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Thread: Condensate drain line as trap prime

  1. #1

    Question Condensate drain line as trap prime

    Help me out here.

    Iím a Home Inspector looking at an arrangement uncommon in my area: updraft condensing furnace with AC coil below it:


    with a condensate drain line connection in the crawl space made directly above the trap of a floor drain:


    the idea, as best Iíve been able to determine, was to make the condensate drain line do double duty as a trap prime.

    Lots of things bother me about this, and I need to know if my concerns are reasonable, and if so should be done to correct any existing problems.

    1) With no air gap on the condensate line and the AC coil upstream of the furnace it appears to me be a big soda straw with one end stuck down the DWV system and the furnace sucking on the other - that if anything obstructs the floor drain at a minimum you could pull sewer gas back past the trap and into the furnace, and that if the DWV system backed up to the point were water rose above the attachment for the condensate drain you could suck contaminated water up as well.

    2) Should that primary condensate drain be right at floor level? Assuming that it needs to be trapped, where would you locate the trap?

    3) Does this arrangement violate the probation against direct connect of the condensate line to the DWV system?

    4) In terms of plumbing issues, am I missing anything?

    - Thanks
    Last edited by michael-thomas; 12-22-2005 at 02:14 PM.

  2. #2

    Default D'oh

    D'oh - it's a counterflow furnace (blowing down). Given that, I'm still interested in plumber's comment on this instalation, and in particular the connection of the condensate line to the DWV system.

  3. #3
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default drain

    The condensate line should be terminated above the overflow level of the trap. Unless there is some condensation 365 days a year the trap can dry out and then the furnace/air conditioner WILL draw sewer gas into the building. Whether the unit could draw backed up water into the furnace would depend on what the amount of negative pressure is in water column inches.

  4. #4
    Master Plumber Dunbar Plumbing's Avatar
    Join Date
    Apr 2005
    Northern Kentucky/Greater Cincinnati Area


    It would of been a better installation if there was a wye branching off in 1.5".....leading to the furnace with the 3/4" piping dumping indirectly into a 2" by 1.5" bell. This would provide an air break, not an air gap and would be used as a indirect waste. Still serves the purpose of keeping the trap seal intact. Since that is a wood floor, rolled goods is required leading to the floor drain opening in order to be an effective opening in case of a fixture overflowing/malfunctioning.
    Read what the end of this sentence means.

  5. #5
    Plumber/Gasfitter dubldare's Avatar
    Join Date
    Sep 2005


    I would have a very serious concern of the A/C drain not being trapped. !!!

    The furnace condensate should be trapped within the furnace, but the drain pan for the coil does not have one.

    The trap for the coil should only serve the coil (not the other condensate lines as well) and should be as close as practical (immediately if possible) to the point of connection to said coil.

    The main purpose for trapping drain pans is to allow for disharge of the condensate, as positive pressure exists on the pan (from the furnace blower). The trap helps mitigate the circulating air from travelling out through the drain. Additionally, it keeps bugs from entering the drain pan. (just think of some roaches camping out in your furnace)

    Condensate wasting is a very grey area, some municipalities prohibit condensate wastes from discharging to the sanitary sewer alltogether, some require neutralization of condensate prior to discharging to the sanitary, and many are silent on this issue.

    My solution for your problem:

    1) resize existing 3/4" pvc condesate drain 'below' floor to 2" minimum (or allow bushing-up to 2" at floor level). The point is to create a receptor for the condesate lines. this will also allow for added protection from sanitary sewer back-up.
    2) the a/c drain will need to be trapped prior to discharging into the receptor mentioned above.
    3) the line from the a/c unit should run independent of the furnace condensate, to prevent fouling of the a/c coil drain pan. (most often, the a/c drain is above the furnace where co-mingled installations are allowed)

    Co-mingling of indirect lines IMHO is poor practice and a potential danger, which is rarely thought about or addressed. I've seen many food establishments and bars where beverage drip pans (and glass washing sinks in bars) are directly connected to ice bin drains prior to terminating at a hub drain. Think of it, a plugged indirect waste line downstream of a ice bin, a glass washing sink upstream of the ice bin will contaminate the ice bin. As a rule I only drink beer from a bottle or a very stiff gin & tonic at bars, lol. Professionally, I do not co-mingle indirect wastes.
    Last edited by dubldare; 12-22-2005 at 09:02 PM.

  6. #6
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default indirect drains

    I have never known an inspector who would allow interconnecting indirect drains. And any plumber who would do it should be demoted back to apprentice.


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