🔍 Key Concepts

• Boiler flareback – what causes unburned fuel to suddenly ignite in the furnace • How residual heat, direct flame, or spark/igniter affect safe fuel ignition • The difference between controlled ignition sources vs. more diffuse or indirect heat sources

💭 Think About

• Which method makes it hardest to precisely control where and when the fuel first ignites in the furnace? • Which method is most likely to allow a buildup of unburned fuel/air mixture before ignition actually occurs? • Think about which source might give a slow, uneven heating instead of a sharp, positive spark or flame at the burner tip.

✅ Before You Answer

• For each option, picture how the flame actually reaches the fuel at the burner nozzle. • Consider which method is most likely to leave unburned fuel in the furnace before ignition takes place. • Ask yourself which method is least direct and least positive as an ignition source at the burner register.