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Air conditioner thermostat wire colors

What to Know About Thermostat Wire Color Codes

One of the most essential items in your home is the thermostat. The ability to control the temperature is what makes your house habitable, even on the warmest summer days and the frigid winter days.

According to the EIAs Residential Energy Consumption Survey, 12% of households in the United States have a centrally controlled air-conditioning unit. 33 million North American houses rely on smart thermostats for temperature control.

Smart heating and cooling systems allow users to save energy through monitoring and effectively controlling usage. They also allow us to restrict the use and reduce the need for human engagement, making energy conservation easier. Smart home market research shows that smart heating and cooling systems save users 50% of their energy consumption.

With the development of smart homes, an increasing number of Americans are gaining control over their home’s HVAC system. Understanding how to wire a thermostat is essential knowledge for every homeowner. If your thermostat fails, you may be able to repair it yourself to save time and money. If you would prefer to use a professional, then this information will help familiarize you with the wiring process.



Wiring a thermostat is basically connecting the correct-colored wires to the correct terminal. Understanding what each wire is for is critical to complete the procedure successfully. If you are unsure of any of the thermostat wiring, it is safer to contact a qualified professional.


 Always check the manufacturer’s instructions before doing any wiring. The wiring for thermostats can be configured in a variety of ways. The most popular ones, which range from 2 to 5 wires are covered below. If you have more than 5 wires, you have more control points or you have a heat pump.

TWO WIRES           

If you have two wires, you almost certainly have a digital thermostat that solely provides heat (i.e. without air conditioning). Typically, the two wires are red and white (see above for information on where they are connected).


This is the most common configuration for a digital thermostat that connects to (and controls) a boiler. There is a 24-volt hot wire, a 24-volt common wire, and a white wire.

 FOUR WIRES         

This is a popular configuration for a battery-powered thermostat or a digital thermostat that solely regulates heat. The standard configuration consists of 24-volt hot (red), 24-volt common (blue), heat (white), and a fan (green).

 FIVE WIRES           

This is the most typical thermostat wiring style, and it applies to systems that regulate both heat and air conditioning. The wires are typically arranged as follows: 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 a different color).



You can also refer to the Conductor Color Chart (see Chart 6, thermostat wire).


Syston Cable offers Thermostat Cables in 18 AWG & 20 AWG from 2 to 10 conductors configurations in UV Sunlight Resistant Riser-rated and Premium Plenum-rated cables.

Visit our webpage for all the Thermostat Cable we have to offer: https://www.systoncable.com/thermostat-hvac-cables/





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The Thermostat Wire Color Code You Need to Know

Whether you’re trying to fix your thermostat, or installing a new smart thermostat, this thermostat wire color code will help you to avoid DIY wiring problems.  However, please note that this is a list of common wire colors used and your thermostat may contain different colored wires. If you have any doubts, get a professional thermostat installation.  

Below are the common thermostat wire colors:  

  • White wires 

  • Yellow wires 

  • Green wires 

The White Wire

The white wire underneath your thermostat connects to your heating system. It terminates at your air handler or furnace. The white wire connects to terminal W in industry-standard thermostats. If your HVAC system has multiple heating stages, then you may find more than one white wire.

Yellow Wires

The yellow wires underneath your thermostat connect to your compressor. They control your air conditioning system. The yellow wires terminate at your compressor contactor via an air handler. The yellow wires connect to the Y terminals on your thermostat.

The Green Wire

The green wire underneath your thermostat connects to the fan of your furnace or air handler. It terminates at the air handler or furnace. The green wire connects to terminal G on your thermostat.

The Orange Wire

The orange thermostat wire links to your heat pump, if you have one. It terminates in your outdoor condenser for reversing valve operation from hot to cold. The orange wire connects to terminal O on your thermostat.

The orange wire only applies to homeowners with an air-source heat pump. Only air-source heat pumps connect to your outdoor condenser. Homeowners with geothermal heat pumps have no use for the orange wire.

Red Wires

Red wires indicate power. You may see an Rc and/or an Rh wire.  

