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  1. #1
    Join Date
    Nov 2008
    Location
    Chapel Hill, NC
    Posts
    1,070

    Why the condensate drain trap?

    I'm having a high efficiency condensing furnace put in our second floor soon. We're going to insulate the condensate line but I'm wondering why the need for a trap? I understand why we do that in a toilet or sink, to keep sewer gases from backing up unto the house. But why a furnace (or A/C) condensate drain that drains out into the open (fresh) air. Seems to me that deleting the trap would eliminate the major freezing concern with a condensing furnace?



    Keith

  2. #2
    Join Date
    Dec 2007
    Location
    Ohio
    Posts
    2,876
    Because the condensation that is being drained from your condensing furnace is coming from your vent pipe.....if there was no trap....your vent gasses would spill out into the living space....
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  3. #3
    Join Date
    Jun 2007
    Location
    south carolina
    Posts
    176
    to allow the water to drain with out it air flowing in the drain line can cause the water to not flow

  4. #4
    Join Date
    May 2009
    Location
    Jonesboro, AR
    Posts
    111
    Check with your hvac installer about the unit. Most condensing furnaces come with an internal trap (ie, a trap inside the system compartment, or just outside it, depending on installation), negating the need for a trap that's external to the system (and don't use two traps). Just like I_bend_metal said, the trap stops flue gases from entering the drain pipe, and allows free flow, or stopping "water slugging". That's also why you have to trap a/c condensate with the evap coil on the negative side of the blower.

    Also, with a condensing furnace condensate that's allowed to drain to unconditioned space, or outside the building is subject to freezing. They can put out quite a bit of water, just like a/c during the summer. Now imagine that during freezing temperatures. You wouldn't (shouldn't) leave a water hose connected during the winter, would you? Once it freezes up, it'll back up and cause a mess, just like a clogged a/c condensate line.

    Jeremy

  5. #5
    Join Date
    Feb 2008
    Location
    Waffleville
    Posts
    10,339
    Quote Originally Posted by sktn77a View Post
    I'm having a high efficiency condensing furnace put in our second floor soon. We're going to insulate the condensate line but I'm wondering why the need for a trap? I understand why we do that in a toilet or sink, to keep sewer gases from backing up unto the house. But why a furnace (or A/C) condensate drain that drains out into the open (fresh) air. Seems to me that deleting the trap would eliminate the major freezing concern with a condensing furnace?



    Keith
    same principle, its an air seal

    heat tape will also remedy a freezing trap

    the trap is on the negative side of the blower. if the trap was not in place, the blower would suck air up through the drain line not allowing the condensate to flow out. thus causing a flooding problem
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  6. #6
    What about an AC unit with a condensate pump, do you still need the trap? I would think the pump would prevent the suction of air through the drain pipe.

  7. #7
    Join Date
    Oct 2004
    Location
    Central WA
    Posts
    1,534
    Quote Originally Posted by DaGoldsteinNY View Post
    What about an AC unit with a condensate pump, do you still need the trap? I would think the pump would prevent the suction of air through the drain pipe.
    Periodically the pump will pump out. No it doesn't go dry, but it will draw it pretty low.

  8. #8
    Join Date
    Feb 2011
    Location
    Ontario
    Posts
    73
    Generally, the purpose of the trap is to use a column of condensate to prevent air moving into or out of the equipment while still permitting the condensate to drain away. The vent on the line must be downstream from the trap, or else it renders the trap useless.

  9. #9

    Waterfurnace Drain

    Hello, Nice site I'm glad I found it.

    WaterFurnace II with electric backup (no gas)

    This Summer I noticed that my original drain hose with trap kept making the drain light come on. I replaced it with a piece of garden hose, no trap.

    It makes a gurgling noise. At first I thought it was pushing air out with the water but a quick test showed that it was sucking air in. Water is draining but it is noisy.

    1. Am I doing damage to the pump not having a trap?
    2. The drain spout is 2 inches off the ground. The trap made the hose rise 3" off the ground then the hose drained into the sump pump. This seemed wrong to me because the water had to try to rise up 1 inch before dropping. If I want it to be more level my only option would be to have the trap at the very end of the hose hanging over the sump pump area.

    I was considering cutting a longer hose and dropping it below the water level in the sump pump because I tested that and it stopped the gurgling noise but not sure that will cause a problem.

    Any suggestions welcome.

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