If you have any vent (open pipe in the shape of a T) on the drain line before the p-trap it needs to be capped; if it's after the p-trap then that's fine. What you're describing is a sign of improper drainage due to a deficient drainage system. Describe your drainage system.

Good advice.

P-trap, p-trap, p-trap.

This is a common problem on air handlers and rooftop units where the coils are upstream of the blower.

Proper trapping is required!!!

The drain for the air handler has two ¾" PVC drain holes of which my HVAC contractor had hooked up reducing fittings from the ¾" down to 1/2". Then he hooked up clear plastic tubing to both drains and combined them into one PVC pipe further downstream. It would have been nice to have them separated so I could see if the main or the backup was flowing. How does having a P-trap make the water flow against the air and into the drain tubing? Does it have something to do with causing a suction or vacuum? Would the P-trap be required only on the main drain where as the overflow drain can be straight out? Does the P-trap need to be cleared out before our MN cold winters come along? I appreciate all of your advice.

Reducing threads in the coil pan drain holes only raises the water level in the higher before draining which makes it worse.

If you coil were to be downstream of the blower. Such as an A-coil onto of an up flow furnace you wouldn't be having this problem.

The P-trap creates a barrier that breaks the suction and as condensate is created it fills the p-trap displacing existing water at a lower level, thus it goes down the drain.

While it is good to have a proper installed overflow drain to backup the main drain, it must also be trapped properly, or it will suck in air and impurities from wherever it is draining to. Many times I have seen these drain into a catch pan or be changed to vinyl tube and looped low to create a trap.

Worried about freezing, I'm guessing that you may have your AC unit in an attic/unheated space?

Rarely does a P-trap freeze under those conditions and tend to have dried out before being exposed to freezing conditions. However, if your duct static pressure is not too high and properly installed, a "running trap" could be used with better results.

Here is a link that might be useful: Types of traps

Thanks for the info everyone, it took me a day or two to think it over but I now understand what you were explaining. Would it work to use 90 deg elbows and a few pieces of straight pipe to make my P-trap. It shouldn't matter if the bottom part of the P-trap is straight because of joining two elbows together. Or does it matter? I thought that since I had the parts lying around I could make good use of them. If that doesn't sound like it will work where would be the best place in Minneapolis, MN to get them locally because Home Depot and other stores like it do not carry P-traps in 3/4 inch. Thanks again for all your advice, I appraciate it greatly!

I've seen traps made as you describe. If you do this, I would monitor it to ensure proper drainage of course. Make sure the vent, if you choose to have one, comes after the trap and not before.

You can make your own, they just tend to catch material because the bend is not as smooth.

Keep an eye on it and perhaps blow it out once in awhile and you'll be fine.

"Keep an eye on it and perhaps blow it out once in awhile and you'll be fine."

Sucking the line out with a shop vac from the open end works very well.
Use a wet rag to make a tight seal between the shop vac hose and the drain line.

Thanks for the info. If the air handler is a sealed system and the air returns are filtered, what kind of material would collect in the condensate?

Airborne particles that pass through or bypass the filter that drain off wet coil and unwanted growth cultures in pan and trap.

(With even a HEPA filter system if it is 100% filtration, it must be blocked)