how to wire
a thermostat
guide
A thermostat is one of the most important appliances in your home. Being able to control the temperature is what makes your house livable, even during the hottest days of summer and the coldest days of the winter.
According to EIA’s Residential Energy Consumption Survey (RECS), 12% of homes in the United States have an air-conditioning unit that is controlled by a central thermostat.
With the rise of smart homes, more and more Americans are taking control of their home’s HVAC system. Knowing how to wire a thermostat is a crucial piece of information for a homeowner. If your thermostat breaks, you should be able to fix it yourself without the need to call in an electrician. Not only will this save you money, but it will also mean that it gets fixed faster. Before you get started, review our electrical safety basics page.
This guide is designed to walk you through the steps to wire your own thermostat, so you address certain heating or cooling issues on your own. If you would prefer to call a professional, then use this guide to familiarize yourself with the process.
DIFFERENT COLORED WIRES AND THEIR
MEANING
Like a bomb-disposal team in a movie, wiring a thermostat is all about connecting the right-colored wires to the right terminal. Understanding what each wire is for is absolutely imperative if you are to do the process correctly. If you are confused by any of this process, it is easier and safer to call a professional.
It is best to take a thorough note of how the wires are connected before you start the process (if you’re rewiring) or use a manufacturer’s photo of the system as a guide. Some models may differ slightly, although the colors outlined below are the most common.
RED WIRE
The red wire should be connected with the Thermostat RH terminal. You can also connect it to the RC terminal with a jumper wire between the RH and the RC. If you only have one R terminal for both heating and cooling, connect it to that.
GREEN WIRE
The green wire controls the fan or the relay controlling the blower. It connects to the Thermostat G terminal.
YELLOW WIRE
The yellow wire controls the air conditioning. The yellow wire should be connected to the Y terminal.
WHITE WIRE
The white wire usually controls the heating and should be connected to the terminal labeled ‘W.’
DIFFERENT
WIRING
VARIATIONS
Before you proceed with any wiring, you should be sure to check your manufacturer’s instructions. There are a few different variations on the wiring for thermostats. The most common ones are covered below, varying from 2-5 wires. If you have more than 5 wires, you have more control points or you have a heat pump, so you won’t be able to follow this process.
2
TWO WIRES:
If you have two wires, you most likely have a digital thermostat providing only heat (i.e. without air conditioning). The two wires are usually red and white (see above for information on where they are connected).
3
THREE WIRES:
This is the usual setup for a digital thermostat that connects to (and controls) a boiler. It has a 24-volt hot, a 24-volt common, and a white wire.
4
FOUR WIRES:
This is a common arrangement for a thermostat that has a battery or a digital thermostat that controls only heat. The usual setup is 24-volt hot (red), 24-volt common (blue), heat (white), and fan (green).
5
FIVE WIRES:
This is the most common type of thermostat wiring, and it covers systems that control both heat and air-conditioning. The usual arrangement of the wires is: red for 24-volt hot, white for heat, yellow for cooling, green for the fan, and blue for common (although the common wire may be another color).
THE WIRING PROCESS
Now you understand the basics of what’s involved in the wiring of the thermostat, you should be able to complete the process yourself. The following points will take you through the process step by step.
1
Safety first
The most important thing throughout this whole process is to stay safe. You are dealing with wiring and electricity, so you need to be absolutely vigilant to make sure that you do everything possible to keep yourself safe.
In most cases, having someone to help you (even if just to observe) will make everything safer. Not only will the second pair of eyes be useful, but if you do get into any danger, they will be on hand to provide assistance. The key rule of safety, however, is: if in doubt, don’t do it. Hiring a professional will cost you a little bit of money, but that’s well worth it to stay safe.
2
Cut the power
For this process, the next important safety step is to turn off the power to the HVAC unit. You don’t want to be doing this process with power in the system for obvious reasons. To cut off the power you can use either the circuit breaker or the emergency cutoff switch.
You should also make sure you know exactly where the emergency cutoff switch is. If you are in need of an electric panel upgrade or electrical repairs, address these before starting this process.
3
Take photographs
A really simple way to make sure the process goes well is to take photographs at each stage. Using your phone’s camera, just take a photo before each step you take. If anything goes wrong, you can review the photos to help you return things back the way they were.
You can also use the photos as a guide for future projects. In addition, if you do need to call in a professional, you can show them the photographs and have them diagnose at what stage it went wrong.
4
Access the thermostat
Once you’re sure you can safely access the thermostat, take the front cover off. If you have a mechanical model, there will be a small adjuster tab in the center. This is known as the ‘heat anticipator’ and should be set somewhere between .1 and 1.5.
Whatever its setting, you will need to return it to that when this process is complete, so take a note of it (don’t assume you’ll remember it – write it down somewhere if you’re not taking photos of your steps).
5
Remove the wires
Firstly, double-check that the power is off, as it’s now time to access the wiring. Unscrew the sub-base from the wall. Underneath this will be a series of wires. Take the wires out of their terminals in the sub-base and wrap them around a pen or a pencil so that they don’t fall back into position.
6
Replace the sub-base
Remove the old sub-base from the wall. If the new sub-base isn’t the same size as the previous one, it doesn’t matter, although you may need to do a quick touch-up of the paintwork to ensure that you’re not left with an unsightly gap.
If you do need to do this, do it now, and then wait for the paint to dry fully before proceeding. You can then install the new sub-base into the position the previous one was in.
7
Drill the mounting holes
Using a drill bit the same size as your wall anchors, drill the mounting holes in the wall. If your sub-base is the same size as the previous sub-base, you can use the same mounting holes that are already present. Once you’ve drilled the holes, push the wall anchors into the holes, leaving roughly 1/16th of an inch remaining sticking out of the hole.
8
Attach the sub-base
Now that you have the mounting holes, you can reattach the sub-base to the wall, screwing the sub-base into the wall anchors. Although it is faster to use a drill, you should take care not to crack the material of the sub-base by screwing it in too tight.
9
Reattach the wires
Now that the sub-base and the terminals are in place, you can reattach the individual wires. Unwind them from the pen or pencil and attach them to the correct terminals one by one. Insert the screws to ensure that they are tightly attached. Use a level to ensure that your sub-base is level.
10
Finish up
Once you’ve done that, you can reset the heat adjustor to its previous level and reattach the front cover. Use a small vacuum to clean up any dust mess you may have made. Then return power to the thermostat and confirm that it works. You should be up and running.
Ultimately, wiring a thermostat is not an extremely complicated process. Knowing how to do it quickly and safely can save you money in a contractor’s hourly and call-out rate.
Understanding how a thermostat works is also good knowledge in general, part of the overall goal of developing your skills and learning how your home works. As long as you can stay safe while you do so, you can use this knowledge in other areas.
This will help you not just maintain your home but improve it. Taking control of wiring and rewiring a thermostat is simply the first step in the process.