How To Identify Wires On A Thermostat? (How To DIY)


Thermostats are devices that sense and regulate the temperature of a space or respond to changes in the surrounding environment by switching heating or cooling equipment on and off. They are primarily used to regulate temperatures in equipment, devices, and homes.

Installing a thermostat can be confusing and will need some experience. However, if you have some technical knowledge, it will not be a problem. If you are looking to install a thermostat in your home, one of the questions you may be asking is –

How To Identify Wires On A Thermostat?

  • The green wire on the thermostat is for the fan.
  • The yellow wire on the thermostat is for the air conditioner.
  • The orange wire on the thermostat is for the heat pump.
  • The white wire on the thermostat is for the heating source. It is mostly used in a furnace thermostat.
  • The red wire on your thermostat is for the power. It is useful in heat or cool calling.
  • The C wire or common wire is always attached to the common terminal. In some thermostats, this wire is not always available. However, you can include it in your thermostat if you want to.

What Color Wire Goes Where On A Thermostat?

In a standard thermostat, there are many sockets, and different wires are being passed into each of the sockets. We have as many as 10-16 sockets in some thermostats, and they are labeled as follows C, R, W1, W2, O/B, G, Y1, Y2, BK, 2x RS, 2x ODT, AUX NO, AUX C, and AUX NC.

Color-coded wiring makes it easy to wire your thermostat.

Find below the different color wires and where they go into:  

  • The Black or Blue wire in a thermostat that goes to the C terminal. It is otherwise known as the C wire, which is the common wire, and It is the wire that completes the 24v electric circuit.
  • The White wire is attached to the W terminal. It usually signifies heat and can be found in gas furnace thermostats, etc. if it has W1 and W2 options, it means it is for two-stage heating.
  • The Yellow wire goes to the Y terminal. It is for cooling and is usually connected to the air conditioner. If it has an option of Y1 and Y2, it is considering a two-stage cooling system.
  • The Green wire is attached to the G terminal. The G terminal is connected to the fan, and it regulates the power input that is sent to the fan.
  • The Red wire goes to the R terminal. The R terminal is the power wire, and it is usually a call for heating or cooling. The wire always has its origin from the transformer.

Other wires that are left in the thermostat are not always very relevant. The thermostat normally would function without them, and so you can wrap the remaining wires up without letting any of them touch each other.

What Are The Wirings On A Thermostat?

Thermostats have different wiring variations, which are dependent on their functionalities. There are two, three, four, and five-wire thermostats, and they all serve different purposes. Your modern digital thermostat will likely have 5 wires. The different thermostat wire variations are explained below:

2-Wire Thermostat

The two-wire thermostat has just two wires which are red and white. The thermostat is used for heating and is always connected to the heating system, usually the furnace. The red wire is the 24V power wire, while the white wire is the heating wire. This type of thermostat is mostly found in old homes.

How To Connect A 2 Wire Thermostat

If your 2-wire thermostat develops a fault and you need to change it, here are step by step procedures to follow:

  • Open the cover of the thermostat and locate the wires. You should expect to see a red and white thermostat fixed to the thermostat with a screw.
  • Unscrew the thermostat and gently remove the two wires from the terminals.
  • When unscrewing, take note of the wire terminals. Usually, the red wire goes to the terminal labeled R while the white wire goes to the terminal labeled W.
  • Replace the old wires with the new wires and reconnect them to their respective terminals.
  • After connecting, test your thermostat to confirm it is working properly.

3-Wire Thermostat

The 3-wire thermostat consists of red, white, and green wires. The third wire in this thermostat serves as a repurposed c wire or common wire. The 3-wire thermostat is connected to the heater and the fan. The G wire goes to the fan while the W wire goes to the heater, and the red wire is the 24V power wire. 

The major difference between this type of thermostat and the 2-wire thermostat is that it has a fan connected to it via the green wire.

How To Connect A 3-Wire Thermostat

  • Open the cover of your 3-wire thermostat and locate the three wires on the thermostat.
  • Take a picture of the thermostat wirings to easily identify the terminal where each wire was originally connected to.
  • Unscrew the terminals and remove the wires on the thermostat while taking note of their respective points.
  • Replace the old wires with the new wires and screw them back into the thermostat board. Connect the red wire to the R terminal, green wire to the G terminal, and white wire to the W terminal.
  • Test your thermostat to confirm it is still working. The boiler, heater, or furnace should be working properly, and your fan should also be working.

4-Wire Thermostat

This type of thermostat is used in HVAC systems where heating and cooling are required. The 4-wire thermostat consists of red, green, white, and yellow wires. The yellow wire is connected to the cooling system. The green wire, which is for the fan, allows for airflow in the HVAC system.

