Usually the water heater will have a 15 amp circuit breaker than controls the 120 volt power to the water heater's relay on the rear. The wall switch is a 12 volt switch and controls only the relay via the circuit board. With the 12 volt wall switch on and the 120 volt circuit breaker off, you should be able to test the control voltage to the relay.
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Jim & Roy Davis
2016 Hurricane 31S
1961 Rampside in tow
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