Hi everyone. I'm guessing this must be old code but I've come across 2 different water heaters that have the T & P valve drained into the slab or into a wall in a townhouse. It is three-quarter inch copper. the houses were built in the early eighties. if I remember right the T & P valve should terminate so that you can tell if it has opened to alert you to a problem or also for testing. my question is should I cut this line when I replace the heater so that the t & p line is easy to see and just place a catch vessel under it so it can be monitored. also there is an AC Evaporator right next to the water heater and I'm wondering if I can drain it into the old T & P line that runs under the slab or if I can make that old line into an open drain and run them both into it from above? Thanks for any suggestions.

Our code requires that the T + P relief valve must discharge in the same room as the heater. Then it can go into a pan that's piped outside, or into a funnel with an air gap.
In your case I'd cut the pipe, add a funnel to the pipe that runs out and drop the pipe from the new heater into the funnel. I usually make them from a 1 1/2" X 3/4" copper reducing coupling.

we have to discharge it to a location where it will not cause property damage in case of a discharge, which usually means through the wall to the exterior of the building. Your underfloor line probably goes to a point outside the building, and since it should always drain "downhill" the end could be buried undergrounw, I have seen it happen many times.