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If you're looking to upgrade your thermostat then you're in the right place. We've created this guide to walk you though how to remove an old thermostat and replace with a new one. Everyone's skill levels are different, so if you are not comfortable messing with electrical wiring or have any doubts, then we highly recommend you seek the services of a HVAC technician.

At SmartHomePerfected we always advise readers not to undertake any work they are not competent in performing and if in any doubt, it is best to seek the assistance of a professional.

While each color wire has a particular purpose, it is still recommended to check the thermostat wiring to your HVAC system to make sure it was wired correctly in the first place!

You should always isolate power to your HVAC system and thermostat wires before working on them, via the circuit breaker box.

Step 1: Identify your HVAC system

HVAC systems can be categorized in many ways. You can identify them by the type of thermostat wiring, the devices connected, or simply by the power they use. If you identify them by power, they are mV, 24V, and 110V. These voltage levels correspond to the connection between your thermostat and the HVAC system, i.e., the control of the HVAC system.

There are standard furnace and air-con systems, heat pumps, hybrid split systems, mini-split and packaged systems, water based boiler systems, electric systems (such as baseboard, radiant or wall heaters) and millivolt systems.

It is important you identify and understand the type of system you have before proceeding.

Types of Thermostat Wiring

In terms of wiring, you need to consider the number of devices connected and how old the system is. Nowadays, HVAC systems are quite efficient, and most can be extended without rewiring the entire setup. These systems use 4 or 5 wire systems for thermostats. Older systems might use the 2 or 3 wires. 2 or 3 wire systems are also commonly used when only one or two devices are connected, such as heat-only systems.

Step 2: Removing your Old Thermostat

Before you remove the old thermostat, there are a few things to keep in mind:

  1. Make sure power to the HVAC system is turned off. If feasible, it’s best to turn the main power off entirely, so you’re completely safe.
  2. Once you remove the base, take a picture of the old wiring. This is so you can identify which wire does what while also helping you out if you need to reconnected your old stat.
  3. Do not reinstall if it’s raining heavily or during any other environmental condition that might cause a short circuit.

Once you’ve taken the steps mentioned above, check how many wires you have connected with the thermostat. You can do this by opening the thermostat or asking your previous installer. Then, just skip to the section of this article that applies.

Step 3: Installing your New Thermostat

Two-Wire Thermostat

Two-wire thermostats are easiest to install and are mostly used for heat-only systems. If you have a line voltage system, you’ll have a two-wire thermostat. In addition, two-wire thermostats are commonly used to control a single device. You’ll find them hooked up to a single furnace, baseboard heater, boiler, or electrical attic fans. They are also common in cabins and recreational vehicles, where you can install them on radiators or electric heaters.

In a two-wire thermostat, one wire is for the power (usually red), and the other is a white or black wire to control the heating. Here’s a step-by-step guide to the process.

  1. Once you’ve turned the main power off, remove the lid. Although you don’t need it in a two-wire system, take a photo just to be safe.
  2. Chances are your thermostat already has red and white/black wires at the back, so just connect the red wire to the R terminal. If this is not the case, check the terminal marked for power/load, and connect the red wire to it.
  3. Connect the black/white wire to the other wire/terminal at the back of the thermostat. If the thermostat has more than two terminals, attach this wire to the W or Wh terminal.
  4. Ensure the connections are tight and insulated, and then push the thermostat back into the wall and install the lid.

Your thermostat is now installed. Turn the power back on and switch on the heating. The same steps will be used for baseboard heaters and boilers. Heaters might have a millivolt system, so a transformer will be used.

Check the specifications of the thermostat for the load/input voltage. It will be mV, 24V, or line voltage (110V for North America). Make sure the voltages on the thermostat and your heating system match. If you have a line voltage system, install a thermostat designed for that. If the voltage levels are different, you will most probably burn the thermostat out or at least trip a circuit breaker.

Three Wire System

Three wires are also used primarily for heat-only systems. The wiring is very similar to the two-wire system. The difference is that now there is a wire to control the fan pumping air into the duct. This wire is colored green and connected to the G terminal.

The advantage of having three wires is you can use a C-wire thermostat. In this case, you can use the G-wire as a common C-wire. However, you’ll lose independent control of the fan. Check if your thermostat needs a C-wire or not, and then follow the steps below.

Turn the power off, take the thermostat cover off, and unscrew the control panel from the wall. Take a photo of the setting, and then remove your old thermostat.

If C-wire not needed

  1. Connect the red wire with the R terminal, the white wire with W or Wh, and the green wire with the G terminal. If the thermostat already has wires, just match the colors and connect.
  2. Make sure the connection is strong and insulated. Then put the thermostat in the wall, screw it, and put it on the cover.

If C-wire needed

You have two options if you’ve got a C-wire thermostat. You can either use an accessory called a common maker or repurpose your G wire as the common wire.

Option A: Use a common maker

There is this device by Fast-Stat, that you can use which takes an input of three wires and outputs four.

  1. Wire your thermostat with the sender and the furnace using the instructions given in the image
  2. Wire the receiver of the common-maker with the furnace/boiler using the diagram given below. If you encounter additional problems, you can read the complete installation guide here.

