Honeywell thermostat backing plate showing wiring connections Thermostat Wire Color Codes
& Thermostat Wiring Tips

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This article describes several tricks of the trade used by professionals when installing room thermostats and wiring them up. It describes the standard wire color conventions for HVAC thermostats.

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Thermostat Wiring Tips & Color Conventions for 24V Thermostats

9 Wiring Tips for Room Thermostats: Helpful Pointers Regarding 24V Thermostat Wiring & Heat-Anticipators

Removing old thermostat - wires extrended (C) Daniel Friedman 3M-22 Filtrete thermostat wiring terminal layout (C) Daniel Friedman

Let's note first that the type of heating system installed determines what thermostat wires are necessary and how they must be connected. Our complete thermostat wiring guide for various types of heating or cooling systems and thermostat brands and models is found at THERMOSTAT WIRE CONNECTIONS. Additional wiring details are at THERMOSTAT WIRING DIAGRAMS if you prefer those.

Also see HVAC SYSTEM TYPES, and see for our complete guide to wiring different types of heating, cooling systems & various brands and types of room thermostats.

  1. The string in a typical recently manufactured multiple element thermostat wire is there to help an installer more easily strip off the main outer jacket without harming or nicking the individual inner wire jackets.

    Once an installer has used a knife or wire stripper to expose enough of this string to be able to grab onto it with his fingers, he can then pull the string at right angles to the thermostat wire, then slicing easily a little further through the main outer jacket without having to risk any further harm to the new just freshly exposed jackets of the smaller inner wires.

    An installer can then simply prune away any of the smaller wires which he may have had to nick before he was able to grab onto the string and to use it to safely strip off the outer jacket.
  2. It's always best to wrap a spare or unused wire around the main thermostat wire outer jacket where the outer wire jacket just starts, so that such wires, if needed, could still be used at a later time or date. Cutting such wires back instead of merely wrapping them back out of the way, may be regretted later on in the job in hindsight. We don't leave bare wire ends because we don't want to risk accidentally shorting out something inside the thermostat or other control device.
  3. Heat-anticipators are typically chiefly found in mechanical (non-digital) thermostats, though some new digital / programmable room thermostats like the 3M-22, discussed at THERMOSTAT SWITCHES, INTERNAL, provide a nearly identical function referred to as an adjustable thermostat swing-rate (cycle rate or thermostat sensitivity).

    Whenever installing or servicing a thermostat with a heat-anticipator, the amperage setting on the heat-anticipator should always be verified as matching the amperage draw on the gas valve or relay that it controls. (See HEAT ANTICIPATOR Adjustment and HEAT ANTICIPATOR MINI AMMETER TEST).

    Not all heating systems and buildings benefit from a heat exchanger but in some cases you want a thermostat with this feature. See Do I need a thermostat with a heat anticipator? for details. In any event, if a heat anticipator is included in a thermostat it should be set properly. Otherwise the relay or thermostat may not function properly, resulting in too-short burner on-time (higher heating costs) or room temperature overshoot (wasted heating dollars).
  4. Color codes for 24V thermostat wires are divided into two groups:

    1. The 5 universal base colors for thermostat wires which are, in order of frequency of use, red, white, green, yellow, and black. These colors are used as follows:
      1. i. Red for the common hot side of the heating transformer
      2. ii. White for the switched heating control circuit.
      3. iii. Green for the switched main blower control circuit.
      4. iv. Yellow for the switched compressor compressor control circuit.
      5. v. Black for the other common side of a transformer so that a digital thermostat would not be entirely battery dependent. Whenever installing or servicing a digital thermostat, If possible, it's always best to try and make sure that this wire is properly attached.
    2. The 5 additional colors for thermostat wires which are generally used for two stage heating and cooling, heat pumps, emergency heat, and other things are usually only found and used in some of the more complex systems. These colors are
      1. orange,
      2. blue,
      3. pink,
      4. purple.
      5. ???

        Watch out: while there are some things that these color wires are more commonly used for than others, not all manufacturers agree on which of these additional colors should always be used for what.
  5. [deleted, redundant]
  6. The temperature actuated on off switch in a thermostat is actually a single pole double throw switch which will have either the heating or the cooling side closed at all times.
  7. Whenever using a digital thermostat, always use one that has the ability to accept batteries, and also, if possible, try to provide non-battery powering to the thermostat via the thermostat's 'C terminal' connection.

    Any digital thermostat (including name-brand thermostats) that does not have the ability to accept batteries, or which does not have it's 'C terminal' properly connected through to a transformer, can cause problems such as requiring frequent battery changes, or possibly may not even work at all in certain types of applications!
  8. Any thermostat wire splices that may be required should always be made in fully visible and accessible locations. Hiding such splices in a wall or other difficult spot is only inviting trouble later on, for either yourself or for another service person. Instead, just use a longer fresh wire in order to avoid having to leave such a splice in a difficult or hidden place.
  9. A properly wired typical hot water boiler (that does not have a domestic hot water heater exchanger attached) should always be wired such that it will automatically completely shut down all circulators and burners during the warmer periods of the year.

    In the past, many of these boilers have been wired to run continuously, even throughout the summer. Such wiring configurations are an extremely costly and inefficient waste of natural gas and/or other natural-energy-resource.

    Note: While disabling the LO/DIFF on an aquastat that controls a heating boiler that does not include a tankless coil, we do not necessarily agree that a boiler should always be wired to shut down in summer; details of this approach and the tradeoffs are found at Optimal LO/DIFF settings save fuel - Ed.

- Thanks to reader " Helpful Pointers" Regarding 24V T, 10/7/2012

Old Timer Tips on Shorting Out Thermostat Wires on Purpose to Identify Which Wire is Which

Tips for figuring out which thermostat wires do what: how to identify (R) (W) (B) wires. These tips are from the 1949 Honey Heating Control Handbook [copy on file] for figuring out the function of thermostat wires whose color cannot be determined (for LOW VOLTAGE series 10 type circuits).

