Why Does My Furnace Knock When It Shuts Off?

Many homeowners concerned about their heating ask, “Why does my furnace knock when it shuts off?” Furnaces, like all appliances, can make noises, but we tend to notice furnaces more because they’re much louder. 

Hearing loud thumping, banging, or rattling noises when turning off your furnace generally isn’t a cause for concern. However, these sounds can indicate more significant problems, so it’s crucial to know the possible reasons why. 

This guide will explore why your furnace might make sounds when it turns off. If necessary, you may need to contact Burr Ridge furnace repair services.

Reasons Why Your Furnace Is Making Knocking Sounds 

A furnace producing a loud bang is a typical issue homeowners deal with. Potential answers to the question, “Why does my furnace knock when it shuts off?” include the following:

A Dirty or Clogged Air Filter 

You can usually attribute a loud banging sound when turning off your furnace to metal contracting. As your furnace undergoes its cycles, it pushes air through your ducts into different areas. A dirty air filter can restrict airflow and trigger warm air to gather in your system. 

Heat backups force your duct system’s metal to expand, bowing outward. As a result, when a heating cycle ceases, your ductwork metal panels and sheet metals cool and contract. These contractions produce a banging or popping noise. 

You can resolve this issue by regularly cleaning or replacing your air filters. The frequency with which you should replace them depends on factors like: 

  • How many people are in your home 
  • The size of your home
  • How much you use your furnace
  • The area you live in 

There’s no set time for when you should replace your air filters. However, you should check them at least once a month during winter. When replacing, use the right air filter size. An improperly fitted air filter can cause leaks and introduce dust into a system.

Filthy Ductwork 

When you turn your furnace off, the accumulation of dust particles and debris in the ducts can also cause loud sounds. These dust deposits restrict airflow, making your home feel stuffy and uncomfortable. 

As ducts heat up and begin expanding, they will contract after finishing a heating cycle, producing a banging sound. You can rectify this concern by regularly cleaning your duct system, allowing air to flow properly throughout your home. 

Alongside improving airflow, cleaning out your ducts ensures better indoor air quality. Poorly maintained duct systems and air filters will make your HVAC systems spread harmful pollutants. 

These pollutants can trigger breathing issues or even lead to mold growth if you neglect them for too long. Dirty ductwork also forces your HVAC systems to work harder, increasing wear and tear and chances for them to malfunction or break down entirely.

Knocking Sounds When You Turn On Your Furnace

Alongside asking, “Why does my furnace knock when it shuts off?” many homeowners ask about banging noises they hear when turning their furnaces on. 

If you hear banging every time you turn on your furnace, dirty burners are the most likely culprits. Accumulating dirt can build up on your furnace’s burners while it operates. Your burners will also collect carbon from their natural combustion processes. 

When left alone, dirty burners may lead to an issue known as delayed ignition. Delayed ignition occurs when dirt or other buildups prevent your burners from lighting gas. The gas then collects inside your furnace’s combustion chamber. 

When your burners finally ignite, there will be a much larger gas reserve. The banging homeowners hear in these instances is the sound of this excess gas igniting. 

A furnace suffering from delayed ignition and dirty burners can lead to severe issues, including a fire. You’ll want to call our professional technicians right away.

What if Your Furnace Is Making Other Noises? 

Besides asking, “Why does my furnace knock when it shuts off or turns on?” you may have questions when your furnace starts making other noises: 

  • Humming sounds: If your furnace is operating correctly, you can usually trace a humming sound to a noisy transformer. However, if you’re noticing problems accompanying this humming sound, you may have a faulty capacitor or interior fan. 
  • Scraping sounds: A damaged motor mount or loose or broken blower wheel can trigger scraping sounds. 
  • Squealing sounds: An inadequately lubricated blower belt can cause squealing sounds. You can also link squealing sounds to a loose fan belt. 
  • Thumping sounds: An imbalanced motor or blower wheel can cause thumping sounds. 
  • Rumbling sounds: You should always treat a rumbling sound as an emergency. Please turn off your burners and gas valves, then call our HVAC team immediately. 

Can You Handle Furnace Repairs Yourself? 

When facing a loud or malfunctioning furnace, treating it as a DIY project can be tempting. However, unless you’re a licensed and insured contractor, you shouldn’t attempt repairs yourself for the following reasons: 

  • You won’t have the necessary tools: HVAC technicians will have the latest tools and equipment to handle the problem. By addressing the issue yourself, you’ll either need to buy new tools or take much longer with the tools you have. 
  • You can cause further damage: Homeowners can quickly damage their furnaces without the proper skills and experience. A minor issue can escalate into significant repairs, costing more time and money. 
  • It can be dangerous: Sometimes, a loud furnace is due to a loose part. Other times, however, there might be a severe fire risk. Don’t put your life in danger just to save a few bucks.
  • You might void your furnace’s warranty: You may void your warranty by attempting to repair your furnace alone. Many warranties require repairs from a licensed and insured contractor. 

Get Help With Your Furnace Today 

When asking, “Why does my furnace knock when it shuts off?” the answer depends on the situation. Ultimately, it’s best to let us troubleshoot the problem. 

Knowing when to replace furnaces requires help from our expert team. Burr Ridge residents can call JC Heating & Cooling today at 708-968-3131!