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Furnace Transformer – What It Is and How Does It Make Your Furnace Work?

Furnace Transformer – What It Is and How Does It Make Your Furnace Work?

The term “furnace transformer” may sound intimidating. It even sounds like a new transformer movie, but the fact of the matter is that this device provides a basic function for any type of furnace.

By going over some of the terms, functions and parts of the transformer, you will be able to get a closer understanding of its overall intention in a furnace.

If you are looking to work on a transformer or just educating yourself on the subject, the following information can help you with your goals, such as installing or replacing the furnace transformer.

Moreover, it will also give you a closer look at how furnaces work and function. Although you will find some differences, this will apply for both residential and commercial furnaces.

What is a Furnace Transformer?

Attached transformer in gas furnace

The basic function of the furnace transformer is to supply power to the furnace itself. This can lower the high voltage capacity to a lower voltage output.

This is required for Arc furnace operations. You will find that some furnaces need 24 volts to operate.

This is meant for the basic control of the furnace, such as timers, ignitions, circuitry and temperature controls. Since the standard voltage output for any house is 120 volts, the transformer will reduce your home’s voltage to the correct amount for use in your furnace.

The voltage on some furnaces may vary, which can include electric and gas furnaces. 

Regulating the Proper Voltage

The importance of a transformer in any electrical device is important. If you don’t regulate the proper voltage according to your circuit breaker or other electrical devices, the entire network will not work properly.


The consequences can result in a tripped breaker, burnt fuses and system shortages.

If you try to force the incorrect voltage, your furnace could be permanently damaged, which will come out costly. The same rule applies for any type of heater transformer.

Buying Your Furnace

When you’re choosing a new furnace, you will find two different types of volts.

Remember to consider the size of the furnace and where it is being installed, such as a residential or commercial structure.

A commercial building will usually require larger furnaces to accommodate the square footage. However, a residential building has a smaller living space, so a standard furnace will do the trick.

Furnaces of all types, such as electric, gas and oil furnaces, will use transformers with 120 or 240 volts. Some options may also use as little as 24 volts.

The volts capacity information can be located within the furnace’s features. If you can’t find a sticker with the information, you can find out the voltage capacity by using a voltage test meter.

Testing voltage in a furnace transformer

It is always important to install the right transformer with the correct voltage. This will give you a successful repair or installation.

Why Does Your Furnace Keep Turning Off?

The symptoms of a bad furnace can include a few things. A furnace that keeps turning off could indicate a problem with the voltage.

However, it could also be a problem with the furnace itself, such as bad controls or a loose wire.

When you’re working on troubleshooting a furnace, you will have to perform a variety of tests to figure out the problem.

Always check the transformer.

In some cases, the transformer could be the problem. However, other things could be affecting the performance of the furnace including a faulty furnace inducer blower causing it not to start up, or a bad thermocouple keeping the burners from lighting.

The first thing to look at is your building’s voltage capacity. If your transformer works with 240 volts but your building works with 120 volts, you have found the problem.

Simply replacing the transformer will resolve the issue.

How Does a Furnace Transformer Work?

It also helps to know the internal mechanics of a transformer. The furnace transformer is comprised of two systems that make up the furnace transformer wiring.

The first system is for the supply voltage and the other is for the secondary voltage. If you are having trouble with your transformer, you may have to deal with the internal wires of these two systems.

A faulty wire could be the cause of a furnace malfunction, so it is essential to properly check these wires. The standard transformers that carry 120 volts will have supply wires that are black and white.

24V furnace transformer

Typical furnace transformer with black and white writing

However, some colors may vary depending on the furnace model. Remember to consult the manufacturer’s instructions if you do have them on hand.

If you don’t, here’s what to do:

The usual setup of the wires are neutral and live wires. The color black will usually be the live wire, and the white wire should be the neutral one.

You can check these wires individually by using a voltmeter. The black lead is used to test the black wire, and the red lead is used to measure the white wire.

Depending on the volts of your transformer, you should get the same reading on your voltmeter. If it doesn’t give you any readings, it means that something is wrong with the wire.

