Tip & How-To about Heating & Cooling

Thermostat Wiring and Terminal Designations Part 2

There is a lot of confusion about terminal designations and thermostat wiring. There are no official standards for wiring colors, or even terminal designations for that matter. To be sure it’s always best to refer to your equipment manufactures wiring diagrams to see how their terminals are meant to be used.

As you can see in Part 1 the most common colors are the red, white, green, and yellow. These four colors are almost always associates with the same terminals. If you have any more wires than that you may have any number of combinations, depending on what the tech decided when they installed your unit. There is no right or wrong way to do anything. As long as the same wire is connected to the same terminals of the two pieces that they are running to and from, then the wiring is correct. It is up to YOU to verify and not just guess at what the installer was thinking.

It is always advisable to make sure that the power is off when working with electricity. Also when checking wiring and even as a good practice when working with your low voltage wiring, always install an automotive ATC fuse holder at the hot side of the transformer secondary. (Load side) This will protect the transformer in case you accidentally short it out.

I need to make sure that I give this caution. Please if you are not familiar with electric and wiring, do not attempt to work on your unit alone. Find someone that is comfortable in their ability to work with electricity. It is very easy to do more harm than good and even experience technicians make mistakes at times. Use every caution and work very methodically and carefully. Haste makes waste!!!

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No cool air. Thermostat AUX light turned on


I suggest you call a qualified service repair man. There are as many ways to connect a heat pump as there are people doing them. Nothing is standard industry wide that has anything to do directly with color codes, so it would be meaningless to go into them here. The terminals may be adapted to standard but but the thermostat manufacturer may not follow any terminal designations.

However you may just have an extra wire touching one of the terminals inside the thermostat. You can check this by removing the cover. Look where the wires attach to the terminal screws on the thermostat. Sometimes a new service person cuts the insulation back too far here or in the outdoor or indoor units and a "whisker" from one wire can touch the other terminals. A whisker is not the normal case try and move it away from another terminal if its is touching there. However, you may have jumpers in place especially if you have W, W1, W2... at any of the 3 sections, meaning the outdoor, indoor and thermostat.

If you can tell me the terminal designations R,Y, W1, W2, E, G, O, (B)... at the thermostat and the color thereon each, then at the indoor unit terminals or wire designations and the color there, then at the outdoor unit and the color thereon each terminal or wire designation there. I might be able to help if the above whisker check you don't find anything.:

Mar 25, 2009 | Heating & Cooling

1 Answer

im setting up a thermostat and im confused about the color wiring


How many wires do you have and what color are they?
Red is power to the thermostat, White generally goes to W1, yellow to cooling Y terminal, Green to G terminal. If you have different colors the set-up may be a little different. You can check at the unit which should have a terminal board which would also indicate which wires go to which terminals.

May 03, 2008 | Heating & Cooling

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