Connecting two traps together to maintain prime

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by Brendan Simons, Sep 4, 2013.

  1. Brendan Simons

    Brendan Simons New Member

    Messages:
    6
    Location:
    Ontario
    I am building a second-floor laundry room. There will be a floor drain under the washing machine, and I'm looking for a solution to keeping water in the floor drain trap without being able to reach it easily for maintenance (and without a trap primer valve, which is proving to be hard to find around here).

    My current thought is to connect the floor drain trap to the trap at the bottom of the laundry standpipe, as shown in the attached sketch. Basically I'll extend the laundry trap depth a bit and install a 1/2" tee below the waterline (but above the trap weir). From there, I'll run a 1/2" pex line down to the floor drain. The laundry trap should drain through the pex line into the floor drain trap until the water level is lower than the tee. I'm hoping that, since the pex line is only 1/2", it won't allow enough flow from the standpipe to flood the 2" floor drain. Can you point out any flaws in the design?
    laundry trap priming diagram.jpg
  2. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,174
    Location:
    Maine
    The short answer is NO!
  3. Brendan Simons

    Brendan Simons New Member

    Messages:
    6
    Location:
    Ontario
    OK... give me the long answer. I'm having a heck of a time finding the "industry standard" way of keeping prime in a floor drain. The local hardware stores don't even sell a trap primer valve.
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    22,015
    Location:
    New England
    You can put a T on the fitting into the washing machine, then use a hose with a valve in-line to limit flow, and use that to run some water into the trap each time the WM is used. If you dialed down the flow to a slight trickle, you'd keep the trap primed as long as you did wash on a regular basis and not impact the time it takes to fill the WM.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,015
    Location:
    New England
    Go to a plumbing supply store...unfortunately, most of them are not open late or on weekends.
  6. Brendan Simons

    Brendan Simons New Member

    Messages:
    6
    Location:
    Ontario
    Thanks, but must I use fresh water to prime the floor drain? This post from Terry a few years ago suggests something like I drew (unless I misunderstand him). I'm just trying to plan the details.
    Last edited: Sep 4, 2013
  7. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    I have done this with a trap primer before.

    The drawing you have shows a floor drain without a vent.
    Everytime the washer discharges, the p-trap of the floor drain will lose water. It may take a day or two before the trap primer would refill the p-trap.
    Put a vent between the floor drain, and where it wyes into the waste line of the washer.

    http://www.terrylove.com/forums/showthread.php?36400-Prime-floor-drain-trap-from-standpipe
    Last edited: Sep 4, 2013
  8. Brendan Simons

    Brendan Simons New Member

    Messages:
    6
    Location:
    Ontario
    OK, I thought I could get away with wet venting the floor drain, but I'll double check.

    Assuming I add the vent, are you agreeing that the two traps can be connected, or do I have to use a trap primer valve?
  9. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    Check all you want, but the answer won't change.
    You make ask some homeowners and I'm sure they will give you the green light. Plumbers won't, because they know it's not legal, and even more important, it doesn't work.

    A washer is never allowed for a wet vent. It's a pumped drain that requires 2"
    Even if it could be wet vented, it would require upping the size of the waste line before you could consider it for venting purposes. It would need to be at least a 3" line.

    Here is one way to prime traps

    [​IMG]

    [​IMG]

    The blue line going into the slap is for the floor drain. There is also a copper line from the splitter that primes the ABS p-trap.
    Last edited: Mar 4, 2014
  10. Brendan Simons

    Brendan Simons New Member

    Messages:
    6
    Location:
    Ontario
    Cool. I'll add the second vent. What about the tee in the stand pipe?
  11. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    I haven't tried that yet. Maybe someone else has.
  12. Brendan Simons

    Brendan Simons New Member

    Messages:
    6
    Location:
    Ontario
    I did a bit more digging. The Ontario Building code (and national plumbing code) Section 7.4.5.5 calls for a trap seal primer or "equally effective means", which isn't very precise. There is an ASSE standard 1044 which governs trap primers connected to "waste line tail pieces from fixtures, such as lavatories, sinks & similar fixtures where grease is not normally generated". I'm not sure a laundry stand-pipe would be acceptable in that standard, and I don't really want to spend $45 to check :(

    In my digging I also found a *lot* of complaints about fowled or otherwise broken trap primer valves. It would sure be nice to build a system without a mechanical valve.
  13. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    Everything fails at some point. We install them with unions to make replacement easy.
  14. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
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    Location:
    Cave Creek, Arizona
    Usually it is done using water from a lavatory or toilet with a flush valve. The tap into the drain/riser has an upturned opening so the water will flow into it. The washing machine riser should NOT have "standing" water in it to flow through your tee. There is a BAP fitting which inserts into a 2" line with a 1/2" tap for an adapter. You could use one at both ends of your piece of PEX and enough water should splash into it to maintain the seal.
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