What is the purpose of pre-purge in a boiler start-up sequence?

• A. To purge the furnace and piping to remove any combustibles before ignition.
• B. To pressurize the boiler.
• C. To heat feedwater to operating temperature.
• D. To test the alarms.

Answer: (A)

During a boiler start-up, the first safety step is to clear the furnace and its piping of any combustible gases. Pre-purge accomplishes this by blowing air through the furnace and connected ducting so that any residual fuel vapors are swept out and vented before ignition. This creates a non-flammable environment, so when fuel is introduced and the burner lights, there’s nothing that could ignite a lingering gas mixture, preventing explosions or dangerous flashbacks.

It’s not about pressurizing the boiler, heating feedwater, or testing alarms. Those functions occur at other stages or aspects of the start-up, while pre-purge specifically focuses on removing combustible vapors to ensure a safe ignition.