Furnace making a loud banging noise

Symptoms

• Loud bang or boom when the furnace first fires
• Popping or banging from ductwork as it heats up
• Noise may repeat every cycle

Likely causes

• CommonDelayed ignition from a dirty burner or blocked portGas builds up briefly before lighting, then ignites all at once with a bang.
• CommonDuctwork expanding and contracting with heatThin metal ducts naturally pop as they warm, especially in a new or recently changed system.
• PossibleDirty burners causing uneven flame spreadSoot or debris on burners disrupts a clean, even ignition.
• PossibleLoose panels or ductwork connections vibratingLoose sheet metal amplifies normal blower vibration into a banging sound.
• Less commonCracked heat exchangerIn rare cases, banging paired with other symptoms can point to a more serious internal issue.

What to do

1. Note whether the bang happens right at startup (points to ignition) or throughout the cycle (points to ductwork).
2. Turn the furnace off and check that all panels are firmly attached and not rattling loose.
3. Replace the air filter if it's dirty, since poor airflow can affect burner performance.
4. If it's ductwork popping, this is usually harmless, but insulating or securing loose duct sections can quiet it.
5. If the bang happens at every ignition and is getting louder, turn the furnace off and call a technician rather than continuing to run it.
6. Do not attempt to clean or adjust burners yourself; this requires disassembly and gas safety knowledge.

Tools and parts

• Flashlight

When to call a pro

Call an HVAC technician to inspect and clean the burners if ignition banging is happening every cycle or getting louder.

Safety