An enterprising Appliantologist was working on a Whirlpool dishwasher with poor wash results, and when he opened up the machine to take a look inside, he saw this:

That's a lot of buildup on that heater! Seems like it indicates that the problem is with the heater. However, our tech noticed that the heater was getting hot enough to make water sizzle when splashed on it, so it was operating at least to some degree.

Additionally, our tech took an amp reading on the heater's circuit while running a test cycle, and he got only 3.5 amps. That seems like way less than you would expect. Are we dealing with some weird power supply issue, and the problem isn't the heater at all?

Let's get more info by looking at the spec block:

Huh? That's odd -- why are there two different wattage ratings for the element depending on if it's wet or dry? Does the control somehow sense if there's water in the unit and adjust the power supply to the heater accordingly? Or does the heater's resistance change when wet? That seems pretty far fetched. What's going on here?

Let's take a little detour into how these calrod heaters are made to answer these questions. This is a diagram of the insides of a typical calrod heater:

There's not actually a lot going on here. You just have three main components: the nickel chromium heating coil, the magnesium oxide insulation, and the steel sheath.

The heating coil is what the electric current actually passes through, and as it does, the coil heats up. It's basically just a specialized resistor.

Magnesium oxide is used for the insulation because it's a bad conductor of electricity, but it's a great conductor of heat. So it lets the heat from the coil radiate out, but it doesn't let current flow into the surrounding sheath.

Here's a cross-section of a real world calrod where you can see the same components:

So there's nothing we can see here about the element that should react to water. But there's something essential we need to know about metals like nickel chromium: their resistance increases with temperature.

The graph above shows this fairly linear rise in resistance as temperature increases. This is just a typical property of metals. They're great conductors at lower temperatures, but as the temperature increases, they lose some of this property.

Why is this? Well, it has to do with how "excited" the electrons in the metal are. On the atomic level, a relatively cool metal has plenty of electrons that are ready to move, but not going anywhere unless voltage is applied to them to make them more.

However, once you heat up that metal, those electrons start to bounce around in random directions due to the thermal energy. This creates what you can think of as a sort of "conflict of interest' when voltage is applied. Sure, plenty of those electrons will still move in the direction the voltage wants them to, but some are still bouncing in random directions due to the heat, possibly contrary to the current flow.

This graphic explains this pretty well:

Why is all this relevant? Well, while the calrod heater being wet doesn't directly affect the circuit current, it does have an indirect effect. That is, water around the heater allows for much better heat transfer than air. Better heat transfer means that thermal energy gets removed from the coil, and less thermal energy = lower resistance. Lower resistance, higher amperage.

Aha! We've figured out why the spec block gives two different wattages for wet vs. dry. A dry heater coil stays hotter, meaning its resistance gets higher, lowering its amperage (and therefore its wattage). That 500 Watt dry heater spec, when we apply a little Ohm's law mojo (I = P/E), works out to a little over 4 amps.

Considering our tech was getting even fewer amps than that, we come to our conclusion: the mineral buildup on the heater is acting as a thermal insulator, making the heater coil get super hot when running. Even when the heater is submerged in water, the buildup is preventing heat transfer. And so you get minimal water heating, leading to subpar wash results.

The customer needed a new heating element and something to soften their hard water issue. Problem solved!

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