As most thermostats that are controlling household appliances such as air-conditioning and heating devices and furnaces need low-voltage power, there should be one or more transformers installed in your house or HVAC system. A transformer is needed to transform the high voltage power available in your house to the required low voltage power for the thermostat. 

But, unfortunately, as with most electrical components and appliances, one of your transformers might just stop working. Generally, you can’t repair a transformer cost-effectively and is it better to purchase a new one.  Then you start wondering whether you would be able to wire the new transformer to the thermostat yourself, or should you call an electrician to come and do the wiring for you. 

It is relatively easy to wire a 24 VAC transformer to a thermostat, even if you are not a professional electrician. In principle, you have to connect the wires correctly and the transformer will supply the low voltage power to your thermostat. You can do the whole installation and wiring in 5 easy steps – even if you are only a DIYer. 

In this article, we’ll look at the steps to connect the wires correctly, and then also discuss a few frequently asked questions in this regard.  

How to wire the 24 VAC transformer to a thermostat

It is rather easy to wire a 24 VAC transformer to a thermostat. Just follow the steps we’re giving below and you’ll be able to install and wire the transformer correctly. You’ll find it easy, even if you are not a professional electrician but only a DIY enthusiast. 

We’ll describe the whole wiring procedure in 5 easy-to-follow steps. 

Step 1 – Identify the wires

Find the different wires that are connected to the 24 Volt Alternating Current (VAC) transformer. Usually, there will be four wires. Two of the wires will normally be black while the other two can be green, yellow or blue. You will later connect the black wires to the 120 VAC input, while the yellow, green or blue wires will be the 24 VAC output wires.

Step 2 – Cut the power to the applicable device

Ensure that no electrical power is going to the device the transformer is meant for.  Don’t just flip the switch on the device or at the wall outlet, but put off the circuit breaker servicing the device. This will ensure that the electrical power is definitely cut when you work on the device. 

Step 3 – Mount the transformer

Your next step is to mount the transformer onto a firm surface. Use the screws provided for this purpose. Sometimes you’ll find that the transformer can be mounted inside the mechanism they are providing the power for. 

Step 4 – Connect the wires

  • Use a wire stripper and strip 3/4 inch of the outer insulation from all four wires of the transformer. You must be able to see the bare copper wire under the insulation.
  • Locate the two 120 VAC power supply wires. One supply wire will be black. That is the voltage wire. The other wire will be white. This is the electrical system’s neutral or common wire (C-wire).
  • Attach the two stripped black wires from the transformer to the 120 VAC power supply wires. You can do this in any order as long as the two black wires of the transformer are attached to the black and white wires of the 120 VAC power supply wires. 
  • Attach the low-voltage side of the transformer (the two wires that are yellow, green or blue) to the thermostat wiring. Remember to check the manufacturer’s wiring specifications as they can differ from brand to brand and even from model to model.

Step 5 – Final step

With your pliers, twist together the bare ends of the wires you’ve connected. Then cover the connections with a plastic wire nut. 

Your transformer is now connected to the thermostat and you can turn on the electrical power to the device.

What is a 24 VAC transformer and what is it used for?

Although you’ve now learned how to wire a 24 VAC transformer to your thermostat, you might still wonder what exactly the use of such a transformer is. 

Have you ever wondered where the low voltage power for the thermostats in your house is coming from? You’ve most probably discovered that most room thermostats on heating and cooling systems use a low voltage input, while your house normally supplies higher voltage power to your appliances. 

The only way to transform the high voltage power available in your house to workable low voltage power for the thermostat is via a 24 VAC transformer.

Step-down and Step-up transformers

This is why a 24VAC (volt alternating current) transformer is sometimes described as a “step-down” type of transformer. The transformer converts 120 VAC to a lower voltage for use in low voltage devices like thermostats. The lower voltage is then sent from the transformer to the thermostat.

Interesting to know that you get “step-up” transformers as well. A step-up transformer converts alternating current from one voltage to a higher voltage. An example of such a transformer is what utility companies use to increase voltages for transfer from the power station to the distribution grid.

Both the step-down and the step-up types of transformers work on the magnetic induction principle.


Q1: What is the common side of a 24 VAC transformer?

A1: There are two sides to a 24 VAC transformer, namely a high side and a secondary, or low-voltage side. The high side accommodates the electrical connection to the feeding voltage, generally a 120 VAC power input from your house. 

The secondary or low-voltage side is the outlet side from where the power that has been transformed to 24 VAC is distributed.

Q2: Can I connect a C-wire to a transformer?

A2: You might have a heat-only system that is connected to your thermostat, and then an easy option to add a common wire (C-wire) is to install a 24 VAC wall plug-in transformer. You just have to insert one wire into the unused “R” terminal on the thermostat, and the other wire into the “C” terminal.

Q3: Are there transformers for direct current (DC) as well?

A3: Transformers are only available with alternating current (AC). You cannot transform DC voltage to a higher or lower voltage. This is because a transformer works on the principle of magnetic flux and magnetic flux is only possible when alternating current (AC) is flowing. 


As almost all thermostats that are used with household appliances, including air-conditioning devices, heating apparatus and furnaces, are low voltage devices, you need a step-down transformer to let them function. It is relatively easy to wire the low voltage transformer to the thermostat, even if you are not a professional.     

We hope that the 5 easy-to-follow wiring steps we’ve shared with you in this article will assist you in wiring your new transformer to the applicable thermostat.

We also believe that the background information regarding transformers, in general, we’ve provided will clarify some of your questions in this regard.

Fortunately, there are excellent how-to videos available on the topic of how to wire your 24VAC transformer to your thermostat.


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