I stumbled today upon the following scenario.

Went to a house for a regular maintenance on A/C. Turned the unit "ON" and took Temp. drop accross Evap. coil, 68F to 55F- only 12F.

Then decided to measure the ESP. Drilled two holes, just below and just above Evap. Coil. The readings were: 1. before the evap. coil = 0.92"w.c. just above the coil= 0.12"w.c.
On the other side - the Return duct showed (-0.12"w.c.)

I could not make sense of it. Is the coil plugged ?

Why below 0.92"w.c. and above 0.12"w.c. ?

Filter was O.K.

Actually, when I pulled the filter - it read 1.10"w.c. just below the Evap. coil.