When you have to replace the thermostat in your home and you don’t have HVAC or electrical experience, you might be afraid to do the wiring yourself. You don’t know which wires carry power and whether you might receive an electric shock. You are also most probably worried that you can cause damage if you wire the thermostat incorrectly. 

Thermostats have hot wires, in other words, wires carrying power. But virtually all home thermostats operate on a low voltage of about 24 volts and there is little risk of electric shock. If you connect the wires incorrectly, there is a chance that damage could be done, but if you connect the wires according to the manufacturer’s instructions, it very seldom happens that the wires are connected incorrectly. 

In this article, we’ll briefly discuss some of the important aspects of the wiring of a thermostat. We hope this will help to give you the self-insurance to wire your new thermostat on your own.  


Home thermostats generally receive their low 24V AC electrical power from a low voltage transformer. Thus, although the wires carry power it is low voltage power. 

The wires providing the power are often called “hot” wires and are providing a source of 24V power that is coming from the HVAC control board. They are labeled Rc (for cooling) and Rh (for heating). 

Little risk of electric shock

As home thermostats normally operate on low voltage there is little risk of harmful electric shock. The only exception is if your system uses a line voltage thermostat. Line voltage thermostats work on 120V and 240V power and the risk of an electric shock when touching the live wires could be great.   

However, it is always good practice to shut off the electrical power to a thermostat – even if it is operating on low voltage – before opening it or working on it. 

C wire

Many people think the C wire is a wire that powers the thermostat directly. That is not entirely correct. To have a circuit that power can flow through, wires need to complete the loop and connect back to the HVAC control board. 

The C wire is an extra wire providing such a return path. It ensures that the thermostat can be powered without disrupting the other wires which are used as electrical on/off switches for the HVAC devices.

What happens if you wire a thermostat incorrectly?

Potentially, incorrect wiring could cause electric shock, a blown circuit breaker, a damaged thermostat unit, a damaged electrical system or even a blown air conditioning or furnace unit.

Normally, it doesn’t take much to install and wire a thermostat, but it only takes one incorrectly connected wire to cause the HVAC system not to work properly. If the thermostat and HVAC system are not working effectively and efficiently, you could end up with high utility bills and/or an uncomfortable temperature in your home. The incorrect installation of the wires can even cause the whole system not to work at all.

It can also cost you money even when the system seemingly runs perfectly. If, for instance, you wire a 2 wire thermostat incorrectly, it can cause the heat to run continuously.

If you still have an older type of thermostat, incorrect wiring would most probably not cause too much damage, if any. But a more modern system that uses, for instance, a serial link to a digital control system, could more easily be damaged with a wrong connection.

Different types of thermostat wiring

For your convenience, we’ll briefly explain what type of wiring is normally used for different types of thermostats and indicate the hot wires. This will help you to check your thermostat to see whether it is correctly wired and assist you to wire a new thermostat correctly. When you find the wiring different from our guidelines, always first check the manufacturer’s instructions before doing anything. 

Digital thermostat solely providing heat – two wires

Generally, if there are only two wires connected, you have a digital thermostat that solely provides heat without air conditioning. Typically, the wires are red and white.

Digital thermostat for a boiler – three wires

If your thermostat needs three wires it is most probably a digital thermostat connected to a boiler. There is a 24-volt hot wire, a 24-volt C wire, and a white wire.

Battery-powered thermostat – four wires

This configuration is for a battery-powered thermostat but can also be for a digital thermostat that solely regulates heat. It consists of a 24-volt hot wire (red), 24-volt C wire (blue), heat wire (white), and a fan wire (green).

 System regulating both heat and air conditioning – five wires           

This is currently the popular thermostat wiring style. It is used for systems regulating both heat and air conditioning. The wires are typically red for 24-volt hot, white for heat, yellow for cooling, green for the fan, and blue for the C wire


Now that you know that although thermostat wires are hot the chance of an electric shock is small and that by following the manufacturer’s instructions you can easily wire the thermostat correctly, you should be able to wire your new thermostat all by yourself. 


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