Plumbing, Electrical, & HVAC

Honeywell Thermostat Wiring Instructions

If in the market for a new Honeywell thermostat, make sure you understand your Honeywell thermostat wiring needs before upgrading.

Q: I installed my new programmable thermostat, but it won’t turn on.

A: If you plan to upgrade to a new thermostat make sure you have the proper number of wires available, otherwise your new thermostat won’t work. If you do need additional wires, you will want to consider how difficult installing new wires will be for your application.

Modern electronic thermostats are no different from any other technology, in that they can do more, but can also seem complicated to install and understand. One comfort to know is the basic wiring hasn’t changed much, with exception of an additional wire to get some programmable digital thermostats operating.

Note: For specific thermostat model installation manuals and wiring instructions, refer to our Thermostat Buying Guide for a complete list and link of manufacturers. Individual thermostat model information is most likely the best place to find particular manuals, and not a general manuals category that can be found within the search function.

SEE ALSO : In-Depth Thermostat Wiring Explained

4 Wire Honeywell Thermostat Wiring

  • G – Fan (green wire)
  • R – 24 VAC / Rc and R or Rh (red wire)
  • Y – Compressor / air conditioner (yellow wire)
  • W – Heat (white wire)

Note: A previous installer may have used a different set of color wires. This will be a non issue to keep track of if a whole new set of thermostat wires is pulled. Otherwise tag the wires before removing from the existing thermostat, or take note of the terminals each wire is attached to.

honeywell_CT87N

 

5 Wire Honeywell Thermostat Wiring

Some programmable thermostats (e.g. Honeywell Prestige wiring requirements) need a minimum of 5 wires, and maybe more depending on what features you want to operate (humidifier, etc). The additional 5th wire will be for common (neutral), which acts the same as it does on an electrical light switch to complete a path and provide power for the display, closing switching relays, etc.

  • G – Fan (green wire)
  • R – 24 VAC / R and Rc (red wire)
  • C – 24 VAC Common (color can vary per manufacturer – may be blue, purple, brown. etc.)
  • Y – Compressor / air conditioner (yellow wire)
  • W – Heat (white wire)

honeywell_thermostat

 

Furnace Electrical Board Connections

Going into your furnace may sound intimidating, but if you have the basic knowledge of wiring, and know how to remove the proper furnace panels to access the electronics, then you should be fine. The COM 24V terminal will be connected to the C (24 VAC common) of the thermostat.
furnace_terminals

 

How to Install Thermostat Wire

If you have easy access to the existing thermostat wires, just remove and follow the same path with the new set of wires. You can use cable ties, electrical tape, or staples (securing wires and cables article). Be sure to take a very conservative measurement on how long your new set of wires should be. You don’t want to start over, if you come up short. Any home improvement store should carry thermostat wiring. Also check local electrical and lighting supply houses.

If wires pass through a floor and up into a wall cavity before reaching a thermostat. Try the following.

  • Start at the thermostat location. (i) A simple trick for pulling new wires to replace existing ones: tape one end of the new wires to the end of the existing wires coming through the electrical box or wall opening. Make sure to tape completely over the wire ends, so no edges are present that can snag something. (image below)
  • Slowly pull the new set of the wires down through the wall from the basement. (i) Use any object larger than the hole in the wall, and tie or fasten to the end of your new wires, so you don’t pull all the wire through the wall. e.g. pencil or flat washer, to act as a stop. Use electrical tape to secure if necessary.

taped_wires
In a situation where the original thermostat wires may have been stapled to an interior wall stud, and not run through conduit, you may be better off leaving the existing set of wires, and cutting them off at the entry and exit points of the wall. Then feed new wires with a fish tape (Klein Tools). Check your local home improvement stores, or local tool rental, for rental availability.

CAUTION: ELECTRICAL HAZARD – Always turn power off before performing any work. Contact a certified HVAC tech if you don’t feel 100% sure of your DIY capabilities.

