Emergency overflow on RV drain plumbing

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by J F, Jun 27, 2021.

  1. J F

    J F New Member

    Joined:
    Jun 27, 2021
    Location:
    Ontario, Canada
    Thinking of a custom RV reno, with an "emergency overflow" on the GreyWater tank (to protect against an overfilled tank from backing-up through the shower drain (the lowest drain) and making a catastrophic mess).

    My idea:

    Shower drain drops straight down (about 15cm) into P-trap.
    Trap arm is as short as possible (a few cm max?) whereupon it feeds directly into straight-inlet of 45-degree wye; 45-degree branch is oriented above the straight path. (The few cm of trap arm is so the wye is not classified as a "crown vent", and so that water exiting the trap keeps flowing horizontally, instead of splashing up the angled branch of the wye.)
    Straight-outlet of wye feeds a relatively short horizontal run (about 30cm, to clear the shower area) at a gentle downslope, into a sanitary-tee; this sanitary-tee drops straight down into a second trap.
    Second trap's arm feeds into a second sanitary-tee, which drains into the GreyWater tank, as normal.

    Back at the 45-degree wye: 45-degree branch is immediately connected to a 90-degree elbow oriented towards the side; this goes to a relatively short horizontal run (about 30cm, to clear the shower area) at a gentle downslope, into another sanitary-tee; this third sanitary-tee dumps "emergency overflows" to the ground below the RV. (See theory, below.)


    Theory of operation:

    Normal shower use keeps both traps primed; first sanitary-tee (above inlet to second trap) prevents air locks between the traps. Second sanitary-tee prevents trap siphoning.

    In case of an overfilled GreyWater tank:
    When backflow rushes towards shower trap: 45-degree wye now acts as a "splitter" (instead of the usual combiner purpose for which they're made) -- First, it acts as a sort-of "wet vent", preventing an air lock between the two traps. (This ensures shower trap water is not "blown upwards" through the shower drain if/when the tank is overfilled!) Next, when the backflow water reaches the wye, the water overflows through the 45-degree branch (and out the side of the 90-degree elbow) before the backflow pressure can exceed the 15cm head height of the shower drain. (approx 1500 Pa (1.5 kPa; 15 hPa; 150 DPa; 0.0015 MPa, 0.015 bar) pressure head)

    The overflow immediately goes out the side of the 90-degree elbow, to the "overflow sanitary-tee" (which was ultimately providing the inter-trap venting for this backflow), and dumps to the ground.

    The vent on this "overflow sanitary-tee" of course also assists the venting provided by the inter-trap sanitary-tee, to ensure neither of the two traps are subject to siphoning.

    Essentially: First trap (below shower drain) seals RV interior from the overflow & drain plumbing. Second trap prevents GreyWater tank vapours from wafting out the overflow. 45-degree wye provides the overflow path, between the two traps. Three vent stacks prevent air locks or trap siphons. 15cm drop from shower drain to first trap is for sufficient pressure head (15 hPa), to ensure backflows dump out the overflow instead of coming up through shower drain.


    Rationale:

    With this arrangement, the two traps stay primed with normal shower use. (If the shower drained normally into the GreyWater tank, and a completely separate emergency-overflow was fitted to the tank with its own trap, nothing would keep the overflow-trap primed!) An overflow without a trap (or a dried-out trap) seems undesirable, as GreyWater tank vapours could then waft out the bottom of the RV, closer to the nose level of humans! (compared to rooftop vents)


    The big question: Will this work?
    The next question: Is this compliant with all applicable codes?
    The final question: If the answer is "no" to either of the previous two questions... is there a better alternative?
     
  2. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    A rough side-view line drawing might clarify.
     
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  4. J F

    J F New Member

    Joined:
    Jun 27, 2021
    Location:
    Ontario, Canada
    Pardon my chicken-scratch handwriting... and attempt at 3D drawing
     

    Attached Files:

  5. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Nice sketch from a communications standpoint. Reading I had thought there was the third possibility of joining the toilet drain, but no.

    I don't necessarily see a flaw. I am not a plumber. I guess the second trap is to prevent smells from the gray water from hitting the outside overflow drain.
     
