There is nothing in the code that sizes a return-air grille other than the 450 fpm that is found in section 603.18 (2012 NCMC).
I propose that a section be added to the code per the following:
"The minimum size of return-air grilles shall be 1 square inch per 2 cfm of air flow."
This gives the inspector a minimum size and the 1 SqIn per 2 cfm is the industry accepted standard to size return-air grilles.
For instance, a 2 1/2 ton AC system has approximately 1000 cfm air flow. Based on the statement above, a 500 SqIn return-air grille would be required. Any combination or number of grilles that equal or exceed the requuired size would be acceptable.
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