A dimmer switch (rheostat etc) is NOT the same as a PWM control nor will they perform the same on a high wattage circuit.
An electric heating element is simply a low ohm resistor that is heavy duty enough to not burn up from the high current. When this resistor has line voltage across it you get current, and this current is high enough that the material gets hot. Imagine overloading an extension cord by 10 times its rating; it will be a heater for a very short time period.
The goal of PWM or a dimmer switch is to limit the voltage applied across the resistive element so you end up with less heat from the resulting current. If you were to put a rheostat in a series circuit with a heating element the total circuit's resistance will not decrease. The resistance will only increase as the rheostat's resistance is increased, the knob is turned. To lower the heat (result of current) in the heating element you must have a higher resistance than the element in the circuit. A large enough resistance rheostat, the 1000W+ kind, will essentially be absorbing more voltage than the heating element, therefore lowering the heat created in the element.
However, by absorbing the majority of the circuit’s voltage the rheostat will also be using the majority of the circuits power. What you end up with is a heating element, the rheostat, inside your control box instead of in your tank. Imagine that a heating element puts off a certain amount of heat, and two of them in series would put off half as much each.
On the other hand PWM accomplishes the same thing by fast switching the circuit. The voltage across the element isn’t reduced at the high frequency that the PWM controller switches but averages out to less voltage at line frequency to the element. I always describe it as a rocket ship that has full on or full off thrust. If you were to “feather” the throttle very fast you could make the rocket hover, neither drop nor rise.
In a lighting circuit, lower power than heating water, a rheostat would work great because it would only have to dissipate the wattage of a bulb, 100W or so. However for a high load circuit you would have very short component life and some unsafe conditions.
I hope this helps you some. Drop me a PM if you have more questions.
