Thermostat Wiring Explained: How To Wire And Install A Thermostat

The thermostat is the essential and most crucial part of every HVAC system. Every homeowner should know about things like how it works and how to replace it.

I have been an HVAC technician for many years now, and I have received so many different calls about thermostat repair. But when I go to the house for repair, it's just an insignificant error done while replacing the thermostat. Homeowners can pay a lot of money to fix an easily avoidable mistake.

The basics of thermostat wiring and how to replace and troubleshoot it is valuable knowledge for anyone to acquire. In this article, I’ll explain the wiring of a thermostat as well as how to replace a thermostat.

What Different Wire Colors Do - Thermostat Wiring Explained

Thermostat Wiring: The Power Wire (R/Red Wire)

Every thermostat will have this Red wire, as it is essentially a 24V AC power wire that connects from the furnace control board to the thermostat. In many electrical systems, the universal color of this power wire is red.

From your furnace, you're getting anywhere between 18 and 24 volts, depending on your system. However, it should be 24V for most systems. That 24V will be coming through the power wire.

What happens is that the power from the R wire goes down to the Y wire and then back to your furnace. That wire tells your furnace to turn on a cooling mode, which is what the Y wire controls.

If you’re looking for your furnace to enter a heating mode, the power from the R wire must be powering the heating wire. From there, it will get back to your furnace or HVAC system and tell it to turn on the heating.

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 used to provide a return path to power the thermostat.

If you have a smart thermostat like a Nest or Ecobee, the common wire allows your thermostat to constantly change 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 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 anything additional.

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 a cooling system. The wire goes to your HVAC system and 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. That will then activate the furnace or burner inside your HVAC system, which is essential for providing heat.

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

The G wire is the fan or green wire. It isn't necessary for most HVAC systems, but almost all have it. The G wire allows you to control your fan so you can turn it on or off whenever you want.

Regardless if you have heating or cooling already, when you activate that system, it will turn on your fan no matter what.

Thermostat RH and RC Wires:

RH stands for the power to the heating, and RC stands for Power to cooling. If you have an RH and RC, then you must place a jumper between R and RC as well as one for R and RH.

Sometimes after replacing the thermostat, people forget to put a jumper in between these two—this could result in heating and cooling not working.

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

The Orange Wire is for the reverse valve cooling, and the Dark Blue/B Wire is used for reverse valve heating.

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, use a screwdriver to take it off.

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

Step 4: Now connect all the wires to the new thermostat and check every option to ensure it’s working. (cooling, heating, fan, etc).

Steps for Installing Different Types of Thermostats:

Steps to install a two-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 wire in the same socket as they were before. Red to R section and White to W section.

Step 4: You need to check that your furnace is working properly.

Replacing the three, four or five wire thermostat?

Removing and replacing the thermostat is the same in all the processes (two, three, four or wires). You just need to find out the wire selection.

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

The four-wire thermostat has four wires included (red, white, green, blue) A four-wired thermostat is used for heat pumps and HVAC (heat and cooling).

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Tips for thermostat wiring

You must attach the proper wires to the corresponding terminals on the back of your new thermostat to install it. The most common systems' thermostat wires follow the following industry-standard color scheme:

Your heating system is connected to the W wire. You will have a white wire if your thermostat regulates your heating.

A yellow wire connects your air conditioning compressor called the Y wire.

The green G wire attaches to the fan.

Rh and RC Your thermostat receives power from one or more red wires. Your thermostat will have a red Rc wire if it is solely used for air conditioning. It will have a red Rc wire and a red Rh wire for HVAC systems. Even though both wires are red, they cannot be switched. The red wires need to be connected to the appropriate terminal in order for the thermostat to function properly.

The C, or common wire, continuously powers smart thermostats. It is typically blue but can also be purple, black, or brown.

Your heat pump is connected by the O, or orange wire (if applicable).

Although it's crucial to understand these industry norms, your wiring might differ. Only use this manual as a reference. You should consult a licensed electrician if you have any questions about the wiring for your thermostat.

Use a magnifying glass to help you see the terminal letters, which can be difficult to see due to their small size.

Remove the current control panel

An older thermostat had its cover removed.

Once you have a basic understanding of wiring, you can get ready to install your new thermostat. Turn your current thermostat to the OFF position first. Turn off the power to your HVAC system at the main breaker box after waiting for any cooling or heating to stop. In accordance with the instructions in the user's manual, remove the thermostat's faceplate or control panel to expose the wires. At this point, avoid removing the wiring or the remainder of the thermostat from the wall.

Find the Common Wire

someone installing a new thermostat

On the thermostat, look for a wire that is attached to a terminal marked with the letter "C." You are now prepared for general installation if you have a c-wire. The common wire, sometimes called the c-wire, is responsible for delivering constant power to your thermostat. C-wires are necessary for programmable and smart thermostats to program a heating or cooling schedule. While some devices can function without a c-wire, most manufacturers advise against it for best performance.

In the absence of a "C" wire:

Look behind the thermostat to see if your electrician or the installer tucked a c-wire into the wall. Although many older thermostats did not require them, c-wires were still available for use.

If you have a furnace, check to make sure. A c-wire connection and lead might be discovered. You do, in fact, have a c-wire, but you need to locate the lead's end close to your thermostat.

Call a licensed electrician if you are having trouble locating the c-wire or need assistance determining the difference between a 4-wire and a 5-wire thermostat. Alternately, look for thermostats that state that a c-wire is unnecessary for operation. Remember that selecting a model without a c-wire will restrict your options and overall functionality.

Label the wires and cut them off

Before you disconnect the wires from your current thermostat, locate and label the wiring:

Snap a photo of your current wiring configuration. Verify that they adhere to the industry's standards outlined in Step 1. You can connect your new thermostat using that as your guide if they do. If not, make your own list on paper with a pencil, and mark each wire with a permanent marker and a piece of painter's tape.

Remove the corresponding mounting screws and the associated wires. Your thermostat should have a free back plate. While you prepare your new thermostat, tape the wires to the wall with painter's tape to prevent them from falling into the hole.

Advice: Occasionally, getting rid of the old anchors may be necessary. Patch the drywall to ensure a tight fit when installing your new thermostat.

Attach the brand-new thermostat.

Place the thermostat's plate against the wall where it will be installed. Mark the screw holes with your pencil and use a level to make sure the plate is level.
Drill guide holes if necessary before attaching anchors to the wall.
Pull the wires through the backplate's hole on the thermostat.
Utilizing the supplied screws, fasten the back plate to the wall. Make sure the thermostat is safe by checking.
Reconnect the wires in accordance with either your unit-specific notes from Step 4 or the industry standard color codes. Verify again that they are connected properly. If necessary, refer to the thermostat's instructions or the picture you took of the old wiring layout.
Put the thermostat cover back on.
Restart the main breaker for the HVAC system. Follow the manufacturer's instructions to set up user programs and connect to Wi-Fi.

Advice: Inspect the wires for wear before connecting them to the new thermostat. If they seem worn, wrap them in electrical tape.

Controlling the comfort system in your home is more practical when you install a new programmable or smart thermostat. With the right information, it's a fairly simple DIY project that anyone can complete. The key to a trouble-free installation is to look for the common wire and map out your existing thermostat wires.