Why is my furnace making a loud banging noise?

A loud banging noise from your furnace typically indicates either delayed ignition (gas buildup causing a small explosion when lit) or expanding/contracting ductwork, both of which need attention.

The most concerning cause is delayed ignition, where gas builds up in the combustion chamber before igniting, creating a loud bang or boom when it finally lights. This happens when the burners are dirty or the gas valve isn't opening properly, causing gas to accumulate before ignition. This is a safety issue that requires immediate professional attention, as it can damage your heat exchanger and poses carbon monoxide risks.

Expanding ductwork is another common cause, especially when your furnace first starts up. As heated air rushes through cold ducts, the metal expands rapidly and can create banging sounds. While less dangerous than delayed ignition, loud ductwork banging often indicates undersized ducts, loose connections, or missing expansion joints that should be addressed.

Other potential causes include a cracked heat exchanger (very serious - can leak carbon monoxide), loose blower motor components, or debris in the blower assembly. In Ottawa's climate, these issues often worsen during our cold winters when furnaces work hardest.

Turn off your furnace immediately and call a licensed HVAC technician. Don't attempt to restart it until it's been inspected. TSSA regulations require that only licensed G2/G3 technicians work on gas furnaces, and delayed ignition issues can be dangerous if not properly diagnosed and repaired.

For immediate service in Ottawa, we can connect you with emergency HVAC repairs and can typically diagnose furnace noise issues the same day. Most banging noise repairs range from $200-600 depending on the underlying cause.