Troubleshooting
Furnace Banging or Booming Noise
Quick answer
Banging on startup is usually delayed ignition — gas pools before lighting and ignites with a pop. Stop running the furnace and call us. It's a real safety issue.
Common causes
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Delayed ignition (boom on startup)
urgentGas pools in the burner box before igniting → boom. Caused by dirty burners or bad ignitor. Don't keep running it.
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Cracked heat exchanger
urgentThermal expansion of a crack makes a sharp pop. Replace the unit.
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Loose blower wheel
moderateBlower wheel set screw loose — wheel hits the housing on startup. Tech tightens it in 5 minutes.
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Expanding ductwork
easySheet metal expands when warm air hits it. Pop or boom from ducts on first cycle of the day. Harmless but annoying — adding cleats reduces it.
DIY first
- 1 Note exactly when the noise happens — startup, mid-cycle, shutdown — and from where (cabinet vs ducts vs registers).
- 2 Don't ignore a startup boom. That's the dangerous one.
Call us if:
- Bang happens when the burner ignites
- You can smell gas before the bang
- Bang is loud enough to feel through the floor
- Bang has gotten louder over time
Other symptoms in this category
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Diagnosis pointing somewhere serious? Call now.
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Mississauga, ON · Greater Toronto Area and up to 2 hours out — London, Kitchener, Barrie, Kingston, Niagara.