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  1. #1
    Join Date
    Jan 2007
    Location
    Houston, Texas
    Posts
    1

    Question To Vent or not to Vent A P-Trap

    Aplication - Small split HVAC systems 1 1/2 ton to 5 ton, residential houses and apartment complex. Closet and attic instalations.

    I have seen 3 ways a p-trap has been installed in air handlers, just a p-trap by itself, a vent after the drain pan but before the p-trap and a vent after the p-trap.

    Question - What are the determining factors regaurding which p-trap constuction is required?

    I have my own common sence ideas but would like to hear other opinions.

  2. #2
    Join Date
    Oct 2006
    Location
    The Beach
    Posts
    717

    Hmm Vent on Drain Line

    We always put the vent on the leaving side of the trap. Make sure the end of vent pipe is higher than the drain pan. If on the other side you are looseing conditioned air. If it is a short run we do not install one. If a long run with very little grade it would be a good idea.
    Blue Fox

  3. #3
    Join Date
    Apr 2002
    Location
    Omaha, NE
    Posts
    1,561
    Quote Originally Posted by lentz View Post
    We always put the vent on the leaving side of the trap. Make sure the end of vent pipe is higher than the drain pan. If on the other side you are looseing conditioned air. If it is a short run we do not install one. If a long run with very little grade it would be a good idea.
    I've never quite understood the need for the vent. Can you explain please?

  4. #4
    Join Date
    Aug 2001
    Location
    forney texas
    Posts
    17,890
    According to NATE all condensate lines get a P-Trap

  5. #5
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,743
    Quote Originally Posted by acmanko View Post
    According to NATE all condensate lines get a P-Trap

    yeah but NATE don't have to come out and clean a trap that was not needed in the first place


    if a trap is needed it should be vented after the trap, before the trap will let air suck in and not let water out if a return side coil.

  6. #6
    Join Date
    Nov 2004
    Location
    Naples, Fla.
    Posts
    1,403

    Traps:

    Condensate traps are intended to allow the condensate to flow from the air handler / coil where the drain is in the NEGITIVE (return) air flow. If you fail to trap it properly the unit will suck air up the drain, displacing the water until it either overflows in the unit or releases with a huge gush (if your lucky).

    The vent after the trap is to prevent a double trap scenario where multiple units are tied together or the condensate line has sags in it. It also serves as a handy clean out.

    Drains in the POSITIVE air flow require additional corrective action.

  7. #7
    Join Date
    Oct 2006
    Location
    The Beach
    Posts
    717

    Hmm Condensation

    Quote Originally Posted by snewman24 View Post
    I've never quite understood the need for the vent. Can you explain please?
    Remember that liquid runs out of pipe a lot better if it does not have to pull a vaccum. So in a long run the water will run out better if air is able come in on the back of the liquid. Turn a Coke bottle full of water up side down and see that it has to gurgle for the water to run out. The vent is really only needed if the run is long and very little grade. If the drain line was almost full of water at some places it would be trying to pull a vaccum.
    Blue Fox

  8. #8
    Join Date
    Feb 2005
    Location
    Richmond
    Posts
    480
    P-traps are mostly used in residential applications on Air Handlers where the coil is under negative pressure. This means that the coil is before the blower, and if a P-trap is not used, the unit will not drain. It may not drain at all, if the return static pressure is high. Vents should always go after the P-trap in negative pressure applications, otherwise you will get the same result if you had no trap. People on this forum have varing opinions on the use of P-traps on positive pressure applications. It's not a bad idea to use a trap here as well, but it will most likley clog with dust being blown into the drain.

  9. #9
    Join Date
    Nov 2006
    Location
    SC
    Posts
    930
    P traps serve several purposes:
    1. They stop the unit from sucking or blowing conditioned air
    2. The trap allows water to flow when the unit is running if it is on a suction side coil (most common).
    3. Keeps critters from walking up your drain line to the inside of your unit to get a drink of water.
    4. If the drain is (illegally in most cases) tied into the sewer system, it prevents poisonous and stinky gas from being drawn into your house.

    I trap and vent every primary drain. The vent ALWAYS goes on the downhill side of the trap, away from the unit.
    I NEVER trap the secondary drain, and don't provide a separate vent. Both are unnecessary.

  10. #10
    Join Date
    Mar 2002
    Location
    Richmond, Virginia
    Posts
    4,264
    I always vent the primary drain after the trap unless the run is very short and I have complete control of the pitch and can be sure there are no sags which will cause the drain to fail.
    There is nothing more frightful than ignorance in action....Mark Twain

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