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A homeowner programs a newly-installed thermostat

How to Change a Thermostat

Tim H.Verified

Lead Building Engineer

Residential and Commercial HVAC

5,194 positive ratings
Before You Start

1. Check the compatibility of your thermostat

2. Turn off circuit breaker box or power panel

3. Grab a screwdriver, needle-nosed pliers, and a pencil

Changing the thermostat in your house does not have to be difficult. Below are the steps on how to change a thermostat and everything you need to know. Doing this yourself can save you money and is relatively simple.

How to choose a thermostat

If you haven’t already bought a new thermostat, you’ll want to consider what kind to get. Thermostats range from simple to complex, with manual, programmable and smart thermostats to choose from. All three work with air conditioning units and all types of furnaces. Which one you choose will depend on how much money you want to spend, how sophisticated you want your technology to be, and how much value you put on convenience.

Manual thermostats are normally found in older homes. They are sufficient for those who are diligent about saving energy and changing the settings. If you are going to be away from home for long periods of time, however, this kind is not recommended.

Programmable thermostats are more energy efficient, saving you money by automatically adjusting the temperature. They are digital and simple to use. These can be programmed to turn on and off, adjust the temperature while you are away, and warm or cool the house before you come back home.

Smart thermostats have Wi-Fi capabilities and allow you to control it from a computer or smartphone. They are programmable and energy efficient. The smart thermostat gives the homeowner better control of the temperature of the home and makes it convenient if you forget to change the settings before leaving home.

Three different kinds of thermostats
A manual, programmable, and smart thermostat, from left to right.

After you choose which thermostat you would like to have, determine what your previous thermostat is. It should be labeled as:

  • 1 Stage: This works with separate heating and cooling units.
  • 2 Stage: This works with multi-stage heating and cooling units.
  • Direct Line Voltage: These are normally seen in older homes and directly hard-wired in the home. They are powered by 110 or 240 voltage.
  • 24mV: This is used for fireplace, floor or wall furnaces.
  • Zoned HVAC: Used when one HVAC unit controls both heating and cooling units but can have different temperatures in different rooms.

How to replace the thermostat

Most thermostats are easy to replace. The following steps will ensure that yours is installed correctly.

First off, you have to determine what type of system you have. Generally there are two types, a heat pump system, and a conventional system. In a heat pump system, different terminals are used. Once you determine this, proceed to the next step.

Once you are ready, before anything else, turn the power off to the furnace or air handler. This is very important. Normally there is a switch near the unit. It will probably look similar to a light switch. Another option is to just use the house circuit breaker.

Next, remove the old thermostat. Remove the cover and any screws, then pull the unit and wall mount away from the wall. Make sure not to pull too hard and rip any wires from the wall.

Afterward, disconnect the wires. Pay attention to the terminal each wire is attached to in case the new thermostat has different colors; it might be best to mark each wire. It helps to wrap the wires around a pencil so they cannot fall back inside the wall. Now you can remove the mounting plate from the wall.

A homeowner mounts a new mounting plate on the wall

Place the new mounting plate against the wall and pull the wires through the holes. Make sure no wires will be pinched between the plate and the wall. Also, make sure the mounting plate is square so that the unit will be level. Now, tighten the screws.

Next, connect the wires. It is vital that all wires are connected correctly as cross-wiring can damage the unit or even start a fire. Each terminal should be marked appropriately for each wire. The following explains each terminal:

  • Terminal G, or green in color, operates the furnace fan blower motor.

  • Terminal W, or white in color, operates the heater.

  • Terminal Y, or yellow in color, operates the air conditioner.

A diagram of a typical thermostat's wiring system


Now it is time to insert the thermostat onto the mounting plate. Make sure it is solid and in place before you let go. Turn the power on and, if you have a programmable thermostat, begin programming. Once you’ve done that, it’s time to test the new thermostat by cycling through some heat and AC cycles to make sure it’s working properly

What to remember

All wiring should be performed with extreme care. If the wires are crossed it could prevent the unit from working correctly or at all. Turning off all power before starting will ensure there is no injury to you or your thermostat or home.

Thermostats are very convenient. Upgrading yours could save you money by reducing the amount of energy your furnace and air conditioning unit uses. The thermostat you choose to install should be compatible with your HVAC unit. Follow all manufacturer’s instructions when installing or replacing a thermostat.

If you run into any problems or have questions, ask a certified HVAC technician. Technicians have years of experience and can give you step-by-step instructions or problem-solving if needed.

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