Why Do Your Ducts Pop or Bang When the Heat Comes On?

Quick answer: Ducts that pop or tick when the heat comes on are usually metal parts warming up: the furnace cabinet and heat exchanger, the metal boots behind vents, or a sheet metal plenum. Ductboard and flex duct in most Central Texas homes don’t tick. A loud boom can mean high duct pressure; a bang in the furnace needs service.

Why do my ducts pop when the heat comes on?

Usually it’s the metal parts, not the ducts themselves. In most homes here, the big box on top of the furnace (the plenum) and the main trunks are ductboard, a rigid fiberglass board. Flexible round duct (flex duct) runs from there to each room. Neither one ticks the way metal does.

The pops come from the metal that’s left: the furnace cabinet, the heat exchanger (the metal chamber that heats the air), the metal boots where each flex run meets a vent, and a sheet metal plenum in the homes that have one. Metal grows a little as it heats and shrinks as it cools, the same reason a car’s hood ticks after a drive.

A single thump when the fan starts has a different cause. The flat sides of the plenum, ductboard or metal, flex in and out as the fan builds and drops pressure, like the lid of a cookie tin.

Why are my ducts louder in winter?

Because the temperature swing is bigger. In summer, cool air enters ducts in a hot attic. In winter, furnace air far warmer than the room hits metal that has been sitting in a cold attic, so the metal moves more. Heat pumps blow air that is warm but not hot, so they tend to pop less.

Is a loud boom from my ducts a problem?

Yes, a boom as the fan starts is worth fixing. A soft tick is normal. A boom usually means the air pressure inside the ducts is too high and the flat panels of the plenum or trunk are getting shoved hard. The usual causes:

• A clogged filter.
• Closed or blocked supply vents (the ones that blow air). Closing vents in unused rooms explains why that backfires.
• Too few or too small return vents (the grilles that pull air back in) for the size of the system.
• Ducts too small for the fan. See why properly sized ductwork matters.
• A fan set faster than the ducts can handle.

High pressure also makes the fan work harder and leaves rooms uneven, so fix it even if the sound doesn’t bother you.

Why does my furnace bang when it turns on?

A bang from inside the furnace right after the burners light can mean delayed ignition, which isn’t duct noise: gas collects for a moment before it lights. That needs a technician soon, not someday. If you smell gas, leave the house and call 911 and your gas utility from outside.

To tell the two apart, stand near the furnace when the heat starts. A thump from the box above it as the fan kicks on is likely the ducts. A bang from the burner area right after the flames light means it’s time to call for service.

What can I do about popping ducts myself?

Start with the filter and the vents. All three steps are safe to do yourself:

• Put in a fresh filter.
• Open every supply vent and keep returns clear of furniture.
• Note when the noise happens: as the burners light, as the fan starts, or while it runs.

Call a pro if the boom stays after that, or for any bang inside the furnace. A technician can measure duct pressure (it takes a few minutes with a gauge) and, depending on the cause, stiffen noisy panels, slow the fan within the furnace’s range, or add return air. Gold Eagle’s technicians fix noisy, high-pressure ducts across the Austin area; you can book a visit online. Loud ducts are often the first clue that the whole system is working harder than it should.

Related guides: Why Is My HVAC System Making Noise? Troubleshooting Guide, Why Properly Sized Ductwork Matters for Your HVAC System and 6 Sounds You Don’t Want To Hear From Your Air Conditioning or Furnace.

About Gold Eagle Services: This guide comes from the team at Gold Eagle Services, a family-run Austin HVAC, indoor air quality and mold remediation company founded in 2017. We’re licensed (TACLA119235 and RCO1783), insured and bonded, and we serve homes across the greater Austin area.

Need help with this in your home? Learn about our ductwork design, sealing and replacement, or call 512-489-4653 to schedule a visit.

Related Gold Eagle service: Home maintenance plan