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Thread: Pan Won't Drain

  1. #1
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    Pan Won't Drain

    I've got an AHU, constant volume, draw through, cw coil that fills the plenum with water. Drain pan has traps but this is a high velocity fan and the water gets sucked out of the traps and into the unit. The only way to drain it is to turn if off. I'm thinking of oversizing the traps. I hate draw through fans just for this reason. What would you do?

  2. #2
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    The trap needs to be below the pan far enough to overcome the negative. It's not how big the trap is, but how far down from the bottom of the pan it is. You need column of water in inches that exceeds the negative you are pulling in the AHU. For example, a Trane M series climate changers require a minimum of 4 inches between the pan and the top of the trap. BTW the legs that come with that AHU will not give you the clearance for the trap. Sort of screwed up, actually alot screwed up. So if you don't put that AHU on a pad, your trap is not going work!

  3. #3
    Join Date
    Nov 2007
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    Pneuma is correct. Measure the static pressure at the condensate pan, then multply the measurement, in " WC by 1.5. That is the minimum distance of the downstream leg of the trap. The leg closest to the unit should be at least 2 x's the static pressure measurement.

    I have seen jobs where they had to cut out of core drill Mech Room floors to get the correct trap depth....no it aint pretty!

  4. #4
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    I'd better get a jack hammer and a concrete saw.
    Thanks fellers.

  5. #5
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    Before you go and tear up the concrete, install a vent line right after the trap. This might help break the vacuum. Just a thought.

  6. #6
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    Here is a reference sheet.
    Attached Images Attached Images

  7. #7
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    One other condition to check is the position of the dampers. If you have return air and outside air dampers that are both closed you will too much negative pressure and pull water out of the pan and it will not drain. I say this because I found this exact problem.
    If I tell you how long I have been in this business, would it really matter?

  8. #8
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    I don't know if the pdf is meant to show the T's as plugged or not but, cured one problem draw thru by installing and leaving open a T at the beginning of the trap. It already had the trap outlet T and didn't have to make the trap any deeper.

  9. #9
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    Quote Originally Posted by Coolmaniac View Post
    I don't know if the pdf is meant to show the T's as plugged or not but, cured one problem draw thru by installing and leaving open a T at the beginning of the trap. It already had the trap outlet T and didn't have to make the trap any deeper.
    Did the same on some units I had. Leave the T uncapped and they're also self priming when it rains, which is a lot in this part of the world.

  10. #10
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    Update

    DS at coil is 5". Trap leg is 10" long and it sticks out about 4" from the unit. Looks to me it was sized correctly. As soon as I prime it, swoosh, into the plenum it goes and splatters all over the place.

  11. #11
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    Turn the blower off before you prime the trap. Otherwise the "prime" goes into the unit as fast as you pour it. It never develops the trap seal.

    If you are still having trouble, measure the negative pressure between the condensate pan area and atmospheric pressure. Post that back here.
    If "I have always done it this way" is a good reason to do it again, how many times do I have to do something wrong - before it becomes right?

  12. #12
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    If the above doesn't help you might try putting a piece of sheet metal to partly cover the upper 2/3 of the primary outlet. Like a flipped copy of the secondary? May allow just water to see the normal press in the trap and give it a chance to move. Moving that much air its a wonder anything makes it to the pan.

  13. #13
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    Quote Originally Posted by cxagent View Post
    Turn the blower off before you prime the trap. Otherwise the "prime" goes into the unit as fast as you pour it. It never develops the trap seal.

    If you are still having trouble, measure the negative pressure between the condensate pan area and atmospheric pressure. Post that back here.
    I have to pull the trap off (no top tee) to fill it. I also shut down the unit and filled it from the inside connection. The measurement I made (5") was the pan area vs atmospheric.
    I know it's been said that size doesn't matter, it's the depth, but I have solved similar problems in the past by over-sizing the pipe diameter simply because I had no more room to increase depth.

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