What you will learn in Thermostat Wiring Colors Code article:

  1. What thermostat wire color is likely to go to which terminal on the thermostat.
  2. Basic electrical safety and standards for wiring color
  3. The function of each wire terminal on the average thermostat. Additionally highlighting some of the differences in those functions from manufacturer to manufacturer.
Sometimes the thermostat is not the problem. Therefore please check this article for a checklist of things to check before calling a heating and cooling repair service.

Wire is available here.

Thermostat Wiring Colors Code | HVAC Control - Safety and Standards

You will always want to follow the thermostat manufacturer’s instructions whenever changing the thermostat. Always turn the power off!You can do this at the air handler and condenser. Always make sure there is no voltage at the transformer before proceeding. The thermostat wire colors below are the most likely colors used for most installations. However, your thermostat wire colors may be different than what is indicated here. There is no official standard for thermostat wire colors. You can consider this the standard wiring colors for thermostat and HVAC equipment. Standards for thermostat wiring colors do not exist except for here and a few other references on the internet. However, these are the most common wiring methods used. This is based on my years of experience in the HVAC field where I successfully wired many thermostats.

For instructions and great tips on how to wire a thermostat please see our how to wire a thermostat page.

Thermostat Wiring Colors Code

Thermostat Wiring Colors Code - High Performance HVAC Thermostat Terminal Designations

Turn the power off before proceeding! Always follow safety instructions especially with electricity. This should be done at the circuit breaker for both the condenser and the air handler/furnace. The power for the condenser and air handler is done at the circuit breaker box or the equipment itself.

It is possible you could cause a dead short in the control circuit and blow the transformer. That could potentially cost you a service call for troubleshooting and repair.

Always make sure you have the right tools and materials for the job. It makes things go so much easier.
Related Link: 4 Wire or 5 Wire Thermostat Wiring Problem – Question from a reader (opens in a new window)

Tracing a Wire to the Source - Thermostat Wire Color Codes

typical wire colors
Terminal Strip in an Air Conditioner Air HandlerIf you find the colors are not matching to the convention described here you can always trace to the source. Most of all, only if you are confident in your skills. Remember, the wires that run from the thermostat are terminated in the HVAC equipment. Most noteworthy, air handlers, boilers, or other equipment have high voltage. Some manufacturers provide terminal strip boards. While in other equipment you will simply find a bundle of wires with wire nuts holding them together. In either case, it is important to find the wiring diagram for the unit. In doing so, you can match up the appropriate wire color from the thermostat to the termination in the equipment.

The red wire should always come from the hot side of the 24-volt transformer. Consequently, the common wire (sometimes black in color) should come from the common side of the transformer. Most noteworthy, the wire colors on the transformer will be different than the thermostat wires. If you do not know what you are doing then please call a professional for a few reasons; as mentioned there is high voltage in the equipment and can be hazardous to your health. Therefore, if you wire it wrong or accidentally touch the wrong wires together you can blow the transformer.

Additional Resources | Thermostat Wiring Colors Code

Thermostat Terminal Designations Explanations

You may also want to check our thermostat category for other detailed articles on thermostats. The category includes other detailed pages about thermostat wiring.

High Performance HVAC Thermostat Wiring Colors

Thermostat Wiring Colors Code

Resource, Learn More HVAC and About the Author

To learn more about heating systems and HVAC use this resource.

If you are purchasing new HVAC equipment don't forget to check out our HVAC Consumers Buyers Guide Category for lots of help.

See the About the Author page for information about the author of this article.

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