Why Do Cartridge Heaters in Kettles and Water Heaters Accumulate Heavy Scale While Inner Walls Have Much Less?

Most homes have to deal with limescale (water scale) building at some point in their daily lives. When you boil water in an electric kettle, a lot of scale builds up on the bottom. Over time, water heaters get a lot of scale on their heating parts. People who are careful notice that scale builds up mostly on the cartridge heater surface, notably the bottom or the heating element itself. The interior walls of the kettle or tank, on the other hand, display much less scale. Why does this happen?

Instead of traditional submerged rods, modern electric kettles have flat cartridge heaters built into the bottom. Cartridge heaters that were immersed directly in water were common in older instant hot water dispensers (boiling flasks). These cartridge heaters were useful, but they were also very dangerous since they caused a lot of fires, which is why universities have tight rules about dormitories.


Scale is a big difficulty with cartridge heaters. As kettles and water heaters are older, it takes longer for water to boil. One big problem with size is that it uses more energy. Scale covers the cartridge heater surface, which stops direct contact between the cartridge heater and water. This greatly lowers the effectiveness of heat conduction.

Calcium carbonate, magnesium sulfate, calcium sulfate, and magnesium hydroxide make up most of the scale. Compared to metals used in cartridge heater sheaths, these compounds have very weak thermal conductivity (about 0.5–3 W/m·K). Copper has a thermal conductivity of 385 W/m·K, and stainless steel/iron has a thermal conductivity of about 79.5 W/m·K. It can be tens to hundreds of times different, so thick scale on a cartridge heater makes the element overheat in one spot to move the same amount of energy. This raises power expenses and could harm the cartridge heater.

When scale covers the cartridge heater, it keeps the heat within, which makes the temperature rise quickly in some places. This could break down the magnesium oxide insulation inside the cartridge heater, make it last less time, or cause safety problems. In places with hard water (like the north), scale builds up quickly, thus you need to clean the cartridge heater often.

The activity of bubbles is intimately related to the establishment of scale. The main parts of scale are calcium carbonate, calcium silicate, calcium sulfate, and magnesium hydroxide that have settled out of tap water. Because the cartridge heater creates heat, bubbles keep forming on its surface as it heats up.

The first bubbles form when air that has been dissolved escapes as the temperature rises and the solubility drops. As the water gets hotter, it starts to boil and makes steam bubbles. These bubbles make a three-phase interface: liquid water, gas bubble, and solid cartridge heater surface. When bubbles break off and evaporate, they cause the calcium and magnesium salts near the nucleation site to become too saturated. This causes precipitation that sticks to the surface of the cartridge heater.

Studies indicate that scaled cartridge heaters generate a greater quantity of bubbles compared to new ones. The rough, porous surface of a scaled cartridge heater makes it easier for scale to form since it has more nucleation sites and is more water-loving. So, the longer a cartridge heater runs with scale already on it, the faster new scale builds up, which makes the problem worse.

Why does the cartridge heater get a lot more scale than the walls of the kettle? Calcium and magnesium ions that form salts are the building blocks of scale. Most stick to the hot cartridge heater surface, while some look like white particles floating in the water (mostly calcium carbonate and magnesium hydroxide), which makes the water less clear.

From the very beginning of heating, the cartridge heater makes localized high temperatures, which are perfect places for bubbles to form. Bubbles form right away on the cartridge heater, which causes scale to build up all the time. In contrast, bulk water only gets hot enough later, and precipitation there needs homogenous crystallization, which takes more energy.

Crystallization theory says that heterogeneous crystallization (on the cartridge heater surface) is energetically good since the solid-liquid interface lowers surface energy. Homogeneous crystallization (in bulk water) needs more energy to make crystal nuclei. Even while quick heating gives out energy, heterogeneous crystallization on the cartridge heater is the main process, which is why there is more buildup there.

The inner walls do build up some scale because they are also places where heterogeneous nucleation happens, but the amount is considerably smaller than on the cartridge heater because there is no localized high heat or severe bubble production.

Use vinegar, citric acid, or industrial descalers to clean the cartridge heater on a regular basis. If you live in an area with hard water, you might want to think about getting a water softener. To cut down on nucleation sites, pick a cartridge heater with smooth, high-quality stainless steel sheaths (304 or 316). Always make sure that everything is fully submerged and keep an eye on the boiling times as a sign of scale buildup.