A loud bang or boom when your gas furnace starts is usually delayed ignition: gas builds up for a moment before it lights, then ignites all at once. Common causes are dirty burners, a weak or misaligned igniter, or low gas pressure. Delayed ignition isn’t something to ignore, because repeated small explosions can crack the heat exchanger over time. A softer pop or ticking sound from expanding ductwork is a different, usually harmless, problem.
Safety first: if you smell gas, leave the home and call your gas utility or 911 from outside. If the bang is loud and repeated, turn the furnace off at the thermostat and call a technician.
Delayed ignition vs. ductwork noise
| Sound | When it happens | Likely cause | Urgency |
|---|---|---|---|
| Loud boom or bang at the furnace | A few seconds after burners should light | Delayed ignition | Call a technician soon |
| Pop, bang or ticking in the ducts | When the blower starts or stops, or as air heats up | Metal ducts expanding and contracting (“oil canning”) | Usually harmless; can be fixed |
| Bang as the blower starts | When the blower kicks on | Ducts flexing, undersized return | Low |
Causes of delayed ignition
Dirty burners
Dust, rust and debris clog the burner ports, so gas from the first burner doesn’t carry flame to the others quickly. Gas builds up, then lights with a bang. Cleaning the burners during a tune-up usually solves it.
Igniter problems
A weak or misaligned igniter takes too long to light the gas. See furnace ignitor not glowing.
Incorrect gas pressure
Gas pressure that’s too low or too high affects how evenly the burners light. Only a technician should measure and adjust gas pressure.
Venting and draft problems
Poor draft from a blocked or poorly designed vent can disturb how the flame spreads.
Cracked heat exchanger (result, not just a cause)
Repeated delayed ignition stresses the heat exchanger. A cracked heat exchanger can let combustion gases, including carbon monoxide, mix with your air. If a technician finds one, stop using the furnace.
What you can do
- Turn the furnace off if the bang is loud or repeated, and schedule service.
- Make sure your carbon monoxide alarms work.
- Replace the air filter.
- Don’t attempt to clean burners or adjust gas pressure yourself.
A furnace tune-up that includes burner cleaning typically costs $75 to $200; an igniter replacement typically $150 to $400.
Fixing noisy ductwork
If the noise comes from the ducts, a technician can add bracing or crossbreaks to flat duct panels, check whether the return is undersized, or adjust blower speed. A slower blower ramp-up on variable-speed systems often reduces popping.
When to replace the furnace
If your furnace is 15 to 20 years old and has a cracked heat exchanger, replacement usually makes more sense than repair. See how long a furnace lasts and furnace replacement cost.
Frequently asked questions
Is a furnace boom dangerous?
Delayed ignition can be. It stresses the heat exchanger and can lead to cracks. Have it inspected promptly.
Why does my furnace bang when it shuts off?
Usually ductwork contracting as it cools. It’s mostly a nuisance and can be reduced with duct reinforcement.
Can I keep using my furnace if it bangs?
If it’s a small occasional pop from the ducts, yes. If it’s a loud boom from the furnace itself, turn it off and call a technician.
Disclaimer: this guide is for general information only. Gas furnaces involve combustion and gas lines. If you smell gas or your CO alarm sounds, leave your home and call 911 or your gas utility from outside.