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What the letters behind your thermostat mean
Updated
Go by the letter printed on the terminal, not the color of the wire. ecobee and Google both say wire colors are not standardized. A red wire is only R if it sits in the terminal marked R.
Safety first. Turn off power at the breaker or the system switch before you pull a thermostat off the wall, then change the set temperature by 5 degrees and wait 5 minutes to confirm nothing starts. If you see thick wires, wire nuts, L1 or L2, or a 120 V or 240 V label, stop. That is line voltage. Put the cover back and call a licensed electrician.
| Label | What it does (per ecobee's label guide) | What it tells you |
|---|---|---|
| R | Power for the heating and cooling system | Normal. Almost every 24 volt system has one. |
| Rc and Rh | Cooling power and heating power. With one R wire, either serves both | A wire from the wall on both can mean two transformers. ecobee's kit does not cover that, and Google advises a pro. |
| C | Common wire. Continuous 24 VAC power for the thermostat | The one that opens up every pick. ecobee notes it is sometimes marked X or B. |
| W or W1, W2 | Heat, and second stage heat. On a heat pump, W2 or AUX is auxiliary heat | W2 means two stage or aux heat. Check the stage limits. |
| Y or Y1, Y2 | Cooling, or the compressor on a heat pump. Y2 is the second stage | Y2 means a two stage compressor. |
| G | Fan | The ecobee adapter kit needs G and Y both present. |
| O/B | Reversing valve on a heat pump | If this has a wire, you have a heat pump. If O and B are separate wires, ecobee recommends a pro. |
| E, AUX | Emergency and auxiliary heat on heat pumps | Heat pump with backup heat. |
| W3, Y3 | Third stage | ecobee does not support three stages and recommends a pro. Google says the Nest Learning (4th gen) supports up to 3 heat stages. |
| S, S1, S2, L | Wired outdoor sensor, indicator light | Not needed. ecobee and Google say they cannot be connected, and the system may still be compatible. |
Labels that mean stop
- L1, L2, 110, 120 or 240 volts, thick wires, wire nuts. Line voltage. ecobee says these terminals come with a danger, warning or caution symbol. No 24 volt smart thermostat fits. See baseboard heat.
- 1 2 3 4 or A B C D. ecobee and Google both call this a proprietary system. Google adds that two wires running both heating and cooling is likely proprietary too.
- Stranded wire. Google says stranded thermostat wire is not Nest compatible as is. Sensi asks for standard 18 gauge thermostat wire.
Two details people miss
- Jumpers. A short wire or staple between Rc and Rh is a jumper. Google and ecobee say not to count it. Only count wires that come out of the wall.
- Take a photo. Honeywell Home and Google both say to photograph the old wiring with the letters readable before you disconnect anything.
Tick your own wires in the wire checker, or compare all seven picks on the roundup page.