This is a photo of the main waste line in a friend's house. All the waste drain lines feed into it, and just to the left of this photo is where the drain pipe exits the house for the septic tank\field. I understand the vent, but why is there an open line on the waste line in this location? AFAICT, in the event of a main line stoppage that trap isn't going to stop the line from overflowing into the basement.

Just a guess here, but I see the lineset for the HVAC there. Could it be the condensate drain and the AC installers didn’t use it?

Was there a washing machine planned for that location? That's what it looks like.

The presence of the P-trap and the direction of the y-fitting both imply that something was meant to flow into the system here. What it was may be known only to the plumber or builder.

Cap it (weld).

It's upstream of the vent, so effectively eliminated, and the cap can be cut off if it ever needs to be employed.

Follow-on thought: if the drain input is not being used, cap it. The P-trap will stop gasses only so long as it has water in it. When the water evaporates, the sewer gasses will come into the house... not good. Stinky, dangerous.

It may be capped with an inside diameter plug that plumbers often use on new construction to keep debris out of the pipe until the fixtures are connected.

Had something like that a while back. It was formally used for the water backflush drain. It was removed as the drain should not have gone into the septic. It was later capped as a source of smell.

But as I'm not a plumber, it's a guess.

This is the most likely answer. My friend has been in this house about four years, and hasn't had a sewer smell issue. Thanks to all for the info.

quote:

Originally posted by a1abdj:
It may be capped with an inside diameter plug that plumbers often use on new construction to keep debris out of the pipe until the fixtures are connected.