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Why Does My Furnace Make a Loud *BANG/BOOM* Noise When It Turns On?

Hearing a loud banging sound when you turn your furnace on can be a frightening experience, and if you’re like most people, you’ll likely be mystified about what caused it.

After all, you don’t know if your furnace is about to break down and leave you without heat or if it's a minor problem that a technician can easily troubleshoot.

If your furnace is making a loud banging or booming noise when it turns on, the two most common causes are:

Keep reading for more information on probable causes and best practices for getting things back to normal.

Need A Furnace Repair? Call MSP!

Having problems with your home heating system? Call MSP Plumbing Heating Air at (651) 228-9200 to schedule a consultation and diagnostic visit, or use our convenient online scheduling form. We’ve been serving homeowners in the Minneapolis and St. Paul area for over a century and are known for providing efficient, friendly service and same-day furnace repairs.

Problem with the Ignition System

Skinny blue flame burning in front of a pitch black background.

Often, the loud banging noise that happens shortly after you turn your furnace on is caused by a problem with the ignition system itself. This is most likely an issue if the banging noise you’re hearing comes from close to the heater itself.

Common Causes

Unexpected and loud noises coming from the furnace itself are often caused by a condition known as delayed ignition. This results from moisture building in the unit due to inactivity — for example, over the summer. Homeowners should schedule a maintenance visit with MSP Plumbing Heating Air before the beginning of the cold season to ensure that their furnace is good to go. Other possible causes of loud noises include the following:

  • Too much air in the gas lines, making ignition difficult
  • Loss of gas supply pressure
  • Dirty or misaligned burners
  • Weak pilot light in older furnaces

How to Fix

Homeowners must resist the temptation to try to fix ignition problems independently. Only trained professionals should deal with this type of problem — after all, the banging sounds you hear, in this case, are miniature explosions, so play it safe and call in an expert. For safety’s sake, turn your furnace off and use a backup heat source, if possible, while waiting for one of our skilled technicians to arrive on the scene.

Expansion and Contraction of Metal Ductwork

HVAC tech wearing a bright yellow hard hat, a black jacket, and red work gloves is working on metal ductwork.

If the banging noises seem to be coming from somewhere other than the furnace itself, chances are good that you’re dealing with issues in the system’s metal ductwork.

Because metal expands and contracts naturally, this could be the source of the popping or banging sounds you hear after you start your furnace. We offer same-day service to ensure the comfort and safety of our customers.

Common Causes

Static pressure is usually behind any issue in the ductwork that causes unexplained and unexpected noises. As defined, static pressure is the resistance to airflow within the ductwork of HVAC systems.

Common culprits include the following:

Dirty Air Filters

Dirty air filters may restrict the air movement inside your unit, generating noise. Furnace filters should be cleaned or replaced regularly, so be sure to schedule routine ductwork maintenance.

Closed Dampers

Closed dampers are another possible reason for airflow restriction that can result in mysterious sounds coming from the vicinity of your HVAC system. Most systems have several dampers, and just one of them being closed usually won’t result in unexplained noises. Additionally, keeping multiple dampers closed may result in increased wear and tear on your system by causing it to overwork.

The Buildup of Air Pressure

Air pressure buildup within your HVAC system can push against the walls of the ducts, causing them to expand, which causes a banging or popping sound several seconds after you turn the system on.

Improperly Installed Ductwork

Improperly installed ductwork often results in bending, which increases static pressure within the system.

How to Fix

Depending on which type of HVAC system you have, an easy fix may be to replace or repair your current air filter — the owner’s manual on your system should tell you which type you have. You should also check your vents to make sure that they are all open and free of any kind of obstruction. If these don’t resolve the issue, it’s time to contact the professionals.

Don’t Wait Until It’s Too Late—Call MSP Plumbing Heating Air to Check Out Your Furnace Today!

Smiling MSP tech wearing a work polo with a white long sleeve shirt underneath, working on the panel of a furnace.

Twin Cities homeowners have relied on MSP Plumbing Heating Air for quick, friendly, and expert service since 1918. With our proven track record of providing superior workmanship and delivering unmatched customer service, you can rest assured that the comfort and safety of your furnace repairs are among our top priorities. Because we know your time is valuable, we offer a convenient scheduling portal on our website that you can access at any time, or you can call us at (651) 228-9200 to schedule a service appointment.