What That Banging Noise in Your Ductwork Really Means (and When to Worry)

When your home's ductwork starts banging or popping, it can feel like your walls are vibrating. But those sudden sounds are more than just a nuisance; they may be a sign that something is wrong with your HVAC system. From pressure imbalances to dirty air filters, there are several common reasons why ductwork bangs. To help you get to the bottom of it, we spoke to an HVAC expert who explained what loud ducts really mean—and when you should worry.

Dirty Air Filter 

One of the most common culprits behind loud ducts is a dirty air filter. Your HVAC system needs the right pressure balance to function properly. There is positive pressure in the supply (air leaving the furnace to warm your home) and negative pressure in the return (air returning to the furnace). "When the air filter gets dirty, the fan can’t pull air through the return and out the supply," says Devin Coyne, HVAC technician for Climate Solutions Services. "The restricted airflow causes low pressure on the return side and high pressure on the supply side, causing the ductwork to constrict and create a banging sound."

Undersized Duct Work 

A more alarming reason your ductwork may be banging is that the ducts were incorrectly sized. "Your ductwork needs to be proportionate to the size of your heater," says Coyne. "If your heater is too big for your ductwork, it causes pressure imbalances in the supply and the return. These imbalances cause the ductwork to expand and contract, which can sound like banging or popping."

Having Too Restrictive an Air Filter 

While an air filter that traps dust and allergens can be a good thing, it is possible to purchase an air filter that's too restrictive. Every air filter has its own MERV rating, which is the number that measures an air filter's ability to trap particles. "Air filters with a lower MERV rating allow more air flow, but collect fewer particles, while an air filter with a higher MERV rating traps more particles, but restricts air flow," says Coyne. "If you choose an air filter that has a very high MERV rating, air won’t be able to flow through the filter, causing banging in the ductwork." 

Blocked Vents 

While many homeowners believe you can redirect air throughout your home by blocking air vents in unused rooms, this is actually a big no-no. "By blocking vents, you are limiting the amount of airflow that moves throughout the ductwork. This can cause pressure imbalances in the ductwork, resulting in banging or popping sounds," says Coyne. "Nine out of 10 times your ductwork bangs, it’s related to an air flow issue."

Troubleshooting the Issue 

If your ductwork is banging, Coyne recommends taking these steps to troubleshoot and potentially fix the issue before it causes additional problems.

• Check your air filter: Check your air filter to make sure it isn't dirty. If it is, change it out for a new air filter, making sure it has the appropriate MERV rating. Coyne recommends changing your air filter every one to three months, depending on the system you have.
• Make sure your vents are open and unobstructed: Check each room in your home to make sure your air vents aren't obstructed or closed.
• Consult a professional: The easiest way to diagnose the source of banging ducts is to call an HVAC professional to inspect your system. While the issue may be as simple as a dirty air filter, it could also be a sign of something more serious.
  

When to Worry 

While you should always get to the bottom of banging ductwork, there is one cause that is more concerning (and costly) than other potential issues. "If you get a new system put in and the banging happens immediately, you need to call a professional to come out and diagnose the problem, as this could be an indicator that the wrong-sized ductwork was installed," Coyne says. "If the ductwork is too small, it will cause your heater to overheat, putting stress on the system and potentially causing it to break." Another time to worry is if you change your air filter and the banging continues, as this may mean there is a bigger issue at hand.