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Top 5 Reasons Why Furnace Transformers Fail

The furnace transformer regulates the voltage, increasing or decreasing it as necessary. It’s an integral part of your heating appliance, and if it fails, the furnace might stop working, or worse, it can blow up, causing a devastating accident and even a fire. But why do furnace transformers fail in the first place?

Here are 5 common reasons why furnace transformers fail:

  1. Wiring problems
  2. Power surges
  3. Old age and worn-out parts
  4. Cooling issues
  5. Accumulation of dirt and debris

In this article, I’ll go over the common causes behind a faulty furnace transformer. After that, I’ll also share a section covering the common signs that the furnace transformer is about to fail, along with what you can and should do with a bad furnace transformer.

1. Wiring Problems

The most common cause of furnace transformer failure is trouble with the wiring. Some amount of vibration should be expected from a normally functioning transformer. However, over time, the vibration can loosen the internal wires. At best, the wires will disconnect, abruptly stopping the furnace from working. The loose wires can create a short circuit which can fry the transformer and even cause an explosion.

This is why it’s super important to firmly attach the wires to the transformer and check in once in a while to ensure the wiring is secure and not loose.

2. Power Surges

Just like all other electrical appliances, furnace transformers are also vulnerable to power surges and voltage spikes. A transformer is rated to handle only a set amount of voltage. As such, if it receives more current than it can handle, it’ll damage the internal components and potentially fry the transformer.

Unfortunately, you can’t prevent a power surge. It usually happens during lightning storms or if your power company experiences a surge. Neither of these is in your control.

However, you can install a surge protector like the Eaton CHSPT2ULTRA Ultimate Surge Protection (available on The device can universally connect to any manufacturer’s load center. Once installed, it’ll protect all home appliances from surges coming through the utility lines and indirect power spikes because of lightning.

3. Old Age and Worn-Out Parts

Under optimal conditions and with annual maintenance, a furnace can last around 15 to 20 years. It’s natural for the furnace and the transformer to fail with old age. However, you can accelerate the aging process if you don’t perform routine maintenance.

Without a professional or technician to service the furnace and the transformer at least once a year, the internal components will wear down. The coils, wiring, and other components inside the transformer will get damaged or begin to rust. If this damage is too extensive, the transformer will fail before its time.

4. Cooling Issues

Transformers generate a lot of heat, which is more of an issue for furnace transformers. If things get too hot, it can affect the internal circuitry, damaging the transformer. As such, to safeguard the device from heat-related damage, many furnace transformers (especially the big ones) use some form of cooling system.

Most systems use forced air to control the temperature of the transformers. Others resort to water cooling systems, and still others use mineral oil as their preferred liquid coolants.

Now, if the cooling system is compromised (either because of a blockage or damage to its components), the transformer will overheat, causing it to fail or even blow.

5. Accumulation of Dirt and Debris

A furnace works by heating the indoor air using the heat exchanger and pushing it back into the room using the blower fan. As air enters the furnace, there’s a risk it’ll also carry dust and debris into the heating appliance. To prevent this from happening, a furnace filter is placed at the point from where the cool indoor air enters the furnace.

Over time, dirt and debris will collect on the filter, which will obstruct the airflow. With limited airflow, the furnace will heat up, which in turn can also heat the transformer to the point that it can fail.

Another possibility is when the furnace filter gets damaged. This will cause the dust and debris to slowly flow inside the furnace.

Eventually, the dust will start gathering on top of the internal components, including the transformer. The dirt will trap more heat inside the furnace, which can cause the transformer to fail.

If you want to replace your furnace filter, make sure you check its MERV (Minimum Efficiency Reporting Values) rating first. Essentially, it’ll tell you the filter’s ability to capture particles within a certain size range. 

For example, if you’re looking for something with a MERV 5 rating, you can get the Filtrete 16x25x1, AC Furnace Air Filter (available on Aside from your furnace, you can also use it for air conditioners and other HVAC systems. However, you’ll need to replace the filters every 90 days. 

Signs of a Furnace Transformer That’s About To Fail

By now, you should understand what can cause your furnace transformer to fail. This should help you fix that issue when it happens and prevent it from occurring again.

However, I think it’d be more useful if you could tell beforehand that the transform is about to fail, which would’ve given you a chance to detect the fault and repair it.

So, I’ve shared a list of some of the common signs and symptoms that signal the furnace transformer is in trouble and needs your speedy attention.

Burn Marks or Visible Signs of Damage

The most obvious and telltale sign that your furnace transformer is about to fail is if you notice any visible sign of damage on the unit. This is why it’s super important to routinely service and maintain the transformer, as it’ll help you quickly suss out any issues with the component.

Potential signs of a bad transformer include burn marks, dents, or bulging. You should also keep an eye for loose wires and dust accumulation.

If there are loose wires, securely attach them to the transformer. And if you notice dust build-up, clean the unit with a vacuum. For this, I recommend the PowerSmith PAVC101 10 Amp Ash Vacuum (available on It’s specifically designed to give appliances like your furnace a thorough clean without damaging its delicate parts.  

