So I still got flooding of my air handler (it's got a catch pan under it so no damage below). Latest theory yields excessive static pressure (vacuum) in the air handler due to too small a return plenum and filter grill. Even though I have the condensate drain line trapped, it cannot drain fast enough with all that neg. pressure in the air handler. Filter grill is a single 24"x24" grill but the return plenum only fits a 20"x20"x1" air filter. I figured this was OK since the 3 ton air handler came shipped with a 16"20"x1" filter. I removed the 16" filter and installed the 20" size in the return plenum box instead. I even lowered the fan speed to low since i wanted the best humidity removal possible...and the unit does not freeze up at that fan speed either.
So how much neg. pressure can a condensate line accomodate before it just can't drain? The trap I built has the downrun length almost 3 times the upslope length. Any ideas?