When using two vertical ducts for a 130 k btu/hr furnace and a 65 k btu/hr water heater, what is the minimum size of the round vertical ducts required?

In determining the minimum size of the round vertical ducts required for venting a 130,000 BTU/hr furnace and a 65,000 BTU/hr water heater, it's essential to consider the combined BTU output of both appliances. The total BTU output is 195,000 BTU/hr.

For venting systems, duct size is critical to ensuring proper airflow and safe operation. The sizing is usually calculated based on the total input rating of the appliances being vented. When dealing with gas appliances like furnaces and water heaters, the venting must support the cumulative BTU input without causing excessive pressure drop or backdrafting.

Industry standards provide guidelines for duct sizing based on BTU capacity. For instances like this, a round duct size of 8 inches is typically used for venting systems that handle a combined output of approximately 195,000 BTU/hr. The 8-inch round duct offers adequate cross-sectional area to carry the combustion gases safely and efficiently, adhering to the requirements of the National Fuel Gas Code (NFPA 54) and other applicable standards.

Selecting a smaller duct, such as 6 inches or even 7 inches, would inadequately accommodate the volume of gases produced by the combined appliances, potentially