You can use the temperature drop across the cooling coil to check your C.F.M.
Did you ever wonder where this came from? "A 20 degree temperature drop across the coil indicates the system is operating properly".
The formula is: S.H.C. / (C.F.M. x 1.08) = Temperature Drop.
When you use a sen heat factor of 0.75 and 400 C.F.M ton, the temperature drop will always be 20.8 degrees.
Example: 4 ton system - 1,600 C.F.M. = 48,000 x 0.75 / (1,600 x 1.08). 36,000 / 1,728 = 20.8 degree T.D (temperature drop).
Same system at 1,300 C.F.M. = 48,000 x 0.75 / (1,300 x 1.08). 3,600 / 1,404 = 25.6 degree T.D.
A 25.6 degree temperature drop could mean 45 degree LAT, which could result in some coil icing. Also, 45 degree is probably below the room dew point and can cause some condensate on the supply grills.
Note: make sure the unit is properly charged and take temperature at the air handler unit.
