confusing waste plumbing

[Matt Fellman]

I can't make any sense of the 2" T coming off the mainline????

Whatever it is, it appears to have been this way since 1979 when the place was built.

[Jim Robinson]

That's a weird one. Sump pump or lift pump below? Even so, the T would be backwards.

[John Kogel]

Yeah, what Jim said, a sewage ejector pump from a sump in the basement, backwards Tee. It would still be wrong with the Tee turned around, BTW.

[Matt Fellman]

Thanks guys... actually, there was an ejector pump below. I was almost thinking of a primer of sorts to keep something in it? But that really doesn't make sense.

It goes against the number one plumbing rule that pipe can never reduce in size (I know, I know, "stuff" always rolling down hill is actually number one :) ) There are two toilets just a few feet upstream and I'd have to think the smaller pipe at the T is totally plugged.

I'm glad to see you guys share my confusion.... I just wrote it up as a non-recognized installation/design and to look into it further.

[Jerry Peck]

Unless that is a vent (and a very large at that) then it is completely wrong ... ALL the waste in that line is going to go down that tee.

[David Bell]

If the picture was upside down it would be ok, but really there must be a check valve in the line up to the backwards t-wye or some serious stink would be going back to the pump.

[Steven Saville]

Likely a vent stack as evidenced by the fixture vent lines in the lower area of photo. Perhaps the homeowner had the original 4" stack relocated to free up an area above the original location in the center of the home.
If indeed a vent stack, no real problem exists.