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A-1 Mechanical Blog

Why Does My Furnace Make Loud Noises When It Turns Off?

A common question we get from homeowners has to do with unusual furnace noises that occur both at startup and once a heating cycle is completed. Although some of these noises are normal and nothing to worry about, some can be quite serious and require immediate attention. 

Whether your furnace makes loud noises when turning off or makes a booming noise when it turns on, let’s go over the most common causes of these sounds. That way, you’ll know which ones are normal and which ones warrant a call for immediate repairs.

Booming Sound at Startup

We don’t want to cause you any unnecessary stress or worry, but if you have a gas furnace, a booming noise at startup is definitely a cause for concern that should be addressed right away. Let’s explain.

A gas furnace essentially runs via the combustion process. When you hear a booming noise, it’s usually caused by a delay in the ignition process. Things that can cause a delay in the ignition process include misaligned burners or the burners are clogged with dirt and dust. This typically happens in furnaces that haven’t received regular maintenance.

A dirty burner might sound innocuous but it can be dangerous. The booming noise you’re hearing is caused by a delay in ignition due to a buildup of dirt on the burners. This dirt causes gas buildup and when it ignites, a small explosion occurs. 

The longer this goes unchecked, the more likely it is that your heat exchanger will accumulate damage. A cracked or damaged heat exchanger will potentially leak carbon monoxide gas into your home. This can make your family members ill and in worst-case scenarios, it can be fatal. Booming noises warrant a call to our team for immediate repair.

Loud Noises When the Furnace Turns Off

There are a few things that can cause your furnace to make noises when a heating cycle is complete and it turns itself off. One of the most common noises is the sound of your metal ductwork contracting and expanding. Let’s go over this process.

  1. Your home’s ductwork channels the heated air by pushing it through the entire duct system and into the rooms of your home. 
  2. When this airflow becomes restricted, the conditioned air can back up in the ductwork, causing the thin metal sheets to expand while the system is on. 
  3. When the furnace has completed its heating cycle and turns off, the metal contracts back into place, making a loud noise usually described as a banging sound. 

If this banging noise is excessively loud or problematic, there are two possible causes: you either have a dirty air filter or some of your vents are closed. Both of these issues restrict airflow throughout the system. 

When air cannot flow unimpeded through the ductwork, it’s not a dangerous situation, but it does mean your system isn’t working as efficiently as it should. Try changing the air filter and making sure all the vents are open. This should help alleviate the severity of the booming sound.

Contact A-1 Mechanical Heating & Cooling for your next furnace repair in Lansing, MI. Comfort and Character You Can Depend On!

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