Why does the relief valve leak on my new water heater?
There are two scenarios,either the relief valve is working properly or it isn’t. Check the pressure. If it is too high the valve is doing its job and you will need to install a regulator. Does this problem happen only when hot or when cold as well? If only when hot and has been sitting unused for at least a few hours an expansion tank will help. If it is hot and leaking, open a hot water faucet and let it run for a few seconds, close it and see if it is still leaking.  If that temporarily solved the problem an expansion tank will help. If it didn’t help it is either too hot or too much pressure.