Thermostat wire colors and terminal letters: what R, C, W, Y, G and O/B control, which colours a maker actually prints, and why the letter wins

Do not trust the colours: the red wire may not be R and the blue one may not be C, so read the letter beside the screw, photograph it, then pick up a screwdriver.

What does each terminal letter control?

The table is Google’s definition of each terminal from its Identify a thermostat wire page, with Resideo’s wiring module table (69-2065EFS) and C-Wire Adapter guide (33-00618) where they print the same terminal. The colour column carries a colour only where a maker page opened for this page prints one; none prints a colour for R, W, Y or G.

If a smart thermostat is on the way, the best smart thermostat comparison starts from these letters, and the C wire guide covers an empty C terminal. Google’s C wire page says why that one matters: the C wire “doesn’t control heating or cooling functions. It only provides constant power to your thermostat.”

Why can you not trust the wire colour?

Because nobody enforces it. Google’s Nest Help page states that thermostat wire colours “often don’t adhere to industry standards, but they can sometimes provide a clue in certain situations”, and its compatibility page says to “enter which connectors have a wire and ignore the color of the wires”. The connector label is what counts.

The research for this page found the same. Sensi’s and Resideo’s wire-colour support articles could not be opened, and the only red-R, white-W, yellow-Y, green-G tables found were on installer and retailer blogs, which this site does not cite. The convention is a habit; no maker document read today prints it.

Every maker’s procedure is built around that. Resideo’s adapter guide has you photograph the control board wires with the terminal labels in frame, then sticker each wire G, R or Rc, W or W1, Y or Y1 before disconnecting only those wires. ecobee’s Essential installation page has you enter each wire in the app and apply its green labels before loosening a terminal.

What is different on a heat pump?

A heat pump adds the changeover valve and backup heat, which is where the lettering gets confusing. Google’s definitions: Y controls the compressor rather than cooling alone, O/B controls the changeover valve, Y2 is the second compressor stage, AUX is auxiliary heat, and E turns emergency heat on or off. W2 can mean second-stage heat, auxiliary heat, or the furnace in a dual-fuel system, depending on the equipment.

O/B is also the one place Google prints colour guidance beyond C: O/B wires are “typically orange”, and a brown wire is “most likely a heat pump wire” for O/B. Resideo’s footnote closes the loop: the wire from O/B on the old thermostat matches O or B at the equipment, not W.

One more trap: Resideo’s guide says the common wire might be labelled B rather than C on certain thermostats, and Google says a blue B wire should most likely go to C. On a heat pump, B can instead be the changeover terminal, so read the old thermostat’s manual first.

Is a 120 or 240 V thermostat the same thing?

No, and the label on the old thermostat decides it. Google’s compatibility page says Nest thermostats are built for 24 V systems and need at least 20 volts; high-voltage thermostats are “usually labeled 110, 115, 120 or 240 VAC”, typically control electric baseboard heat, and are incompatible, though a professional can sometimes fit a step-down transformer. Google also lists millivolt heaters, solid fuel (coal, wood chips, pellets, biomass) and proprietary labels such as 1, 2, 3 or A, B, C as incompatible.

The letters on this page describe low-voltage control wiring; a line-voltage thermostat switches the heater’s supply current directly, a different job with a different thermostat. No maker’s line-voltage installation guide could be opened for this page, so Google’s is the only maker wording here. ecobee’s Power Extender Kit page draws the same line from the accessory side: 24 VAC systems with at least four wires, not millivolt, high-voltage, dual transformer, fan coil or communicating systems.

What should you do first?

1. Turn the power off at the breaker or the furnace switch: step 1 in Resideo’s adapter guide, and ecobee’s first instruction too.
2. Photograph the old thermostat’s terminals with the letters legible in the frame (Resideo), then sticker or tag every wire by letter, never by colour (Resideo, ecobee).
3. Read the label. 110, 115, 120 or 240 VAC means line voltage: stop here and shop for a line-voltage thermostat instead.
4. Note the letters that carry a wire, including any unused wire in the bundle, and enter those letters, not colours, in the maker’s compatibility checker (Google).
5. While the furnace cabinet is open, check the filter: the furnace filter change interval and filter arrow direction guides cover what a new thermostat cannot fix.

Frequently asked questions

Sources

1. Google Nest Help, Identify a thermostat wire (manufacturer or vendor document)
2. Google Nest Help, Nest thermostat compatibility (manufacturer or vendor document)
3. Google Nest Help, Learn about the common or C wire (manufacturer or vendor document)
4. Resideo, Honeywell Home C-Wire Adapter installation guide 33-00618 (PDF) (manufacturer or vendor document)
5. Resideo, THP9045 Wiring Module instructions 69-2065EFS (PDF) (manufacturer or vendor document)
6. ecobee, Installing your Smart Thermostat Essential (manufacturer or vendor document)
7. ecobee, Power Extender Kit (manufacturer or vendor document)