When flexible duct is used in place of galvanized steel duct to provide combustion air, the duct diameter shall be increased by what amount?

When flexible duct is used in place of galvanized steel duct to provide combustion air, the duct diameter shall be increased by what amount?

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

Answer: (B)

When flexible duct is used for combustion air, the duct’s resistance to flow is higher than that of rigid galvanized duct. To ensure the appliance still receives the required amount of combustion air, the duct diameter must be increased by one inch. This modest upsizing compensates for the added friction and potential restrictions in flexible duct, helping maintain adequate airflow to the appliance. Increasing by two inches would be overkill, leaving the size the same would risk under-supplying air, and “two duct sizes” isn’t a standard method.

When flexible duct is used for combustion air, the duct’s resistance to flow is higher than that of rigid galvanized duct. To ensure the appliance still receives the required amount of combustion air, the duct diameter must be increased by one inch. This modest upsizing compensates for the added friction and potential restrictions in flexible duct, helping maintain adequate airflow to the appliance. Increasing by two inches would be overkill, leaving the size the same would risk under-supplying air, and “two duct sizes” isn’t a standard method.