Thermostat Wiring: How To Wire Thermostat? (2,3,4,5 Wire Guide)

Thermostats are the ultimate control hubs for HVAC devices such as furnaces and air conditioners. Thermostat wiring is a useful skill to know if you have to replace an old thermostat or just check if something is wrong with the new thermostat. With a little help, you can learn how to install a thermostat on your own.

LearnMetrics has designed this guide as that little help. We will cover everything you need to know about thermostat wiring; including where does each color wire goes.

Obviously, there are many different kinds of thermostats out there. 100% of guidelines will not work for 100% of thermostats. There are, after all, many different types of thermostats with different wiring out there: Honeywell thermostat wiring, nest wiring, old thermostats, and so on.

We will cover the most common thermostat wirings for 2 wire, 3 wire, 4 wire, and 5 wire thermostats. At every stage, we will point out what 2 wire or 5 wire thermostats are used to wire, for example, to get a bigger picture of where those color wires go and how they enable the functions of air conditioners and furnaces.

In all cases, we will use this basic Honeywell thermostat for reference:

honeywell thermostat wiring
This is a standard Honeywell thermostat; perfect for learning the ins and outs of thermostat wiring. The protocols apply to Honeywell thermostat wiring as well as for other thermostat wiring.

We will also try to address all the common questions regarding thermostat wiring, such as:

“What color wire goes where on a thermostat?” (Check the diagram below)

“How many wires does a thermostat need?” (At least 2)

“What is R and RC on the thermostat?” (Color coding diagram below)

“What is the black wire for on a thermostat?”

“What if there is no C wire for the thermostat?” (Older thermostat; you can leave it without or add it)

Let’s take a systematic and practical approach here. We will follow this general order to learn how to wire a thermostat:

  1. Remove the thermostat control panel, and expose the colored wires.
  2. We will learn what each color wire means. Understanding color coding will give you a good understanding of how the thermostat actually functions.
  3. Replacing the old thermostat with a new one.

An important note: Keep in mind that dealing with electrical circuits can be harmful to yourself. That’s why always follow the appropriate safety measures when working on live electrical circuits. For more information about the wires and their ampacity, consult the AWG gauge chart with wire sizes for specific amps here.

Let’s start right off with exposing the color wires:

Removing The Thermostat’s Control Panel (Getting To The Color Wires)

The first step in thermostat wiring is actually getting to the wires. You probably have your thermostat mounted on the wall. Exposing the wires is easy; it starts with removing the thermostat’s control panel.

Before you start: Turn the power switch to the device’s thermostat is connected to off.

Most control panels can be removed just by nudging it from below or above a bit. Some Honeywell thermostats can have some screws as well; just unscrew them to expose the wire terminals.

Beneath the control panel, you will find the base and the wires sticking out of the wall. With the basic Honeywell thermostat, you will find:

  • 8 wire sockets to the left.
  • 8 wire sockets to the right.

Before we take the flathead screw to unscrew these, let’s first familiarize ourselves with the crucial part of any thermostat wiring: Wire codes and colors.

Here’s where the thermostat wires go:

Thermostat Wiring Color Codes

As you can see on the thermostat base, you have 16 sockets, with letters C, R, W1, W2, O/B, G, Y1, Y2, BK, 2x RS, 2x ODT, AUX NO, AUX C, and AUX NC. With 2 wire, 3 wire, and 4 wire thermostats you don’t have all of these. That’s normal. You might also notice that all sockets don’t wire in them. That’s standard as well.

For each of these thermostat terminals, there is a designated wire, characterized by a color:

thermostat terminals with codes and color wires
Thermostat consists of 16 sockets with codes (C, R, W1, W2, O/B, G, Y1, Y2, BK, 2x RS, 2x ODT, AUX NO, AUX C, and AUX NC) and designated color wires.

If your thermostat doesn’t look exactly like this, or even anything like this, don’t worry. We just need to get a grip of what each color wire is and what it does.

Here is the full-length article about all thermostat wiring color codes for 3-8 wire thermostats. Let’s look at the short summaries each of these wires in turn color-by-color:

Thermostat Black Or Blue Wire (“C” Wire)

The black wire or blue wire (either color is used) is the C – “Common” wire. What does the C wire do? C wire is connected to the transformer and completes the 24V electric circuit. Newer thermostats have a continuously looped 24V circuit; in older models, the loop is completed only when the power is required (when you turn the AC on, for example).

More often than not, the black or blue wire connected to the C terminal is found in the newer “smart” thermostats. The older thermostats may not have the “C” wire; they function on-demand while the new ones function all the time (continuously 24/7).

That’s why the newer digital thermostats consume energy even when the furnace or air conditioner is not running.

Thermostat Red Wire (“R” or “Rc” Wire)

Red wire or “R” wire is the power wire. They originate in the transformer (air conditioners; in the air handler) and provide the 24-hour volt AC power.

All air conditioner thermostats have a red wire for power. You may also have Rc or Rh terminals for dual transformer systems; these are special cases in thermostat wiring.

Thermostat White Wire (“W1” or “W2” Wire)

White wires are for heating. You will find them in gas furnace thermostats, for example, but won’t find them in air conditioner thermostats.

W wires go directly to the source of heating; that may be a furnace (gas, electric, oil, you name it) or heat pumps (mini-split heat pumps included).

W2 is designated for two-stage heating. Most heat pumps include second-stage heating and require the white W2 wire.

Orange Wire For O And Dark Blue Wire For B (“O/B” Wire)

“O” or the orange thermostat wire is for reverse valve cooling. Most bigger heat pump manufacturers like Trane, Goodman, Lennox, and so on have the orange wire that goes to the condenser (located in the outdoor unit of heat pumps).

