Yesterday my Rheem hot water heater with automatic ignition stopped heating the water. I turned the thermostat on the unit up from 1/4 hot to 3/4 hot and the automatic ignitor turned on, the burner turned on, and the heater ran and made the water hot. A few hours later I turned the thermostat back to 1/2.

This morning, no hot water. I turned the thermostat back up to 3/4, the ignitor turned on, the burner turned on, and the heater worked again. The water is hot.

Is this a temperature sensor problem? Thermostat problem?

Please help.