If too much primary air is used, the flame will _____?

• A. flicker
• B. stabilize
• C. shorten flame
• D. lift off of the burner

Answer: (D)

Too much primary air makes the fuel–air mix at the burner too lean and increases the velocity of the air jet. That fast-moving jet can carry the flame away from the burner, causing it to lift off the burner face. When the flame lifts off, combustion becomes harder to sustain at the burner and heat transfer can drop. Flickering would indicate an unstable flame still attached near the burner, while a stabilized flame means the air–fuel balance is proper. A shorter flame is usually due to other factors, like fuel flow or different air distribution issues, not simply excessive primary air.