🔍 Key Concepts

• Air register doors (item 12) control combustion air for each individual burner, not for the whole furnace at once. • Relationship between combustion air, flame stability, and excess air for a burner that is lit versus one that is idle or secured. • How air flow to an idle burner can affect furnace draft, heat loss, and risk of flame entering the idle burner register.

💭 Think About

• Think about what you want the air register doors to do when a burner is actually firing. Do you normally want maximum air, or do you usually trim the air to match the fuel for best combustion? • For a burner that is not in use, consider whether you want furnace gases and hot flame fronts to be able to flow back through that open passage, or whether you want to isolate that burner from the furnace. • Ask yourself if the air register for one burner should depend on what the other burners are doing, or if each burner’s register is adjusted according to that burner’s own firing condition.

✅ Before You Answer

• Verify which choice treats the lit burner as needing controlled (throttled/adjusted) air rather than just "wide open" all the time. • Verify which choice fully isolates an idle burner from the furnace to prevent heat loss and possible flame impingement or flashback. • Eliminate any option that suggests the same air register position for burners that are lit and idle, since those conditions clearly need different air flow.