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  1. #1
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,066

    School me on condensate p-traps!

    Name:  ptrap.jpg
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    In the picture, I have a refrigerated HCR Air Door for a open shop in refrigerated cooler box.

    The air door drain line connects with the cooler box drain line on the left side of the picture.

    After the drain lines connect, they drop down several feet and there is a p-trap going into a floor sink.

    BUT my problem is, the Air door drain line is attached to the negative pressure side of the blower compartment. And it has been sucking air through the cooler box drain lines from above, causing any water from the air door coil to be sucked out and whipped up into the blower compartment, causing it to fill with water and leak.

    SO I added the P-trap you see in the picture today, figured, if that trap is full of water, the air door won't be sucking through my drain line anymore.

    But I'm wondering if I need to install a vent line, before or after the trap?
    If so, why?
    Is my current p-trap just going to act like a straw without a vent?
    I mainly work of refrigerators, and we don't have vents on our drains. But I know most A/C units have vents. Is it because the high negative pressure?


  2. #2
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,066
    I think I got it figured out...

    Since my drain is attached on the negative side of the blower...

    My P-trap must have a vertical offset measured in inches GREATER than the suction side of my blower measured in inches.

    I have about a 6" vertical offset on the trap. So my blower would need to pull over 6"WC in order to suck water back up outta my trap.

    Am I on to something?

  3. #3
    Join Date
    Sep 2002
    Posts
    12,548

    Condensate trap design -

    The reason for trapping a condensate drain is that the drain line will be under either positive or negative pressure. So the pressure will either want to blow air out through the drain line or suck air in through the air line. Neither of which is a good thing.

    These pressure differences are measured in inches of water column pressure. It is conveniently fortunate that a trap in a drain line will contain . . . . inches of water.

    To create a successful drain line trap all you have to do is to make it deeper in inches than any possible positive or negative pressure which the air handler may generate.

    Easy huh?

    PHM
    --------




    Quote Originally Posted by Phase Loss View Post
    Name:  ptrap.jpg
Views: 947
Size:  42.4 KB

    In the picture, I have a refrigerated HCR Air Door for a open shop in refrigerated cooler box.

    The air door drain line connects with the cooler box drain line on the left side of the picture.

    After the drain lines connect, they drop down several feet and there is a p-trap going into a floor sink.

    BUT my problem is, the Air door drain line is attached to the negative pressure side of the blower compartment. And it has been sucking air through the cooler box drain lines from above, causing any water from the air door coil to be sucked out and whipped up into the blower compartment, causing it to fill with water and leak.

    SO I added the P-trap you see in the picture today, figured, if that trap is full of water, the air door won't be sucking through my drain line anymore.

    But I'm wondering if I need to install a vent line, before or after the trap?
    If so, why?
    Is my current p-trap just going to act like a straw without a vent?
    I mainly work of refrigerators, and we don't have vents on our drains. But I know most A/C units have vents. Is it because the high negative pressure?

    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  4. #4
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,674
    If the negative pressure in the air handler is high enough, the trap may never actually fill with water.

    I've seen a few cases where, in spring, air handlers with high return statics need the traps "jump started" by placing a hand over the end of the trap and allowing it to fill up for a second.

    One that is done, you're good to go until the trap dries out again.

  5. #5
    Join Date
    Apr 2010
    Location
    San Diego, CA
    Posts
    218
    If you've got 6" of vertical offset above the trap outlet, and 3" of trap below the outlet, then you're good for up to 6" WC. It should be vented on the outlet side, but I bet it works just fine as is. The other drain line up above will cause the pressure on the outlet side of the trap to equalize to atmospheric pressure, just not as quickly as it would with a vent right at the trap.

  6. #6
    Join Date
    Apr 2010
    Location
    San Diego, CA
    Posts
    218
    Bah, do we not have an edit button? My post above should read, "then you're good for up to 5" WC." The vertical offset is WC + 1", the trap is supposed to be 1/2 of the vertical offset. So 6" above and 3" below is good up to 5".

  7. #7
    Join Date
    Feb 2012
    Location
    Durham, NC
    Posts
    73
    I recently had this same problem on a york rtu. It boggled my mind for a while. The installers put the vent pipe before the trap. The negative pressure from the return wouldn't allow water to drain from the evap pan. Ended up I had to move the vent pipe after the trap and capped off the original vent. Luckily, it was in a warehouse because it was pouring gallons of condensate inside.

    They haven't called back so I assume all is well!

  8. #8
    Join Date
    May 2012
    Location
    Upper Michigan
    Posts
    3,588
    Quote Originally Posted by jpsmith1cm View Post
    If the negative pressure in the air handler is high enough, the trap may never actually fill with water.

    I've seen a few cases where, in spring, air handlers with high return statics need the traps "jump started" by placing a hand over the end of the trap and allowing it to fill up for a second.

    One that is done, you're good to go until the trap dries out again.
    X2 I learned that trick the other day from our lead tech. He said he was on many calls where they have water leaking from an rtu, they shut the unit down call a tech and by the time he gets there and starts the unit it already drained and filled the trap.

  9. #9
    Join Date
    Dec 2006
    Location
    In a Mess
    Posts
    1,008
    how did ya make out?
    I've seen a few of these out there never had this issue, but not sure if i had a shared drain line, but definatly have smaller P traps than the one in this pic?

    Mike
    Sig removed by mod. G-Rated site

  10. #10
    Join Date
    Jun 2013
    Location
    So. Cal
    Posts
    58
    Each unit needs a trap, and I always vent them after just to make it easier to prime them and clean out, then stick a piece of maybe 3" pipe going up. I swear I fix 4-5 drains a week that are not vented, not trapped, or vented on the unit side of the trap. Wshp are the worst. And you MUST vent the secondary drain on all air handlers also!!!!!!!i last week I had to fix a new install that flooded a converence room due too multiple errors, primary drain straight out into a condensate pump, secondary out 2" then 90 down into pan, installer drilled out secondary hole "well I knocked out both holes and one was smaller than the other so I used a uni bit and made it bigger" (7 years with the company...) and they installed one of those pipe clamp condensate sensors on top of secondary pan pipe instead of bottom and it was installed so there was an air bubble when it filled up....... Man what a jacked up day.... And brand new unit had bad contactor.

  11. #11
    Join Date
    Feb 2008
    Location
    California
    Posts
    2,066
    Quote Originally Posted by MikeySq View Post
    how did ya make out?
    I've seen a few of these out there never had this issue, but not sure if i had a shared drain line, but definatly have smaller P traps than the one in this pic?

    Mike
    I added a vent after the trap.

    Never leaked water again!

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