In an ante-room area open to the attic (something like a storage closet without a back wall), I found a GOODMAN (HOUSTON, TX) HVAC unit mounted upright on a wooden box (obviously a plenum of sorts...no visible auxiliary overflow pan).

Finally found the pan in an adjacent air return (on the stairway no less), that is under the "box" which is under the HVAC unit. Auto shut-off switch in the pan. Like standing in a 30mph wind tunnel inside the return...never stood inside a return in operation...good unit.

No auxiliary drain to the exterior and no visible pan (as if the owner would ever go into the attic or open the air return to check for an auxiliary pan).

The installer made a "donut" pan to go around the bottom of the HVAC unit (under the coils) without interferring with the return air... and added another shut-off switch in the new pan....impossible to run another drain pipe from that area of the house to the exterior. Seems to be working. Comments?

Thanks,

Don