I have a washing machine that drains into a sink standpipe.

I also have a clothes dryer that drains to sewage or greywater plumbing.

When I connect the two via a T-piece with the dryer inlet at the side and the washer inlet at the top, even with a siphon internal to the T-piece, backpressure further down the drain leads to backflow from the washer to the dryer, which if operating, thinks that its drain outlet leads to the internal water tank, which has become full, and stops so the tank can be emptied, thus interrupting drying.

Right now, I've had to separate the two outlets, with the dryer outlet moved to the sink, but this is less than ideal, as we may want to store water in the sink without it overflowing, and I aam anticipating adding another appliance which I would not want to have drain to the sink at all.

The dryer barely has enough pump pressure to pump water into the sink, any back-pressure leads to the dryer assuming a full tasnk and shutting down, so common backflow preventers that need even a slight bit of pressure to allow forward flow won't work.

The appliance I am considering adding to this system is a Catgenie, a black-water-outputting device, with a low flow and also a low output pressure, so I would certainly not want that draining direct to the sink.

How can I prevent the washing machine outlet from backflowing to the dryer and catgenie, prevent the catgenie from backflowing to the washer or dryer, and prevent backflow from any of those 3 devices to the sink?

I am looking for a minimum time and cost solution, as I can't afford to go to a plumber unless I have no other choice, and would prefer a non-permanent solution that isn't 'plumbed in'.


I am going to need some sort of backflow preventer. When the washer drains to the standpipe, sometimes grey water flows into the sink basin, depending on if there is a partial blockage downstream.

With the catgenie connected and the washer running, the possibility of black water backflowing into the sink basin currently exists, which, while tolerable for grey water, is obviously unacceptable in this situation.

Any backflow preventer is going to have to work at very low pressure, since everything is effectively gravity fed.

So... I need a backflow preventer for the sink basin, and also for the dryer and catgenie. They need to be near-zero forward pressure devices. Mains pressure backflow preventers will not work. Something like a float or gravity valve, probably.

  • Do you mean a Wye because a Tee would create back pressure even if the pipes are sized correctly
    – Ed Beal
    Commented Jun 30, 2018 at 14:49
  • @EdBeal, I had plastic tubing coming from the T side branch and pointing downwards internally, so that water from the washer should actually create a vacuum that sucked water out of the dryer hose. However in practice, back-pressure originating below the sink caused backflow into both the dryer branch and the sink basin.
    – Monty Wild
    Commented Jun 30, 2018 at 16:19
  • ... a clothes dryer that drains? Commented Jun 30, 2018 at 19:14
  • @DanielGriscom, yes, a clothes dryer that drains. It is a heat-pump condensing type, that doesn't dump heat and water vapor into the air, but instead traps water, and releasescit to a drain if possible
    – Monty Wild
    Commented Jul 2, 2018 at 1:46
  • So the dryer discharges this condensate with a pump to discharge to an elevated level? Or is it just gravity discharge? Which then suggests a floor drain or a pumping setup is needed.
    – user68386
    Commented Jan 9, 2019 at 19:27

1 Answer 1


If these appliances are supposed to drain into a standpipe, then they need to be connected in a way that allows them to drain in open air.

Get a double wye, of the appropriate size to fit on the standpipe. Then put one drain into the left wye, the other into the right wye, and leave the center wye open.

If the standpipe opening is large enough to accommodate both drain lines, and still have open space to "breath". Then you should just be able to put both drains into the standpipe, without adding any other fittings.

  • 1
    Also if your stand pipe is plumbed to the sewer or septic you need to make sure your stand pipe has a trap installed in it to prevent sewer gas and odor from entering the living space.
    – Alaska Man
    Commented Jul 1, 2018 at 17:25
  • @Alaskaman that's true. I just assumed the standpipe was properly plumbed.
    – Tester101
    Commented Jul 1, 2018 at 17:53
  • Could you give me a link to a product that matches your description? I'm having trouble envisioning what you mean, and US and Australian terminology sometimes differs.
    – Monty Wild
    Commented Jul 2, 2018 at 10:00
  • @MontyWild double wye
    – Tester101
    Commented Jul 2, 2018 at 12:14
  • Needs air gap +1
    – Mazura
    Commented Jan 10, 2019 at 2:59

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