The Rc Wire 

 Rc wires are for air conditioning systems or dual transformer systems. Dual transformer systems refer to a setup with a cooling and heating transformer. Rc wires connect to the RC terminals on your thermostat.

The Rh Wire

The Rh wire connects to your heating system as opposed to your cooling system. This wire may be red without an H attached to it, depending on if you have a dual transformer setup. The Rh wire connects to the RH terminal on your thermostat.

The Blue or C Wire

Blue wires are also called C wires because they are the Common wire. C wires are necessary for any smart thermostat that needs to be connected to a power source 24/7, regardless of your heat pump type. (It’s important to determine that your heat pump is compatible with your thermostat before you decide.) 

C wires or varying colors apply to every thermostat, but blue C wires belong to thermostats attached to a heat pump. Blue wires are for heat pumps. This wire connects to terminal B on your thermostat.

Got Your Wires Crossed?

There’s a lot to keep track of when it comes to thermostat wires! Avoid DIY wiring mistakes by calling Aire Serv® to help you get it right. We assist homeowners with thermostat installation and repair. Call (855) 679-0011 or request an appointment online to get started.


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How to independently connect the refrigerator thermostat?


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Let's start with the fact that the thermostat in the refrigerator serves to turn off / turn on the refrigeration compressor. When a working refrigerator is first turned on, the thermostat contacts are closed and a command is given to turn on the compressor. You can set the temperature in the refrigerator by turning the knob - the degree of cooling varies, as a rule, from +8 degrees to 0 degrees Celsius, a lower temperature is achieved by turning the thermostat knob clockwise until it stops.

Refrigerator thermostat assembly

The thermostat mechanism is a lever system that controls electrical contacts. Externally, the thermostat is a small box with a handle, on one side of which there is a tube filled with freon, and on the other side - contacts for connecting to an electrical circuit.

The number of contacts can vary from 2 to 6, and the length of the tube filled with freon can be from 0.8 to 2.5 meters. It depends on the additional functions of the thermostat, the temperature regime and the number of connected refrigerator modules (light, defrost, indication). It is not recommended to disassemble the working thermostat to study the internal structure.


The principle of operation of the thermostat is quite simple. The end of the capillary tube of the thermostat is located in the cooling zone and is attached to the evaporator of the refrigerator. The lever mechanism of the thermostat, which is located in the box, acts on the contact group during cooling - the thermostat opens. When the temperature rises, the thermostat returns to its original position - the power contacts close.


Externally, the breakdown of the thermostat (temperature sensor) manifests itself in two ways. This may be a banal shutdown of the refrigerator compressor from the electrical circuit (the compressor does not turn on, there are no sounds, there is light in the refrigerator), or it may be a change in the temperature regime in the refrigerator compartment (freezing or high temperature).

In the first case, there is a high probability of damage to the galvanized capillary tube of the thermostat, which is subject to corrosion in the aquatic environment, as a result of which the thermostat's lever mechanism simply stops working. In the second, it is necessary to understand what specifically caused the violation of the temperature regime - corrosion, sticking of the thermal relay contacts or a violation of the internal factory settings of the sensor. The answer can only be given by a specialist - a refrigerator repairman.

Installation location

Faulty thermostat needs to be replaced. Replacing a broken thermostat yourself is quite simple if you get to the place of its installation. This is where the difficulties arise.

In modern refrigerators, the thermostat control is usually located on the front panel and is located at the top of the refrigerator, but can also be located inside. The cooling module of the refrigerator (evaporator) is hidden under the plastic casing and is located at the rear.

To install a new thermostat yourself, you must remove the broken thermostat.

  • To do this, de-energize the refrigerator by unplugging the power cord.
  • Depending on the model of the refrigerator, remove the plastic casing cover that contains the broken thermostat.
  • Mark the wiring diagram with a marker.
  • Remove the capillary tube of the broken temperature controller from the mounting (placement) place.

Install the new thermostat in reverse order.

