How to Identify Each Wire for an HVAC for a Thermostat

If you're looking to do any repairs or maintenance to your thermostat or HVAC system, you may need to determine which wires are which. This can be difficult if you no longer have access to the manufacturer's instructions. Fortunately, thermostat wires are often color-coded according to common rules for ease of use, especially in the case of forced-air furnaces, central air conditioning and heat pumps. Proper safety techniques must be followed when engaging in any kind of electrical work.

Technician installing heating system
credit: Dragan Smiljkovic/E+/GettyImages
How to Identify Each Wire for an HVAC for a Thermostat

Thermostat Wiring Basics

Most thermostats send commands to your HVAC system – heat or air conditioning – via low-voltage wires inside a command box. This box enables you to set the temperature and configure heat or air conditioning programs depending on your habits and needs.

Occasionally, either the thermostat box or HVAC unit itself may require some maintenance. This may require familiarity with the various colored wires that connect the two. In addition, if you are replacing the thermostat or HVAC system, you'll likely have to disconnect the wires and reattach them to the appropriate junctions in your new unit.

Electrical Safety Basics

Before beginning any electrical project, no matter how small, be sure to turn power to the area off at the level of the circuit breaker. Inform anyone present of the task you are working on and request that they not turn the power back on until you have finished. Using high-voltage rubber gloves and insulated tools is also a safe bet. If you are unsure at all what you need to do for any project involving electricity, it's best to involve a professional.

How to Identify Thermostat Wire Colors

Once your thermostat and HVAC unit are turned off, turn off the circuit breaker and follow the safety instructions as outlined above.

Next, remove the front panel from your HVAC unit. Though old thermostat wiring may not adhere to modern color-coding trends, it's likely that newer units do. These wires will likely be labeled on your HVAC unit with a series of letters that indicate what they do. The letters also correspond to the colors of the wires.

R, a red wire, is for power and is a 24-vac transformer. Rc, also a red wire, is the power for cooling. Rh is red, as well, and is the power wire from the heat. A wire marked C may be either blue or black and is the common wire for the 24-vac transformer. A white wire marked W is the primary heat cell relay. E, which is likely a brown wire, is an emergency heat relay wire. Y, a yellow wire, is the primary cool call relay wire, and Y2 is the secondary cool call relay wire. A green wire marked G is the fan relay, while O is orange and the change-over relay to the cool relay. B, a blue wire, is the change-over relay to the heat relay. You may also have wires labeled A for heat and cool active wires, L for an emergency heat active lamp, X for system malfunction lamp or P for defrost control lamp.

To identify a Honeywell thermostat's wires, you should be able to follow these guidelines as most models follow the same patterns.

Danielle Smyth

Danielle Smyth

Danielle Smyth is a writer and content marketer from upstate New York. She holds a Master of Science in Publishing from Pace University. She owns her own content marketing agency, Wordsmyth Creative Content Marketing (, and she enjoys writing home and DIY articles and blogs for clients in a variety of related industries. She also runs her own lifestyle blog, Sweet Frivolity (