A blower type warm air funace must have its air supply duct sized for its Btu output rating of the furnace. What is the minimum unobstructed cross sectional supply duct area required for a 180,000 Btu furnace?

A blower type warm air funace must have its air supply duct sized for its Btu output rating of the furnace. What is the minimum unobstructed cross sectional supply duct area required for a 180,000 Btu furnace?