🔍 Key Concepts

• Boiler furnace purging before light-off • Difference between combustible vapors and inert vapors/oxygen • Effect of air register doors open vs. closed on clearing the furnace

💭 Think About

• Ask yourself: what is the main safety goal just before lighting an idle boiler—what are you trying to remove from the furnace to avoid a flareback? • Think about how air actually flows through the furnace when the forced draft blower is on—what happens if the register doors are open vs. closed? • Which combination of gas type (combustible vs inert) and register door position would best ensure any dangerous mixture is swept out before ignition?

✅ Before You Answer

• Verify which type of vapors are actually dangerous in a flareback situation. • Check whether an effective purge requires a clear air path through the furnace (consider the position of the air register doors). • Confirm that the blower operation and register position chosen would maximize removal of unburned fuel/combustible mixture, not safe or inert gases.