While watching videos about draining tankless water heaters, I noticed none seem to have this beige pipe with odd silver cab at the end. Does anyone know what this is and its purpose?
That is what we call a "pop off", or pressure relief valve. If the water pressure gets too high, that valve will open and drain into your waste water system. Its there so your tank, or hot water system, does not blow up due to getting too hot.
I believe that's the pressure relief valve. Where does the plastic pipe go to