I was doing a mock inspection today and came across this in the boiler room.
To me this looks like a extension pipe for the pressure relief valve but I don’t see a valve.
If this is the extension pipe it ends on top of the hot water tank.
Michael, you may want to expand the picture in view so all can see what it is hooked up to.
That may help in responses.
It just looks like a pipe for an overflow off a tank of some sort. 
That helps a lot, now some plumbers might chime in.