On Sun, 19 Apr 2020 19:55:15 +0100, NY wrote:

"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.

Perhaps the venting wasn't so good in those days? Although I don't see why, the vent pipe sticking up to roof height has always been pretty much the same.

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!

Sounds sensible to me. When the mind is doing nothing during your walk, it's going to think about something.

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.

That makes sense, a smaller version of the big pipe that's usually placed just after a toilet. When the sink finished emptying, the water flowing through that steep pipe and filling it's whole diameter, would suck the air behind it. But the extra pipe allowed air in from higher up instead of pulling on the u-bend water.

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.

I don't see the point in that. If the downpipe became blocked, all that does is make loads of water pour out of the box instead of just spilling from the gutters over a wider area. I'd say it makes the problem worse.