Thermostat Wiring [In-depth Guide]

The modern era has made life easier for mankind. Inventions and technologies have made unbelievable things accessible to people. The thermostat is one of those inventions that bring the temperature regulating tasks at your fingertips.
When technologies ease life, they have got science in them which requires proper check and maintenance along with installation under supervision. Thermostats have got their power supply system as the latest technology has brought smart thermostats which require an exclusive and continuous power supply.
Before start wiring any thermostat, the installing person should be aware of the type of thermostat to ensure whether installing and wiring it without a technician’s services is a wise idea or not. E.g. if you are wiring an HVAC system or an air conditioner, wiring system would be different from the wiring of the thermostat of a Heat Pump system. For wiring a thermostat, a person should also have a doubt-free mind which will restrict you to have second thoughts during the process. Start the process only if you do not have any doubts and confusion related to installation.
Thermostats for different systems have different wirings and understanding those wirings is necessary before installing the device. Here are some common types of wirings that might be available with your system.

5-wire thermostat wiring:

This type of wiring is usually found in air conditioning systems or gas furnaces. It has 5 different color wires, each for a different terminal and function.

Red wire for 24-volt hot
White wire for heat
Yellow wire for cooling
Green wire for the fan
Blue wire is the common wire (common wire can be of any other color)

4-wire thermostat wiring:

This type of wiring is commonly found in battery operating thermostats or digital thermostats with only heating systems. These thermostats have 4 wires described as below:

Red wire for 24-volt hot
White wire for heat
Green wire for the fan
Blue wire is the common wire (common wire can be of any other color)

Cooling wire of yellow color as in 5-wire heating system is not found here because this wiring is for heat only thermostat which does not have dual temperature controls.

3-wire thermostat wiring:

This type of wiring is for thermostats that controls a boiler. Since boiler does not need a cooling function or the fan function, these two wires are not found here leaving three wires mentioned as below:

Red wire for 24-volt hot
White wire for heat
Blue wire is the common wire (common wire can be of any other color)

2-wire thermostat wiring:

This type of wiring will be found in heat only thermostats that do not need a cooling wire or a fan wire. Only two wires are found described as below:

Red wire for 24-volt hot
White wire for heat

Sometimes, thermostats may have more than 5 wires, this means that your device has some more functions or control points. Before wiring the thermostat, you obviously will need a thermostat. And for that, you will be needing to make a choice of thermostat that suits best with your routine and your device system. For example, if you are a stay-at-home parent, you may not need a programmable thermostat because programmable thermostat is best for those who want to come back to their home at the desired comfort level and desired temperature.
Additionally, you should be aware of the fact that installing the thermostat is a risky job. Cases have been seen where people have wasted their time, money and of course thermostats because they were not pro to the task but at the same time, people have performed this job successfully. So, before starting with the wiring process, you should be clear with your capabilities so that you can save your expense of buying another thermostat in case of failure or incurring high long term running expenses in case of mismanagement with wirings.
Before start wiring the thermostat, the power supply should be turned off with the furnace, air handler, heat pump, air conditioner, boiler or any other device. If you are replacing a thermostat and the device is at its place, take a picture of the old wiring so that it can be used as a reference. Now start detaching the wires from the old thermostats (one at a time) and attach them with the right terminals at the new thermostat. Repeat the process till you have done this with all the wires.
Once you are done with the process and you have adjusted the settings on the thermostat, you need to turn on the circuit so the power supply will tell you whether your installation was successful or is there a need to call the thermostat’s company for the troubleshooting.
For a furnace or air handler, you have to turn off the power supply first. Then you need to look for the housing or wire bundle that is attaching the thermostat from the furnace, you will see wires here. Wires can be of various quantities and different colors. Now take a picture as a reference to see which color wire will be fitted with which terminal. Now since the furnace is uncovered, use the photo you have taken to wire the thermostat. Once you are done with all the wires and all the terminals, turn the circuit on, adjust the thermostat for settings of heating or cooling. If the device is working properly, you can cover the furnace. If it isn’t, this means that there is a need to call the expert of HVAC.

