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 it? 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 better? 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 help?
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.