When flexible duct is used instead of galvanized steel duct to provide combustion air, the duct diameter shall be increased by one inch.

When flexible duct is used instead of galvanized steel duct to provide combustion air, the duct diameter shall be increased by one inch.

• A. Shall be increased by two inches
• B. Shall be increased by two duct sizes
• C. May remain the same
• D. Shall be increased by one inch

Answer: (D)

Flexible duct adds more resistance to air flow than rigid galvanized duct because of its folds, seams, and rougher interior surface. That extra friction and potential for turbulence reduce the amount of combustion air that reaches the appliance at a given duct size. To ensure the required amount of air is delivered for safe and proper combustion, the duct size must be increased when flexible duct is used. This sizing adjustment helps maintain the necessary airflow and prevents issues like incomplete combustion or backdrafting, which could lead to hazardous conditions. The other options either overstate the needed increase or suggest no change, which would not meet the requirement to compensate for the additional resistance of flexible duct.

Flexible duct adds more resistance to air flow than rigid galvanized duct because of its folds, seams, and rougher interior surface. That extra friction and potential for turbulence reduce the amount of combustion air that reaches the appliance at a given duct size. To ensure the required amount of air is delivered for safe and proper combustion, the duct size must be increased when flexible duct is used. This sizing adjustment helps maintain the necessary airflow and prevents issues like incomplete combustion or backdrafting, which could lead to hazardous conditions. The other options either overstate the needed increase or suggest no change, which would not meet the requirement to compensate for the additional resistance of flexible duct.