If your furnace makes a loud bang, pop or booming sound right after it shuts off, the noise may be coming from the furnace itself or from your home’s ductwork.
One of the most common causes is sheet-metal ductwork expanding and contracting as it heats up and cools down. But a loud bang can also be related to airflow problems, a dirty filter, improperly sized ductwork or a mechanical problem with the furnace. The timing and location of the sound can provide important clues about what is happening.
Your ductwork may be expanding and contracting
Metal ductwork changes slightly in size as its temperature changes.
When your furnace is running, warm air heats the ducts and causes the metal to expand. After the furnace shuts off and the ducts begin to cool, the metal contracts again. Sometimes a section of ductwork suddenly flexes back into position, producing a loud bang or popping sound. This is especially common with large sections of sheet-metal ductwork and may be more noticeable during very cold weather when there is a greater temperature difference between the heated air and the surrounding space.
Restricted airflow can make the problem worse
Your furnace needs to move a certain amount of air through the heating system.
When airflow is restricted, pressure can build inside the ductwork. That pressure can cause thin sections of metal duct to flex while the blower is operating. When the furnace shuts off and the pressure drops, the duct can snap back into its normal position with a noticeable bang. One of the first things to check is your furnace filter. A heavily clogged filter can restrict airflow and increase pressure within the system. Replacing a dirty filter may reduce the noise if restricted airflow is contributing to the problem.
Closed or blocked registers can affect airflow
Closing too many heating registers can also increase pressure inside your ductwork.
Homeowners sometimes close registers in unused rooms in an attempt to save energy, but your heating system was designed to move air through a certain amount of ductwork. Closing several registers can change the pressure inside the system and may contribute to popping or banging noises. Make sure your supply registers and return-air vents are open and not blocked by furniture, rugs, curtains or other objects.
The ductwork may not be properly supported
Ductwork that is loose or poorly supported can move when the blower starts and stops.
A duct may shift against a floor joist, wall, support strap or another section of ductwork, creating a bang that sounds much more serious than it actually is. An HVAC technician can inspect the ducts and determine whether additional supports, bracing or other adjustments could reduce the movement.
The ductwork may be undersized
Sometimes the problem is not the furnace at all.
If the ductwork is too small for the amount of air the furnace is trying to move, pressure inside the system can become excessive. That can cause ducts to flex, vibrate or make popping noises as the blower cycles on and off. Undersized ductwork can also contribute to weak airflow, excessive blower noise and uneven heating throughout the house. Determining whether your ductwork is properly sized requires more than simply looking at the ducts. An HVAC technician can evaluate airflow, static pressure and the design of the system.
The furnace blower may be involved
If the bang seems to come directly from the furnace rather than from somewhere inside the walls or ceilings, the blower assembly may need to be inspected.
A loose blower wheel, worn motor mount or another moving component can sometimes produce noise when the blower slows down and stops. Mechanical noises coming from inside the furnace should not be ignored, particularly if they are getting louder or occurring during every heating cycle.
Pay attention to when the bang happens
The exact timing of the noise matters.
A bang that happens several seconds after the furnace shuts off is often associated with ductwork cooling, contracting or changing pressure. A loud bang when the furnace first starts is a different problem and may have a different cause. For example, a gas furnace that makes a boom or bang when the burners ignite could have an ignition or burner problem and should be inspected promptly. If you are not sure whether the sound is happening during startup or shutdown, stand near the furnace during a heating cycle and listen carefully.
Is a banging furnace dangerous?
A popping duct is usually more annoying than dangerous. But you should not assume every loud furnace noise is harmless.
Call for furnace service if:
The banging noise suddenly appeared
The sound is becoming louder
The bang seems to come from inside the furnace
The furnace is also overheating or shutting down unexpectedly
Airflow from the registers seems unusually weak
You hear rattling, grinding, scraping or other mechanical noises
The furnace makes a loud boom when the burners ignite
These symptoms can indicate problems that should be diagnosed before they become more serious.
Can a technician stop ductwork from banging?
In many cases, yes.
The solution depends on what is causing the noise. A technician may find that the system needs a clean filter, better airflow, duct reinforcement, additional supports or adjustments to the blower or duct system. Sometimes the noise comes from one specific section of duct that flexes every time the blower stops. Reinforcing that section may substantially reduce or eliminate the sound. The important thing is identifying the actual cause rather than simply trying to silence the duct.
Need help with a noisy furnace?
An occasional pop from metal ductwork as it heats and cools may be normal, but loud or repeated banging should be investigated, especially if the noise is new.
If your furnace bangs when it shuts off, makes unusual noises while running or is not heating your home properly, Roberts Brothers Comfort can inspect the furnace and duct system, identify the source of the noise and recommend the appropriate repair. We provide expert furnace repair service throughout Ohio’s Miami Valley region including Casstown, Covington, Fletcher, Piqua, Rosewood, Saint Paris, Sidney, Tipp City, Troy, Urbana, West Milton and all the surrounding areas.
