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Originally Posted by Weedy1
With ^^^ being said, I have often wondered how a resi contractor determines the actual air flow of an existing installation where the homeowner doesn't want a new furnace? Do any of the contractors do traverses or static pressure curves to determine the true operating point of the blower?
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New furnace or not is not relevant. A contractor first needs to determine what the air handler/furnace is capable of, then he needs to determine maximum and minimum air flow the duct system may be capable of.
There is some differing options as to what static pressure variable should be used to determine duct sizing. Many of our local contractors insists on using a static pressure factor that ices coils and trips limit switches. I have talked to dozens of homeowners that are completely frustrated with equipment that just wont function properly due to under sized ducting. They do this obviously to save money, along with the lightest gauge tin they can get away with. At best you get a noisy and inefficient system, at worst the system just doesn't function or fails prematurely.
So to answer your question, it is very difficult to determine the exact airflow without x-ray vision or some background info of what is in finished walls and ceilings. It is not difficult however to determine if you meet the minimum requirements of the equipment you wish to install simply by measuring the exposed duct work at the furnace, counting the registers/returns and checking air flow at the registers. A simple slide type duct sizing calculator and a little math is all that is needed.
The rule of thumb is a minimum of 350 to 400 CFM per ton of AC.