What should a technician check if a furnace makes a loud bang upon ignition?

A loud bang upon ignition in a gas furnace is often caused by a delay in ignition, which can lead to a build-up of gas in the combustion chamber. When the gas finally ignites, it can create an explosive sound due to the sudden release of pressure as the gas ignites all at once.

Checking the gas pressure is essential because if the pressure is too low, the burner may not ignite properly, leading to a delay in ignition. Alternatively, if the gas pressure is too high, it can cause an excessively rich fuel mixture, which also contributes to delayed ignition. Proper gas pressure ensures that the furnace ignites smoothly and efficiently, thus preventing the loud banging noise.

While checking other components like the thermostat, blower motor, or electrical connections is important for the overall operation of the furnace, they are less directly related to the specific issue of a loud bang upon ignition. The immediate concern is the gas pressure, as it directly influences the ignition process and is crucial in preventing such occurrences.