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

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