It can be scary when your furnace starts up and you hear a loud rumbling sound. Is it a major issue that could be dangerous to your home and family or something simple with a bad sound? Learn about the common causes of furnace rumbling and how you can prevent them in the first place.

Pilot Light Problem

Some furnaces have a pilot light to ignite the burners. This is a small flame that continually burns inside the burn chamber so that it’s ready when your thermostat calls for heat. These are especially prominent in furnaces produced before about 2010.

A problem with the pilot light can cause this rumbling sound while the system is running or even once it completes a heating cycle. The issue could be a flame that’s too high or too low or a flame that is surrounded by soot. If you suspect a problem with your pilot light, it’s best to call a professional repair technician to prevent personal injury and damage to your system.

Dirty Burner

Your furnace produces several byproducts as it burns its fuel, one of which is carbon. This is the primary component in soot, which can collect in your unit.

Normally, the vast majority of the carbon is vented outside with the exhaust. However, if there’s a problem with your unit’s exhaust system, that carbon will collect and build up in your system as soot.

This soot will collect on your burners, which inhibits their ability to work properly. As it builds up, it’ll cause the burners to burn less intensely. This causes your system to emit a rumbling sound.

In this case, both the rumbling sound and the dirty burners are actually symptoms of a different problem. This is why it’s important to have a professional address the issue to ensure that you’re getting to the root of the problem and not stopping at the symptoms.

Draft Inducer Motor Trouble

Some furnaces, including most high-efficiency units, have a special fan to help evacuate the exhaust from the system. This is a significant safety modification for furnaces that prevents a buildup of exhaust gases that can lead to gas exposure or explosion risks. These special fans also draw in the air needed for the right fuel-to-air mixture.

There are two primary causes of these fans not working properly, both of which lead to rumbling sounds. The first is when the draft inducer fan gets dirty, which naturally happens as it moves the exhaust gases.

The second cause is when the motor gets loose. This also happens naturally as the fan motor runs due to the vibration it creates. The best way to prevent both a loose and dirty inducer fan is through routine maintenance.

Wearing Motor Bearings

Both the fan inducer motor and the circulating fan motor have motor bearings to allow it to spin. As these wear, it can cause the motor to start emitting a low rumbling sound. However, when they fail completely, you’ll likely hear a loud scraping or screeching sound.

You can extend the life of your motor bearings by ensuring they are properly lubricated. Some units have sealed fan motors, which means no one can lubricate the bearings. However, with unsealed motors, you want to ensure they are lubricated as part of routine maintenance.

Loose Housing Panels

The panels that make up your furnace’s housing are made of sheet metal and secured to the frame with screws. The entire unit creates vibration as it runs, which naturally loosens screws and mounting bolts over time.

When they become loose enough, they’ll create a loud banging sound. However, when they first start loosening, they may create a rumbling sound.

This is fairly easy to identify by placing your hand on the panel to see if it stops the sound. If it does, simply tighten the screws on the panel. Work your way around the entire unit so that no other panels start rumbling immediately.

Cracked Heat Exchanger

The heat exchanger is a series of metal tubes that the exhaust flows through before entering the flue to vent outside. Its job is to transfer the heat from the exhaust to the air circulating through the system.

These tubes can become damaged and cracked, leading to a very dangerous situation in your home. One of the components of furnace exhaust is carbon monoxide, so a crack can lead to carbon monoxide exposure and poisoning.

In most cases, a cracked heat exchanger will emit a rattling sound when the furnace first cycles on and is warming up. However, it may produce a low rumbling sound in some cases. The most common cause of a cracked heat exchanger is your system overheating, often caused by restricted airflow.

Other signs of a cracked heat exchanger include additional soot, potentially around your vents, or a strong smell that is similar to formaldehyde. Given the potentially serious consequences of a cracked heat exchanger, turn your system off and call a repair technician immediately if you suspect that yours is damaged.

Unburned Fuel

One of the more concerning rumbling noises your furnace may produce is one that shakes your home. While this is fairly uncommon, it is indicative of a very dangerous situation that needs immediate attention.

The common cause of this drastic rumbling is unburned fuel building up in the burn chamber. This eventually causes an uncontrolled explosion, which can cause furnace damage or worse. These events are called puffbacks, and they have several potential causes, most relating back to poor furnace maintenance.

The most common cause is a small leak in or around the burn chamber. This allows fuel to slowly accumulate and then explode when the burner ignites. They can also be caused by a problem in the exhaust system, dirty burners, and dirty igniters.

Preventing Furnace Rumbling

The best way to prevent your furnace from rumbling is to ensure you maintain your system properly. The first step is changing your furnace filter regularly to prevent airflow restrictions that may cause the system to overheat.

Next, make sure to get professional furnace maintenance every year. The best time to get this is in the fall so that you reap all of its benefits, like season-long improved efficiency.

During this maintenance, a technician either checks for or resolves many of the causes of furnace rumbling. They will clean your burners and igniter whether it’s a pilot light or other igniter. While they’re in the internal parts of your furnace, they’ll inspect the heat exchanger to look for any cracks.

They’ll also clean and lubricate the circulating fan and draft inducer motors, and they’ll tighten the mounting hardware. If you have a pilot light, they’ll ensure it’s properly calibrated to the correct height. Once they’re done, they’ll check the screws holding the housing panels in place to ensure they’re tight as well.

The HB Home Service Team has served the residents of Baltimore since 1914. Our carefully assembled team of experts not only provides heating and cooling installation, maintenance, and repair but also a host of plumbing, electrical, home safety, and indoor air quality services. Call to schedule your furnace maintenance or repair appointment with one of our experienced technicians today.

HB Home Service Team

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