backup | Second-stage furnace heat, or backup/aux heat on a heat pump | Two-stage furnaces, heat pumps with backup strips |
| W3 | Heat, stage 3 | Third heating stage on some multi-stage furnaces or boilers | Rare — some multi-stage/hydronic systems |
| Y | Cooling (single-stage) | Calls the compressor for cooling (or, on a heat pump, for both heating and cooling) | On any system with a compressor |
| Y1 | Cooling, stage 1 | First-stage compressor call on multi-stage systems (same job as plain Y) | Two-stage/variable equipment |
| Y2 | Cooling, stage 2 | Second-stage or full-capacity compressor call | Two-stage/variable equipment |
| G | Fan | Runs the indoor blower | Yes, on forced-air systems |
| O | Reversing valve (cool) | Energizes the heat-pump reversing valve in cooling, on most brands | Heat pumps only |
| B | Reversing valve (heat) | Energizes the heat-pump reversing valve in heating, on Rheem/Ruud and similar brands | Heat pumps only |
| O/B | Reversing valve | Single shared terminal for the reversing valve wire; you set O or B in the thermostat menu | Heat pumps only |
| E | Emergency heat | Manual override: disables the compressor, runs backup heat alone | Heat pumps only |
| AUX | Auxiliary heat | Backup heat that runs alongside the compressor automatically (often the same physical circuit as W2) | Heat pumps only |
| L | Indicator light | Typically lights an indicator for aux/emergency heat or a system fault | Uncommon — some furnaces/air handlers only |
| S1 / S2 | Outdoor or remote sensor | Carries outdoor-temperature (or remote-sensor) data to the thermostat | Only on thermostats with a wired sensor input |
| K | Combined fan + common | Honeywell-specific: one wire carries both G and C signals via an adapter module | Only with a Honeywell K-terminal model + WireSaver adapter |
| * (accessory / ACC) | Accessory | Connects a humidifier, dehumidifier, or ventilator | Only if the thermostat supports one accessory device |
Colors are not on this list on purpose. Wire color is a convention some installers followed, not a standard anyone enforces. Go by the terminal letter, always. Full color-by-color breakdown and multimeter checks: thermostat wiring color code guide.
Rc, Rh, and the jumper: the terminal pair that confuses the most people
Rc and Rh only exist because some homes run two 24V transformers, one for heating equipment and one for cooling equipment, instead of one shared transformer. On a single-transformer system, the one R wire lands on either R or Rc (interchangeably, depending on the thermostat), and a jumper bridges Rc to Rh so that one 24V feed powers both circuits.
Two R wires means two live transformers: each wire gets its own terminal, and the jumper must come out, or you risk shorting two out-of-phase 24V feeds together. This is common enough, and wrong often enough, that it gets its own page: thermostat Rc/Rh jumper wiring covers the jumper, the slider-tab equivalent on newer Honeywell models, and how to test which situation you have with a multimeter.
W2 isn’t one thing
W2 means something different depending on what’s attached to it, which is why it shows up in more searches than almost any other terminal:
- On a furnace or boiler with two heating stages, W2 is the second, higher-output heating stage. W1 (or plain W) handles normal demand; W2 adds capacity on cold days.
- On a heat pump, W2 is usually the backup/auxiliary heat terminal: electric resistance strips (or, on a dual-fuel system, the furnace) that run alongside the compressor when it needs help. This is the same physical role as the AUX terminal on many thermostats; manufacturers label it either way.
Same letter, two different jobs, decided entirely by what kind of equipment is on the other end of the wire. If you’re not sure which applies, check the equipment’s own wiring diagram, usually printed inside an access panel.
Terminals that only show up on some systems
A handful of letters on the table above are easy to miss because most homes never wire them:
- S1/S2 carry data from a wired outdoor or remote-room sensor, not a 24V switching signal. Only thermostats built for a wired sensor input have them; the Honeywell T10/T10+ Pro is one example, using them for an outdoor-temperature probe.
- K is a Honeywell-only shortcut: paired with an add-on module, it lets one wire carry both the fan (G) and common (C) signals, which frees up a conductor on a 4-wire system that has no C. Only T-series Honeywell thermostats have the terminal at all.
- L usually drives a physical indicator light for aux/emergency heat or a fault condition, not a heating or cooling call itself. Whether your equipment has it (and what triggers it) depends on the furnace or air-handler control board.
- The accessory terminal (often marked ACC or *) connects one extra device, most commonly a whole-house humidifier or dehumidifier. It doesn’t control heating or cooling and won’t appear on a basic thermostat.
None of these are required for a working system. If your old thermostat’s sub-base doesn’t have a matching terminal for one of these, the wire simply wasn’t there to begin with.
How many terminals you actually need
Terminal count isn’t the same as wire count. A basic heat/cool system needs five terminals at most (R, C, W, Y, G); a heat pump with backup heat and two compressor stages can use nine or more (R, C, Y1, Y2, G, O/B, W2/AUX, E). Match what’s physically on your old thermostat’s sub-base, letter for letter, and you’ll never need more terminals than wires. Full breakdown by wire count, with a diagram for each: thermostat wiring guide.
| 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. |
FAQ
What do the letters on thermostat wiring mean?
Each letter marks one job in the 24V control circuit: R is power, C is common, W is heat, Y is cooling, G is fan, O/B is the heat-pump reversing valve, and E/AUX handle backup and emergency heat. The full table above covers every letter, including the less common ones (S1/S2, K, L, accessory).
What is Rc on a thermostat?
Rc is 24V power dedicated to the cooling circuit on a system with two transformers (one for heating, one for cooling). On a single-transformer system, R and Rc are the same terminal, usually bridged by a jumper. Full explanation: Rc/Rh jumper wiring.
What is W2 on a thermostat?
Second-stage heat on a furnace or boiler, or backup/auxiliary heat on a heat pump. Which one it means depends on the equipment attached; check your furnace or air handler’s wiring diagram if you’re not sure.
Does every thermostat use all these terminals?
No. A basic heat-only or heat/cool system uses five terminals at most. The rest (O/B, W2, E, AUX, S1/S2, K, accessory) only appear on systems with a heat pump, multiple stages, a wired sensor, or an accessory like a humidifier.
Is the wire color the same as the terminal letter?
No, and don’t assume it is. Wire color is a common convention, not an enforced standard; always match by the letter printed next to the terminal, not the color of the insulation. See the color code guide for the full color-by-color breakdown and how to verify with a multimeter.
What is the accessory or “*” terminal for?
It powers one extra device the thermostat controls beyond heating and cooling, most often a whole-house humidifier, dehumidifier, or ventilator. Not every thermostat has one, and it only supports a single accessory at a time.
⚠️ 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.
Last reviewed: July 2026. Terminal definitions cross-checked against Honeywell Home, Google Nest, and Resideo/Honeywell T10 support documentation, plus iFixit and HVACR School for the less common terminals; always confirm against your equipment’s own manual before wiring.
