Installing a New Smart Thermostat ─ Know Your Wires
Installing a new thermostat was pretty simple in the 1970s. There were just two wires to worry about: power and ground. They were color-coded white and black so there could be no mistakes. Today’s thermostats are a lot smarter and more complicated. If you intend to install a smart thermostat on your own, make sure you know your wires.
Thermostat wires are still color-coded today. However, they may not match the colors of the wires already embedded in your walls. Homes with older wiring might present with different colors. I’ve seen some situations in which all the wires were the same color!
Perhaps this is why Vivint Home Security recommends professional installation over DIY. Here’s the deal ─ mixing up the wires could fry a sensitive component in your HVAC unit. Then you would be looking at an expensive repair bill.
I Learned the Hard Way
I learned this lesson the hard way. Several years ago, I decided to replace my traditional thermostat with a programmable model. I was well aware of wire colors and what they meant. But there were two wires in the new thermostat with colors that didn’t match my existing wiring.
Understanding the seriousness of the situation, I spent hours researching online. Once satisfied that I had figured out the correct wiring configuration, I went to work. It turns out I had it wrong. Powering the system back on led to the HVAC unit briefly starting and then shutting back down. Several hours later I was looking at an expensive repair bill to replace a small, burned-out circuit board.
The strange thing was that the first technician who showed up to look at my system couldn’t figure it out, either. He had to call in a more seasoned technician who had experience with older wiring. That was a big red flag for me. When I get ready to replace the thermostat in my new home with a smart model, I’m going to have a professional do it?
What the Colors Mean
Now that you know my story, let’s talk about what the colors mean. Note that what I am about to explain is general in nature. There are exceptions to every rule. Don’t use my explanation as a guide for replacing your own thermostat. If you don’t know exactly what you are doing, find someone who does.
Red Wires
Wire colors tend to have subsequent letters that designate their purpose. Let’s start with the red ones. A red wire marked with the letter ‘R’ is the power wire. This would be standard for a heating-only unit. An HVAC unit with both heating and air conditioning could have two red wires:
- Rc – Power for the cooling system
- Rh – Power for the heating system
Note that systems with dual transformer setups might label their power wires with a single ‘R’. If you are not sure of your current HVAC setup, take the time to figure it out before you start looking at wire colors.
White Wires
White wires are generally the wires connected to air conditioning systems by way of their compressors. These are control wires. They connect to the ‘Y’ terminals on a thermostat. Remember that systems with multiple cooling stages might also have multiple ‘Y’ terminals.
Green Wires
Green wires are intended for fan operation. They connect to the ‘G’ terminal on a thermostat. Multiple green wires might be present on a system featuring multiple fan speeds or a more advanced air handler.
Orange and Blue Wires
Red, white, and green wires are the most common for modern thermostats. Some systems also have orange and blue wires as well. Both are related to heap pump operation. They can vary in terms of function, so it’s important to thoroughly understand heat pump operation before connecting these wires.
Black Wires
Rounding things out is the black wire. It is the ‘common’ wire that provides constant power to a thermostat. It should not be confused with the black ground wire found in a typical 120-volt setup. Also, note that the common wire is sometimes dark blue.
Compatibility Apps Make It Easier
Today’s smart thermostats are even more complex than the programmable thermostats I tried to install years ago. I can’t imagine trying to install one today without help. Fortunately, any DIY enthusiast who insists on installing his own thermostat can probably find help in a compatibility app.
A compatibility app is a mobile app developed by a thermostat manufacturer. When you’re ready to install, you pull up the app and select the colors your current system utilizes. The app will determine whether your new thermostat is compatible or not. If it is, the app will also provide a wiring diagram.
How to Do It Safely
I will go on record as saying it’s better to hire a professional to install a new smart thermostat. But if you are going to do it yourself, make sure you do it safely. Here’s the process:
1. Shut Off the Power
The first step is to shut off the power. Do so for your own safety as well as protecting your HVAC system. Trying to install a new thermostat with the power still on could lead to getting zapped by an uncomfortable shot of electricity. You could also damage sensitive components in your HVAC system.
2. Identify Your Wires
After removing the thermostat cover and unscrewing the unit from the wall, identify your wires by color. Do this before you detach the thermostat from the wiring. Note that your current thermostat might have letter designations on its terminals. This can help you better understand your existing setup. It’s also helpful to label the wires before you detach them.
3. Install the New Thermostat
Once you’re sure of the wiring configuration, detach the old thermostat and install the new one. Screw it to the wall, put the cover on, and turn the power back on. Provided you did everything right, you should be good to go.