A small repair is going on, but what is this other pipe coming off the drain pipe near the p-trap? enter image description here

  • Is there a dishwasher? Is there a crawlspace below or a basement? This could be waste from a dishwasher. Apr 10 at 19:34
  • Thanks Jim. The dishwasher attaches elsewhere nearer the sink. This is an apartment building, so only apartments below me.
    – Boyd
    Apr 10 at 19:53
  • 3
    From the picture it looks like water would sooner flow down that then out the p-trap outlet? Is it actually horizontally lower than the p-trap outflow? Apr 10 at 20:32
  • Yes Fresh, it's lower than the p-trap outflow, so water goes down it. Why have two drains?
    – Boyd
    Apr 10 at 20:55
  • 1
    Since you live in an apartment, you should ask the landlord/management. They'll have a far better chance of answering than we will since we can't see the other end of this pipe.
    – FreeMan
    Apr 11 at 0:38

2 Answers 2


Possibly an alternative form of "trap primer" for a floor drain (or other rarely used device with a trap) that is being kept from drying out.

Each time you use this sink, a little bit of water goes to that trap.

  • That seems like it (thanks also P2000). Why there would be one in my apartment on the 6th floor is beyond me.
    – Boyd
    Apr 11 at 21:33

Pumped Drain

There are several plumbing appliances that could connect to the drain as pictured, especially if it has a drain pump incl. a pan pump, sump pump, condensate pump:

  • dishwasher (dismissed already)
  • laundry machine (less likely, since larger diameter required)
  • hot water tank pressure relief
  • heater/furnace condensate
  • air conditioner condensate

If you have any of these appliances you could see if they have a pump or relief vent attached to similar copper pipe.

Pumped drain lines do not require down-flow gravity.

The other 1/2in pipe may be a hint: is it your kitchen hot water supply?

Grey Water Primer

Alternatively, as suggested in the answer by Ecnerwal, it could be a trap primer.

I had never seen one like this before, but with a bit research it turns out they are not a DIY-jimmy-rig. They are called a "Grey Water Primer".

Here is an example by siouxchief

enter image description here

If you indeed have a floor drain you could test this suggestion by blocking the P-trap entry after that T-coupling, and pouring some water into the opening so that a copious amount flows down this primer line. You would hear or see it arriving in the floor drain. If you pour some herbal tea or soap water, you would smell it in the floor drain.

Alternatively, you can pour water in the floor drain and listen with your ear closely to that kitchen P trap. This is a bit more difficult due to its location under the cabinet.

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