Hi this tank has a drain at the top, and when I'm not using hot water, water flows out of the drain at the top of the tank. (Sorry this is how I am able to explain it). When I use hot water, the drain stops draining water for a few hours. [Note pic not of actual malfunctioning tank]
I climbed up to the roof cavity to take a look and it seems that when I use hot water for kids' bath, the level in the tank drops a 3 inches. Then when the water tank heats up the level rises until it constantly runs out the overflow drain. (I haven't re-checked to see if the "floating ball valve" is leaky however)
Why would the water rise and fall in the first place? Is my water heater thermostat broken?