“B” is for reversing valve heating. Some producers like Rheem energizes the reversing valve when the heating mode is on in heat pumps. A dark blue “B” wire is needed for the t-stat terminal.

Thermostat Green Wire (“G” Wire)

Green wire is for the fan. Mini-splits have an indoor air handler with a fan; the power input that goes into the fan is regulated by the green or “G” wire.

Yellow Thermostat Wire (“Y1” And “Y2” Wire)

Y terminals are for the connection to the compressor relay. It most often wires to the air handler (indoor split-system unit). Y1 is for ordinary or one-stage cooling; this is what most of us have at home. Y wire with “Y1” code is usually yellow-colored. You can read more about what size wires you need for air conditioners here.

“Y2” is only for air conditioners with second-stage cooling. This terminal is only required if you have:

  • 2 compressor.
  • Two-stage compressor.

Other Wires (BK, RS1, RS2, ODT1, ODT2, AUX NO, AUX C, AUX NC)

Some other terminals that rarely get used at all the ones on the right side of the thermostat. In the future, we plan to create a separate article details that the specific function all of them have.

Do keep in mind that with so many different thermostats, each wire code can have a different color wire. “O” wire, for example, is not necessarily always orange. If a technician that installed your thermostat used a different color, you might have a problem reconnecting the wires. That’s why this next section is so important:

Removing The Old Thermostat (Take Photo)

If you’re planning to replace the old thermostat, don’t immediately remove the old thermostat. First of all, make sure you will be able to correctly wire the new thermostat.

HVAC technicians know the color-coding by heart; they can just remove any Honeywell thermostat, replace it with the new one and reconnect the terminals.

If you are DIY thermostat wiring, here’s the easiest thing to do before removing the old thermostat:

Take a picture.

If you unscrew the terminals and remove the wires right away, you may have problems reconnecting the wires to the new terminal. Make sure you have a picture of how the wiring to the old thermostat looks like before continuing.

Make Sure To Spead The Wires (You Don’t Want To Lose Them In The Wall)

When you have taken the photo of the old thermostat, you can remove it. But do be careful:

The thermostat holds the wires out of the wall; the wires are suspended on the thermostat. If you remove the thermostat right away, the wires will be lost inside the wall. You will have to either:

  • Fish the wires out of the wall.
  • Smash the wall to be able to reach the wires.

To prevent these two scenarios, make sure to spread the wires. Take each wire and bend it outwards. Here’s the logic behind this:

  • Hole in the wall is about 2×2 inches.
  • Wires themselves are about 1×1 inches.
  • Spreading them apart will create at least a 2×2 inch crosssection, and the wires will be suspending on the wall (instead of on the thermostat).

When you have done that, you can safely remove the old thermostat.

Installing A New Thermostat (Reconnecting Wire By Wire)

After you remove the old thermostat, take the new thermostat are put it in the exact same place as the old thermostat. The wires you have fixed on the wall have to be pushed through the hole in the new thermostat.

Now you start reconnecting the wire by wire. Having a photo of the old thermostat is very helpful here. There are two ways of how to know where each wire goes:

  1. Use the photo and reconnect the wires accordingly.
  2. Use the color codes and reconnect the wires.

If you are going by color codes, you can check what each wire means in the section above. Here’s a quick basic summary:

  • Red wire: Power (24V input in most cases).
  • Green wire: Fan.
  • White wire: Heating.
  • Blue wire: Cooling.
  • Rh: Heating power.
  • Rc: Cooling power.

To reconnect the wire, simply put the wire on the right terminal and tighten down the set screw. Pull a wire a bit to make sure the wire is properly fixed.

Example: Reconnect the red wire to R, tighten the screw, check, and move to the new wire.

After you have reconnected all the wires, put on the control board, and check if everything is fine.

Now that we know the basics of how to properly wire a thermostat and the basics of color-coding, let’s look at how to wire 2, 3, 4, and 5 wire thermostats in step-by-step guides:

2 Wire Thermostat Wiring (Furnaces)

The most basic thermostat has 2 wires; usually a red and a white wire. Two wire thermostat wiring is used for furnaces only and usually doesn’t need a “C” or “Common” wire. That’s why we only need two wires. This is a 2 wire thermostat wiring color code:

  1. Red wire for power (24h).
  2. White wire for heating.

Wiring a 2 wire thermostat is pretty straightforward.

Here the step-by-step of how to DIY 2 wire thermostat wiring:

  1. Remove the control panel of the old furnace thermostat.
  2. Take a note of where the wires go; usually, red R wire will go to R, and white wire will go to Rh or W1. You can also take a photo.
  3. Unscrew the two wires from the terminals.
  4. Remove the motherboard of the old 2 wire thermostat and put the new 2 wire thermostat in its place.
  5. Reconnect the red and white wire, tighten down the set screw, and put the control panel back on.
  6. Test the 2 wire thermostat wiring by turning the furnace on.

If you have properly reconnected the 2 wires, the new 2 wire thermostat should control the furnace in the same way your old 2 wire thermostat did.

3 Wire Thermostat Wiring (Boilers, Heaters)

3 wire thermostats are most commonly used to control the heaters; boilers and hot water heaters in particular. The 3 wires you will find usually have codes R, G, and W.

The difference between 2 wire and 3 wire thermometer is the “G” or green wire that is usually used for fans. In the 3 wire thermostat wiring, the green wire serves as a repurposed C or “Common” wire. Here is the 3 wire thermostat wiring color code:

  1. Red wire for power (24V).
  2. White wire for heating (connected to W or W1 terminal).
  3. Green wire as a repurposed C wire.

