Thermostat wiring looks intimidating, but it is only a handful of low voltage wires, each switching one function. Knowing what each terminal does helps with a thermostat swap, and helps you tell a technician exactly what you see. One rule first: turn off power to the furnace or air handler before touching any wires.
Colors are a convention, not a rule
Installers usually use standard colors, but nothing forces them to. Always go by the letter on the terminal where each wire is connected, at both the thermostat and the furnace or air handler control board. Photograph the wiring before disconnecting anything.
What each terminal does
- R, Rh, Rc: 24 volt power from the system’s transformer, usually a red wire. Rh is power for heating and Rc for cooling on systems that use separate transformers. On most homes with one transformer, R, Rh and Rc are joined.
- C: the common wire, often blue or black. It completes the circuit so a thermostat has continuous power. Most smart thermostats need it.
- W or W1: first stage heat, usually white. On a gas furnace it calls for heat. On a heat pump system, it may control auxiliary heat.
- W2: second stage heat, on two stage furnaces, or auxiliary heat on some heat pump setups.
- Y or Y1: first stage cooling, usually yellow. It turns on the compressor in the outdoor unit. On a heat pump, the compressor runs for both heating and cooling.
- Y2: second stage compressor, on two stage systems.
- G: the indoor fan, usually green.
- O/B: the reversing valve on a heat pump, often orange or dark blue.
- AUX or E: auxiliary or emergency heat on some heat pump thermostats.
Y1 vs Y2
Two stage air conditioners and heat pumps have a compressor that can run at low or high capacity. Y1 calls for low stage, which handles most of the year. Y2 calls for high stage on the hottest days. If you replace a two stage thermostat with a single stage one and connect only Y1, the system will run at low stage and may not keep up in a heat wave. If you connect Y1 and Y2 together, it runs at high stage all the time and loses the comfort benefit.
The heat pump O/B setting
The reversing valve switches a heat pump between heating and cooling. Manufacturers energize it in one mode or the other:
- O energizes the valve in cooling. Most heat pumps use this.
- B energizes the valve in heating. A smaller number of brands use this.
Most thermostats have a single O/B terminal and a setting to choose which. If it is set wrong, the heat pump heats when you ask for cooling and cools when you ask for heat. That is the most common mistake after a DIY thermostat swap on a heat pump. Check your equipment manual. See heat pump not cooling.
Auxiliary heat settings
On heat pumps, the thermostat decides when to bring in electric backup heat. Wrong settings, or no outdoor temperature lockout, can run the heat strips far more than needed and raise your bill. See heat pump using too much electricity.
Common wiring problems
- No C wire for a smart thermostat. Solutions include using a spare conductor, running a new cable, or an adapter kit.
- A loose or broken wire, often at the terminal screw, causing intermittent operation.
- Two bare ends touching, which shorts the circuit and blows the 3 amp fuse on the board. See a furnace blowing its fuse.
- Rodent-chewed wire in the attic.
- Communicating systems that use a data connection instead of standard terminals. A generic thermostat will not work the same way on them.
What we do
We verify the wiring at both ends, configure staging, O/B and auxiliary heat correctly, add a C wire where needed and fix any shorts. Our diagnostic is a flat $69 and it is credited toward the repair. See thermostat replacement cost and thermostat not working.
Frequently asked questions
What do the thermostat wire colors mean?
By convention, red is R for power, white is W for heat, yellow is Y for cooling, green is G for the fan, and blue or black is often C, the common wire. Colors are not guaranteed, so go by the terminal letters.
What is the C wire on a thermostat?
The common wire, which completes the 24 volt circuit so the thermostat has continuous power. Most smart thermostats need it.
What is the difference between Y1 and Y2?
Y1 calls for the first, lower stage of cooling. Y2 calls for the second, higher stage on two stage systems.
What does O/B mean on a thermostat?
It controls a heat pump’s reversing valve. O energizes it in cooling and B in heating. Setting the wrong one makes the heat pump run in the opposite mode.
Why is my heat pump heating when set to cool?
Often the thermostat’s O/B setting is wrong, especially after a new thermostat was installed. A stuck reversing valve can also cause it.
Can I wire a thermostat myself?
On simple systems, many homeowners do. Turn off the power, photograph the existing wiring, and match terminals. Heat pumps, multi-stage and communicating systems are easier to get wrong.