Option B: Refurbish G wire as the common wire

If you do not want to use a common maker, you can use the green wire as a common wire. The disadvantage is that you’ll lose independent fan control. So the fan will turn on only if the heating is on, too, and it will remain on continuously. Check out this guide from ecobee support to make the connection.

Four Wire Thermostat

Four-wire thermostats are quite common. These systems are quite flexible and offer extended control over your devices. Dual-stage cooling/heating devices also use the four-wire system. Many modern thermostats are compatible with this system, e.g., Nest’s Learning Thermostat and ecobee’s Smart Sensor Thermostat.

A four-wire system means there is no common wire, and now the question is, what kind of thermostat do you have? If you have a C-wire thermostat, you’ll need to either set up the fifth wire or make some adjustments using certain kits. However,  if you have a thermostat that does not need a c-wire, setting it up will be easier. Check your thermostat’s specifications and skip to the section that applies.

If C-wire not needed

If your thermostat does not need a C-wire, you can connect it directly without making adjustments. Check what color wires you have on your existing thermostat. The colors will most probably be red, white, green, and yellow/blue. If there are other colors, you might have a unique system, or the previous wiring is not done according to color standards. In that case, you’ll have to check the devices to see which wire represents which function.

If you have the mentioned colors, then look at the color code and follow the instructions below.

Red Power R/Rh/Rc
White Heating W
Green Fan Control G
Yellow/Light Blue Cooling Y
  1. After you’ve turned off the main power, remove the base and unscrew. Once the thermostat is out of the wall, take a photo of the previous wiring template.
  2. Connect the wires of the new thermostat by matching the colors or if your thermostat has no wires, connect them directly to the terminals.
  3. Put the thermostat back in the wall and screw it in.

If C-wire needed

For a thermostat that needs a C-wire, a 4 wire system won’t work on its own. That is because the thermostat needs to be placed in a position right between the power and your HVAC system. So, in this case, you’ll need to create a fifth wire.

The best way to handle this situation is to install a fifth C-wire between your thermostat and the HVAC system. However, that would require a technician’s help. If you don’t want to go that route, you can use several kits, which convert a 4 wire system to a 5 wire system. We’ve given a list of some of the most popular devices on the market below.

However, it is better to check with your manufacturer and see if they already have a kit, specifically for their thermostats. For example, if you have bought the Nest Learning Thermostat, you can get Nest’s power connector to install it without a C-wire. There are similar devices for ecobee’s thermostats as well. Honeywell also has a few kits available if you don’t have a C-wire.

If your manufacturer does not have this device, you can use some of the generic ones on the market. Here are some of them. For detailed information on the C-wire and how to get around it, give our other article a read.

5+ Wire Thermostat

This is the most common type of wiring in modern homes and can be used for any HVAC system. Most modern thermostats are made for the 5-wire installation, so you can use most devices on the market if you have 5 wires coming out of the wall. There is a list of wire colors given below. Check which wire you have.

RedRPowerProvides power from the transformer (24V usually)
GreenGFan ControlTurns the blower fan on/off for air in the ducts
YellowYCoolingAir Conditioner (AC relay specifically) 
BlackAnyDepends on your system
WhiteWHeatingConnected to a Heater, or the heat pumps

Note: If you have more than five wires, no need to panic. Just take a look at our thermostat wire color guide to connect the correct colors/terminals.

  1. Turn the main power off, and remove your thermostat from the wall by unscrewing and removing the base/lid.
  2. Take a photo of the current wiring before you disconnect the old thermostat.
  3. Check the colors of the wire, and connect the wires by matching the colors. If your thermostat does not have wires, connect the wires to the correct terminals.
  4. Install the thermostat back in the wall and turn your power on.

Common Questions Relating to Thermostat Wiring

What is a line voltage thermostat?

Thermostats are powered by mV, 24V, and 120/220V systems. Thermostats that connect directly to the voltage in your wall socket are called line voltage thermostats. For North America, the line voltage is 120V. For most of Europe and Asia, it’s 220V while it varies in the rest of the world.

What kind of wire should I use?

You’ll come across two types of wires for thermostats, 18-gauge and 20-gauge. These standards are based on the thickness of the wire. Regarding quality, the low-loss copper wire will do just fine and ensure it is not stranded. You can use the 20 gauge wire for a smaller distance (up to 200 feet). If the distance is larger, go for the 18-gauge!

If I don’t have a C-wire, can I leave it open?

If your thermostat requires electricity to operate, you will need to connect a C-wire like most modern thermostats. If you leave it open, your system won’t turn on since the circuit won’t be complete. If you don’t have a C-wire system, there are remedies.

Many manufacturers such as ecobee and Nest provide you with an extension kit that’ll let you power your thermostat externally. In addition, there are a few devices that are specifically made to bypass the C wire.

The Verdict

The first step to wiring a thermostat is recognizing what kind of system you have. Then you can see if your thermostat is compatible, and if not, then fine some ways to work around it. Make sure you’ve always taken all necessary safety precautions, and if you can’t figure out wiring even after doing some research, it is better to call a technician and let them handle it.

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