 

26 Comments

  1. 1

    Hi Ellen,
    With so many wiring variations, the best option is to compare the thermostat wiring directions with your heating and cooling device and then use this article guide and additional links for further assistance on terminal descriptions. If you don’t have the manual for your baseboard heater, you may find it at http://www.manualsonline.com/

  2. 2

    I’m replacing a real old Honeywell round thermostat for electric baseboard heat with a new programmable one (which has a lot of slots for different colored wires)… I only have 1 wire that comes out of the wall that splits in 2… 1 ribbed(neutral?) & 1 smooth(hot?)…which slots does which go into? B? W?

  3. 3

    Hi Tim,
    The furnace board and thermostat terminals that the existing wires connect to will tell the complete story. Color of the wires should be standardized but a previous installer could have used whatever color to connect “X” function. So, color alone doesn’t tell me what is connecting to what function.

  4. 4

    I am replacing an older thermostat with a Honeywell wifi programable one. My problem is the number and of wires the first old one has. I don’t want to miss something. My old one has six wires connected, a yellow, brown, green, white, red, and blue what goes where?

  5. 5

    Hi Don,
    The furnace panel and thermostat terminals your existing wires connect to are needed as colors are not always standardized.

  6. 6

    I purchased a CT31A Honeywell non programable thermostat to replace a programable thermostat. The new CT31A has terminals for 4 wires red, white, yellow and green. The old programable one has 5 wires red, white, yellow, green and black. What do I do with the 5th wire?

  7. 7

    Hi,
    If you only have 4 existing thermostat wires then I don’t believe any of the newer digital thermostats can function as they are designed to have the 24v Common wire to complete their electronic circuit. This article provides much more depth on electrical wiring. http://www.electrical-online.com/thermostat-wiring-explained/.

    You asked which Honeywell thermostats work with only 4 wires? I would contact Honeywell or look through each manual and see what their wiring requirements are. The manuals can be found in pdf form in each thermostats product page. eg. http://yourhome.honeywell.com/home/Products/Thermostats/7-Day-Programmable/Prestige+HD+7-Day+Programmable+Comfort+System.htm

  8. 8

    Hi, I have an older Honeywell that I would like to replace with a newer wifi programmable thermostat. I looked at the wiring that goes to the older Honeywell and I have the following:

    W1 – going to W1 connector
    W2 – going to W2 connector
    2 Red wires – both going to the Rh connector

    My question is what Honeywell product supports this setup?

  9. 9

    Hi Tim,
    Wiring in reverse will not be a problem. The 24C is required in some thermostats to complete the circuit path for their electrical design. But if your thermostat doesn’t require the 24C then there will be no harm leaving it unconnected. For future reference label this wire by the thermostat, cap it off with a wire nut so there are no electrical short issues, and just tuck it away where it can easily be accessed.

  10. 10

    Hi Denny
    First off, It’s been a year since the last question, I don’t know if you still answer questions but if you do….Thanks for providing a place for DIY’ers to get quality answers for their heating problems..

    I have a Lennox lp gas furnace with off peak electric and A/C. My current non-programmable thermostat uses 5 wires (the C 24v common wire), the new Honeywell programmable uses 4 wires (saying don’t hook up the C wire) Will this cause a problem?

  11. 11

    Hi Rodney,
    With so many thermostats on the market I don’t have any definitive troubleshooting knowledge of each one, especially when not seeing the products in person and having the ability to do a first hand inspection. If you did the wiring correctly and the blower motor for the AC is turning on, I’m not sure why the call for heat doesn’t work as well, the furnace blower motor is used for both operations. I can only suggest re-checking your wiring on both the thermostat and furnace electrical panel, and going through the settings again. There is always the possibility of a faulty product too.

    I suggest looking into Amazon or other reviews of your exact product, maybe someone else had a similar issue and solution.

  12. 12

    I just installed the RTH5100B with the 4 wires to correct terminals. The fan does not come in auto heat. It comes on with AC auto. It is a gas furnace with Ac. What could be the problem?I have tried all the different settings.