Here is how to wire a 3 wire thermostat:

  1. Remove the control panel of the old thermostat.
  2. Take a picture of the 3 wires. Note the color – red, white, and green – and the terminals – R, W or W1, G.
  3. Remove the motherboard of the old thermostat completely, and secure the wires. You don’t want them to hide deep inside the wall.
  4. Put the new motherboard in the place of the old one, and pull the wires through the 3 wire thermostat.
  5. Correctly reconnect the wires – red to R, white to W or W1, green to G – and tighten the terminal screws.
  6. Put on the control panel, and test if your boiler or hot water heater connected to the 3-wire thermometer functions properly.

Knowing how to wire a 3-wire thermostat is fairly easy compared to 4 and 5-wire thermostats:

4 Wire Thermostat Wiring (Heat Pumps, HVAC)

4 wire thermostats have a bit more flexibility. Smart thermostats like Nest and Ecobee thermostats require 4 wire thermostat wiring to function properly.

In addition to heating (2 wire thermostats) and C or fan (3 wire thermostats), 4 wire thermostats include the cooling wire, usually in blue or yellow color. Here are the wires in 4 wire thermostats with terminal codes and color codes (also referred to as 4 wire thermostat wiring color code):

  1. Red wire for power (24V).
  2. White wire for heating (connected to W or W1 terminal).
  3. Green wire for fans.
  4. Blue or yellow wire for cooling (connected to Y).

These thermostats are typical for heat pumps: HVAC devices that can cool and heat. To produce the airflow, a fan is needed (green wire).

Here is how you can wire a 4 wire thermostat yourself:

  1. Take the control panel off and expose the wires in the old 4 wire thermostat.
  2. Take a photo of the wires; you can also label where each one goes, but taking a photo is much easier.
  3. Take off the motherboard and secure the wires; if you don’t hold them, the four wires will get lost in the wall.
  4. Screw on the new motherboard, and pull the 4 wires through the hole.
  5. Reconnect the 4 wires to the appropriate terminals – red to R, white to W or W1, green to G, and blue or yellow to Y – and screw them in place. Pull each wire to make sure that it’s fixed in place.
  6. Turn on the heat pump or any other HVAC device that connects to the 4 wire thermostat.

Let’s look into one of the most common thermostats. Here is the Honeywell 5 wire thermostat standard wiring:

5 Wire Thermostat Wiring (Any HVAC Device – Air Conditioners, Heat Pumps, Furnaces, etc.)

5 wire thermostat is basically a 4 wire thermostat with a “C” or “Common” wire. The newer digital thermostats for HVAC devices all require a 24V C wire connection to function. 5 wire thermostats are the most versatile thermostat; they control anything from smart air conditioners, heat pumps, furnaces, and so on.

Here are the 5 wire colors and terminals codes (5 wire thermostat wiring color code):

  1. Red wire for power (24V).
  2. White wire for heating (connected to W or W1 terminal).
  3. Green wire for fans.
  4. Blue or yellow wire for cooling (connected to Y).
  5. Black wire for “C” or “Common” wire.

Here is what a wired 5 wire thermostat looks like:

wired 5 wire thermostat with black, red, white, green and yellow wire

Let’s look at how to replace an old 5 wire thermostat with a new one:

  1. Take the control panel off and expose the wires in the old 5 wire thermostat.
  2. Take a photo of the wires; you can also label where each one goes, but taking a photo is much easier.
  3. Take off the motherboard and secure the wires; if you don’t hold them, the four wires will get lost in the wall.
  4. Screw on the new motherboard, and pull the 5 wires through the hole.
  5. Reconnect the 5 wires to the appropriate terminals – red to R, white to W or W1, green to G, and blue or yellow to Y, black to C – and screw them in place. Pull each wire to make sure that it’s fixed in place.
  6. Turn on any smart device that connects to the 5 wire thermostat, and try to use the smartphone app or remote control to check if everything works as it should.

These are the basics of thermostat wiring. If you have just a bit of technical skills, you will probably be more than able to DIY thermostat replacement.


87 thoughts on “Thermostat Wiring: How To Wire Thermostat? (2,3,4,5 Wire Guide)”

  1. Man, I am soooo confused right now. When I first got in the new house I didn’t have AC, I later had it installed. Now my thermostat went out and behind it is only 2 wires. I thought 2 wires didn’t have an AC unit. There’s one wire to R and another to C. I have a Honeywell THX9421R5021WW that’s gone bad and wanted to switch it with something that doesn’t require that stupid ‘hub’ that honeywell makes you get in order to use the wireless functions. If anyone has any advice/help I could really use it.

    • While it normally takes 4-6 wires to run a modern thermostat with air conditioner and a common wire to run the timeclock in the thermostat, Honeywell figured out how to do it with just 2 wires. This is because your home was probably built with just a furnace and 2 wires to the thermostat, many homes are built that way.

      So Honeywell produced a couple of products that can talk to the thermostat, using just two wires to power the thermostat, and then have a control box back at the furnace that has the 4 or more wires to run the fan, cooling, heating stage 1 and stage 2, cooling stage 2 if required.

      If you don’t understand this, it is best to call a professional and tell them your problem, so perhaps they can bring out the proper thermostat with them to solve your problem. Otherwise they will show up, and then spend $90 an hour to go chase parts while you pay them to drive all over town.

      There are several solutions to your problem. Some might just want to install a new 6 or 8 wire thermostat wire to your thermostat. (the proper but usually more difficult way) This might mean relocating your thermostat. Or they might bring a product like the one that you have, to wirelessly connect the thermostat to the furnace, and then use the builder installed 2 wires to power the thermostat.

      It is also possible to install a battery powered thermostat, I installed one back in 2016, and it stops working once the AA battery is dead, so that is not so great if you don’t know they need to be replaced every once in a while. The homeowner did not want the thermostat in the hallway, and it had to be relocated to another place.

