120v Thermostat Wiring Diagram

Thermostats are essential components of HVAC systems that help maintain a comfortable temperature in homes, buildings, and other spaces. A thermostat serves as the control center for heating and cooling systems, ensuring that the temperature remains within a specific range. In this article, we will discuss the wiring diagram for a 120v thermostat, including its installation, wiring steps, frequently asked questions, and tables.


To install a 120v thermostat, follow these steps:

Step 1: Turn off the Power

Before starting the installation, turn off the power to the HVAC system. This will help prevent electrical shock and damage to the components.

Step 2: Remove the Old Thermostat

Remove the old thermostat from the wall using a screwdriver. Make sure to disconnect the wires from the terminal screws.

Step 3: Mount the New Thermostat

Mount the new thermostat on the wall using the screws provided. Make sure to level it and mark the locations of the screw holes.

Step 4: Connect the Wires

Connect the wires to the new thermostat according to the wiring diagram. Typically, a 120v thermostat has four wires: red, white, green, and yellow. The red wire is connected to the R terminal, the white wire to the W terminal, the green wire to the G terminal, and the yellow wire to the Y terminal.

Step 5: Test the Thermostat

After connecting the wires, turn on the power to the HVAC system and test the thermostat. Set the temperature to a desired level and monitor the system for any issues.

Wiring Steps

The wiring diagram for a 120v thermostat is as follows:

Wire Color Terminal Label Function
Red R Power
White W Heating
Green G Fan
Yellow Y Cooling

The red wire is connected to the R terminal, which provides power to the thermostat. The white wire is connected to the W terminal, which controls the heating system. The green wire is connected to the G terminal, which controls the fan. The yellow wire is connected to the Y terminal, which controls the cooling system.


What is a 120v thermostat?

A 120v thermostat is a device that controls the temperature of an HVAC system using 120 volts of electrical power. It is commonly used in residential and commercial buildings to maintain a comfortable indoor temperature.

What is the difference between a 120v and 240v thermostat?

The main difference between a 120v and 240v thermostat is the voltage they use. A 240v thermostat requires twice the voltage of a 120v thermostat, making it suitable for larger HVAC systems. Additionally, a 240v thermostat may have different wiring and installation requirements compared to a 120v thermostat.

How do I know if my thermostat is 120v or 240v?

You can determine the voltage of your thermostat by checking the rating on the back or the installation manual. Additionally, you can consult a licensed HVAC technician who can help you identify the voltage of your thermostat and HVAC system.

Can I install a 120v thermostat myself?

It is possible to install a 120v thermostat yourself if you have experience with electrical wiring and HVAC systems. However, it is recommended to hire a licensed HVAC technician to ensure proper installation and avoid potential safety hazards.

What should I do if my 120v thermostat is not working?

If your 120v thermostat is not working, check the wiring connections, batteries, and circuit breaker. Additionally, make sure the settings are correct and the temperature sensor is working properly. If you are unsure how to fix the issue, consult a licensed HVAC technician.