I watched ask this old house and they said you have to test your emergency t/p valve once a year. I never knew that before. When I tried it, it was hard to open and then after it let it close, it started to leak just as Tom said on the show. Luckily, it is only about a $15 part that is pretty easy to replace though there is more water in the top of the tank than I thought and I made a bit of a mess. Even though I've never had a water tank explode, I feel better knowing that this relief valve is working properly.

Thanks!