  2. Thank you I have been trying to fix mine for about a month now and I would of never thought of yellow and blue could either go in the y circuit thank you for the info I really needed help on this one

    • Hello Duane, sounds like a 3-wire thermostat setting. For specific information, you should consult an HVAC professional near you. It’s really difficult to help in these cases online because everybody has a specific situation.

  3. Installing new thermostat Honeywell smart with 5 wires.the blue wire was not connected so I connected that to the blue connection.
    The furnace has only 4 wire connection with the blue wire not connected where do I connect the blue wire. I. Am thinking white white connection is that right.
    Thank you for your reply

    • Hello Ignacio, the blue wire in a 5-wire Honeywell smart thermostat can double as a C wire or as a t-stat terminal wire. It’s really hard to solve this online; it would be best if you called somebody for a hands-on look.

    • We have a boiler only, and our old Honeyell Thermostat only have the Red and White wire!! I want to covert to a newer Thermostat that requires a C-wire, will this work?

      • If you buy a thermostat with AA battery power, it should work with the two wire set up. However some winter night, the battery will run out, and leave you in the cold. So keep changing those batteries every fall, and use the older ones for something that is not nearly as important, like the remote control for your TV until the battery is dead.

        Look for a thermostat that has 3 or 4 of the AA batteries, and then says for 2 wire heating systems.

  4. My existing Honeywell thermostat T8112D1005 has only 3 wires R (Red),G (Green),W(White) wires only connected in the panel,remaining two wires I noted yellow and blue were not connected. No this thermostat display stopped working, so now I purchased new Honeywell Thermostat RTH9585WF has 5 wired connection mentioned with C wire requested connection.In the new set up do I need to connect Blue wire not connected previously to C (Common ) point or G(Green)wire existing to C (Common) section? In existing old system only 3 wires connected (R,W,G).Please advise.

    • Hello there, sorry for the PHP error, we’re trying to fix that. The new Honeywell thermostat should follow the same wiring as the old one. Connect the 3 wires – R, G, W – as with the old one, and try if that works. If not, it is probably the C-wire thermostat, and you’ll have to connect the C wire as well.

  5. Can’t figure this out. Helping my neighbor put in a WiFi thermo. 5 wire bundle. At the furnace/with ac, 4 wires connected to control board: RWGY to same color wire. Blue not connected. At the Thermostat (Honeywell RTH230B) R-to-Rh, W-to-W, G-to-G and Blue wire-to-Y, and Y-wire not visible (cut). Therefore only three wire connected RWG. AC works. I am confused. Any thoughts on how this is working?

    I was planning on connecting B wire-to-C furnace control, then the rest of wires normally, but I am confused about what will happen when I do actually connect the Y wire.


    • Hello there, it seems like a complex thermostat wiring situation. We don’t have the full view here; for safety reasons, it would be best if you consult with a hands-on electrician in this case.

  6. I have a gas heater that’s old with 2 wires only. Red and white but my new thermostat doesn’t have white. How do I hook it up? What it has is R, C, Aux/E, Y, G, O, and B. Please help

    • Hello James, in almost all cases, heating works like this: You connect the white wire to the W or W1. Since the gas heater is powered by gas, you probably only have to connect that wire not for electric input to the heater itself, but to the fan that blows the hot air out of the heater. Green wire or G is used for the fan. You can try connecting the white wire to G and check if it works.

    • The replacement thermostat that you bought is for heat pump use only. Or it might be for both heat pump and not heat pump, and has a second way to connect it to the Aux/E for your electric heat set up. If it has a “Emergency heat” switch on the bottom, then connect the red to R terminal, and white to the Aux/E terminal, and set the switch to Emergency Heat, and it will work normally as a heating thermostat.

  7. I have an old thermostat (1997)with the letters H, Rc, G, Y, W, Rh.
    My new thermostat has 7.5, Fur, Hs/Hp, Cool, Fan, Gnd, Hi fan

    Both are Dometic. The customer service would not help with converting. Please help, this shouldn’t be so hard.

    • Hello Karen, you can check the article. We have specified what every thermostat wire is meant for. For example, ‘Cool’ is the ‘Y’ wire, Fan is the ‘G’ or ‘Green’ wire, and so on. The converting process is not a piece of cake since there are different color wires that can mean the same thing.

    • My Dometic RV A/C has wiring like you described in your second thermostat. However the original thermostat was not set up for 2 speed fan, like a RV thermostat. You need another thermostat set up the way your original one is set up.

      So Rh and Rc can be wired together, and G is for running the fan, Y is for cooling, W is for heating. At Home Depot or Lowes, they have thermostats with all of these wire connections. If you have a fifth wire to act as common, then connect that wire to the C terminal at the thermostat and the furnace. If you only have 4 wires, and no ability to install another one, then you should look for a thermostat that is powered by AA batteries, and can run without the common wire. It should state that on the outside of the thermostat, near the wiring and application guide.

    • Hello Tracey, in general, you can almost always upgrade to a thermostat with more wires. 2-wire to the 3-wire thermostat is the standard upgrade when transitioning to a wireless thermostat.

  8. Hi there, I currently have a Lennox iComfort thermostat, but it is broken, so hoping to replace it, but the wiring is confusing.

    I have 4 wires: C, i-, i+, and R. I _think_ the two i-wires are for communication with my system.

    Any suggestions on what I should be doing with the i-wires?

    • Hello Andrew, i- and i+ are usually communication wires. Through those, your thermostat sends signals to Lennox iComfort thermostat. In this instance, it is probably best to call an electrician.