Frequently Tripping Circuit Breaker

Do you notice the circuit breaker connected to your furnace tripping frequently? If yes, that could signal the furnace transformer is going bad. The circuit breaker trips when it senses a higher current than normal going towards your electrical appliance. 

When the circuit breaker trips, it cuts the connection between the power source and the appliance, protecting it from the high voltage and the potential damage a trip can cause.

Now, as I said before, the furnace transformer increases and decreases the voltage according to the heating requirement. If the transformer is faulty, it can draw more current than necessary, causing the circuit breaker to trip.

As such, if you notice that only the circuit breaker for the furnace is frequently tripping, the transformer might be going bad and you should look into it.

Excessive Vibration and Humming Noises

A normally functioning transformer will create some humming noise. However, it shouldn’t be louder than the sound made by the blower fan inside your furnace. The reason the transformer hums is because the lamination core is in a constant process of expanding and contracting as it regulates the voltage.

Now, if there’s an issue with the voltage or some transformer component, the iron core can expand and contract very rapidly, which eventually causes the unit to fail. You can know this is happening if you notice a drastic increase in the loudness and frequency of the humming sound accompanied by excessive vibration.

Loud humming noises and vibrations can also indicate a malfunctioning furnace capacitor.

As such, this abnormal behavior isn’t absolute proof that there’s something wrong with the furnace transformer. But it definitely signals that there’s an issue that warrants a deeper investigation.

Furnace Not Working Properly

If you have a bad transformer, the furnace might stop working properly. Again, the furnace transformer plays a pivotal role by increasing and decreasing the voltage in the furnace. It raises the voltage when the furnace needs to generate more heat and then reduces the voltage when the room reaches the desired temperature, and heating isn’t necessary.

Now, if the transformer is malfunctioning, the furnace might keep on heating when the voltage doesn’t come down, or it might stop heating altogether because of low voltage.

In most cases, what’ll likely happen is that the furnace will detect that the transformer is faulty and just stop working. This is because a faulty transformer can cause severe voltage fluctuations, damaging various components inside the furnace. As such, the device will stop working until the bad transformer is replaced with a working one.

That said, a furnace that’s not working properly could mean a lot of things. A bad heat exchanger, blower fan, or control board can all cause issues with the furnace.

As such, you can’t just blame the transformer if the furnace stops working. Instead, you should start thoroughly troubleshooting the device, examining each component for faults and defects.

How To Check if the Furnace Transformer Is Good or Faulty

By now, you have a good idea of the different causes of a faulty transformer. You also know certain signs that indicate that the furnace transformer may be malfunctioning. But how can you know for sure whether the unit is working or defective?

Well, for that, you’ll need to use a multimeter and run some tests on the transformer. In case you don’t have a multimeter, I recommend getting the AstroAI Multimeter 2000 (available on It’s affordable and accurate enough to test common household appliances.

Next, you’ll need to remove the furnace transformer to test it.

To do this, follow the steps below:

  1. Unplug the furnace from the power outlet to avoid any electrical issues. If there’s no plug, switch off the circuit breaker connected to the furnace.
  2. Locate the transformer inside the furnace by referencing the user manual. It’s usually inside the blower chamber.
  3. Remove the panel covering the area where the transformer is located.
  4. Take a picture of the transformer. This is to record the particular wire configuration, so you know where each wire goes when you refit the transformer.
  5. Disconnect the wires connected to the transformer.
  6. Unthread the screws mounting the transformer in place, and remove it.

With the transformer in hand, you’ll need to run a few continuity tests with the multimeter.

For reference, here’s a short 2-minute YouTube video on how to test for continuity:

For a good and functional transformer, the following should be true:

  • There’s no continuity between the primary binding and secondary binding.
  • There’s continuity in the primary binding.
  • There’s continuity in the secondary binding.

If even one of these statements is false, the transformer is faulty, and you should replace it with a new one.

What To Do With a Bad Furnace Transformer

If you have determined that your furnace isn’t working because of a faulty transformer, replacing it with a new and working one should fix all issues. But first, you’ll need to buy the replacement part. I recommend checking the user manual or contacting your dealer to know what type or model of transformer you need for your furnace. Alternatively, you can also walk into a repair shop and ask a technician for help.

Once you have the replacement in hand, remove the old and faulty transformer and put the new one in its place.

I’ve already provided a step-by-step guide on removing a transformer from the furnace. You can check it out for reference.

Alternatively, if you prefer video guidance, you can take a look at this 2-minute YouTube tutorial on replacing furnace transformers:

If your furnace was acting up because of a bad transformer, installing a new and working one should fix the issue. But if the problem persists, I recommend calling a professional HVAC technician to help you troubleshoot the problem.


  • Jake Alexander

    Jake is a freelance writer from Pennsylvania who enjoys writing about science and sports. When he's not writing for Temperature Master, he can be found watching the NFL or playing basketball with his friends.

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