Features of connection

Do not confuse different thermostats that look similar to each other. Some can only work at positive temperatures, others are designed only for freezers. The use of a thermostat that is not designed to operate a refrigerator (freezer) can lead to incorrect operation of the equipment and failure of expensive elements (compressor).

Therefore, be sure to check the wires connected to the thermostat. It's one thing if you find your own replacement thermostat, of the same manufacturer or brand, and another if you use an analogue.

By the way, the wires connected to the thermostat have the following purpose:

  • orange, red or black — connects the thermostat to the compressor;
  • brown - phase wire leading to the outlet;
  • white, yellow or green - leads to a light indicating that the refrigerator is on;
  • striped yellow-green - ground.

Starting from the size of the contacts (the width of the flat conductive contacts has 2 standards - 4.8 and 6.3 mm), locations, thermostats can differ in the settings of the contact groups (power or low-current) and purpose (medium temperature or freezing). For example, the use of an outwardly similar temperature sensor K57-2.5 instead of K59-2.5 will lead to frosting in the rear wall of the refrigerating chamber and a change in the temperature regime of the refrigerator.


All thermostats have the so-called operating temperature range (for example, for the RANCO K-59 thermostat it is -32/+6), which the thermostat is designed to maintain.

Thermostats on the outside of the case or inside have 2 adjustment screws responsible for the adjustable temperature range within the operating range (this is approximately 4-18 degrees) and for the response difference (usually 2-8 degrees). Be careful - a simple adjustment of one screw shifts the on/off operating temperature range. For example, the normal factory setting of the thermostat in the extreme minimum position (in the extreme position when rotating counterclockwise) is set to thresholds --- minus 10 / plus 3.5 degrees Celsius. Turning the adjusting screw responsible for the temperature range shifts these settings ---> for example, to position minus 5 / plus 8.5 degrees or minus 15 / minus 1.5 . As a result, high or low temperature in the refrigerator and spoiled food, etc. And if the second screw is additionally adjusted, then it is very difficult to restore the factory settings after intervention - often, it is necessary to replace the thermostat with a new one.

Caution. Spinning is not recommended. The threading of the adjusting screw in the thermostat may not be structurally limited (especially for Chinese counterparts), as a result, the screw may fall out of the thread during adjustment - it will not work to insert the screw back without disassembling the entire thermostat.

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How to connect a heat pump thermostat? Everything you need to know

A heat pump drives a compressor for heating and cooling. Your heating and air conditioning system is updated by connecting a heat pump to an internal heating tape. installed with an additional heating cable.

The thermostat manufacturer's algorithms indicate the optimal time and intervals for the heat pump to operate, the possible heat levels for your model, the heating rod and the speed of the internal fan. The control settings for your system can be found in the "Thermostat Configuration" menu, accessed via the switches, specific to each thermostat model. Information about the parameters can be found in the ~!phoenix_var9_1!~~!phoenix_var9_2!~ manual and heating tape.

Things you will need to connect the heat pump thermostat


Wire stripper


Plastic cable ties

Connection options for the heat pump thermostat

90 heating and cooling depends on the time of year and the thermostat setting. In addition, ~!phoenix_var38_1!~ have backup heating methods for two reasons.

When the heat pump system starts to defrost, it needs a backup heat source.

The main backup or backup heating method for heat pumps is electric band heaters. The second most common backup heat source is a gas stove. Finally, these two different methods may require a separate heat pump thermostat connection.

Labels for heat pumps

Name and connection of cable C - old cable C? This is a common thread that causes indigestion and sleepless nights for many people. The reason for this is that most modern thermostats require a C-wire to work. the 24 VC cable comes from the control transformer and the aforementioned cable R. 4-wire HVAC control transformer. One side is also for the mains voltage in the transformer, and the other side is for the 24 V control voltage coming from the other side of the transformer.

To get the complete circuit, you need source, path, and load. In this case, the transformer is the source, wire R and wire C are the paths, and the thermostat is the load.

