Thermostat Wiring Explained (How To Wire And Install A Thermostat)

thermostat wiring

The thermostat is the basic and most important part that every HVAC system has. And every homeowner should know about things like how it works and how to replace it.

I have been an HVAC technician for about 6 years now and I got so many calls about thermostat repairing. But when I go to the house for repair it’s just a tiny error done while replacing the thermostat. Most of the people end up paying 100-200 bucks for a tiny error.

So you must learn about the basics of thermostat wiring and how to replace and troubleshoot it. So In this article, I’ll explain all about the thermostat wiring and how to replace the thermostat.

If you have any questions or suggestions make sure to ask them in the comment section.

Thermostat Wiring Explained

Thermostat Wiring: The Red Wire (R wire/R terminal)

Every thermostat will have this Red wire. It is basically a 24V AC power wire that connects from the furnace control board to the thermostat. In most of the ac or furnace systems, the universal color of this wire will be red. 

So from your furnace, you’re getting anywhere between 18 and 24 volts depending on your system it should be 24V for most systems. That 24V coming through the red wire or the power wire.

So what really happens is that the power from the R wire goes down to the Y wire and then back to your furnace. And that tells your furnace to turn on cooling which is what the Y wire means.

And if you want to heat, the power from the R wire would go down to the heating wire and it’ll get back to your furnace or HVAC system and tell it to turn on the heating.

So the R wire is the most important and it is the only one that will have power on it until your thermostat has turned on.

Thermostat Wiring: The Common/Blue Wire (C/Blue Wire)   

In most of the furnace/ac systems, the universal color of the common wire will be blue. The C wire is basically used to provide a return path to power the thermostat.

Basically, if you have a smart thermostat like a nest or ecobee the common wire allows your thermostat to constantly charge without turning on the heating or cooling or the fan.

Because that thermostat has a tiny little battery in it that needs to stay charged up in order to communicate with Wi-Fi signals to your network.

So what the R wire does with the C wire is it creates a circuit that can charge the battery without activating any other things. 1

Thermostat Wiring: The Cooling Wire (Y/Yellow wire)

The yellow wire is the cooling wire and it goes into the Y terminal of the thermostat. The yellow wire is used for cooling.

It controls your air conditioner and this wire goes all the way back to your HVAC system connects to a terminal on your HVAC system board. It controls your air conditioner compressor and condenser. 

Thermostat Heating Wire (W/White wire)

The W wire is the heating wire. The heating is activated whenever you get R (power) to W and that will activate the furnace or burner inside your HVAC system which will then provide you with Heat.

Thermostat Wiring : The G Wire (Green/Fan Wire)

The G wire is the fan or green wire and that actually isn’t required for most HVAC systems. But almost all have it what the G wire does is it allow you to control your fan to turn it on or off whenever you want.

Regardless if you have heating or cooling going normally when you activate cooling or heating it’s going to automatically turn on your fan no matter what.

Thermostat Rh and Rc Wires:

Rh stands for the power to heating and Rc stands for Power to cooling. If you have an Rc and Rh then you must place a jumper between R and Rc for cooling and R and Rh for heating.

Sometimes after replacing the thermostat people forgot to put a jumper in between these two and this may result in heating and cooling not working.

Orange Wire (O wire) and Dark Blue wire B:

The orange wire is for the reverse valve cooling and the dark blue/B wire is used for reverse valve heating.

Check Our Tutorial Guide On How To Bypass a Thermostat?

Steps For Installing a New Thermostat:

Step 1: First thing you must do is turn the power off so that your wires don’t have any current in them.

Step 2: Remove the thermostat from the base. If you have a clipped thermostat just pull it out. It’ll easily come out. If you have a screwed one then use a screwdriver to take it off.

Step 3: Now you want to take a picture of all the wiring. Because in case if you ended up forgetting the wire placement it’ll help you.

Step 4: Now connect all the wires into the new thermostat and check every option if it’s working or not (cooling, heating, fan, etc).

How to install and connect a 2 wire Thermostat?

This type of thermostat is used for furnaces only and they have only two wires to work with. The power wire (Red wire) and the White wire (heating wire).

Steps to install the 2-wire thermostat:

Step 1: Turn your power OFF and then remove the thermostat cover.

Step 2: Now you need to check which wire goes into which socket or do you have a different color code than the universal ones.

Step 3: Remove the old thermostat and replace it with the new one. Connect both the wire in the same socket as they were before. Red to R section and White to W section.

Step 4: Now you need to check if your furnace is working properly or not.

Replacing the 3, 4, or 5 wire thermostat?

Removing and replacing the thermostat is the same in all the processes (2,3,4,5 wires). You just need to find out the wire selection.

The 3 wire thermostat is probably used to control the boilers and hot water heaters. It includes 3 wires red, green, and white. You just need to put the same wire in the new thermostat.

The 4 wire thermostat has four wire included i.e red, white, green, blue. 4 wired thermostat is used for heat pumps and HVAC (heat and cooling).

Scroll to Top