FAU blower suction not allowing condensation drainage?

Hi all,
I have a heat pump A/C system in my townhouse. What I've noticed is that if I leave the A/C on all day, eventually water starts leaking out of the unit and into my closet one floor below the unit.
When an A/C technician came over we took the door off the unit and poured tons of water into the runoff tray inside, but none of leaked out. It all properly drained out the primary condensation line. We couldn't figure it out!
Finally one day after the A/C had been running a lot, it began leaking again. I quickly went up to the FAU (is that what this thing with the blower and filter and radiator is?) and took the door off to see the source of the leak. It was coming out of the runoff tray.
The weird thing is, the second I took the door off with the A/C running, the whole runoff tray emptied via the primary condensation drainage line.
What I've determined is that the fan inside the FAU is spinning so fast that it's creating a vacuum inside and it's not letting the water drain out properly. That's why when I took the door off and the air pressure equalized, it all drained out.
Now I manage to avoid more water damage by turning off the A/C after it's been running for a while to allow the air pressue to equalize and to allow drainage.
However, I'd like to get this fixed so that I don't have to worry about it. Why would this happen? Is it a design flaw? What can I do?
Thanks!
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snipped-for-privacy@NOSPAM.org wrote in message snipped-for-privacy@4ax.com

the condensate drain line MUST have a trap in it as well as a vent that's downstream of the trap. If you do the trap correctly, you'll end up with a vent that you can pour bleach in once a year, directly into the trap. At the rate you're going, you're going to have a serious mold problem next.
notice how the second T fitting is open to the atmosphere at this link.....thats the vent.......
http://209.224.198.110/pdf/Sept_Oct_Issue/design_details.pdf
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Just needs P or S trap....without it, the water will not drain correctly...

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Change air filter. Provide more air return ducts.
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Christopher A. Young
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On Wed, 29 Oct 2003 12:24:51 -0500, "Stormin Mormon"

Could also be a partially blocked drain pipe. I have the problem here in Arizona of spiders filling up the external outlet, so I added a fitting so I can blow it out periodically with a garden hose.
...Jim Thompson
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