Can I safely open the pressure relief thing myself? I hear a hissing and have water coming out of the drain pipe. Will that help?

Yes, but it may not reseal due to a build up of mineral deposits. It is generally not recommended to to open the PRV for this reason. Since yours is already leaking, give it a try. Nothing to loose. May have to replace.

The T&P valve will leak due to high water temperature or high water pressure and if it fails itself, you need to check the water pressure and temperature before you remove or replace the T&P valve.
As Harold says if there is build up of mineral deposits opening the T&P valve will not help you need to replace it, if you want change it come back and we will tell you how to replace it, good luck.

John

Both answers are "spot on". If working the handle up and down doesn't stop the hiss then the valve must be replaced. Good luck, Tom

If you see water coming out of the relieve valve drain pipe it may indicate your pressure or water temperature is too high.

You will have to find out which one is the problem, correct it and replace T&P valve.

Milo and Afaroo (John) both basically said, If you see water coming out of the temperature and pressure relief valve drain pipe it indicates your pressure or water temperature is too high...

Just to be clear, when you do see water coming out the T&P pipe it could certainly be the issues mentioned here, but most of the time when I find a leaky T&P valve it is simply defective and needs to be replaced.

In my opinion, you should at least test the water temperature and if the temperature is within normal limits (120-130F) just replace the T&P valve. If the T&P valve drips again, you would then go out and get a hosebib pressure gauge and test for pressure issues.

Most likely the T&P valve is just defective and needs to be replaced.

Good luck!

Mark