Thermostat and home wiring FAQ

By convention, red is R (24 V power), white is W (heat), yellow is Y (cooling), green is G (fan), blue or black is C (common), and orange is O/B on heat pumps. Installers don’t always follow it, so wire by the letter each wire was connected to, not its color. See the thermostat wire colors chart.

Y (or Y1) calls for cooling: it turns on the compressor in the outdoor unit. A second cooling stage uses Y2. W calls for heat and G turns on the indoor fan, so a cooling call normally sends power to both Y and G. On a heat pump, Y runs the compressor in heating too. Details on the Y wire page and the G wire page.

R (also Rh or Rc) is the 24 V AC power from the transformer in your furnace or air handler. C is the other side of that transformer. The thermostat switches R out to W, Y and G to turn equipment on, and uses R and C together to power itself. See R wire and C wire.

Both come from the same 24 V transformer. R is the “hot” side and C (common) is the return. Connected together through the thermostat, they give it continuous power. They must never touch each other directly: that shorts the transformer and usually blows the 3 or 5 A fuse on the furnace board. With power on, you should measure about 24 to 28 V AC between them.

O/B controls the reversing valve on a heat pump, which switches it between heating and cooling. Most brands use O (energized in cooling); Rheem and Ruud use B (energized in heating). You set O or B in the thermostat’s settings. If heat mode blows cold air, this setting is usually reversed. See O/B explained.

On many older Trane, American Standard and Carrier systems, B is the common (C) wire, not the reversing valve. If your old thermostat had wires on both O and B, B is almost always common and goes on C. On Rheem and Ruud heat pumps B is the reversing valve. See the B wire page or the blue B wire setup.

Rh is power from the heating transformer and Rc from the cooling transformer. Most homes have one transformer, so the old thermostat has a single R wire and a short jumper between Rh and Rc. Leave that jumper behind: smart thermostats join Rh and Rc internally. If both terminals have their own wire from the wall, you have two transformers and must keep them apart. See Rh and Rc.

W2 is a second stage of heat. On a two-stage furnace it runs the higher heat setting; on a heat pump it usually runs the backup (auxiliary) electric heat, and may be labeled AUX. See W2 and AUX.

K is used with Honeywell’s wire saver (C-wire adapter). It carries the Y and G signals on one wire, so a four-wire cable can be used to bring a C wire to the thermostat. The module at the furnace splits K back into Y and G. Amazon’s C-wire adapter works the same way. See the K terminal page.

The C (common) wire completes the 24 V circuit so a smart thermostat has steady power for its screen and Wi-Fi. Without it, the thermostat has to draw small amounts of power through the heat or fan wires (“power stealing”), which can cause clicking, a blank screen or dropped Wi-Fi. See what a C wire is.

Look for a wire on the old thermostat’s C terminal (sometimes labeled COM, or B or X on older brands). If C is empty, check the wall opening for an unused wire folded back. At the furnace board, a thermostat-cable wire on C is a C wire even if it isn’t connected at the thermostat end. The no C wire guide walks through each check.

For most models, yes, or an adapter that provides one. ecobee, Honeywell Home and Amazon thermostats need a C wire or their adapter. Google Nest usually runs without one on a furnace with AC, but Google says heat-only, cool-only and heat pump systems need a C wire or the Nest Power Connector. Tick your wires in the wiring tool to see what each model needs.

Usually. If you have four wires (R, W, Y, G), each brand has an option: the ecobee Power Extender Kit (in the box), the Honeywell wire saver, Amazon’s C-wire adapter, or the Nest Power Connector. You can also use a spare wire in the wall, a plug-in 24 V transformer, or run new 18/5 cable. See no C wire options and the 4-wire setup.

The Power Extender Kit (PEK) is a small module ecobee includes with the Premium and Enhanced. It mounts at the furnace, connects to R, W, Y, G and C there, and sends power plus all four signals up your existing four wires. At the thermostat, the wire that was on G goes to the PEK terminal. See the ecobee Premium wiring guide.

Measured between R and C with the power on, a healthy system reads about 24 to 28 V AC. Zero volts usually means the furnace door switch is open, the service switch is off, or the fuse on the control board has blown. Our multimeter guide shows how to test it.

A typical furnace with AC uses five: R, C, W, Y and G. Four wires (no C) works with an adapter for most brands. Heat pumps usually have five to seven (adding O/B and AUX), and two-stage systems add Y2 or W2. Only two wires (R and W) is a heat-only system, which needs a different approach. Browse wiring by setup.

Four wires is usually R, W, Y and G: a furnace with AC but no C wire. Each wire goes to the same letter on the new thermostat, plus the brand’s C-wire adapter where needed. If the four are R, C, W and Y, there is a C wire but no fan wire, which works with most models. See the 4-wire wiring diagram.

Five wires on R, C, W, Y and G is the standard furnace-and-AC setup and works with every major smart thermostat: each wire goes to the same letter. If the fifth wire is O/B instead of C, it is a heat pump without a C wire. See the 5-wire wiring diagram.

Six wires is most often a heat pump with a C wire and backup heat: R, C, Y, G, O/B and AUX (or W2). It can also be a two-stage furnace or AC. The letters tell you which, so check them before buying. See heat pump with a C wire and two-stage heat and cool.

