Hot water heater is overflowing
Hey guys, I think my hot water heater from time to time has overflowed a little bit (few drops here n there), but recently, I turned it up all the way to the max HOT since we now have 3 people showering in the morning at the same time. Of course, this didn't help and the water overflowed pretty badly. I turned it off, let it cool and drained some hot water from my faucets. Now I've turned it back on and set it warm and watched it. It didn't overflow. After a few more showers, it overflowed again, but not as badly (about a car wash size bucket full of water). Also, the water is only hot for about 10-15 minutes if I shower at night with no one else showering. If all of us shower, it gets pretty cold pretty fast.
It’s a gas water heater, about 6 years old now.
What are some things I can do or test to see if something is broken on it?
And what does the little lever on the left side of the water heater do? It looks like a pressure reliever or something.
Thanks guys!