Thermostat Wiring: A Guide for Beginners


By William Chutney

Published: October 24, 2018

Before reading this guide, if you are in the market for a smart thermostat, check out our guide to buying the best smart thermostat. Wiring a thermostat is not exactly an easy job, even for experienced handyman. If they do not have sufficient experience with the particular type or sometimes, even the particular model in question, they will have a tough time. As you can imagine, if you are just a beginner with no experience or knowledge about wiring thermostats, this can seem like a very daunting and confusing task. If you have any experience with this, then you probably already know that the hardest part of the job is figuring out the function and place for all those wires of varying colors. Even when you are done figuring out that color code, there are the letters and numbers on each of the connectors that you will have to deal with next. To help make things a lot easier for beginners and even people with intermediate experience, let’s now go through the basics of the color code first.

White Wire

There should be just one white wire when you have a single stage heating system and it is supposed to be connected to the auxiliary heat on the HVAC system.

Yellow Wire

There should also be just one yellow wire in single stage cooling solutions which you will connect to the system’s compressor.

Red Wire

Depending on the system and the model, there can be two red wires in there. The one that is meant to be connected to the heat should be marked as “RH,” while the red wire that is to be connected to the cooling should be marked “RC.” Note the demarcations and connect accordingly. If there are no demarcations, just plug one into the heating and the other into the cooling without worrying about which is which.

Orange Wire

If your model has a heat pump, the orange wire is supposed to connect with it and there should only be one of those.

Green Wire

The green wire in any system usually connects with the fan/blower without exception.

Black Wires

If you are wondering where those black wires are supposed to go, don’t worry about them too much because most people do not have any use for them and therefore, you won’t likely have to connect the black wires to anything. For the sake of general knowledge though, know that they are generally meant for outdoor heating up in the North.

Blue Wire

Finally, we have the blue wire, which should be marked as “C.” C stands for Common here and it also happens to be one of the most common causes for confusion during the wiring. Some HVAC systems may not come equipped with a C wire by default and some thermostats just don’t work without the blue wire!

Warning with the C Wire

Generally, none of the other wires will shock you during the wiring, but if any of the open wires touch the C wire, it will definitely shock you because the common wire is responsible for supplying a continuous flow of 24VAC power to the thermostat. So, be careful not to let any of the wires touch the blue wire while you are in the process of wiring your thermostat. The safest thing to do, however, would be to turn off the power altogether.

What If Your System Doesn’t Have a C Wire?

As mentioned earlier, some systems do not have a common wire and that can be a big problem. In those situations, the technician will likely need to install a new C cable because thermostats without a common wire could be hard to find. Choose an 18/5 C wire for the installation, irrespective of whether you have a HVAC system or just a heating unit. It is true that an 18/3 wire is enough for a heating unit, but 18/5 is still recommended. If you really want to skip the C wire installation, a smart thermostat without the common wire (ecobee3 is a good example) is your best bet.

Beware of Preowned Systems

There is no telling how the technician wired a preowned system, so always beware of them. If you connected everything as recommended here and it still doesn’t work, the problem could be that the person before you didn’t follow the universal color code. The issue could be addressed by testing out each wire separately to figure things out. Once you do figure it out, do not forget to mark it accordingly, or at least note the configuration down somewhere for future reference.

The Abbreviations

When your thermostat has letters such as G, Y, W, etc. printed on the terminals, don’t get confused because each letter indicates the color of the wire it is supposed to be connected to. For example, G = green wire, W = white wire and so on. This should make the job even simpler in most cases, except in situations where the universal color code for wiring is not followed or changed.

It can be a bit confusing to see both a Y1 and a Y2 label on some of the newer models, but just use Y1 and ignore the Y2 if all you have is one stage cooling. If you have a second stage of cooling though, that’s where the Y2 connector comes into play. Similarly, ignore the W2 connector unless you have a two-stage heating system.

Tools of the Trade

A professional usually carries all the tools necessary, but just in case you want to do everything on your own, you might want to have the following list of tools checked:

  • Small screwdrivers: flathead and Philips head
  • Wire stripper
  • Needle-nose pliers
  • A drilling machine with the appropriately sized bit
  • Wall anchors provided (buy some if they were not provided with the thermostat)
  • Small level
  • A pencil for marking the spots

This May Not Work in Some Situations

What you just read should come in handy while installing most thermostats available in the market, but it may not be applicable with some of the models. The good news is that this is pretty much the industry standard, so you are probably covered. In any case, it’s probably a good idea to go through the manual or instructions provided (if any) first.

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