When your HVAC system is not working and it seems as if it is a thermostat-related problem, the issue can sometimes be resolved by checking and repairing the wiring and connections at the back of your thermostat. However, when you took your thermostat off the wall to try to fix a problem, you found that a red wire had become disconnected. Now you have two questions. Firstly, you wonder whether a thermostat has to have a connected red wire, and secondly, if so, what is the red wire for.

Every thermostat needs a red wire to function because the red wire’s main function is to provide 24-hour volt AC power from the transformer to the HVAC system, including the thermostat. Without a connected red wire the HVAC system cannot work. Thus, the red wire must be connected to your thermostat as it is literally the “lifeline” of your HVAC system.  

In this article, I briefly discuss the function and characteristics of red wires and for your convenience, also give a summary of the other wires you might find at the back of your thermostat.

Main function and characteristic of red wires

All thermostats have a red wire. The color red indicates that the wire is providing volts to the system. Without a connection to the red wire, your thermostat will not have a power source. There are different types of red wires that a thermostat could have, but in principle, a red wire is the power source for your thermostat. 

Usually, black and red wires and white wires with black or red tape indicate a “hot” wire. The term “hot” means that the wire is carrying a live current from the electric panel to its destination, including your thermostat. In thermostats, red wires – especially Rc wires – are carrying volt AC power from the transformer. It carries power 24/7 unless the power source has been shut off.

More about red Rc and Rh wires

Your thermostat will have a red Rc (“Red Cool”) wire if it is dedicated to air conditioning only. If your thermostat controls heating and cooling systems, it will have a red Rc and a red Rh (“Red Heating”) wire. But remember, although both wires are red, they are not interchangeable.

The Rc wire connects to the cooling system and the RC terminal on the thermostat. The Rh wire connects to the RH terminal on your thermostat and it is the connection to your heating system.

Some thermostats don’t have an RH wire, but only an R wire. An R wire is similar to either the RC or the RH wire. If your system doesn’t have an RH wire, the R wire will be controlling the heating system.

Although the term ‘red wire’ is usually used to indicate that the wire is a live electrical wire, it not necessarily means that working with the live wire is dangerous. In your HVAC system, including the thermostat, the wire only utilizes 24 volts. This is not enough volts to shock you, but it is always a good policy to cut all power supply to your HVAC system and thermostat when you want to work with the thermostat wires, especially the red wire.

Other wires connected to a thermostat

One can’t actually talk about red wires in a thermostat if you don’t know something about the other wires as well. Basic knowledge of the functions of thermostat wires and how they should be connected can prevent an uncomfortable cold night if a wiring problem occurs when no technician is available and you have to rectify the problem yourself.

For your convenience, I’ve summarized the main functions and connections of the different colored wires (including the red wires). Remember, although every thermostat has at least one red wire, all thermostats do not necessarily use all the wires I’m going to mention.

White wire

The white wire underneath your thermostat is the connection to the HVAC heating system. It connects to the “W” terminal on the thermostat and runs up to the air handler or furnace.

Yellow wire

The yellow wires connect to the “Y” terminals on your thermostat and connect the thermostat to the compressor. The yellow wires control the HVAC cooling system.

Green wire

The green wire which is connected to the G terminal on your thermostat is connecting the fan of the furnace or air handler.

Orange wire

The link to your heat pump, if your system has one, is made with the orange wire. The wire is connected to the “O” terminal of the thermostat and terminates in the outdoor condenser where it is used to reverse valve operation from hot to cold.

Red Rc wire

Without red wires, specifically Rc wires, the thermostat, and HVAC system cannot function. The red wires are responsible for 24-hour volt AC power from your system’s transformer. Rc wires are only for air conditioning systems or dual transformer systems and connect to the thermostat’s R or Rc terminals.

Red Rh wire

Another wire also responsible for 24-hour volt power from the transformer is the red Rh wire. The Rh wire connects to your heating system and connects to the RH terminal on your thermostat.

Blue wire

The blue wire is the Common wire (C-wire) and is necessary for any thermostat that needs to be connected to a power source 24/7.


Q1: Are the red Rc and red Rh wires running to your thermostat interchangeable?

A1: If your thermostat only controls air conditioning, it will have a red Rc wire. For separate heating and cooling systems, it will have a red Rc and a red Rh wire, but although both are red, they are not interchangeable.

Q2: Is it dangerous to touch a thermostat’s red wire?

A2: Although it is red and carrying power, you will not be shocked if you touch the wire because it brings only 24 volts of power from the transformer. It is, however, a good policy to cut all power to the thermostat and HVAC system before working on any electrical wiring.


It can be said that the red wire is the most important wire in your HVAC system and thermostat. If other wires are not connected or have become disconnected over time some functions can be interrupted while other functions might still be executed. But when the red wire has become unconnected the whole system stops working because no power is coming to the thermostat and HVAC system.


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