I'm removing a stackable washer/dryer and installing a vanity sink for my daughters art studio. All the lines are in place, hot & cold water lines and vent pipe (washer's waste water pipe), however as I opened the wall to relocate the valves and tap into the washer's vent line, I came across the following.

Current Install & Planned Upgrade

As you can see in the sketch, "Current" shows an existing p-trap from the washer's vent line, in the wall. I've read various "not a good idea to have two p-traps on the same line" scenarios due to potential lack of pressure and possible backwash, etc.

Any how, what I'd like to do, and know if it's within code, "Upgrade" is add a vent line for the vanity on the existing vent pipe, just above the washer's vent line?

A bit off topic but... I also noticed that there's only one shut-off valve (hot water line) on the water heater, and nothing for the cold one; is this normal? How do you go about turning the cold water off to run new lines?

Water-Heater Tank

  • 1
    There's no sketch in your post. To turn off the cold, you turn off all of the water. The IPC calls for a shutoff valve (technically a "full open valve") where the water first enters the structure, and I assume the UPC has a similar requirement. That's the valve to turn off. Trace the cold water feeding your hot water tank back toward the street and you'll eventually find it inside the house somewhere. Unless your install doesn't meet code.
    – popham
    Jan 30 at 7:29
  • Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking.
    – Community Bot
    Jan 30 at 11:23
  • In some (normally) warmer places the shutoff and meter (if public water) may be outside the house. I live in a place where that would be insane due to freezing. Edit your post and use the "sun and mountains" icon above the edit box to insert your picture. LEAVE THE CODE IT INSERTS ALONE. Look below the edit box for the preview of how it will post. On many computers you can also drag and drop an image.
    – Ecnerwal
    Jan 30 at 13:20
  • What size is the washer drain pipe? If to current codes (2") an additional P-trap is unlikely to be a problem.
    – Ecnerwal
    Jan 30 at 14:31
  • 1
    I think you meant two P-traps on the same drain. The upgrade is two drains and each drain requires it's own P-trap before the main drain/vent. If removing the washer, then can cap that drain line. P-traps require water added every so often to not dry out.
    – crip659
    Jan 30 at 22:50

1 Answer 1


Your photo appears to show the shutoff at the water heater on the COLD line. It's quite normal to have only one valve at the W.H. on the cold line, because the hot water piping is necessarily downstream of that valve. You can always use the main shutoff on the water service where it enters your building to shut off all piping.

Your schematic of the "UPGRADE" drain piping looks fine, as long as the washer drain is sized 2", and the section of wet vent between the washer santee and the sink santee is sized 2" (one size greater than normally required)

  • If that drawing is to scale, then the standpipe looks shorter than 18".
    – popham
    Jan 30 at 23:07
  • @crip659's comment is correct about how the old washer's p-trap is going to go dry and then become a problem.
    – popham
    Jan 30 at 23:11
  • @popham -No, the sketch is not to scale, it simply shows what the plan is, the before and after. Jan 31 at 0:23
  • @crip659 -I originally thought of just connecting to the washer's vent line but I read where multiple p-traps on the same line could create a future problem, thus a separate line for the sink; the Upgrade. I want to leave the washer hook up as it stands, if she later changes her mind, less work later on. Your comment... If removing the washer, then can cap that drain line... is this "you can cap" or "you can't cap. You continue to say that "P-traps require water added every so often to not dry out." Why should I be concerned with the p-trap drying out? Jan 31 at 18:24
  • @popham -Yes, I get that. This is why I'm going to cap the washer's vent line. If this is the concern for the p-trap drying out, wont capping the washer's vent line prevent this from happening or is there more to the story? Also, in answer to your previous question photo of pipes in the wall the vent pipe has 2" marked on its surface; but to be honest, I haven't measured them. since the initial install was done in the rough, I'm assuming it passed inspection. Jan 31 at 23:54

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