The short answer
Most thermostats in Northern Virginia homes follow a color habit: red for R (power), white for W (heat), yellow for Y (cooling), green for G (the fan), blue or black for C (common) and orange for O/B (a heat pump’s reversing valve). It’s a habit, not a rule. Google’s wire guide says colors often don’t follow industry standards, so go by the letter each wire is landed on, and take a photo before you disconnect anything.
What each terminal does
Swipe to see all columns →
Swipe to see all columns →
| Terminal | What it does | Usual color |
|---|---|---|
| R | 24 volt power from the transformer in the furnace or air handler | Red |
| Rh and Rc | Power for heating (Rh) and cooling (Rc) on systems with two transformers. A jumper joins them when there’s only one. | Red; a second R wire has no set color |
| C | Common, the return path that gives a smart thermostat steady power | Blue or black, but it varies |
| W or W1 | Heat, first stage | White |
| W2 | Heat, second stage, or backup heat on some heat pumps | No set color |
| Y or Y1 | Cooling, and the compressor in both modes on a heat pump | Yellow |
| Y2 | Second stage cooling | No set color |
| G | The indoor fan | Green |
| O/B | The heat pump’s reversing valve, which switches it between heating and cooling | Orange; Google says brown is most likely a heat pump wire too |
| E or AUX | Emergency heat, or the backup heat strips on a heat pump | No set color |
| L | A fault signal from some heat pumps | No set color |
| S | Wired outdoor or remote sensors | No set color |
The trap is the letter B. On some older systems B meant common, not the heat pump’s valve. Google’s wire guide says a blue wire on a B terminal should most likely go to C, while a brown wire most likely belongs on O/B. When a label and a color disagree, the furnace or air handler’s board settles it: whatever terminal the other end lands on is what the wire is.
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Rh, Rc and the jumper
Many thermostats have two R terminals: Rh for heating power and Rc for cooling. Most homes have one transformer feeding both, so there’s one red wire and a small jumper joining Rh and Rc. With one R wire, Google says it can go on either. Sensi says to cut its Rh and Rc jumper only when there are two separate R wires, which means two transformers. Cut it by mistake? Sensi’s fix is to rejoin the two terminals with a short piece of wire.
Nest thermostats don’t use jumpers at all. Google points out that a jumper only joins two terminals on the old thermostat and never runs into the wall, so it stays behind with the old one.
Heat pumps: O or B
A heat pump uses one wire to tell its reversing valve which way to run. Most brands energize O in cooling; a few, Rheem and Ruud among them, energize B in heating. Set it wrong in the new thermostat’s setup and the heat pump blows cool air in Heat and warm air in Cool. That’s a settings fix, not a repair. For the heat pump troubles that aren’t the thermostat, start with aux heat vs emergency heat and a heat pump covered in ice.
Before you take the old one off
- Turn off the power at the breaker for the furnace or air handler. Google says to do that first.
- Check the colors. If two or more wires share a color, Google says to stop and get a pro, because there’s no telling them apart by sight.
- Take a photo of the wires and the letters they’re on. Google calls that photo an essential reference later.
- Label each wire with the letter it came off, not its color. Sensi’s stickers fold into a flag around each wire, and Sensi says colors don’t always match the letters.
- Keep the wires from falling into the wall. Wrap them around a pencil if you have to.
Google also notes that in rare cases a wire is on the wrong terminal at the old thermostat. If the new one won’t run something, check where that wire lands on the board at the furnace or air handler.
The wire itself
The cable in the wall is low voltage thermostat wire, usually 18 gauge, sold by how many conductors are inside: 18/5 has five, 18/8 has eight. If you’re running new cable, run more conductors than you need today. A spare is what makes the next smart thermostat an easy swap.
Two thick wires with wire nuts are something else. That’s a line voltage thermostat, 120 or 240 volts, running electric baseboard heat. Google says those systems aren’t compatible with a Nest, and none of the colors in the table apply. A thermostat with no C wire at all is its own problem, with its own fixes in the thermostat C wire guide and C wire adapters.
Communicating systems are different
Some systems, like Carrier Infinity, don’t use R, W, Y and G at the thermostat at all. Their control talks to the equipment over four terminals marked A, B, C and D. A regular smart thermostat can’t just take those wires over the way it would on a standard system. Check the wall plate before you buy. Carrier’s furnace codes are in Carrier furnace codes.
When it’s a call
Two wires share a color and nothing’s labeled. The new thermostat blows cool air in Heat. There’s a wire you can’t place, or one too few. The furnace fuse blew when the new one went on. That’s a wiring problem at the equipment, and it’s ours.
When we come out for a thermostat problem, the technician checks the whole system, not just the thermostat: the equipment, the wiring at both ends, the filter and the drain. The diagnostic is a flat $99, the repair is priced in writing before anything starts, and if the problem really is the thermostat, we say so. If the thermostat itself is done, a replacement runs $300 to $640 here, wired and set up for the furnace or heat pump it runs.
Every brand’s codes and warning lights are in one place: error codes and warning lights. Words on the screen instead of icons? See what thermostat messages mean.
Model details are the makers’ own, from Google’s wire guide, its compatibility page, Sensi’s labeling steps and Sensi’s jumper page. Last checked November 11, 2026.
Book it
Book a visit online and put what the screen says in the note, or call (703) 997-9757, answered 24/7. Book from goswiftpro.com/cookiesoffer by November 30 and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
What color is the C wire on a thermostat?
Usually blue or black, but colors aren’t a rule. Google says thermostat wire colors often don’t follow industry standards, so check which terminal the wire lands on at the furnace or air handler. C is common.
What does the red wire do on a thermostat?
Red is usually R, the 24 volt power from the system’s transformer. Rh and Rc split it into heating and cooling power on systems with two transformers.
What if two thermostat wires are the same color?
Stop and get help. Google says to contact a pro when two or more wires share a color, because you can’t tell them apart by looking.
What’s the difference between O and B on a heat pump?
Both run the reversing valve. Most heat pumps energize O for cooling, and some, like Rheem and Ruud, use B for heating. Set it wrong and the heat pump cools in Heat mode.
How much does it cost to fix thermostat wiring in Northern Virginia?
At SwiftPro Heating, Cooling & Plumbing the diagnostic is a flat $99, and the repair is priced in writing first. A thermostat replaced, wired and set up, runs $300 to $640.
