Why Does My Furnace Make a Loud Noise When It Turns On?

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Under normal circumstances, you don’t hear much noise from your home’s furnace as it starts, runs, and stops. Should you notice unusual sounds when your furnace cycles, this noise is likely cause for concern. Why does my furnace make a loud noise when it turns on? Mission Mechanical explains the reasons why furnaces may have a noisy startup and what is necessary to fix these problems.

1. Delayed Ignition

A loud banging sound that occurs a few seconds to one minute or so after your furnace starts up is most likely an issue called delayed ignition. The loud sound you hear is actually a small explosion within the combustion chamber.  Continued delayed ignition issues can cause damage to your furnace, leading to a cracked heat exchanger which can be a very dangerous problem. You’ll want to have this furnace problem repaired right away.

How does delayed ignition happen? When dirt and soot accumulate on the furnace’s burners, they can become clogged. As gas enters the furnace, the dirty burners are unable to instantaneously light the fuel. Gas builds up inside the furnace until the burners are able to ignite. Once the burners do ignite, all the gas inside your furnace combusts, causing a mini explosion and a loud bang.

2. Expanding Air Ducts

When heat starts moving through the furnace and into the ducts shortly after a new cycle starts, the warm air comes into contact with the cold metal of your ductwork. As the metal warms, it expands. This can produce a loud popping or banging noise that usually occurs a bit away from the furnace unit itself.

Expanding ducts can be a normal part of heating system operation. Sometimes, they can be caused by issues such as poor duct system design, unsecured ductwork, or even closed vents. Check the vents throughout your home, making sure all are open and uncovered. If you suspect an issue with your duct system is responsible for making loud furnace noises, contact your HVAC company to assess your ducts and make repairs or modifications as necessary.

3. Ignition Problems

Clicking noises at the onset of a heating cycle indicate a problem with the furnace’s ignition and tell you the system is not lighting correctly. This may be caused by one of the following issues:

  • A faulty thermocouple fails to sense a flame, which shuts the gas valve to prevent a leak.
  • The pilot flame is out or is not in the proper position due to poor mixing of air and gas, which causes the thermocouple to close the gas valve because a flame is not detected.
  • Gas valve doesn’t open because it is not energized due to a bad valve, faulty ignition board, or a poor connection between components.

When you hear clicking as your gas furnace attempts to start, turn off your system and call your HVAC company. A professional will need to find the cause of this noise and make repairs.

Contact Mission Mechanical for Furnace Repair in Indianapolis

If you’ve been asking, “why does my furnace make a loud noise when it turns on?,” you probably need furnace repair. Call Mission Mechanical for fast, reliable furnace repairs in the Indianapolis area.

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