Two wires (R and W) means a heat-only system, usually a boiler or older furnace. The brand C-wire adapters need four wires, so they don’t help. Your options are a plug-in 24 V transformer, running new cable, or a thermostat designed for two wires. If the two wires are thick with wire nuts, it is line voltage instead. See the 2-wire heat-only setup.

Y runs the compressor for both heating and cooling, O/B switches the reversing valve, G runs the fan, and AUX (or W2/W1) runs backup heat. E, if present, is emergency heat. After wiring, choose heat pump as the equipment type and set O or B in the app. See heat pump thermostat wiring.

Aux (auxiliary) heat is the backup heat in a heat pump system: electric strips in the air handler, or a furnace on a dual-fuel system. It comes on during defrost, in very cold weather, or when the house is far below the setpoint. If it runs often on mild days, check the settings. See aux heat running too much.

Emergency heat turns the heat pump off and heats with the backup heat only. Use it when the heat pump is broken or iced over and waiting for repair. In normal weather leave the thermostat in Heat mode, because electric emergency heat costs much more to run. See E and emergency heat.

Most often the O/B setting is reversed, so the heat pump cools when asked to heat. Change it from O to B (or the other way) in the thermostat’s settings. Also confirm the thermostat is set up as a heat pump, not a furnace with AC. See settings to check after installing.

On a standard 24 V system, yes: it is a common DIY job with a screwdriver, once power is off at the furnace or breaker. Take a photo of the old wiring and label each wire by its letter. Stop and call a professional if you find thick wires with wire nuts, a breaker trips, or a fuse blows. See how to replace a thermostat.

Not a standard one. Baseboard heaters use line-voltage (120 or 240 V) thermostats, and connecting a 24 V smart thermostat to them destroys it and is a shock hazard. Use a smart thermostat built for line voltage, rated for your heaters’ voltage and total wattage. See line voltage vs low voltage.

Residential thermostat cable is normally 18 AWG. It is sold as 18/5, 18/7 or 18/8, where the second number is how many conductors are inside. Buy one or two more conductors than you need, so a spare can become a C wire later. See thermostat wire gauge.

Check the furnace door is fully closed (its safety switch cuts power), the furnace service switch and breaker are on, and the small fuse on the furnace control board isn’t blown. Then measure R to C for about 24 V. A smart thermostat without a C wire can also go blank during long run times. See the troubleshooting checklist.

The click means the thermostat is sending the cooling call, so look at the equipment. Check the outdoor unit’s breaker and the disconnect box next to it, then measure Y to C at the furnace board while cooling is on. Voltage there with no outdoor unit running points to the contactor, capacitor or the wire outside. A clogged condensate drain can also trip a float switch that cuts the call. See thermostat clicking but AC not turning on.

This is usually the compressor protection delay. After the compressor stops, or after power comes back, thermostats wait a few minutes (often about five) before starting it again so it doesn’t restart against high pressure. Leave this protection on. See settings to check after installing.

Most often the wire isn’t fully seated in its connector. Turn the power off, strip about 6 mm (¼ inch) of fresh copper, straighten the end and push it in until the button or tab stays down. If it still isn’t detected, check the wire for voltage with a multimeter. See the multimeter guide.

With the breaker off and the cover removed, look for white wires joined together under a wire nut at the back of the box and not connected to the switch. Those are neutrals. If only one cable enters the box and its white wire goes to the switch, it is a switch loop with no neutral. Our neutral wire checker walks you through it.

Yes, if you choose a model designed for it. Most smart switches need a neutral to power their radio, but some no-neutral dimmers, such as many Lutron Caséta models, work without one, usually with a minimum bulb load. Check the box with the neutral wire checker before buying.

Ring’s wired-power video doorbells accept 8 to 24 V AC (5 to 40 VA); the Wired Doorbell Pro and Plus need at least 16 V. Wired Google Nest doorbells need 16 to 24 V AC and at least 10 VA. Many older homes have a 10 V transformer, which is too low for these. Enter your transformer’s rating in the doorbell transformer checker.

The common terminal is the darker (usually black) screw. At one switch it gets the hot wire from the panel; at the other it sends the switched hot to the light. The two brass screws are for the travelers that run between the switches. Flip the switches in our 3-way switch wiring simulator to see the power flow.

A GFCI needs power to reset, so first check for a tripped breaker or another GFCI upstream that has tripped. Most GFCIs made since 2006 also refuse to reset if the line and load wires are swapped. If it trips again right away, there is a ground fault on that circuit. See GFCI outlet wiring.

It grades the Google Nest Learning Thermostat (4th gen), ecobee Smart Thermostat Premium and Enhanced, Honeywell Home T9 and Amazon Smart Thermostat against your wires, using each brand’s installation manual. It also flags line-voltage, millivolt and proprietary systems that none of them support. See the smart thermostat guides.

The tools are free and need no account. A photo is sent to our AI model only to read the terminal letters and wire colors, and isn’t saved; you can tick the wires yourself instead. Details are in the privacy policy, and how the site is funded is on the affiliate disclosure.