How do i measure the proper size air return vent and ductwork for a new A/C system?

I have a 60+ year old house, two story with basement. There is no current A/C system and I want to install one but am running in to a wall, literally, with supplying an air return from the sexond story.

How do i measure the proper size air return vent and ductwork for a new A/C system?
the generall rule of thumb on sizing an a/c is 400-500 sq feet per tone. depending on how well imsulated. with old house id say use the 400 number, you will need probably a 4-5 ton unit. the thing is, unless you have a refrigerant license and a legit a/c tech, you cannot purchase the a/c yourself. as far as return air,run a 14" return flex duct into a ceiling mount return air box aprox 20x25 into hallway on second floor. as for suply ducts, depends on size of a/c used and size of rooms, There is a chart called a DUCTULATOR that hepls u determine what you need per room.. but honestly consult a qualified A/C service company and have them come do a work up of your house. trust me, its better to do it right first time, then fight it forever.
Reply:From my ten years in the business and 2 years of owning my own company I have found that the best rule of thumb is 600 sqft. area per ton of cooling and 400 to 500 cfm (cubic feet per minute) of volume per ton.





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