Is there a rule of thumb for duct sizing?
In another thread, I posted that the main supply duct coming out of my 4-ton unit is 8 X 10 inch galvanized duct and a reply said it was "WAY undersized for 4 tons." I was assured the Man J and D were done by the reputable HVAC sub, so I am perplexed that it could be WAY off. Is there a rule of thumb or "napkin calculation" for duct sizing that I can use to determine if this is WAY off? If you have a 4-ton system, what size ducts do you have? Thanks!