first, if you have an air conditioner in your attic, then there should be TWO drip pans, and TWO drain lines.
- one of the drip pans should be built in to the air handler/coil unit
- the second drip pan should be at least as big as the entire air handler and coil unit, and positioned under the air handler to provide coverage for the entire unit
- both pans should have INDEPENDENT drain lines
- ideally, the second drip pan should have a float sensor unit it to shut off the system BEFORE it overflows.
either drain should be able to completely empty whatever amount of water you have in the pan within seconds. if it can't, then you have a clog somewhere.
also, the large drip pan below the air handler should typically be completely dry. it's there as a backup in case the drain line for the air handler clogs up. if you have any amount of water in there, then you have a problem somewhere else.
so, than being said, do the following:
- verify that both the air handler and the external drip pan have independent drains
- get a wet/dry shop vac and suck out both drain lines
- pour bleach down both drain lines
- if you don't have a way to pour bleach down the drain connected to the air handler, make one. a simple way is to cut the existing line at a vertical run, splice in a T, then make an L shaped piece out of some tubing and an elbow. leave the top part unglued so you can remove it. see drawing.
- test your newly cleaned drain lines. remember, both drain lines should be able to quickly remove any water you pour in them. if they drain slower than your bathroom sink, you have a clog somewhere.