R Cable Identification and Connection - The R cable is the 24V hot cable of the 24V control transformer. This is the way to activate the control relays and connections in your system. The thermostat is just a switch and power from the transformer comes through this device.

Wiring problem C

C-wire usually dark blue.

This means if you don't have the right amount of wires in your thermostat, you will need to connect a new thermostat wire to your new thermostat. In other words, you have the right amount of wires for your new thermostat to work properly. A heat pump thermostat requires more connections than the air conditioner thermostat. It also provides C-wire.

Other names and wires for R-wire clamps

You can't have two transformers powering one thermostat. If you only have one transformer, you probably have a jumper (or copper tape) between the right and RC terminal . If you don't have a jumper, the red wires will be disconnected. The filament transformer has a red wire that ends on the right. Also, the cooling system control transformer has a red cable that ends at the remote control.

Some people may wonder why in this situation they should use one common stream, or C-stream. A big question, especially from a newbie who started his career in the climate industry. This shows that they are afraid. This is because all neutrals are (in most cases) the same. Unless it's too expensive, I advise the buyer to always rewire the transformer. Most transformers in any system can handle the VA power required to power all controls in both systems, and wiring is easy (at least , for me).

Other control terminals and cable designations for heat pumps

Identify the cable terminal Y. The wire in this color is usually yellow and goes to the Y terminal. In the heat pump it is used for cooling. The compressor contactor is switched on in the condenser.

Wire Label O - Refers to the non-return valve in the condenser unit. The reversing valve switches the condenser from heating mode to cooling mode and vice versa, depending on the thermostat setting.

Adjust cable clamp W - this color is usually white and matches the W clamp. In the case of a heat pump, this is an additional source of heat. works with ventilation equipment/ovens and condensers. Condenser defrost control panel automatically defrosts the system. When you enter defrost mode, backup heating is turned on. In most cases, when replacing a thermostat, you do not have to worry about connecting a condenser.

Connect an additional thermostat to the heat pump thermostat.

In some cases, you have a system that needs to be deployed. Most residential multi-level systems are two-level. In this case, you need a thermostat with a W2 connection and a Y2 connection. The colors of these connectors are different, but in most cases W2 is black and Y2 is blue . Depending on the system, W2 can be used for emergency heating, a backup heat source. It is sometimes referred to as a backup heater and is used when there is a problem with the condensing unit.

In some cases, the thermostat terminal is labeled Aux/E/W2.

If your old thermostat used two separate wires and connected them separately to the Aux and E terminals, use the extra wire and wire to tie these wires together. Then connect the auxiliary cable to Aux/E/W2*.* Always follow the manufacturer's instructions connection.Manufacturers may have different connection instructions to get it right.

This is rare, but still possible. If you have a traditional air conditioner or heat pump and have an RC-RH bridge installed, you only have one adapter and you don't have to worry about two transformers in your system.

If you have problems with operation, be sure to make sure that you have the same connections from air handling to thermostat and condenser unit. Also, use our other resources (other articles) to troubleshoot thermostat problems.

Hints for connecting a heat pump thermostat

The table above will help if you have a question about which thread color goes where. Thermostat cables will come in handy. Colors like red for power and go to station R. As with other colors and stations. Depending on who finally connected the thermostat, they might not follow the color code of the wires.

Before removing the old wires, take a picture of the wires and stand on the bottom base of the old thermostat. Use your phone to take a picture for future reference.

Always make sure the power is off before attempting any wiring. If you forget to do this, the transformer may burn out or trip the LV circuit breaker. Or blow the low voltage fuse.

For a complete list of colours, nature and purpose, see Heat Pump Thermostat Wire Colors.

Your temperature may differ from what is described here. The heat pump circuit diagram above covers about 90% of the heat pump thermostat.

Note. Some thermostats may have an "Emergency Temperature" function that, when installed, turns off the heat pump. It then activates the heat from the tape, which becomes the main heating source. This function should only be used for a limited period of time, as energy costs are usually higher than heat pump systems. Used station E.

Please note the following features found on most modern programmable thermostats.