furnace check

How To Check If Your Furnace Transformer Is Bad 

Furnaces are made up of several parts, and one of those parts is called a transformer. A furnace transformer is a component that increases or reduces the power output of the alternating current (AC) and moves it within the heating system. 

A furnace transformer uses a magnetic field to produce a current and transmits electrical energy from one circuit to another. Its key responsibility inside a furnace is to monitor and control the incoming current to meet the system’s requirements. 

Like any other component in the furnace, such as the heat exchanger, the ductwork, or the blower motor, a transformer is a crucial part of a properly functioning furnace. Therefore, a homeowner owner should know the basics of the unit and how to check if a furnace transformer is bad. 

Here’s a complete guide on the things to look for if you suspect your furnace transformer is bad. 

Bad Furnace Transformer Symptoms

Like many things, a bad furnace transformer will have symptoms you can look out for to catch it before it causes damage. 

Here is how to check if your furnace transformer is bad.

1. Circuit breaker keeps tripping

If you didn’t know your furnace transformer could affect your circuit breaker, it can. In fact, it is one of the most common bad transformer symptoms. 

When your furnace transformer isn’t working correctly, it will trip the circuit breaker to avoid the usage of excess power voltage. So, if your circuit breaker keeps tripping, don’t ignore it. It’s best to resolve any issues as soon as possible to avoid more furnace complications in the future.

The role of a transformer inside any electrical appliance is critical, and your furnace system will not work properly if you don’t control the proper voltage from your furnace circuit breaker. Attempting to force the wrong power output could cause severe damage to your furnace, which is expensive to replace. The damages could include burned fuses and even system outages.

2. Your heater is not working

You get out of bed on a cold morning, walk to your house thermostat, and adjust it. You know your furnace is set to a comfortable temperature, but it never turns on. You return to your thermostat and start it again—still nothing.

If, after trying over and over and putting on warmer layers, it seems like there’s nothing you can do, you may have a bad transformer. 

The furnace transformer is likely preventing your furnace from heating your home because it is faulty. Address this as quickly as possible to avoid major damage to the furnace.

Even with an adequate power supply, your furnace might not work if the transformer is faulty. The unit does this to defend itself from electrical problems that can cause severe damage to the system. This said it’s difficult to confirm that the furnace is malfunctioning because of a defective transformer unless you are a professional. 

So, if your furnace isn’t working, contact a reputable HVAC contractor to get it inspected.

3. Humming and vibrations

The furnace transformer comprises steel plates made of laminated layers, which produce noise as they contract and expand. Therefore, it is normal to hear some humming sound coming from your furnace transformer. 

However, an extensive noticeable vibration or buzzing, combined with a broken fan, can be a key indication that your furnace transformer is faulty. Sometimes vibrations are strong enough to make connectors lose, which can lead to the failure of the furnace transformer.

So, if you experience humming and vibrations from your furnace transformer, these are bad transformer symptoms that you shouldn’t ignore. Remember, heat transfer cannot occur if your furnace transformer does not function properly.

Note: Sometimes (for less than 10% of cases), the HVAC transformer is situated in the outdoor unit of an air conditioner. Therefore, when we talk about the symptoms of a bad furnace transformer, note that they are similar to bad AC transformer symptoms.

Main Causes of a Bad Furnace Transformer

In addition to learning how to check if a furnace transformer is bad, it’s also good to know how it happens. What causes a furnace transformer to malfunction? 

1. Dirty filters

The heat exchanger in a furnace heats the indoor air and pushes it into the room via the blower fan. Air may bring debris and dirt inside the heating device when it reaches the furnace. 

A furnace filter is positioned at the point where cool indoor air flows in the furnace to stop debris and dirt from accumulating.

Over time, debris and dirt will accumulate on the filter, obstructing airflow. When there is insufficient airflow, your furnace heats up, which can cause the transformer to collapse. 

You can learn how to check a furnace transformer yourself to see if it has dust. Otherwise, contact a professional for regular maintenance.

Also, if your furnace filter becomes clogged, dust will slowly flow within the furnace. The dust will eventually accumulate on the upper edge of the internal parts, such as the transformer. Too much heat will be trapped inside the furnace due to the dirt, which can potentially cause the transformer to malfunction.

2. Power surge leading to burning transformers

The possibility of a transformer blowing up and burning is not likely to happen, as the firewall constraints installed on furnace transformers effectively lower the risk of fires and explosions. However, in some cases, furnace transformers can blow up. 

Furnace transformers are responsible for transferring power between circuits and switching power from one voltage to another. However, voltage levels can suddenly rise because of faulty mechanisms or accidents. If the furnace transformer experiences a power surge, it will blow up and burn. You will see some of the bad furnace transformer symptoms in the event of a burn.

Transformer burns and explosions can result in serious injuries and property damage. Your furnace also will not work until it is repaired or replaced. 

3. Bad or Faulty Contactor 

A faulty contactor will, in most cases, lead to a bad furnace transformer. In fact, it usually leads to a blown transformer. 

When you have a bad contactor, the entire heating unit is put under immense stress. And when the strain becomes unbearable for the unit, the transformer will blow. Again, while burns and explosions are unlikely due to your furnace’s internal safety parameters, it is possible. Get your furnace checked regularly to avoid it.


If your furnace transformer isn’t working properly, you may need to get it replaced. For transformer replacements or any other HVAC needs, book an appointment with the professionals at Anderson Air. 

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