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.

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.

 

12 Comments

  1. 1

    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.

  2. 2

    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.

  3. 3

    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.

  4. 4
    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?

  5. 5

    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.

  6. 6

    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

  7. 7

    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).

  8. 8

    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.)

  9. 9

    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.

  10. 10

    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?

  11. 11

    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.

  12. 12

    How often should the thermostat be replaced?

Leave a Reply

Your email address will not be published.

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>