How to properly extend the TPR Pipe on a water heater
You can buy a replacement TPR drain tube for about $5 at any home improvement store.  Simply unscrew the old drain line, measure and cut the new line, then screw the new line in place.  You'll need two wrenches, and some PTFE tape (or whatever your preferred pipe dope is).


Place one wrench on the TPR valve, to prevent the valve from moving (a pipe wrench works well).
Place the other wrench on the drain tube.
Use the first wrench to hold the TPR in place, when you use the other to unscrew the drain pipe.
Cut the new pipe to length.
Apply PTFE tape (or your preferred pipe dope) to the threads of the pipe.
Use the two wrenches to install the new pipe.


NOTE: This answer assumes a side discharge, threaded TPR valve.



Technically, you cannot use SharkBite® connectors on the temperature relief valve drain. Temperature relief valves can be set as high as 210°F, while SharkBite® connectors are only certified to 200°F. However, your local building department may allow it, so check with them to see if it's acceptable.