How To Connect A 4-Wire Thermostat

  • Open the thermostat covering and secure the wires by holding them to avoid getting lost in the wall.
  • Take note of the positions the wires are connected to. You can use a camera to take a picture.
  • Unscrew the terminals and remove the wires.
  • Replace the old wires with the new wires and screw them into their respective terminals.
  • Test to ensure the cooling system and the heating system is working well. Also, ensure that the fan is providing a proper inflow of air.

5-Wire Thermostat

This is the most common thermostat in modern housing, and it usually consists of red, green, yellow, white, and blue or black wire. It is used in HVAC systems with air conditioners, furnaces, boilers, etc. it is similar to the 4-wire thermostat but with a fifth wire called the common wire.

How To Connect A 5-Wire Thermostat

  • Open the thermostat cover and locate the wires.
  • Position the wires to avoid getting lost in the wall.
  • Unscrew the terminals and remove the old wires while taking note of their respective positions.
  • Replace the old wires with the new wires in their appropriate terminals while ensuring the blue or black wire is connected to the common or c terminal.
  • Test your thermostat for workability. You can also test the relays and smart devices connected to the thermostat to ensure it is fully functional.

Guidelines To Follow When Installing Or Replacing Wires On A Thermostat

Replacing wires on a thermostat can be tactful if you are trying to do it by yourself at home. As thermostats are connected to electricity, you should follow the guidelines below to ensure your safety.

  • Make sure you wear your safety kit. This includes a good electric safety boot and gloves to prevent electric shock.
  • Make sure you disconnect the thermostat from the electric supply. You should short the circuit in your home entirely from the mains to prevent the inflow of electricity.
  • When the thermostat is open, make sure you locate and hold the wires to a fixed position to prevent them from going back into the wall.
  • Use a camera to take pictures of the thermostat settings at every point to avoid mistakes when reconnecting. You can always look back at the picture as a reference.
  • If you get lost, do well to call an experienced technician or an electrical engineer.

What Happens If You Wire A Thermostat Wrongly?

If you happen to wire your thermostat wrongly, you may experience some faults which range from little damages such as a blown fuse to more serious problems like an electric shock or equipment damage. Below are a few things that may happen when your thermostat is wrongly connected

  • A wrongly wired thermostat can result in electric shock.
  • You may experience overheating your thermostat if you mistakenly connect the wires wrongly.
  • If you have a circuit breaker installed in your home, a wrongly wired thermostat can blow up the fuse in your circuit breaker due to excess voltage.
  • Wrong connections can also result in total damage to the thermostat and its connected units.
  • If you are experiencing poor heating up or cooling up of your home, it might just be that your thermostat is not working properly due to wrong wiring.

Related Readings:

How to Identify Wires in a 3-Way Switch?

How To Find A Reverse Light Wire?

How To Identify Neutral Wire With Multimeter?

How To Add A Neutral Wire For Smart Switch?

What Happens if You Switch Neutral And Ground Wires?

How To Test If Wire Is Live/Hot Without a Multimeter?

How to Identify Line and Load Wires? – 4 Ways

Why Does My Thermostat Have Six Wires?

Some thermostats may have up to 6 wires. This is usually because there is an extended socket for second-stage heating and second-stage cooling. They are usually represented as Y1 and Y2 and W1 and W2. The thermostat may also have an option for a heat pump, but they do not usually have all three points.

What Color Is The Common Wire In A Thermostat?

The common wire in a thermostat is always the wire with the blue or black color. They are sometimes not seen in old thermostats but are usually a part of modern standard thermostats. The common wire is always connected to the transformer and normally serves as a completion to the 24V circuit.

What If There Is No C Wire For A Thermostat?

If you don’t find a C wire on your thermostat, you do not have to panic. Infact, the truth is that thermostats can work even without a C wire. You may only experience little fluctuations in temperature. Old fashion homes had thermostats without a C wire, and it still worked perfectly. 

However, if you check your thermostat and you don’t find a C wire, you should further check to make sure it is not rolled up and tied to the back of the thermostat. If you still can’t find it, you may need to put one.

Is Rc Wire The Same As C Wire?

The RC wire on a thermostat is not the same as the C wire. While the C wire is common, the RC wire serves a different purpose. Some thermostat has just the R wire, but if you happen to have one that has the RC wire, then it is for the air conditioning system. 

It usually means that the thermostat has a dual transformer and contains both heating and cooling.


Thermostats are often referred to as the “brain” of a home’s heating and cooling system. It is an electronic device with a number of wires that connect directly with the HVAC system in the home, ensuring that it is working properly. Some thermostats have more than three wires, and it can be difficult to know which wire is which. Follow the guide in this article if you are trying to identify the wires in your thermostat.

Conclusively, if you experience an electrical issue or misconnection with your thermostat, it is advised that you contact an experienced engineer or electrician.

Scroll to Top