  13. 13

    First off, thermostat wires are all low voltage, meaning they are only signal wires sending 24v or less, so there is no risk of electrical shock. But if you have no electrical knowledge or experience you may be best off having someone that does survey what you have and what is needed for the upgrade.

    If you have an existing HVAC technician or think you will need one soon for a new furnace and/or air conditioner, you can contact an HVAC company as well, and have one of their techs do the upgrade, while hopefully building a reliable and solid relationship in the process.

  14. 14

    I have an older thermostat and am trying to assemble a programmable thermostat. The current one only has 2 wires connecting it and the programmable one had 3 spots for wires . I know nothing about electricity or wiring and wondering if I need an electrician to install this.

  15. 15

    Hi Tom,
    If you’ve already had a service tech on site that can perform tests and are dealing with the manufacturer, they are going to be better able to advise than us.

  16. 16

    We live a condo with 4 zones. It is a gas burner. The service man advised us our relay was bad. I sent away for a Belimo matching relay. Our Loft zone keeps calling for heat. I changed the relay correctly. While waiting for part I discounted the 3 wires from the unit and rotated counter clockwise for no heat.
    After installing new unit same problem occurred. I then disconnected all the wires to the thermostat and still the heat kept coming on. Not until I disconnected the3 wires from the Bilimo relay did the heat shut off.
    What is causing the heat to come on? Please help.
    The Belimo company said he was sending the same relay although the numbers may be different now.

  17. 17

    Hi,
    As straightforward and standardized as some things should be, advising without actually seeing what one is working with may lead to issues as there are variances between products. Following your manufacturers wiring instructions, take a look at your furnace electrical panel and connect the furnace terminal wires to the correct thermostat terminals. This article adds further explanation to thermostat wiring. http://wiki.xtronics.com/index.php/Thermostat_signals_and_wiring

    From my understanding you have a 5 wire setup that requires you to remove the R and RC jumper.

  18. 18
    Herb Shallcross

    My old round Honeywell thermostat has a W terminal with a white wire, Y terminal/Yellow wire, G terminal/Green wire, R terminal/Red wire and an RH terminal/Blue wire. My new Honeywell RTH2300 programmable thermostat has W, Y, G, R, and RC terminals.

    Does the diagram above mean both the RH and R wires go on the R terminal, or should I tape off and abandon one of them or take the jumper between R and RC and put one of them on the RC terminal, and if so, which one?

  19. 19

    I found the best option for my needs with this same situation was to add a new set of 5 wire thermostat wiring. Read the section of our article on why the common wire is needed. For me this wasn’t an very difficult task as our basement ceiling is fully exposed and I could follow the existing wires to the furnace, then feed the new wires by taping to the existing wires and pulling up through the conduit to the thermostat wall opening. Not sure of your situation?

    The batteries you mention are just power failure backup to save the settings.

  20. 20

    changing out old thermostat have 4 wires w g y r no common had batteries trying to install new wifi programable dont need wifi what the wife bought what to do about no common

  21. 21

    Hi Mark,
    You mention a “T” furnace terminal. Lennox Furnace? If so, yes, the low voltage “T” terminal on the furnace can connect to your thermostat C (common terminal).

  22. 22

    Furnace has R, W, Y, G, and T terminals, but only R, W, Y, and G at the thermostat. Is the T terminal the same as the C (common) that I need for new programable thermostat? (I have ac unit too.)

  23. 23

    Hi Lori,
    Yellow is normally for air conditioning. Do you have an air conditioner? Best to go into your furnace and see what terminals the existing wires connect to. You can use any color wire as long as it connects the correct thermostat and furnace board terminals.

  24. 24

    I have a green, red, white and blue wire from my old thermostat. All of the programmable directions require a yellow wire. Is one of the wires interchangeable with the yellow wire?

  25. 25

    Hi Bobby,
    There is no timetable on replacing your thermostat from my knowledge. Just replace when the unit goes bad.

    If your current thermostat is old enough that it contains mercury, please contact your local waste disposer for proper disposal.

    If you are in the market for a new thermostat, please view our Thermostat Buying Guide.

  26. 26

    How often should the thermostat be replaced?

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