Thermostat wiring diagrams for HVACs (Heating, ventilation and air conditioners)

Different installation processes and methods are followed for different types of HVAC systems. Installing thermostats for air conditioners are way different from installing thermostats for heat pumps. So before starting with the installation process, you should be clear with the type of HVAC you are using.
Secondly, on the preparation stage, you must have all the necessary tools that are required while wiring a thermostat. Requirements of tools again vary on the type of thermostat and type of HVAC under consideration but some of the common tools you will require are listed below:

A flathead screwdriver
A small screwdriver
Some wire pliers
Wall anchors (if not provided)
A drill (if you are using your wall anchors)
Wire nuts
Plenty of bright light

Wiring procedure:

Before starting the installation process for an air conditioner’s thermostat. The power supply should be cut down to ensure that no electricity is going to the thermostat. If you have broken the circuit, but still are unsure about power cut down, turn the thermostat on to confirm that electricity is not reaching the thermostat. This is a safety measure that should be followed plus it is a protective measure for the thermostats because not killing the power supply can result in blowing thermostats while installation.
Once power cut off is assured, uncover the device. Now everything should be noted down for referencing when you install the new one. Settings on the thermostats would be different from the digital and non-digital thermostats. You can always take a picture for having a note of the existing settings. If the non-digital or mechanical thermostat is being replaced with a digital one, you must know that the new thermostat will adjust the settings automatically. For having a better know-how about this, you should check the installation guide provided.
Once you have noted the wiring, unscrew the thermostat from its base that is called a wall plate or backplate. Again you should be aware of the wiring that may match the following list:

Red wire should be attached to the thermostat RH or thermostat RC terminal. It can from the R terminal. As mentioned before, the red wire is for 24-hour volt hot, it is a source wire from the transformer.
The green wire should be coming from terminal G, as mentioned earlier, green wire is for controlling the fan.
Yellow wire should be attached to terminal Y and this wire controls cooling of the air conditioner.
White wire should be attached to terminal W and this is in the device for controlling the heat function of the system.

Now remove all the wires from the terminal and if you find that the attachment of wire with alphabetical and coloring order is not there and is somewhat different. You should mark the wires and label them to ease yourself further. The next step is to unscrew the sub-base that is the wall plate. But hold on! What if removing the wall plate remove the wires? Are you sure that the wires will not fall? Are you sure that the length of wires is enough to hold them on? You need to answer these questions. Try to pull the wires to see whether the length is available enough that they can be pulled further. If yes do so. If no, just hold the wires and unscrew the plate carefully and then tie the wires to save them from falling back.
You can now screw the new wall plate of the thermostat. Check for the size of the thermostat and compare it with the older one to see if you’ll need any change in the design of paint of the wall or wallpaper. Do the mandatory touch up before screwing the new plate and fix the wall plate at the level. Maintaining the level is important as it impacts the temperature. You can use any ruler or leveling tool for your assistance.
You have to fix the wall anchors now by drilling the mounting holes. If the kit has bits with the anchors, their use is recommended. If inserting the bit is not possible with hand, you may use a screwdriver for this purpose. After that, untie the wires of the thermostats and pass them through the new base. Screw all the hole of the base but not tightly. Make sure the level and then tighten them with a screwdriver.
With the help of the color codes mentioned above, you can start fixing them with the terminal. If the color codes vary with the above lit, you must have labeled them. In that case, start following labels and attach wires. Attaching the wires tightly is important and also making sure that wire is not touching anything other than the terminal is also necessary. This is the toughest part of installing a thermostat. If wires are properly attached, you are almost finished with the task.
Place the thermostat to the wall plate. Some thermostats have built-in screws while others snap in the position with the grip of the base. Tighten the screws if provided or else put in in the grip of the base. Set the thermostat with the old settings. If you have replaced a mechanical thermostat with a digital one, you do not have to worry about the settings, thermostat will set automatically.
Cover the thermostat from the front, restore the power supply and start checking the functions of the heating and air conditioning. Then switch off these two functions to check the fan-only function. If all the functions are running properly, your installation is successful.
As a responsible citizen, a person should not feel happy when a new thermostat has been installed successfully by himself but does remember his civil duty to dispose of the old thermostat properly as thermostats contain mercury which is hazardous for human health and environment. State provides the platforms to dispose of such materials so the person disposing it off should look for those sites.

