What is the proper way to test the heating element(s) in a water heater? I have heard they can be tested with the power either on or off? Is one way better than the other?
The heating element in an electric water heater works the same as the heating elements in ovens, toasters and the stove top. The elements heat up because they highly resist the flow of electricity. This "resistance" can be likened to friction, causing an increase in heat. If the element is broken... meaning the electricity does not have a compete path to follow through the element... the element will not heat up.
Of course, if there is no electricity reaching the element you may have a different problem... perhaps an "off" or broken circuit breaker, a defective thermostat or even a loose wire. Therefore, testing involves troubleshooting not just the element but also its electrical connections and switches, too.
Testing the element with the electricity "on" is both unnecessary and dangerous... dangerous because all electrical work with live wiring is dangerous and usually unnecessary. This case is no exception because you cannot test the element directly with the power on and the wires connected... only "indirectly". An explanation is in order.
Let's say you decide to test the element with the power on. If the power is on you cannot test the element "directly" via resistance, since it is not possible to test for resistance with a multimeter where there is voltage across the same two terminals... you will fry your multimeter! And testing for resistance is the only direct test for element function.
However, if you set your multimeter to test voltage (for residential 1000v is fine) instead of resistance and do not get a voltage reading, one of two things is occurring. Either (1) there is no electricity across the terminals or (2) the element is working fine. These two statements may seem contradictory, but make sense when you understand how a multimeter tests for voltage. If the electrical connection across the element is complete (high resistance) then a multimeter cannot read the electricity. Multimeters can only take voltage readings across "open" circuits. Therefore, if you touch the multimeter probes across the element's terminals and get no reading it means that the element is working fine... open circuit... or there is no electricity... also open circuit. So, to finish testing, one of the electrical wires must be disconnected to determine if there is electricity reaching the element.
You can see why it is easier to just keep the power off, disconnect one of the wires from the element and test for resistance across the element. You immediately know if the element is functioning without any "confounding" factors!
copyright G.G. Alonzy
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