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The pressure drop is negligible but what can be done is to compare the size of return air gap with any grille of same size and same CFM and check the noise level and pressure drop across.
Basically we consider it as a free return so there is no pressure drop and if there any it will be negligible.
The pressure drop in Air return gap or ( Air plenum ) is taken as 0.05 in wc (12.5 Pa ) which is a negligible value as compared to the conventional Return air duct pressure drop.
10 to 15 pa is believed to be ideal pressure drop form return air gap usually the air gap is neglected as it is free return so no need to consider
