There is only the one condensation drain for the 2, A/C units which are located in the attic. It drains through an overflow pipe which hits the ground from 20' up and right outside the window near the head of my recliner. Very irritating.
There must be something wrong with either the location of the backup condensation drain (BCD) or its placement on the basin drain (PBD), or ???.
I have attached a rough drawing showing it as it exists. I think that I have proved that it is the problem, as I have disconnected the A/C condensation drain hose from the basin drain and the external drip, drip, drip, has stopped.
So, what do I need to do, and I am forever in your debt.





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Let me see if I can figure out how to post the image. Grrrrr

Those condensate drains are notorious for changing the liquid water to a snot like substance and can cause a stoppage. You need to flush periodically by using a garden hose while plugging any vent it may have.

the connection to the basin is the PRIMARY drain where the condensate SHOULD be flowing


the outside drain through the eave above your bedroom is the (emergency) backup drain to prevent a flood if the primary is obstructed

it is so located to be CLEARLY visible when, not if but when, the primary drain fails


you have a stoppage in the primary drain which usually is 'fixed' by 'back blowing' with air


THEN


the primary drain MUST be periodically serviced, usually by pouring a bleach solution INTO the evaporator coil pan located at the 'air handler' section of whatever HVAC system is in operation

'your' system is evidently in the attic and the secondary pan/drain is actually doing its job of saving your ceiling from water damage due to the primary drain's obstruction

HOWEVER

it would be even safer if a float switch to shut down the unit in the event of said obstruction was actually installed and functional

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638



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locate the drain hole in the pan in the attic. cover the sink with a big, heavy towel.
put a garden hose into the drain hole in the attic and blow water into the drain.
that should clear it.

easy does it..

the (access to) the inlet of the hole in the drain pan would be INSIDE the casing of the air handler

here is a PROPER installation (from my attic)

note the multiple access points for service and/or rodding and/or brushing



the pipe labelled 'primary drain' (in the OP's) case would go to the basin tailpiece

the secondary pan drain labelled 'pan safety switch' (higher outlet than the primary) can be EITHER piped out with an alarm OR 'plugged' with a cut-off switch

the pipe labelled 'external safety pan' is REQUIRED (as is the EXTERNAL pan) for an A/C air handler located in the attic as a 'last resort' fail safe (should the pan in the air handler leak) to prevent ruined ceilings below - said pan MUST be piped to the exterior terminating in a clearly visible outlet - usually a short piece sticking down from the eave

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638