Frustrating condensate drain issue.

[chazysciota]

Hello, I am hoping that someone can help me with this issue I'm having with my heat pump A/C (split system). The air handler is in the attic, and I seem to be having trouble with the drain. When the temperate rises over about 85F, the A/C starts blowing warm air. If I go and suction the drain with a shop-vac, a decent amount of clean water gushes out and after about 15-30 minutes, the unit starts blowing cold air again. In hot weather, we have to do this at least once a day. So something is causing the drain to back up, but I can never seem to suck the clog out. Is there a better way to clear * Someone mentioned that I should have someone "cover the bypass in the attic" while I vacuum the drain, to create more suction; But I'm unclear on what that means exactly.

So, my question is what would cause the drain to back up only in extreme temps; and what can I do to clear the drain * A technician looked at it a few weeks ago, and said that apart from being very old (17 years) and having dirty and rusted evaporator coils, everything seemed to be in working order. Indeed, when the water is sucked out of the drain, it blows cold. I realize the unit will have to be replace sooner or later, but if all I have is a drain problem at the moment, then I'd like to get that cleared up. Can anybody *

EDIT; I should mention that there does not appear to be any water overflowing into the emergency pan. There is a float-switch in the main condensate line, that presumably stops the backup from overflowing.

[nomadpeo]

agree with the duke. an electric air handler pulls air through the coil, creating a negative pressure. for that reason, air can also pull through the main drain, preventing water from draining out. a trap in the main drain creates a liquid seal preveinting that from happening. without it, the water won't leave the main drain pan until the fan turns off. during running time, the pan will fill, then overflow into the secondary drain pan. there should not be any water in that pan. however, if there is and it doesn't drain out for some reason (it has its own drain line terminating out the eave of the house, usually in fron of a window so you can see it dripping), the float switch will open the compressor signal and stop water production, and of course cooling as well.