[header] How to turn on a water heater [title] Shut off the water supply and gas valve or circuit breaker. [step] Turn the gas valve to " off, " or make sure the breaker switch for the water heater is off at the circuit breaker. To turn the water supply off, twist the valve for the cold water supply line that enters the tank (usually from above). 
[substeps] The breaker switch for your water heater should be labeled, but if it isn't, go ahead and turn off the main power. [title] Drain and flush the tank to clean it out.