Watch out: DANGER OF DEATH BY ELECTROCUTION: don't try this experiment if your thermostat uses line voltage (120V). The procedure Honeywell described was ONLY for low voltage (nominally 25V) equipment.

Watch out also about shorting thermostat wires together on newer HVAC equipment using circuit boards. That advice is only for old, simple series 10 type control circuits. As we report in the article above, on some modern heating and air conditioning equipment shorting wires can blow a circuit board component or fuse. If you've already made this mistake, check for a blown fuse on a circuit board in your furnace or boiler controls.

  1. With the three thermostat wires properly connected at the heating equipment end (we hope you didn't disconnect them there as well) where they operate a relay or valve, hold the three bare wires together. The heater should run.
  2. Now remove one of the wires. If the burner stops, this is the (W) or white wire. Attach this wire to the W terminal of the thermostat.
  3. If the burner does not stop, hold all 3 wires together again and try another wire until the white wire is found.
  4. After the white wire (W) has been identified and labeled and secured to the (W) terminal, one of the remaining two wires will start the burner when shorted to the white (W). The one that causes the burner to start is the blue (B) wire. Connect it to the (B) terminal.
  5. The remaining wire is the red wire and should be connected to the (R) terminal.

Shorting Out Thermostat Wires by Accident - Leads to Loss of Heat

Watch out: do not short any wires together. Turn off power & confirm it is off. Don't forget to turn off all electrical power involved with your heating system before working on thermostat wiring, and confirm that power is off where you are working by using an appropriate test instrument such as a VOM. Shorting thermostat wires to ground or together is likely to damage components in the thermostat itself or in other heating system components, as this reader's report below illustrates:

I had a no heat situation since replacing my programmable thermostat. I did some checks that the thermostat company said and the thermostat is good. But now my furnace won't come on at all. I can hear the transformer humming. It is a Miller model #M3RL-080-ABW in a four year old manufactured home.

Apparently I touched two live wires together when doing the thermostat installation. I know, I should have turned off the breaker, (power), before working. That is the lesson of the week for sure!

I blew a small purple, two prong, auto style fuse in the electrical board on my furnace. What I did not know is the fuse was marked with a "E" instead of a number, but it is a 4 amp fuse, found at your local automotive store. I was without heat all weekend, even though it could have been on Friday evening.

I'm glad I pulled the fuse and looked to see the metal inside was broken. Otherwise I might have been without heat until I could get a HVAC guy to come out to my house. -- S.R.

Note: your fuse marked with an "E" was probably being read upside down and was a 3-amp fuse. Using a fuse one amp greater incapacity is probably not too dangerous, but it's best not to overfuse as doing so risks equipment damage. Certainly a larger fuse is asking for wiring or device burn-up should a future short circuit occur. - Ed. & reader comments

Since shorting thermostat wires can damage a circuit but can also blow the low voltage transformer, also

Reader Question: possible shorted thermostat wires

Our furnace thermostat failed after an adjacent water heater installation. I noticed that wires were pinched between the water heater and the gas pipe. After I loosened the safety straps and freed the wire, the thermostat works. (I reset the furnace). Were squished wires the cause of a temporary short and will the wires keep working if they are undisturbed? - Larry 8/2/11

Reply: check for shorted wires and for thermostat transformer damage as well as for blown fuse

Yes it's surely possible that your thermostat wires were shorted. Now a short in those wires sometimes lets you off the hook with no trouble, since the wires and thermostat are basically an "on-off" switch that calls for heat (or no heat).

But a thermostat wire short at the transformer could burn out the low voltage transformer that powers the thermostat and heating controls or maybe even damage the thermostat itself.

Find and check the low voltage transformer (often it's mounted near your furnace, on an electrical junction box, or it may be integral in your heater control unit). See LOW VOLTAGE TRANSFORMER TEST

Question: reader comments on shorting out thermostat wires & fusing requirements

Greetings, not sure if anyone saw this or not, but whoever S.R. is above in the description about shorting out wires seems to maybe have missed the fact that the "E" they refer to is actually a "3" amp fuse and not a 4 amp fuse.

My HVAC installer of 30 years who recently installed 2 furnaces for us and is installing my heat pump agrees that it is a 3 amp fuse. We purchased an extra one just in case. Putting a 4 amp in there may not be good for the board as it may allow too much current for a relay or something and you may fry a piece on the board which is most likely not cheap. I hope this helps - D.M. - 3/12/12


Thanks D.M., We posted your important fuse size comment right in the article
at Shorting Out Thermostat Wires - Leads to Loss of Heat.

Reader Question: Where does the white wire go?

On a York roof mount heat and air system thermostat wire white go to the com terminal or the white terminal - Gary


Not sure for your system which wire goes to which terminal - take a look at the wiring instructions on the appliance or in the installation manual to be sure. If you give me the product name and model number we can help find the installation and wiring instructions for a certain answer. I didn't find a customer service telephone number for York but the company's website includes consumer help and is found at

Another reader offered: on a York roof mount heat and air system thermostat wire white goes to the com terminal or the white terminal


Continue reading at THERMOSTAT INSTALLATION STEPS or select a topic from the More Reading links shown below.

Or see THERMOSTAT WIRE CONNECTIONS detailed room thermostat installation & wiring guide for each heating or cooling system type and each thermostat wiring specification


Or see THERMOSTAT WIRING TIPS & COLOR CODES & methods for identifying which thermostat wire is which if yours have lost their labels or have unclear color codes.

Suggested citation for this web page

THERMOSTAT WIRE FUNCTIONS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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