The most common reasons for an excessively hot gas water heater include a defective gas valve, a defective thermostat and a short in the heating element. The most effective solutions are replacement of the defective parts and adjustments to the heater's electrical wiring scheme.
The primary indicator of a defective gas valve is that the burner continues running when the thermostat is at its lowest setting. The best course of action in this situation is to turn off the gas supply, remove the faulty valve and replace it. This task requires professional assistance.
A heater's thermostat is part of its gas valve assembly. Thermostats are more reliable than gas valves and rarely need replacement. Many water temperature problems are due to improper thermostat settings, not a problem with the device itself. In such cases, adjusting the thermostat temperature fixes the problem. If this does not work, the thermostat may be faulty.
Diagnosing a faulty thermostat involves turning a household faucet to its hottest setting and comparing the temperature of that water with the thermometer display. A discrepancy greater than 20 degrees indicates the heater needs a new thermostat.
Ungrounded heating elements are prone to shorting and constant burning. Connecting the heating element to a grounding wire fixes the problem. This procedure requires assistance from a professional electrician.