Home Thermostat Wiring How to Do It Right: Wiring Guide

Smart thermostats are becoming popular day in day out. They aid in controlling the temperatures (heating and cooling systems) in your house in accordance to your preference. Further, they easily connect to your home’s Wi-Fi network to enable you to control it remotely from your smartphone or tablet. But how do you get them to work?

Thermostat wiring is a common phenomenon for thermostats. This is because without wiring, your thermostats may fail to work properly. And it starts with identifying the type of wiring your thermostat needs or one that is in existence already. After identification, the next thing is installation and this is very easy.

You will hardly need a professional to handle this. Thus, this article explains how to wire home thermostats in the right way.

What is Thermostat Wiring?What is Thermostat Wiring?

Before the evolution of smart thermostats, there was no need for wiring. All you had to do was switch the thermostats on and off because there was no constant supply of power. Nowadays, thermostats need their own stable power source because of their Wi-Fi compatibility and LCD displays.

There exist various types of thermostat wiring that you can choose from; millivolt, 110/240v or 24v. These are the common ones, 24v systems being the most commonly used. In the case of the 24v, there is the C Wire, otherwise known as common wire that makes it possible for power to continuously flow to the thermostat.

Why C Wire?Why C Wire

Generally, C wires supply constant power to your thermostat thereby ensuring that you enjoy maximum comfort. It is important to note that not all thermostats require a C wire in order to function but most of them do. In its place, the thermostats can get power from any of the other wires, most likely the red one. However, the R wire does not supply continuous power. This is why the C wire comes in to complete the circuit.

How will you know that your thermostat needs to use a C wire?How will you know that your thermostat needs to use a C wire

Simple! You can check the manual that comes with your thermostat for such information. In addition to this, you can check if the wiring system in your thermostat has labels. The labels have different wires connecting to them. If you see a label ‘C’ and there is a wire that connects to it, then this is the C wire on your thermostat.

In case you do not see this wire, do not stop looking yet. It may be somewhere tucked into the wall because it is not in use at the moment. The next place to check is the furnace. First, turn off the power supply to your furnace and open it up.

Look for the wire labels inside the furnace and locate “C”. If you find it, then you are lucky. If not, there are ways to sort this out.

What to do when you cannot find the “C” wireWhat to do when you cannot find the “C” wire

  1. As long as your thermostat and furnace have a C terminal, you can get a new wire and run it between the new thermostat and your furnace.
  2. Get a brand new 18/5 wire and run it entirely from your furnace to the new thermostat. Nonetheless, you need to have an already existing “C” terminal on your HVAC system. This is slightly easier than option 1.
  3. Buy a C wire adapter only if there are at least four wires on your HVAC system and your thermostat. This adapter will allow the four wires to coordinate with a thermostat that has to work with a C wire. Try the Venstar Add- A- Wire kit for a start. It is not only easy to use but also budget friendly.
  4. Find a smart thermostat that comes with a power extender kit which makes it possible for it to operate without a C wire. This will only work if your HVAC has the following;
  5. 3 wires comprising Y (Y1), G or R (RC/ Rh).
  6. 4 wires comprising W (W1), Y (Y1), G or R (Rh/ RC).

Interpretation of colors in a Thermostat wiring systemInterpretation of colors in a Thermostat wiring system

When you open up a thermostat or your HVAC unit you notice that there are different color labels and wires as well. It is important to understand what each of these colors mean to enable you to carry out wiring in the right manner. We have already discussed one of the wires (C) which is usually blue in color.

a) White wire

This wire connects to the heating system in your home and attaches to terminal W.

b) Yellow wire

It connects to the air compressor and therefore controls the whole air conditioning system. It attaches to the Y terminals of your thermostat.

c) Orange wire

If you own a heat pump, then this wire will connect to it. It helps to cool the reverse valve also.

d) Green wire

This one connects to your furnace’s fan or air handler. It attaches to terminal G on the thermostat.

e) Rc or red wires

They deliver 24v from your transformer all the time. They connect to the RC terminal on the thermostat. RC wires only deal with air conditioning systems.

f) Rh wire

This wire connects to your heating system and not the cooling system. The label may either be ‘R’ alone without ‘H’ or just Rh. This depends on if your setup is that of a dual transformer. Normally, it connects to the Rh terminal on the thermostat.


These terminals are on the right side of the thermostat and it is very rare for one to use them for wiring a thermostat.

Home Thermostat WiringHome Thermostat Wiring

Before you start wiring, you should know that there are different wire variations that you can use for thermostat wiring.

  • Two wires

When you see two wires, red and white, just know that it is a digital thermostat. It only delivers heat and there is no air conditioning.

  • Three wires

A digital thermostat that attaches to a boiler usually has a three wire setup which includes; white wire, 24v hot and 24v common.

  • Four wires

This occurs in digital thermostats or battery operated ones that only control heat. They are; green, 24v common which is blue, white and 24v hot which is red.

  • Five wires

Of all the wire variations, this is the most common. If your thermostat controls air conditioning and heat, then it will have this kind of wire setup. The wires include; yellow, green, and 24v hot which is red, white and common wire which is blue.

Steps to follow when wiring a Home ThermostatSteps to follow when wiring a Home Thermostat

1) Cut off power supply

Before you do any wiring, you must ensure that the power supply to your thermostat is off. This is to prevent any electricity hazards. Locate where the circuit breaker is and turn it off. Also, it is important to put on safety gear that will protect you against any dangers.

2) Remove old thermostat

If you want to replace your old thermostat with a smart one, you will need to remove it off the wall first to give room for the new one. Use a screwdriver to loosen the screws that hold the thermostat to the wall plate.

3) Take pictures or label the wires

After removing the old thermostat, you will see wires of different colors. For the purposes of correct wiring, you need to mark the previous wire connection to your old thermostat. Use a pencil and tape to mark the wires according to the labels or you can even take pictures of the whole connection for later referral, this is easier.

At this point, you should ensure that the wires do not fall back into the wall. Do this by bundling them together using a tape or roll the wires around a pencil.

4) Mount new plate

Get your thermostat’s plate and drill it onto the wall making sure that it is straight by using a level. Ensure that the holes in the wall match with that of the wall plate.

5) Connect wires

Now you are ready to connect the wires from the wall to your new thermostat. You can now refer to the photographs or the marked labels and wire them appropriately. The terminals should match appropriately and this is what indicates that you are doing it right.

The easiest method is reconnecting via the wire by wire method. If you do not want to use the pictures, you can follow the color codes and do the reconnection. Each color code has a meaning and we highlight this in the article.

6) Put back thermostat to the wall plate

After wiring, ensure that you carefully push back the wires into the wall and put back the thermostat onto the wall plate. Tighten the screws. Turn back on the power supply and check to see if your thermostat will work.


Wiring a thermostat is a simple DIY process only if you are sure about the terminals or the color codes. Not all thermostats need a C wire to operate but if you are planning to upgrade to a smart thermostat, there may be a need for the wire. We hope that this article comes in handy for you as you carry out your home thermostat wiring.

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