  6. J F

    J F New Member

    Joined:
    Jun 27, 2021
    Location:
    Ontario, Canada
    The toilet drains to its own BlackWater tank; completely separate from the GreyWater system.

    There's also a bathroom sink and kitchen sink that drains into the GreyWater tank. For simplicity, I omitted these from my sketch, but these two fixtures have their own traps and vented sanitary tees (just as one would expect in a house), all draining into the GreyWater tank.

    I've noticed in a few RVs now, separate GreyWater vent stacks are brought together high up in the walls or even the roof, to a final single opening. I'm assuming this single opening is to prevent wind / air currents from circulating through the pipes and spreading the stink? (Or is it just a cost saving measure? Anyhow, the GreyWater tank holds the waste from washing dishes, so food scraps -- combined with whatever else gets washed down the sinks -- can make the tank contents smell like a kitchen garbage bin. Kind-of gross.) So yeah, the second trap is to prevent my "emergency overflow" idea from inadvertently creating a vent that can waft tank odours.


    Motivation for this:

    I've personally had the tank level indicator fail to warn the tank was reaching it's upper quarter... and then disaster struck: The shower drain spewed up grimy stinky water when the stopper was pulled from the kitchen sink and the water tried to drain into an unknowingly overfilled tank. Some blankets that were temporarily tossed in the shower (space is at a premium in an RV!) ended up getting ruined too.

    Turns out, from reading RV forums, this is a common thing. (Tank level indicators becoming unreliable, and then surprise floods in the shower.)
     
  7. Jeff H Young

    Jeff H Young In the Trades

    Joined:
    Feb 27, 2020
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    92346
    Actually like your chicken scratch sketch pretty clear. I wouldn't expect overflowing on to ground to be desirable or Rvs wouldn't back up in shower. Codes on RV don't know never heard of a permit or inspection on old units except mobile homes or manufactured homes not rv's , assuming its kind of unenforceable but somewhere there is a law telling us how to plumb
     
  8. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
  9. Jeff H Young

    Jeff H Young In the Trades

    Joined:
    Feb 27, 2020
    Location:
    92346
    funny they call it overflow prevention. its more of an overflow allower, allowing raw sewage to dump in yard, I'm aware of those and people leaving a loose jim cap at cleanout allowing sewage to spill on ground and not in house. I guess it should be allowed for the black water in an rv as they do homes. not real crazy about it . but I have left a loose jim cap on homes upon owner request
     
  10. J F

    J F New Member

    Joined:
    Jun 27, 2021
    Location:
    Ontario, Canada
    That's an interesting point. Correct me if I'm wrong, but as I understand in the world of fixed buildings, a national plumbing code (or maybe just a guideline at this point?) exists, but then it's up to municipal governments to enforce it? (as an actual code) (Case in point: Some cities require air gaps on dishwashers; others don't.)

    I'm not sure how things work with the plumbing in travel-trailers and motorhomes. As far as I know, vehicle safety devices and compliance with transportation law is at a federal level... at least for stuff like brakes, bumpers, exterior lighting, seat belts & air bags, etc.

    Since the whole purpose of an RV (motorhome, travel-trailer) is to camp/live outside of your home city, it would seem silly for aspects of RV design to be regulated at a municipal level. ...So is it provincial/state? Federal? ...Or no building codes exist for something on wheels?
     
  11. Jeff H Young

    Jeff H Young In the Trades

    Joined:
    Feb 27, 2020
    Location:
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    JF, seems silly but an RV is also a home they do have regulation and a buyer in the USA can get a tax write off on interest paid as it can be a home. there are some standards I think they must have fire extinguishers etc. you got me on who regulates it or if a homemade is exempt. and federal or state in the USA wouldn't seem to matter in Canada.
    I would think as a private owner assuming you aren't re selling it your main concern would be function and safety. the double trap might allow the overflow dump to not act like a open vent from tank or to a sewer connection , and function properly but kind of guessing as its not very conventional, and again not so sure I can or should recommend it or tell you its legal. in practice moving the vehicle etc the trap likely lose its seal
     
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