===== Duct Sizing Rules of Thumb =====

General rule would be that 1sq" of filtered return will move 2 cfm.

Figure unit tonnages at 450-CFM per-ton of cooling.

Therefore our 2 1/2 ton unit upstairs needs 1125 CFM of air movement.

A ton of refrigeration (12,000 BTUs per ton) is the amount of 
heat required to melt a ton (2000 lbs.) of ice at 32 degrees Fahrenheit.