What size drain pipe is required here?

[Jadnashua]

I'm at my mother's house, and the drain line from her kitchen has always been prone to clogging. It is tied into a 2" line that also drains a tub and a shower after it before it dumps into a 4" main line.

The slope on this line never was really correct, so it tends to back up from the kitchen sink. I noticed when I took a shower this morning that there was about an inch of standing water before I was done...it drains, but slowly. It had always been working, but I'm not here all that often. I have an alternate path I'd like to use for the sink drain, but I need to know what would be allowed.

Below it, there's a 1.5" drain that is dedicated to a laundry pump for the washing machine that runs about 2' lower than that sink, tub, shower line, to a 4x2 (fitted to the 1.5") fitting. If I Y in the kitchen sink to this line and after the junction increase the size, can I get by with 2", or does it need to be larger? It goes, as I said, into a 4" pipe. It would be easier to reuse the 4x2" Y (after reaming out the reducing collar), than taking that out and trying to fit a new pvc piece into the donut of the cast iron fitting. FWIW, both of those things have their own AAV on them, as this 60+-year old house originally had lots of S-traps that I've mostly redone. This runs into a septic system, if that matters.

 

[Jadnashua]

I really could use some guidance on this. I know 'normally' a WM drain pipe would be 2", but in this case, it goes into a sump and pump, and the outlet of the pump is run with 1.5"...I figured increasing it to 2" after joining the kitchen sink in would be okay...it does run into a 4" pipe, so I could go larger if required, but that's another section of pipe to buy. The WM drains into a sink and the sink's drain goes to the sump.

 

[Sponsor]

 

[Terry]

The 4x2 wye that now has a 1.5" pipe can be piped with 2" which is twice the volume of 1.5". The pumped line should be piped to a 3" or 4" line. You can't mix gravity and pumped on the small lines. Have you snaked the shower?

 

[Reach4]

I'm at my mother's house, and the drain line from her kitchen has always been prone to clogging. It is tied into a 2" line that also drains a tub and a shower after it before it dumps into a 4" main line.

The slope on this line never was really correct, so it tends to back up from the kitchen sink. I noticed when I took a shower this morning that there was about an inch of standing water before I was done...it drains, but slowly.

"it" is the shower, I presume. Since you are also talking about the kitchen drain, I am thinking you are implying that the drain from the shower shares a drain pipe with the kitchen.

If you bring in a drain cleaning specialist, that might get the drains working her for another 20 years or so. Get her a strainer for the kitchen sink bowl to keep food out. Make sure the transition from the kitchen vertical drain to the horizontal is a long sweep.

Has the septic been pumped in the last several years?

 

[Jadnashua]

The shower only gets used (it's upstairs) where there are guests. I think that after a few showers, the height of the fall helped clean the line out some, but I'd still like to separate the kitchen sink from the same line and combine it to the WM's pumped line out of the basin. I think that the pumping action would help keep the line clean, and the extra fall to that lower line would speed it up a bit before it gets to that Y.

It runs into the main outlet to the septic system which is a 4". I could change the Y to a 4x3 verses the 4x2 that's there now (after reaming out the reduction fitting), but would rather make the line 2" verses 3" if that would be okay.

So, sounds like combining the kitchen sink's 2" and the pumped line and then continuing it in 2" would be okay?

 

[Terry]

So, sounds like combining the kitchen sink's 2" and the pumped line and then continuing it in 2" would be okay?

It's not okay to mix the pumped with the gravity on a 2" line. If you have a back up on the gravity line. My inspectors like to see those being separate.

 

[Jadnashua]

So, if I bump it up to a 3" line, all would be well at the Y?

 

[Terry]

3" works.

 

[Jadnashua]

Thanks...now getting a new piece into the donut of the CI may be the next challenge!

 

[Jadnashua]

Well, finally got around to doing this...it's a wonder that the kitchen sink drained at all for all of the grease and crud in there! The larger pipe, flushed by the washing machine, should keep this viable for a long time. It was a bear getting the old pipe out of the donut. I ended up smashing it into pieces with a hammer. Sanded a slight bevel on the new pipe, slobbered a bunch of liquid dish soap on it, and was able to carefully hammer it in...I'd left it long so I'd have enough to cut off if the hammer messed things up, but it worked out fine. I thought that there'd be enough slop so I could cement all of the fittings, but couldn't (I don't usually have good luck with repair couplings on plastic pipe)...only had to make one trip to the store to get a banded coupling to put the last thing together. As is normal, took way too long, but cheaper than paying someone, and I had the time.