  9. I have a very old two wire thermostat (circa 1978). Can I just attach the two wires (white and red) to the R and W of a Honeywell RTH2218 Programmable Thermostat? Is it sufficient to just cut power to the heating system before doing this? (No AC)

    • Hello Pat, yes, before you start any thermostat wiring, you have to cut the power. Attaching white wire to W and red wire to R is on a Honeywell thermostat is exactly right.

  10. I only have two wires, red and white in my old thermostat. The white is connected with C and the red is with R, also jumper connected with Rc (i have heating and AC). how can i connect with Nest learning thermostat? it seems it doesn’t support jumper connection. thanks

    • Hello Daniel, you would need a 2-wire thermostat. 2-wire Nest learning thermostat usually has the red wire connected to the Rc, and the white wire connected to W1. The Nest jumper cables are a bit different; you can read more about how they work here and you will be able to use both R and Rc.

  11. Very useful but I have a problem. My Honeywell thermostat use only w/g/y/r.
    -There’s a jumper between w/y and Yellow wire goes on Y.
    -Green wire goes on G
    -White wire goes on O
    -There’s a jumper between Rc and R and Red wire goes on R.
    No wire goes on B or C.
    The thermostat is Honeywell TH3110D1008 and I would like to use a Nest Thermostat.
    My heat/ac unit (on the roof) is all electric.
    How should I connect the wires on the Google Nest Thermostat?
    There are only Y C W G R *OB.
    Do I need to add a C wire from the unit to provide electricity to the thermostat?
    I dunno

    • Hello Antonello, that’s quite a common question. With a Nest thermostat, you don’t need a C wire (in practice, you would only need it if you have power issues like the battery being drained often). B and C are usually used to connect the C wire (‘common’ wire). Alright, with a 4-wire Nest thermostat, you (most commonly, it’s difficult to get 100% correct wiring without looking at the thermostat and the wires) connect the green wire to G, white wire to *OB, white wire to W, and yellow wire to Y. The Nest thermostat will have G, OB, W, Y.

      In any case, if you are in doubt, you should always consult an electrician for on-site consulting. Most thermostat and wire combinations had to be done on-site to ensure everything works fine.

    • You have a heat pump. The wiring indicates this from the original thermostat. It is important to not program the Nest to dual fuel, as everything is run on electric.

      Your red wire is connected to the R terminal on the Nest. The yellow to the Y terminal, white wire to the O/B terminal. Nothing to the W terminal. The G gets the green wire.

      Then tell the Nest that you have a heat pump. Not dual fuel. That the O/B is energized in cooling. Then test the cooling. If the OB is energized in cooling, the air will come out cold. If the OB is energized in heating, the air will come out hot. Then change the nest programming if required.

      Then it should work fine. Unless you shorted the wires and need to replace the fuse on the heating unit on the roof. Best to shut off power to the heating unit while working on the thermostat wiring so the fuse will not blow.

  12. I hope someone can help me with this: I’m installing a Nest Learning thermostat. I have 2 red wires (R, Rh), a green (G), yellow (Y) and white (W1). All wires are coming to the thermostat from my air conditioner’s air handler. I notice on the terminals of the air handler that the red wire (Rh) coming from my heating system is connected to the C terminal (on the air handler).
    Does they suggest that the Rh wire should be connected to the C terminal of the thermostat?
    When I connected it to the Rh terminal the Nest thermostat ran out of power after about 3 days running which suggests the battery drained, so I must need power to the thermostat to keep the battery charged.

  13. I have only two wires in my old thermostat wires….both are white in color.
    Does it make any different which is connected to the R and the W terminals?

    • Hello Kirk, having two wires with identical colors is not customary for thermostat wiring. The easiest way to check that is just to try it; if it doesn’t work, just switch the wires and troubleshoot again.

  14. I have a 1981 ICG furnace with a black and white wire to my mercury thermostat. Is the Black wire my power wire and would be connected to my R terminal in a digital thermostat?

    • The black wire in the thermostat is usually the “C wire” or “common wire”. A newer thermostat with a continuous loop it’s usually connected to a C terminal. These older mercury thermostats are a bit tricky. Honestly, we don’t have sufficient knowledge to adequately advise on how to upgrade from a 40-year-old mercury thermostat to a digital thermostat.

    • Oh my gosh. I read the other answer, and thought when I went to school, your system was only 3 years old, and modern. Any milivolt or nest thermostat should work fine with your system. Even one marked 2 wire heating thermostat. If you want programmable and automatic set back, get one that is powered by 3 or 4 AA batteries.

      Connect one wire to the R terminal, the other to W terminal. The wire colors do not matter in your case. With a Nest thermostat, they have a connection to charge them with a cell phone charger cube, and small connector (not the apple one). Once fully charged, it might recharge itself using power from your 24 volt thermostat during the off cycle. OR it might require recharging every month or two.

    • Hello Nelson, that information should be in the manual. Usually, you use a W terminal for heating appliances. If this is not the case, Honeywell will most likely suggest an alternative thermostat wiring.

  15. I have a 3 wire thermostat but the way I see the wires Red wire on the R, White wire on the W, but the green is connected on the Y is that right??? There is also a jumper between R and RC I am trying to put a ecobee or nest smart thermostat but I don’t know if it will work can you please advise.

  16. Have google nest thermostat. Says I need a c wire. Only have a red and white wire that’s existing. Want to replace with new wire instead of using an additional c-wire. What should I get to replace existing 2 wire. Thank you

    • Hello Jeff, with Nest Thermostat E or Nest Learning Thermostat, it’s advisable to have a C wire, yes. Usually, you would connect the red wire the Rc, and white wire to W1. Hope this helps.

