Why does condensate miss the pan?

Hi, forgive me if this has been answered already, but I did not find anything when I did a couple of searches.

At my work, we have several York heat pumps that are roof mounted on a flat roof building. They are all between 3 to 5 ton units.

We have a recurring problem with one of them: after several hours of AC use, water will drip from the air supply duct. Unfortunately, there is a fairly expensive piece of electronic test equipment below, so we would like to solve the problem. I talked to the technician who performs our maintenance and repair on the units about this problem and he does not seem to know how to fix it. He blamed the problem on the condensate line drain and said that his company could replace all the lines for us for just under $500. Since he was going to use PVC, I declined and decided to do it myself.

I replaced the line to this unit with new 3/4 inch PVC and created a water trap as instructed on the label outside the unit. I also did this for several others at the same time. The other units are draining well, but this particular unit has not produced any condensate out of the drain.

I opened the unit up enough to look inside where the condensate pan is and there was not a drop of water in the pan. The drain is completely clear and I ran a wire through to verify. But the insulation pieces on either side of the condensate pan were soaking wet. WTF! Again, the pan is dry but the insulation just to the side of the pan is wet. The pan does not appear to be damaged or leaking (steel, no apparent rust).

The cooling coil is mounted vertically just above the pan, but none of the condensate appears to be hitting the pan. What would cause this to happen?

Any answers or even speculation would be appreciated. Thanks.

The drain in the unit is plugged. Dirt, leaves, dead mouse, etc.

There are several possibilities here.
First when you installed the new trap, did you prime it?
You have a negative pressure coil, and air will be sucked in through the condensate line, and keep the water from flowing out.

If the coil is dirty, it prevents the water from flowing down the fins, and into the pan. Most is then pulled off of the coil by the blower motor, and ends up in the bottom insulation.

If your coil is freezing up periodically due to a low refrigerant charge, or dirty filters,...........same thing! The blower pulls the condensation off of the coil, and floods the unit.

If none of those reasons are the cause, then you most likely have a crack, or holes in the drain pan somewhere.

Last idea is a "sloped" roof curb that the unit is setting on. It should be level, or slightly slanted towards the condensate opening.
I HAVE seen curbs that were so out of level in the wrong direction, that the water poured over the opposite end of the pan, without ever reaching the drain connection.

Hope this helps.