What is the primary purpose of purging a furnace before igniting it?

The primary purpose of purging a furnace before igniting it is to remove flammable gases. This step is crucial to ensure safety during the ignition process. If flammable gases such as unburned fuel accumulate in the furnace, igniting them can create a dangerous explosion hazard. By thoroughly purging the furnace with air or an inert gas, the concentration of any potentially hazardous gases is significantly reduced, allowing for a safer ignition of the burner.

The other options, while they may represent important aspects of furnace operation, do not specifically address the critical safety concern related to the ignition process. Increasing temperature, reducing pressure, or stabilizing combustion are important for optimal operation and efficiency but are not the immediate focus when preparing for ignition. Therefore, removing flammable gases is the primary goal that directly relates to preventing hazards during the start-up of the furnace.