"Commander Kinsey" wrote in message
newsp.0jb2xvjcwdg98l@glass...
Why did drainpipes used to have a box on the wall, some sort of overflow
if blocked? See link below for an example photo.
https://www.dropbox.com/s/1jry8zas14bmivc/box.jpg?dl=0

It might have been to avoid sewer gases from flowing up the drain pipes from
sinks if the water in the U trap got blown/sucked out. I'm not sure why it
was fairly common (or even universal) for older plumbing but is very rare on
modern plumbing, even on houses with external drain pipes.

I will make a confession. When I was very little (maybe around 5 years old)
I used to have an infatuation with plumbing, and knew various houses on my
walk between home and school not by who lived there but by the pattern that
the pipes made on the wall. Embarrassing to admit such nerdy behaviour!
There were some houses that had steeply-sloping pipes from sink/bath
directly into the vertical soil pipe, but which had a second pipe that
branched off the sloping pipe and entered the soil pipe higher up. That
always intrigued me. I wonder if it was a way of avoiding the U-trap water
from being sucked out.

It was even done for rainwater pipes: https://goo.gl/maps/6zvySgaiNRRny41w5
shows the pipes from three gutters draining into a common pipe, with two of
them going into a hopper and another going into a side branch of the down
pipe.