Primary / Secondary Drain issue

Old 08-30-08, 08:11 AM
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Primary / Secondary Drain issue

I have a unit in the attic, The primary drain has a vertical pipe that was open at the top and not capped. While sealing my ducts, I notices this pipe was venting cool air out into the attic.

So, I capped it on both units.
Cut to a year later, and I have this huge rust stain outside my house because the pan is full of water. Today, I go oup there to look at things, and Just for kicks, I un-cap the top of the vertical pipe on the primary, and the dripping into the pan stops instantly.

Should that pipe (circled in red) be capped or un-capped? I have seen online where others have that capped but why does capping this cause the secondary drain to start draining? I am pretty confused at this point.
Here is a picture of my unit:
Old 08-30-08, 09:46 AM
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A problem I see here is the lack of a trap, which should look similar to the ones you have in sinks (the P-trap or drain trap). You may put the plug back but not before adding a trap here.
You have a drain but no trap.
BTW, the size of these PVC lines is huge!
Old 08-30-08, 12:03 PM
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Never cap that pipe opening. If you cap that opening, it creats pressure in the pipe so the water won't go down anymore. that is why you start see the water going into your secondary drain pan. If you worry about that little cold air blow into the attic, you can cap half the opening. but do not cap the whole thing. sometimes, it is also served as the overflow if the primary is plugged so the water won't go back to your unit. good luck.

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