Which of the following conditions is NOT good for burner operation?

Flames that touch the burner housing indicate an improper combustion condition, which is detrimental to burner operation. This situation can lead to overheating of the burner components, potential damage to the equipment, and a risk of fire.

In contrast, a well-defined flame shape suggests that the burner is operating correctly, allowing for efficient heat transfer and stable combustion. A blue flame color is a sign of complete combustion, indicating that the burner is functioning properly with the right air-to-gas ratio. Stable gas pressure is essential for maintaining consistent burner operation; fluctuations in gas pressure can lead to unstable flames and variations in heat output, impacting both safety and performance.

Thus, among the options presented, flames that touch the burner housing represent an unsafe and inefficient operating condition.