If a furnace makes a loud bang upon ignition, what is the technician likely to check first?

In situations where a furnace makes a loud bang upon ignition, a technician would likely check the supply gas pressure first. A loud bang can indicate that an excessive amount of gas has accumulated in the combustion chamber before ignition, leading to a small explosion as the gas ignites. If the gas pressure is too high, it can cause more gas to enter the chamber than the system can handle safely.

In this context, ensuring the correct supply gas pressure is crucial for safe and efficient furnace operation. Adjusting the gas pressure can prevent dangerous conditions, such as delayed ignition or backdraft, which could lead to larger explosions or mechanical failures.

Although checking the ignition sequence is also important, it typically comes after verifying that the gas supply is at the correct pressure. Similarly, while thermostat settings and fan motor operation are relevant to the overall performance of the heating system, they are less likely to relate to the immediate danger indicated by the loud bang during ignition.