Proper Maintenance Makes a Difference

You may find the need to perform maintenance on your furnace from time to time. This can include your transformer as well.

As discussed before, checking your transformer wires can be a part of troubleshooting and proper maintenance. If a wire is worn or even torn, you can replace it.

Check the secondary voltage:

You can check the secondary voltage as part of your maintenance checkup. Checking the secondary voltage is the same process as testing the supply voltage.

It may get complicated throughout this process because the colors will vary a bit more. The usual colors for the secondary voltage are red and blue.

At this point, it is ideal to follow the manufacturer’s instructions to find out what each wire does.

You’ll follow the same steps with the voltmeter by testing the red lead to the red wire and the black lead for the blue wire. If the voltmeter does not give you a reading, you will need to replace your furnace transformer.

Getting to Know the Furnace Transformer Housing

Before you begin the transformer replacement, make sure you know the inner workings of the house voltage. The supply and secondary voltage can also be referred as the primary and secondary coil.

Both of these coils are wrapped around a metallic core. This improves the overall conductivity of the voltage.

The role of the primary coil is to create magnetic lines of force as the current alternates and changes directions.

Electricity is generated into the secondary coil by the magnetic lines passing through and providing the output for the transformer. If you want to optimize your transformer, you can use a magnetized core.

How to Replace a Furnace Transformer

Before proceeding with the replacement, it is extremely important that you turn off all power going to the furnace. You can turn off the power at the circuit breaker.

1. I cannot stress the importance of turning off the power enough. In fact, when I work with anything electrical, my OCD compels me to keep checking if the power is truly in the off position.

I also go the extra mile of barricading the area around the circuit breaker with chairs. I mean, not really, but not a bad idea. However, you can inform others not to turn on the power because you will be working on the electrical parts of the furnace.

2. Once you are certain the power is off, you can open the furnace cover to find the transformer.

3. You can use wire cutters to cut the four wires connected to the transformer.

4. Next, you can use a screwdriver to remove the transformer. You may also need a socket wrench depending on its attachment.

5. Connect your new transformer. If your new transformer came with instructions, make sure to read them carefully. Remember to connect the correct wires for each of the supply and secondary wires.

You can use the old wires as a guide. Once you are done using the old wires as guides, you can connect the new ones to the furnace.

You can use wire nuts to complete this process.

6. Finally, test the newly installed transformer with a voltage tester. The video below can help you if you need it.

Remember to turn the power back on at the circuit breaker. Replace the furnace cover back on, and you should be finished. 


Q: Why does my circuit breaker keep tripping?

A: The usual issue with this is the difference in voltage. Remember that the building’s voltage and the transformer need to be the same to work properly.

Q: Do I need to turn off the power to work on the transformer?

A: Yes, anytime you are working with live electrical wires, you will need to turn off the power from the circuit breaker.

Q: Why do the furnace transformers fail?

A: Most of the time, the furnace transformers will stop working because it couldn’t convert the right voltage to start the ignition, controls and timers. This can be caused by a faulty wire in the transformer or just an old furnace.  

Q: What does a furnace transformer do?

A: The furnace transformer reduces your home’s voltage to use inside your furnace. In other words, it regulates the voltage power to accommodate your furnace power output.

Q: What can cause a transformer to blow?

A: This can be a very serious issue, which is why the transformer is an essential part to any electrical device. A furnace transformer can blow due to a surge of electricity to the furnace’s core. A lightning strike can also be the cause of an electrical overload coming directly from the transformer.

Q: What systems compose the transformers?

A: Two systems compose the transformers, which are the supply and secondary voltage systems.

A Quick Overview

Whether you need to fix or replace your furnace, remember to check the furnace transformer for any malfunctions.

If you are experiencing power shorts or your breakers are tripping, chances are the furnace volts could be the problem.

It is always important to follow the instructions when you are replacing your transformers. It is equally important to turn off the power to the furnace when you are working with any live electrical wiring.

It is also essential to connect the corresponding wires to their locations, such as the neutral wires. Remember to follow each step carefully so that you can have a successful replacement.

By following all these tips and educating yourself on furnace transformers, you are sure to get the results you want. I feel like watching a Transformers movie now.   

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