bathroomdrainplumbingsink
As title states, this sink drain in a bathroom has me confused as to the purpose of the higher set pipe above the trap. My assumption is that it's a vent, but I don't understand why it's here. I'm pretty sure this was a previous DIY job given that all the fittings are glued together. Any insight would be much appreciated.
Replacing the whole lot is certainly the way to go. The layout is daft and it seems held together with wishful thinking.
What to do? Something like what you have in mind but IMO the dishwasher should join in immediately above the p-trap.
Many companies sell complete packages for what you're trying to do.
E.g..
(source: plumbers-mate-sales.co.uk)
Or this one listed as "space saving"
(source: scene7.com)
Unfortunately, once you've done all of that I think you're still going to to find that the waste pipe comes out if the wall a little to high up.
Best Answer
Most likely, this is not a vent, or more precisely, this tube will not work as a vent.
F.e., this tube will not prevent that the siphon/trap will lose its water if a low pressure sets in downstream the trap.
Venting is normally done on the other side of the trap (downstream).
It looks more like a drain of a washing machine, dish washer, a condensing water line of an A/C or a gas/oil heater etc.
Maybe behind that wall there are hints of (former) installations?