Thermostat’s wire colors and codes:

Terminal	Terminals function	Colors of wire
Terminal R	Terminal R has the red wire that comes along the transformer and the transformer is found in the air handler unit of split air conditioning systems but it might be present in the condensing unit.	Color of wire for this terminal is red usually but this may be varied if the thermostat is of the latest manufacturing design and avoid conventional color coding.
Terminal RC	If your HVAC system has two transformers for heating and cooling, then the RC transformer will be responsible for the power for cooling.	Red color red wire is attached to this terminal usually but this may be varied if the thermostat is of the latest manufacturing design and avoid conventional color coding.
Terminal RH	If your HVAC system has two transformers for heating and cooling, then the RH transformer will be responsible for the power for heating.	Red color red wire is attached to this terminal usually but this may be varied if the thermostat is of the latest manufacturing design and avoid conventional color coding.
Terminal Y	This terminal in air conditioning systems is responsible for cooling function and goes to the compressor. Wire for this terminal is made for air handlers in the split system and its spliced part goes to the condensing unit.	Yellow color wire is attached to this terminal usually but this may be varied if the thermostat is of the latest manufacturing design and avoid conventional color coding.
Terminal Y2	This terminal I found in those air conditioning units that have two staged cooling. If the system has a two-staged compressor, this terminal would be required for the additional compressor.	Usually, a light blue color wire is designated to this terminal but as mentioned before, choice of color of wires is the manufacturer’s decision, it may be of a different color. Before installation, this should be studied from the user manual and installation guide.
Terminal W	This terminal is designated for heating. So all heating units like a heat pump, oil furnace, gas furnace, boiler or electric furnace, have this terminal which is straightly wired for heating.	A white colored wire is usually seen with this terminal but this may be varied if the thermostat is of the latest manufacturing design and avoid conventional color coding.
Terminal W2	If your gas furnace has dual heating options like low heat and high heat, you probably have two-stage heating systems that will require an additional terminal W2 for the additional heat option.	Brown colored wire is usually seen for terminal W2 but as mentioned before, choice of color of wires is the manufacturer’s decision, it may be of a different color. Before installation, this should be studied from the user manual and installation guide.
Terminal G	This terminal is designated with fan function which is usually in the air handler unit of the split systems or it can be used to energize the indoor blower fan.	Green colored wire is associated with terminal G but this may be varied if the thermostat is of the latest manufacturing design and avoid conventional color coding.
Terminal C	This is the common wire that is necessary for the system to complete a 24-volt power circuit of the thermostats is electricity operating.	This wire is usually seen in black or blue color but no universal or standard color has been fixed for this terminal. As mentioned before, the choice of color of wires is the manufacturer’s decision, it may be of a different color. Before installation, this should be studied from the user manual and installation guide.
Terminal E	This terminal has an emergency function. Thermostats have a sense of emergency which alerts when additional heat is required in case of failure of heat pump condenser. Additional heat is supplied through the backup heat source that many heat pumps have. This terminal is not found in every thermostat.	This terminal does not have any universal color. Its color is the choice of the manufacturer. Even having this terminal or not is also the choice of the manufacturer.
Terminal X or Aux	This terminal also uses a back-up heat source for the supply of supplement heat.	This terminal does not have any universal color. Its color is the choice of the manufacturer.
Terminal O or B	Terminal O has the function of reverse valve for cooling while terminal B has the function of a reverse valve of heating. For heat pumps, where cooling is the normal position, a reversing valve is used to bring power back down. Terminal O and B cannot be used together.	For terminal B, dark blue colored wire is present while it is also there for the common wire in some thermostats. For terminal O, an orange-colored wire is designated.

Some thermostats come with extra wires that might not have any terminal to be attached to. The installation process never demands to cut them off, instead, hold them with the thermostat so that anytime in the future, if any of the wire is found to be bad or non-working, this spare wire can be used. At that point in time, inserting an additional wire would not be needed.

Smart thermostats:

It is a commonly known fact that thermostats these days are getting more and more technology-oriented and it has been proved by the launch of Bluetooth thermostats and Wi-Fi thermostats that require a smartphone for operating the system. These are attractive for the users because they and energy-saving thermostats as they can be operated for the times you need a comfortable environment and desired temperature at the home.
These devices have different designs but wiring these thermostats is not much different from a non-smart or conventional thermostat. Wiring for these thermostats will be the same till the step where Wi-Fi will need to be connected with the thermostat. This step will be different so instructions from the installation guide should be followed to connect your thermostat with the Wi-Fi.

Some useful tips for placement of thermostat before wiring:

Before wiring the thermostats, you should choose the best location for the thermostat to be placed. Thermostats should always be placed on an interior wall as the exterior wall have an impact of extreme temperatures that vary for the prevailing and desired temperatures of the room inside. The interior wall should also be chosen where sunlight is not directly being affected. The interior wall should not contain any drafty doors and windows nor should the thermostats be facing bathroom in such a way that direct steam of heat affects the device. If you think that you are not capable enough of installing or wiring the thermostat yourself, take an expert’s service.

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