A blower type warm air furnace 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 furnace 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?