On 19/04/2020 20:46, charles wrote:
In article op.0jb2xvjcwdg98l@glass,
Commander Kinsey wrote:
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

I still have them. It provides an air break to prevent feedback.


There is some suggestion on certain web sites that it is to increase
flow rate.

--
Adam