A homeowner's furnace makes a loud bang upon ignition. What should the technician check first?

In a situation where a homeowner's furnace makes a loud bang upon ignition, it is crucial to assess several factors to diagnose the problem correctly. The loud bang can be indicative of delayed ignition, which occurs when gas builds up in the combustion chamber before igniting, leading to a sudden explosion.

Checking the supply gas pressure is essential because if the pressure is too low, it may cause improper gas flow and delay ignition, resulting in a bang.

The vent system is also vital to inspect since improper venting can lead to a buildup of flue gases, which may exacerbate ignition issues.

Lastly, burner alignment must be examined. Misaligned burners can lead to uneven combustion and can contribute to delayed ignition, causing a loud noise upon startup.

By identifying and checking all these potential issues—supply gas pressure, vent system integrity, and burner alignment—the technician can pinpoint the cause of the bang and implement the necessary repairs. This comprehensive approach ensures the furnace operates safely and efficiently, minimizing the risk of hazardous conditions.