Learn what the different thermostat wiring colors mean

and how to service and repair control wiring.

On your gas furnace, oil burner, heat-pump or air-conditioner.

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I get many inquiries about what the different wire colors mean and what they do and how troubleshoot problems where nothing happens or some parts will work and others will not.

    First I am not into disclaimers because I assume you are smart enough to know not to mess with stuff that you are not familiar with or competent at doing. However I must make an exception here.

    Control wiring on hvac equipment while harmless looking and 24 volts ac wont shock you (it will bite) can cause very bad things to happen if connected incorrectly. You could destroy your equipment at best or burn down your home or business at worst. Among the control wiring can be high voltages from 120/240 volts in a residential furnace to 480 or 600 volts in commercial equipment that can kill you instantly or burn you in a flash.

    So if you are unsure of what you are doing or your abilities please leave it to someone who understands control wiring and electricity. If you need to ask for a wiring diagram then it is a good bet that you should seek extra help because the knowlege required to do the job safely far exceeds the knowlege required to read and follow a diagram.

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Keep in mind that more than one disconnect may be required to remove all power from a piece of equipment. If your unit has service switches find them first. Or turn off the circuit breakers or pull the fuses, then check with a meter or test lamp to be sure the circuit is dead.

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With all that said lets get to the terminal designations and colors. These designations are for 24 volt control wiring used in most residential and light commercial equipment, line voltage stuff can be different. Never assume that a wire color is correct for the function it should perform; check it out first  The same goes for high voltage control wiring and motor terminals .

• {Terminal name}, {color}, {function}
• (R), Red, hot side of transformer.
• (C) Common side of transformer (See B)
• (Y), Yellow, Compressor activity (cooling or cooling and heating on a heat pump).
• (W), (W1), (W2) White, "Heat"  (gas burner, oil burner , electric heat, (auxiliary heat on a heat pump including defrost output from the outdoor unit to activate electric heat and turn on the AUX. heat lamp). Note: some thermostats require a jumper from "W" to "Y" for heat pump operation.
• (G), Green, furnace blower fan. (needed for air conditioning, heat pumps and some electric furnaces). NOTE: on most thermostats the "G" and "Y" are connected together at all times when the fan switch is in the "Auto" mode!
• (O), Orange , Energize to cool (used for reversing valve on heat pumps)
• (B), Blue or Orange, Energize to heat (used on some systems, Rheem/Ruud is notorious for this).
• (B) or (X), Blue, brown or Black, common side of transformer. Needed on some electronic thermostats or if you have indicator lamps. Do not confuse with (B) Reversing valve (energize to heat) above. York and Trane like to use (B) as common.
• (E), blue, pink, gray or tan, emergency heat relay on a heat pump. Active all the time when selected, usually not used.
• (T), Tan or Gray, outdoor anticipator reset.

Used on GE/Trane/American Standard and some Carrier Products.
• (W2), Pink or other color, second stage of heat (may be same as (W) on heat pump or fossil fuel system).
• (Y2), Blue or Pink, second compressor stage.
• (L), Blue, brown, tan or gray service indicator lamp.
• Numbers, see manufacture's diagrams but can be stages of electric heat especially on York/Borg Warner/Coleman-Evcon, Frasier Johnson/ Air-pro.
• (X2), Second stage of Electric Heat on GE/Trane/American Standard. See link above. Also used as indicator lamp or misc. contacts on other systems.
• Learn more about a Heat Pump Thermostat.

    I cannot say it enough times: You should never go after your thermostat as a cause of the problem until you verify with test lamps or a meter. To do so you are not solving the problem but creating more and risk blowing a fuse or the control transformer. On most systems you can get to the control wiring at the air handler or furnace and take your reading there. You should only replace or change a thermostat after you verify that it does have a problem.

    If your system does not have a fuse protecting the secondary side of the transformer add one before proceeding! An automotive ATC fuse holder and a 3 amp fuse should do the job. If you have a Trane unit and you blow the control fuse, replace it with and automotive 3 amp ATC fuse.

Link to White Rodgers that has a good look up table of different manufacturers. You will notice that Lennox's color codes are TOTALLY WEIRD to put it gently.
Link to White Rodgers Clip art that has pictures of their different control products.

    I would highly suggest identifying each wire by connecting the hot lead to each function wire with clip leads and a 3 amp fuse and recording the function before installing your new thermostat. While this is more trouble it may save you from ruining your day. 

Did you consider Solar Energy?

 If your outdoor unit turns to a block of ice in the heating mode read this.

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