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

Thermostats are the ultimate control centers for hvac devices like 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’ll cover everything you need to know about thermostat wiring; including where each colored wire goes.

Reading: How to connect thermostat to furnace

Obviously, there are many different types of thermostats. 100% of the guidelines will not work for 100% of the thermostats. After all, there are many different types of thermostats with different wiring: honeywell thermostat wiring, nest wiring, old thermostats, etc.

We will cover the most common thermostat wiring for 2, 3, 4 and 5 wire thermostats. At each stage, we’ll indicate which 2-wire or 5-wire thermostats are used to wire, for example, to get a broader idea of ​​where those colored wires go and how they enable functions on air conditioners and furnaces.

In all cases, we will use this basic honeywell thermostat as a 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 in each place on the thermostat?” (see diagram below)

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

“what is r and rc in the thermostat?” (color coding diagram below)

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

“what if there is no c-wire for the thermostat?” (old 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 colored wire means. understand the color coding will give you a good understanding of how the thermostat actually works.
  3. replacing the old thermostat with a new one.

an important note: be aware that dealing with electrical circuits can be harmful to you. therefore, always follow the proper safety measures when working on live electrical circuits. For more information on wires and their ampacity, see the AWG chart with wire sizes for specific amps here.

let’s start by exposing the colored wires:

removing the thermostat control panel (getting to the colored wires)

The first step in wiring the thermostat is actually getting to the wires. you probably have your thermostat mounted on the wall. exposing wires is easy; It starts with removing the thermostat control panel.

before you start: turn off the power switch until the device thermostat is connected.

Most control panels can be removed by simply pushing them in from the bottom or top. Some honeywell thermostats may also have some screws; simply unscrew them to expose the wire terminals.

Under the control panel, you’ll find the base and cables sticking out of the wall. with the basic honeywell thermostat, you will find:

  • 8-wire plugs on the left.
  • 8-wire plugs on the right.

Before we grab the flat head screw to unscrew them, let’s first familiarize ourselves with the crucial part of wiring any thermostat: the wire codes and colors.

This is where the thermostat wires go:

thermostat wiring color codes

as you can see on the base of the thermostat you have 16 sockets, with the letters c, r, w1, w2, o/b, g, y1, y2, bk, 2x rs, 2x odt, aux no, aux c , and auxiliary nc. with 2-wire, 3-wire and 4-wire thermostats you don’t have all of this. that’s normal. you may also notice that all plugs are not connected. that’s also standard.

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

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let’s look at each of these cables separately:

black or blue wire from thermostat (“c” wire)

the black wire or the blue wire (any color is used) is the c – “common” wire. what does wire c do? wire c connects to the transformer and completes the 24v electrical circuit. newer thermostats have a 24v continuous loop circuit; On older models, the cycle completes only when power is required (when you turn on the AC, for example).

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Most of the time, the black or blue wire connected to the c terminal is found on newer “smart” thermostats. older thermostats may not have the “c” wire; they work on demand while the new ones work all the time (continuously 24/7).

That’s why newer digital thermostats draw energy even when the heat or air conditioning isn’t running.

red thermostat wire (“r” or “rc” wire)

The red wire or “r” wire is the power wire. They originate from the transformer (air conditioners, at the air handler) and provide the 24-hour AC power.

All air conditioning thermostats have a red wire for power. can also have rc or rh terminals for dual transformer systems; these are special cases in thermostat wiring.

white wire from thermostat (“w1” or “w2” wire)

white wires are for heating. You’ll find them on gas furnace thermostats, for example, but you won’t find them on air conditioning thermostats.

w wires go directly to the heat source; which can be a furnace (gas, electric, oil, whatever) or heat pumps (including mini-split heat pumps).

w2 is designed 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 orange thermostat wire is for reverse valve cooling. Most of the big heat pump manufacturers like trane, goodman, lennox etc have the orange wire that goes to the condenser (located on the outdoor unit of the heat pumps).