    • I have installed a Nest to a two wire heating system. it has the ability to recharge using a phone microcharger on the back of the thermostat. So it is removed from the wall and recharged every couple of months. It might be able to recharge itself using the 24 volt furnace power during the off cycle, and run for years between recharging. Or require recharging every couple of months.

      Ask Nest and tell them you have a two wire furnace, and hopefully they will send the right one. Then connect one wire to the RH terminal and the other to the W terminal. IT will work fine, until the internal battery dies, and that might be 3 months or 10 years, depending on if it works right or not.

  17. I have Honeywell thermostat about 10 years old for heat and air conditioning but it only has three Wires and the connections are labeled 1 2 3 but the colors of the wires are red white and blue. Which smart thermostat will work for my system?

  18. I was just on a chat with Honeywell and do not yet have an answer to my question. I am installing a Honeywell RTH9585WF Programmable Thermostat. I have an oil fired boiler with 3 wire type 1311 White Rodgers hydronic zone valves w/ single circulator pump no AC. Thermostat is a White Rodgers SPDT has wires labeled W, R and Y. I have determined that R is 24 volt that supplies power to the valve motor through the Thermostat. W wire opens the zone valve (call for heat) and Y wire closes the valve. (Motor driven valves @ .4 amp new thermostat is rated 1.0 amp max so I should be OK concerning the load.) The neutral wire from the transformer is connected to one side of the zone motor. I know that I need to pull at least a 4th neutral wire in the wall to get continuous 24 volt power to the thermostat terminals R and C. Will my valve open and close if I match W, R, and Y on the new install? Y is AC, but will that pull my valve shut?

    • Hello Joe, thanks for a detailed description of your wiring problem. As a general rule, don’t forget the take a photo of the old thermostat; it’s easiest to copy that wiring. Let’s go through wires one by one:
      R wired is correctly wired; the 24-volt continuous supply to the motor.
      W wire openings the zone valve (heating) and Y closes it (running a 0.4 amps thermostat on a 1 max rating is adequate as well, no worries here).
      The valves, in general, should work fine once you match the W, R, Y. If everything is correctly wired, the Y wire connected to an air conditioner should keep the valve open.
      You have already done the best thing here; call Honeywell to help you out. They know the specifics for all Honeywell programmable thermostats. If you don’t mind me asking, what did they advice for the RTH9585WF model? Does it have some specific thing that should all be aware of?

    • You would need a thermostat with automatic change over, and the ability to power shut the valve once the call for heat is over. That is a pretty rare system. Most will allow heating, say it will turn on the heat at 70F and off at 71F. Then let the cooling energize at 74F to close your hot water valve. That might be to much heat into your room before the heating valve allowed to close by energizing the Y terminal.

      However most magnetic thermostats that do have a W and Y terminal also have a manual changeover from heating to cooling, and that would not work either.

      I guess I am out of options on your situation, and I don’t normally work on hydronic systems. You might end up replacing your valve with one that is powered open and spring return (to closed).

  19. Excellent Article!
    Thanks for all the free information. I can tell from the comments that there are many different possible wiring configurations depending on the installer and system used. Hopefully I will be able to figure out how to replace the ancient thermostat in the house I’m currently closing on. I think the existing system is most likely a 4-wire setup for a 1970s heat pump, but I guess I will find out when I move in and open up the thermostat.

    Do you have a recommendation for a particular smart thermostat? Regardless of the thermostat I end up buying, I assume that I will end up running a common wire to the new thermostat if one is not already there, just so I don’t have to constantly deal with batteries.

    • Hello Jordan, thanks, it means a lot. Well, in recent years, Nest thermostat are quite popular; this would be my go-to choice if I would be replacing a thermostat.

      • I agree with the Nest thermostat, and my power company will help pay for a Nest thermostat, IF I wanted to install one. I have a programmable thermostat that the power company can not shut off on a high energy use day. I think they can control certain Nest thermostats. (not sure about that). Your power company might give you a rebate if you agree to allow set back on hot days and cold nights if you install a Nest thermostat they might provide at a very low cost.

        The Nest might work without the need for a C wire, if you get the right model. They normally have a charge point for the battery that connects to a phone charge cord.

        It might be possible to pull in a new 6 or 8 wire thermostat wire while pulling out the old one, or using a “Fish tape” or “Fish Stick” to pull the new wire into your current or perhaps a new thermostat location. Yet if the builder stapled the thermostat wire to the wall, it might be impossible to pull in a new one using the existing wires.

        Fish stick is a modern way to pull wire, the stick is about 4 or 5 feet long, can be attached together for longer runs, and you push it through the wall to guide the wire behind it. You might be able to check out a video on Youtube by looking for wire pulling fish stick.

  20. Old thermostat had 7 wires YGCOW2E(brown)R. Using Honeywell t5 pro, I connected every wire to the exact slot in the multiconnector. My question is does the W2 wire from old tstat go into the W2 slot on the new one or does it go into the W slot?

    • Hello Harrison, generally, you connect all tstats into exact same slot as with the old thermostat. Of course, there are exceptions. Honeywell T5 PRO doesn’t have a W2 slot, it has a W – Heat Stage 1 (Heating) slot. In most cases, the W2 (if it’s for heating) should go into the W slot but to tell you that precisely, we would have to see all the wires. Hope this helps at least a bit.

  21. I have a 4 wire HVAC in a older home the wires to the thermostat are color coded as follows. Red,blue, green and white. Can you confirm what color wires are for which function. Thank you.

    • Hello Michael, of course, here are the general functions for each of these wires:
      – Red thermostat wire is the heating.
      – Blue thermostat wire is the common wire or C-wire.
      – Green thermostat wire is for the fan (usually for the indoor air handler).
      – White thermostat wire is for heating.

      Hope this helps you correctly wire a 4 wire thermostat.

