Symptoms
- Distinct boom or thump specifically at ignition, not during the rest of the cycle
- May be getting louder or more frequent over time
- Sometimes felt as a vibration through the floor
Likely causes
- CommonDirty or partially clogged burner portsDebris on the burners delays and unevens ignition, letting gas build up first.
- PossibleCracked or misaligned igniter causing delayed light-offA weak spark or glow takes longer to ignite the accumulated gas.
- Less commonGas pressure issue from the utility or a regulatorIncorrect gas pressure can affect how quickly and evenly burners ignite.
- Less commonCracked heat exchanger allowing abnormal combustionA more serious internal issue that needs prompt professional inspection.
What to do
- Replace the air filter in case restricted airflow is affecting combustion.
- Note how loud and how frequent the boom is, and whether it's getting worse over successive heating cycles.
- Avoid running the furnace repeatedly to 'test' the boom; instead schedule a technician visit soon.
- Turn the furnace off and use backup heat if the boom is severe, very loud, or paired with any burning smell.
- Have burners professionally cleaned and inspected rather than attempting to clean them yourself.
When to call a pro
Call an HVAC technician promptly to inspect and clean the burners and check for a cracked heat exchanger.
Safety
Use caution
A worsening ignition boom can indicate a combustion safety issue; turn the furnace off and call a technician if it's loud, worsening, or accompanied by any smell.