“b” is to reverse valve heating. some producers like rheem energize the reversing valve when heating mode is activated on heat pumps. a dark blue “b” wire is needed for the t-stat terminal.

green thermostat wire (“g” wire)

the green wire is for the fan. mini-splits have an indoor air handler with a fan; Power input going into the fan is regulated by the green or “g” wire.

yellow thermostat wire (“y1” and “y2” wire)

terminals and are for connection to the compressor relay. most of the time it is connected to the air handler (split system indoor unit). y1 is for ordinary or one-stage cooling; this is what most of us have at home. the wire y with the code “y1” is usually yellow. You can learn more about the size of wires you need for air conditioners here.

“y2” is only for air conditioners with second stage cooling. this terminal is only needed if you have:

  • 2 compressors.
  • two-stage compressor.

other cables (bk, rs1, rs2, odt1, odt2, aux no, aux c, aux nc)

some other rarely used terminals at all on the right hand side of the thermostat. in the future, we plan to create a separate article detailing the specific function all of them have.

Please note that with so many different thermostats, each wire code may have a different colored wire. the “o” wire, for example, is not always necessarily orange. If a technician who installed your thermostat used a different color, he may have trouble reconnecting the wires. That is why the following section is so important:

removing the old thermostat (take photo)

If you plan to replace the old thermostat, don’t remove it right away. First, make sure you can wire the new thermostat correctly.

HVAC technicians know the color code by heart; they can simply remove any honeywell thermostat, replace it with a new one, and reconnect the terminals.

If you are DIY thermostat wiring, this is the easiest thing to do before you remove the old thermostat:

take a photo.

If you unscrew the terminals and remove the wires right away, you may have trouble reconnecting the wires to the new terminal. make sure you have a picture of what the wiring from the old thermostat looks like before proceeding.

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make sure you run the wires (you don’t want to lose them in the wall)

When you’ve taken the photo of the old thermostat, you can remove it. but be careful:

thermostat keeps wires out of the wall; the wires are suspended in the thermostat. if you remove the thermostat right away, the wires will be lost inside the wall. you will have to:

  • pull the wires out of the wall.
  • break the wall so you can reach the wires.

To avoid both of these scenarios, be sure to extend the cables. take each wire and bend it out. here is the logic behind it:

  • the hole in the wall is about 2×2 inches.
  • the wires themselves are about 1×1 inches.
  • separating them will create at least a 2× section 2-inch cross section, and the wires will be suspended from the wall (instead of the thermostat).

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

install a new thermostat (reconnect wire for wire)

After removing the old thermostat, take the new thermostat and place it in the exact same spot as the old thermostat. the wires you fixed to the wall should go through the hole in the new thermostat.

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

  1. Use the photo and reconnect the wires as appropriate.
  2. Use the color codes and reconnect the wires.

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If you use color codes, you can refer to the meaning of each wire in the previous section. here’s a quick basic summary:

  • red wire: power (24v input in most cases).
  • green wire: fan.
  • white wire: heating.
  • blue wire: cooling.
  • rh: power heating.
  • rc: cooling power.

To reconnect the cable, simply place the cable on the right terminal and tighten the fixing screw. pull on a cable a bit to make sure the cable is properly attached.

example: reconnect the red wire to r, tighten the screw, check and move on to the new wire.

after reconnecting all the wires, place the control board and check if everything is ok.

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

2-wire thermostat wiring (oven)

The most basic thermostat has 2 wires; usually one red and one white wire. Two-wire thermostat wiring is used for furnaces only and typically does not need a “c” or “common” wire. that’s why we only need two cables. this is a 2-wire thermostat wiring color code:

  1. red cable for power supply (24h).
  2. white cable for heating.

Wiring a 2-wire thermostat is pretty straightforward.

here is the step by step of how to make a 2-wire thermostat wiring:

  1. remove the control panel from the old oven thermostat.
  2. note where the wires go; usually the red r wire will go to r and the 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 base plate from the old 2-wire thermostat and put the new 2-wire thermostat in its place.
  5. Reconnect the red and white wires, tighten the set screw and replace the control panel.
  6. Test the 2-wire thermostat wiring by turning on the oven.

