Some areas of the US will not allow the temp/pressure valve to be vented inside. at least not any more. When one goes on temperature it can be really dramatic. It will vent until the temperature/pressure is low enough for the valve to reset. Diameter of the vent depends on the length of the pipe and number of elbows. PVC is a really bad idea. If the relieve valve goes cause of temperature then the PVC is going to melt. All of the drains I have seen are usually in copper pipe. I remember seeing a really old installation done in galvanized pipe.
Check with a pro in your area