In rec.crafts.metalworking Existential Angst wrote:
Awl --

Ahm semi-re-plumbing the lines to the HW heater, and I noticed what appears
to be some sort of vent/pressure relief ditty coming off the *CW inlet to
the HWH*. From the CW pipe to this gadget is copper, from the gadget to the
wall is 1/2 black pipe. The 1/2 pipe goes thru an outside wall, altho I
have yet to find anything outside!

I'd be hardpressed to imagine that this thing still works, whatever it was.
I'm sure it's 25++ years old.
It's not an anti-water hammering device, afaict. Is it safe to just get
rid of this thing? City water pressure is barely 60 psi, I have gauges
plumbed into a couple of lines, very little variation.
It would make my life a lot easier if I could ignore/get rid of this thing.

it is some sort of weird attempt at a drain line going outside?