  22. I want to change out my Lennox heat pump stat for a Honeywell Smart stat. I have 9 wires. Wiring references are different on 2 stats. What to do?

    • Hi Harry, calling an electrician or an HVAC expert would make sense in this case. Wiring a 9-wire thermostat is quite complex and it’s very hard to DIY that.

  23. I have a Honeywell thermostat and it has 1. Which is white 2. Which is yellow R which is Red and C which is blue no green Air Handler has same 1. White twisted with another White and 2. Yellow twisted with another Yellow that goes out to the Outside compressor/Heat Pump. how do I hook up a thermostat that needs a green wire?

    • Hi Brandon, the green wire is most often used for powering a fan; that can be a fan inside the furnace or a fan inside the air handler (mini splits). Connect it to the ‘G’ terminal on the Honeywell thermostat.

        • Hi Brandon, you should check if you have an air conditioner or a furnace that requires a fan (green wire). If not, a thermostat without green wire is OK. If you have a fan, you need a green wire. Hope this helps.

  24. Hi —-just call me “Confused”. Just purchased an older home (early 70’s). Replacing the White-Rogers mercury thermostat with a non-programmable Honeywell. 4 wires out of the wall. Green to G. White to O. Red to RH with a jumper to RC. Yellow from the wall to W with a smaller gauge yellow originating from the back of the wall plate going to the Y with a jumper to the other yellow at the W position. Red to RH with a red jumper to RC. There is a blue wire that goes from the mercury tube itself and is soldered to the control plate at a position that appears to connect it to the O position on the wall plate. There is no blue wire from the wall itself. There are also a yellow and a white wire connected to the mercury tube that are soldered to the control plate that ultimately connect to the A position on the wall plate. Now I am out of breath from this long-winded description!!! I have tried several different wiring possibilities. I don’t know if it is a wiring issue or a thermostat programming issue. My AC/heater is a geothermal heat pump. How should this be programmed ? Please don’t tell me to consult an HVAC expert. Give me just a little advice. I am a 63 year old woman, but I am usually very handy with household dilemmas. Actually I am having an “emotional moment” just because I needed to ask . (No control issues here) Any advice (other than calling someone else) would be greatly appreciated. Thank You !!!

    • Hi Carla, thank you for such a descriptive comment. Convert those old mercury thermostats to new Honeywell thermostats is not exactly easy, as you have figured out yourself. And, I have to admit, the first advice would be to consult an HVAC expert. 🙂 Now, a little advice to point you in the right direction would usually be the smart choice. However, we are talking about geothermal heat pump here. This is an expensive over $10,000 HVAC device that is both complicated and valuable. You don’t want to damage it by trying to get the wiring right. If you don’t get it right, you may need repairs that will cost $1000s. In short, it pains me to say this because you have expressly requested for a DIY advice, but in this case calling a HVAC guy to wire the thermostat and, more importantly, check if everything is OK makes a lot more sense than wiring it yourself.

    • Are you trying to install a Nest thermostat? You have a heat pump, and it seems simple enough. For a heat pump, the red wire normally goes to the Rh and Rc terminals, and many Nest say to just connect to one, they internally connect Rh and Rc. That is the red wire done.

      The white wire was connected to the O terminal, and should be connected to the O/B terminal of your new thermostat. So that is 2 wires done.

      The G wire is connected to the G terminal, green wire. Three wires done!

      The yellow wire was connected to the yellow or white position, and is connected to the same one on the new thermostat. Don’t connect the yellow and white together on the new thermostat.

      Program the thermostat for heat pump NOT dual fuel, and energize O/B in the cooling mode. Test the cooling, and if heat comes out, program the O/B for energized in the heating mode.

      Seems simple enough for me. But I have been doing this since 1983.

  25. I recently installed a Wind Chester electric furnace as a standalone for some extra heat in my basement..My question is can I run this on a 2 wire set up or do I need to use a 5 or 6 wire set up..furnace comes with a wire bundle with the white, green, yellow, red, brown, and dark blue.

    • Hi Bryian, you would need 1 wire for furnace heating and 1 wire for the furnace blower. That means the 2 wire set up will be enough. Those 5 or 6 wire sets include air conditioning and so on; you don’t have to worry about those. Hope this helps.

  26. I have a honeywell RHD6350D programmable thermostat. It has four wires, plus one wire that is jumpered to two terminals. They are labeled:

    Not used
    Y (a blue wire)
    R (red wire), jumpered with a bare wire to terminal marked RC

    What does the R wire do, and why is it jumpered to the RC terminal

    • Hi Tom, R or Red wire is used to power a HVAC system (via a transformer). The jumper to Rc is responsible for powering the cooling device (air conditioner). Rh would be responsible for powering the heating device (furnace, most often). Hope this helps.

  27. 12/26/2022 – Could really use some quick help. I’m replacing a White-Rodgers 1F81-261 with a White-Rodgers 1F80-361.
    Old terminals were: W, W2, R, G, Y, Y2(unused),C(unused).
    5 wires at thermostat: Blue, White, Red, Green, Yellow
    New terminals are: W, RH, RC, B, C, G, O, Y

    Which wires go where?????

    This is for a TraneTUD060R9V3K4 Variable speed, 2-stage Furnace system.

  28. Hi! I have a Coleman electric furnace, with a single stage electric air conditioner.
    I have the following wires connected at the thermostat: W,Y,G,O,R (bridged to Rc) and C.
    I am not sure why I have an O wire, since it’s supposed to be for heat pumps?

    • Hi Peter, you are right, the O wire is used for heat pumps (to switch from heating to cooling, and reverse). Do you have to switch from the furnace (heating) to the air conditioner (cooling)? That might explain why you would need an O wire for switching. Otherwise, it’s quite unusual to see an O wire if you don’t have a heat pump. Hope this helps.