If you’ve reconnected the 2 wires correctly, the new 2 wire thermostat should control the oven the same way your old 2 wire thermostat did.

3-wire thermostat wiring (boilers, heaters)

3-wire thermostats are most commonly used to control heaters; boilers and water heaters in particular. the 3 wires you’ll find are usually coded r, g, and w.

The difference between the 2-wire thermometer and the 3-wire thermometer is the “g” or green wire that is usually used for fans. In 3-wire thermostat wiring, the green wire serves as a “common” or reused C wire. here is the 3-wire thermostat wiring color code:

  1. red wire for power (24v).
  2. white wire for heating (connected to terminal w or w1).
  3. green wire as a reused c wire.
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Here’s how to connect a 3-wire thermostat:

  1. remove the control panel from 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. completely remove the base plate from the old thermostat and secure the wires. you don’t want them hiding inside the wall.
  4. place the new baseplate in 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. place the control panel and test if your boiler or water heater is connected to the 3 – the wire thermometer works correctly.

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

4-wire thermostat wiring (heat pumps, hvac)

4-wire thermostats have a little more flexibility. smart thermostats, like nest and ecobee thermostats, require 4-wire thermostat wiring to work properly.

In addition to heat (2-wire thermostats) and C or fan (3-wire thermostats), 4-wire thermostats include the cooling wire, usually colored blue or yellow. here are the wires on 4-wire thermostats with terminal codes and color codes (aka 4-wire thermostat wiring color code):

  1. red wire for power (24v).
  2. white wire for heating (connected to terminal w or w1).
  3. green wire for fans.
  4. blue or yellow wire for cooling (connected to y).

These thermostats are typical of heat pumps – hvac devices that can cool and heat. to produce the airflow, a fan (green wire) is needed.

this is how you can connect a 4-wire thermostat yourself:

  1. remove the control panel and expose the wires on the old 4-wire thermostat.
  2. take a picture of the wires; you can also label where each one goes, but taking a picture is much easier.
  3. remove the motherboard and secure the cables; if you don’t fasten them, the 4 wires will be lost in the wall.
  4. screw the new base plate 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 into place. pull on each wire to make sure it is locked in place.
  6. turn on the heat pump or any other hvac device that connects to the 4-wire thermostat.

Let’s look at one of the most common thermostats. here is the standard 5 wire honeywell thermostat wiring:

5-wire thermostat wiring (any hvac device: air conditioners, heat pumps, furnaces, etc.)

The 5-wire thermostat is basically a 4-wire thermostat with a “c” or “common” wire. newer digital thermostats for hvac devices require a 24v c wire connection to work. 5-wire thermostats are the most versatile thermostats; They control anything from smart air conditioners, heat pumps, furnaces, etc.

here are the 5 wire colors and terminal codes (5-wire thermostat wiring color code):

  1. red wire for power (24v).
  2. white wire for heating (connected to terminal w or w1).
  3. green wire for fans.
  4. blue or yellow wire for cooling (connected to y).
  5. black cable for “c” or “common” cable.

This is what a 5-wire hardwired thermostat looks like:

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

let’s see how to replace an old 5-wire thermostat with a new one:

  1. remove the control panel and expose the wires on the old 5-wire thermostat.
  2. take a picture of the wires; you can also label where each one goes, but taking a picture is much easier.
  3. remove the motherboard and secure the cables; if you don’t fasten them, the four wires will be lost in the wall.
  4. screw the new base plate and pull the 5 wires through the hole.
  5. reconnect the 5 wires to the corresponding terminals (red to r, white to w or w1, green to g, blue or yellow to y, black to c) and screw them into place. pull on each wire to make sure it is locked in place.
  6. turn on any smart devices that connect to the 5-wire thermostat and try using the smartphone app or remote to check if everything works as it should .

These are the basics of thermostat wiring. if you have a bit of technical savvy, you’ll probably be more than capable of replacing the thermostat yourself.

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