Gas and Oil Home Heating Furnaces - Condenser drain leak

Discovered that the drain line (not the emergency overflow) from the coil in the attic is backed up and leaking onto my ceiling. Where does this drain line go and how do I remove the clog? Is it an a/c problem or plumbing problem?

There is about 8 feet of pvc from the coil box in the attic, and this line drains into another larger diam. pvc that is full of water. I've tried to trace it into the bathroom but can't figure out where it goes or how to unclog it.
Thanks,
Brian

Not sure what you have here. First this over flow water from the unit should go into the drain pan that is under the unit there not on the ceiling thats why its there. I think I would put a pan under the 8' pipe and cut it say at 4' take a shop vac and suck on both the ends there and then blow .Hope that this pipe goes into a P trap before it goes into that vent pipe if thats what you have. you can put the pipe back with just a Pvc coupling.;) ED