  29. very useful but didn’t match with my case.
    I have a Totaline P286-1600 thermostat which control my furnace and air conditioner with 2 wires only. The 2 wires T1 and T2 from the wall connect to the Totaline which only have two terminal RH and terminal Y2. If I want to replace it with a nest learning thermostat 3rd generation, Which two terminals should I connect to the nest?

    Thank you

    • Hi Andy, on the Nest learning thermostat (3rd gen), the white wire connected to W1 is for heating; this should be your furnace wire. The air conditioner wires on the Nest thermostat are connected to Y1 or Y2. Hope this helps a bit.

  30. Very informative article. A comment and few questions (I am a ECE Prof / researcher with experience limited to few installations and so on).
    While reading the install manual for Honeywell RTH6580WF (old unit: Radio Thermostat CT32), I learned not to look at wires using the colors but by the terminals to which they are connected.
    Question: I am a senior citizen who just moved into a rental home in Tucson and decided to go for the WiFi thermostat (free from TEP) but find the furnace is at the attic (with the control unit and the heat exchanger / air blower there) and the condenser coil for the AC on the outside of the home. I have no access to the manuals to know which system is in the house. The CT32 had five wires connected including the C wire but also uses 4 AA batteries and it’s manual says “the CT32 runs on 4 AA alkaline batteries and/or the C wire if
    available. If you do not have a C wire you can run a new wire from the HVAC or
    use a standard 12-24V [AC or DC] wall transformer. A constant power source is
    required when using a radio module”. It is interesting they say a 24V DC can be used and in other place they indicate the polarity of connection is immaterial (The CT32 can be externally powered with a power source rated from
    12V to 24V, AC or DC, at 100ma or greater. If used, connect to the C
    and RH terminals (no polarity)). Polarity doesn’t matter when using DC too?
    I wired my new RTH6580WF but when the AC was turned on, there was no display and of course the AC is not kicking in. I checked the voltage between various terminals and found G-R, W-R, Y-R showed 26.6V AC but nothing between C-R. So, did the previous installer just inserted the brown or cyan color wire into C even though it was not connected at the controller side? Still the CT32 was controlling the system and C is needed only as an alternate for battery power? What is the difference in the new thermostat that AC won’t even trigger? Since I am seeing 26.6V at R, can I connect another 24V transformer between C and R or I need to get into the attic at this old age to connect the other end of C to the controller? I read about a Power Extender Kit (PEK) for Ecobee thermostat but not sure I need to do something similar here.
    I know this is long winded but I would appreciate any technical thoughts. TIA.

    • Hi there, thank you for explaining the problem very well. Alright, let’s maybe start with this to put things into context: there are 3 types of thermostats.
      1) Manual thermostats.
      2) Programmable thermostats. The Honeywell RTH6580WF you have is an example of a Wi-Fi programmable thermostat.
      3) Smart thermostats. Ecobee and Google Nest thermostats are the best examples here.

      Old manual thermostats don’t require a C-wire. Programmable and smart thermostat do; this C-wire needs as 24V power source. Any other source – be it 12V battery, or even 110-120V AC voltages, requires a transformer/inverter that will turn any electricity into 24V DC source.

      Now, it is incredibly difficult to check what is wrong with a thermostat wiring if you don’t have hands-on approach. Especially when people buy or rent an older house that has various HVAC appliances, have already upgraded old manual thermostat to newer versions, and so on, it is always recommended that you call a HVAC expert or an electrical expert to come and check if everything is wired as it should be wired. It is a bit of work to reverse engineer all the wirings on various appliances with no manuals, but these guys have mileage and will troubleshoot everything quite easily. So, the advice here is to call a guy. Hope this helps.

  31. I found this so interesting. Thank you. I think I missed what brought me here so hopefully I can ask correctly.
    If you have a thermostat that has 6 pins and change to a 3 pin, you just have to use the R, W and G as I understand?
    It’s my Mom’s thermostat, I walked into it just hanging off the wall torn apart with a few wires.
    The neighbor started to put up the new one. He didn’t have a clue going from 3 to 6 and left. Said he’d be back but didn’t. Being it was torn apart I wasn’t able to take a picture.
    She lives in a motorhome and the thermostat controls
    the AC and furnace.
    Both are Dometic thermostats. Any help would be appreciated.

    • Hi Suanne, if you have 3 wire thermostat, the most commonly used 3 wires are R (red wire) for powering the thermostat, W (white wire) for heating, and G (green wire) which is basically a repurposed C wire. The if have leftover pins, you don’t need to connect them; you only connect the 3 wires you have. Hope this helps.

  32. Hello,
    a great article, thank you. I have a slightly different question – I have an old two wire gs heater (connected to W and R) . The thermostat is old and falling apart (complete with 3 AA batteries). My idea is to set the thermostat to be permanently on and control the heater with a relay connected to a micro controller with a heat sensor and timer. So,the question is: can either wire control the heater with a simple on / off relay? Excellent article, nice and clear and very helpful.

    • Hi Stephen, thank you. The old manual thermostat cannot do that; the new programmable thermostat should be able to do that. The best option here is just to get a new programmable thermostat (2-wire should cost less than $100) and you can control the heater via the micro-controller, heat sensor, and timer. Hope this helps.

  33. old 5 wire thermostat colors are red,yellow,white,blue,brown no green!? the old tthemostat was removed before i got home so any help would be appreciated. thank you!!

    • Hi Doug, the green wire is usually reserved for indoor air handler fan (mini splits, heat pumps). In old thermostats, you won’t see green wire since heat pumps are a bit of a modern invention that didn’t exist few decades ago. Hope this helps.


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