Why Does My Furnace Make a Loud Banging Noise When It Turns On?
Few things are as startling as a loud "bang" or "boom" coming from your basement the moment your furnace kicks in. While some noises are a normal part of your home’s heating cycle, a sudden, explosive sound can be a major warning sign. At Hatco Heating & Air Conditioning, we help Ajax and Durham homeowners distinguish between simple "house noises" and critical mechanical failures that require immediate attention.
Common Causes for Banging & Booming
1. Delayed Ignition (The "Boom") This is the most common cause of a loud "boom" or "bang" right at startup. If your furnace burners are dirty or the igniter is failing, gas builds up in the combustion chamber for a few seconds before finally lighting. When it does ignite, that buildup causes a miniature explosion.
The Risk: Repeated delayed ignition can crack your heat exchanger, a repair that often leads to a full furnace replacement and carries a risk of Carbon Monoxide leaks.
2. Expanding & Contracting Ductwork (The "Pop") If the noise sounds more like a metallic "pop" or "thud" throughout your house, it’s likely your air ducts.
The Cause: As hot air rushes into cold metal ducts, the metal expands. When the furnace turns off and the metal cools, it contracts back.
The Fix: While often harmless, excessively loud popping can indicate undersized ducts or a severely clogged air filter that is causing pressure imbalances.
3. Dirty Burners Over time, dust and soot accumulate on the burner ports. This obstructs the flow of gas, leading to uneven ignition and the startling "banging" sound you hear. Annual maintenance is the best way to prevent this buildup.
What Should You Do?
Listen Closely: Does the bang happen exactly when the flame should light, or a few minutes later in the vents?
Check Your Filter: A clogged filter can cause pressure issues that make ductwork "pop" much louder than usual.
Shut It Down: If you hear a loud "boom" accompanied by the smell of gas, turn off the furnace at the switch immediately.
Schedule a Safety Check: Delayed ignition is a safety hazard that won't go away on its own.
Stop the "Bang" with Expert Furnace Service in Ajax.
Don't ignore the warning signs of a failing furnace. Hatco’s TSSA-certified technicians specialize in burner cleaning and heat exchanger inspections across Pickering, Whitby, and Oshawa. As an authorized Lennox and Rheem dealer, we have the tools to restore your system’s quiet, safe operation.