AC Thermostat Wiring: R, C, Y, G and W Terminals Explained

Most thermostat swaps that go wrong do not fail because of a bad thermostat. They fail because someone moved a wire by its color instead of its terminal letter. AC thermostat wiring in a conventional furnace-plus-central-air system uses a small set of low-voltage 24-volt control wires, and each one tells the equipment to do one thing: power, cool, fan, or heat. Once you know what R, C, Y, G and W mean, replacing or upgrading the thermostat is a manageable job.

In a typical system, R is 24-volt power from the furnace transformer, C is the common return that powers smart thermostats, Y calls the outdoor air conditioner, G runs the indoor blower, and W calls for heat. Two-stage systems add Y2 and W2. Heat pumps use different logic with an O/B reversing valve, so this guide covers conventional furnace and central AC setups only.

The safe approach is to label every wire by the terminal it came off, photograph the old wiring, and work only with the power off. Below you will find each terminal explained, how to handle old thermostat labels, the step-by-step swap, and where the job stops being DIY.

AC Thermostat Wiring Terminals, One by One

Terminal letters are a near-universal HVAC thermostat wiring standard. Wire insulation colors are only a convention, and installers break it constantly, which is why the letter printed next to each screw matters more than the color.

• R (Rh / Rc): 24-volt power from the transformer, usually a red wire. Rh powers heating, Rc powers cooling. With one transformer, Rh and Rc are joined by a jumper or an internal setting.
• C (Common): the return side of the transformer, often blue or black. It gives the thermostat continuous power, which smart models need.
• Y (Y1): calls the outdoor condenser for cooling, typically yellow.
• G: turns on the indoor blower fan, typically green.
• W (W1): calls the furnace for heat, typically white.
• Y2: second-stage cooling on a two-stage condenser, often light blue or another color.
• W2: second-stage heat on a two-stage furnace, often brown or another color.

A basic single-stage AC unit thermostat wiring setup uses four or five wires: R, Y, G, W, and ideally C. If you have only two wires, R and W, you likely have heat-only.

Low Voltage vs Line Voltage: Know Which You Have

Central AC and furnace thermostats run on low-voltage control wiring, typically 24 volts from a transformer inside the furnace or air handler. The wires are thin, often bundled in a single sheathed cable with several colored conductors. Line-voltage thermostats, used with electric baseboard heaters, run on 120 or 240 volts and have thick wires, often with wire nuts. Those are a different job and are not interchangeable with 24-volt thermostats. If your thermostat has heavy wires or says 120V/240V on it, stop and call a licensed electrician.

Tools and Materials

• Non-contact voltage tester
• Small Phillips and flathead screwdrivers
• Phone camera for photos of the existing wiring
• Masking tape or the wire labels included with most thermostats
• Wire stripper for 18-gauge thermostat wire
• Drill with small bit and drywall anchors for the new base
• Level
• C-wire adapter or power extender kit, only if your new thermostat requires C and none exists

Step-by-Step: Replacing a Furnace and AC Thermostat

1. Turn off power at the breaker. Switch off the breaker for the furnace or air handler, and the outdoor condenser breaker too. Many furnaces also have a service switch that looks like a light switch near the unit; turn it off as well.
2. Confirm power is off with a non-contact voltage tester at the furnace service switch area and verify the thermostat display is dead. Never probe or measure voltage on powered circuits.
3. Remove the old thermostat faceplate and photograph the wiring with terminal letters clearly visible.
4. Label each wire by terminal, not by color. Tape a label marked R, C, Y, G, W, Y2 or W2 to each conductor before disconnecting it.
5. Disconnect and remove the old base. Wrap the cable around a pencil so it cannot fall back into the wall.
6. Check the cable for an unused conductor. Many walls have a spare wire folded back into the sheath. It may serve as a C wire if it is also connected to C at the furnace control board.
7. Mount the new base, level it, and feed the wires through.
8. Connect each labeled wire to the matching terminal. If the new thermostat has separate Rh and Rc and your old one had a single R, follow the manual: usually R goes to Rc or Rh with the jumper in place.
9. Configure the thermostat. Set the system type to conventional furnace and air conditioner, not heat pump, and enter the number of stages.
10. Restore power at the breakers and service switch, then test fan, heat and cool modes. Allow a few minutes for built-in compressor delay before the condenser starts.

Smart thermostat wiring follows exactly the same terminal logic. The main difference is that most smart models need a C wire for steady power, so plan for that before you buy. Using a non-contact voltage tester during setup keeps the job safe and predictable.

Two-Stage Thermostat Wiring

Two-stage equipment runs at a lower capacity most of the time and ramps up on hot or cold days. For 2 stage thermostat wiring, the thermostat needs Y1 and Y2 terminals for cooling, W1 and W2 for heating, or both. Wire Y to Y1, the second cooling wire to Y2, W to W1 and the second heat wire to W2. Then set the thermostat to the correct number of heating and cooling stages. If the furnace control board is set to handle staging on a timer, it may use only W1; check the equipment label and manual before assuming both wires should be connected.

Reading Old Thermostat Labels

Old thermostat wiring, especially on mercury-bulb and early electronic models, may use labels that no longer appear on new units. Here is how common legacy markings usually translate:

• 4 or V: often R, the power terminal.
• M: sometimes Y, the cooling call, on older units.
• F: fan, equivalent to G.
• H: heat, equivalent to W.
• X or B (on a conventional system): may be the common, same as C. On a heat pump, B has a different meaning, so confirm your system type.

Mercury thermostats contain a sealed glass bulb of mercury. Do not throw one in the trash; take it to a household hazardous waste drop-off or a retailer take-back program. Furnace thermostat wiring behind these old units is usually just two to four wires, and it may lack a C conductor entirely.

Troubleshooting After a Thermostat Swap

• Blank screen: no C wire, a tripped breaker, the service switch still off, or a blown low-voltage fuse on the furnace control board. Check breakers and switches first.
• Fan runs constantly: G and R may be swapped, or the fan setting is on rather than auto.
• Heat comes on when set to cool: W and Y are reversed, or the system type is set to heat pump.
• Outdoor unit will not start: the Y wire is loose, the condenser breaker is off, or the compressor delay is still counting down.
• Thermostat reboots repeatedly: power-stealing through a system without C. Add a C wire or an adapter kit.
• Short cycling: poor thermostat location near a supply vent or in sunlight, or incorrect cycle-rate settings.

If the equipment still misbehaves with correct AC thermostat wiring, the fault is likely in the furnace or condenser itself, which is a job for a technician.

Cost to Replace a Thermostat

Cost ranges vary widely. Basic non-programmable models are inexpensive; programmable units cost more; smart Wi-Fi thermostats sit higher still. A C-wire adapter kit adds a modest amount. Professional installation for a straightforward swap usually involves a service-call fee plus labor, and costs rise if the technician must run a new cable, add a C wire at the control board, or configure two-stage or zoned equipment.

When to Call a Licensed Professional

Swapping a thermostat on existing low-voltage control wiring is within DIY reach. Everything beyond that belongs to licensed trades. Call a licensed HVAC technician for anything involving refrigerant, the condenser’s high-voltage wiring, the furnace control board, gas valves or burners, or if your system is a heat pump and you are unsure how it is configured. Running a new thermostat cable through walls is usually fine for a handy homeowner, but new line-voltage circuits, outlets or any 120-volt or 240-volt work should be done by a licensed electrician. Some jurisdictions require permits for HVAC equipment changes, so check with your local building department before replacing equipment rather than just the thermostat.