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An immersion water heater is a type of electric water heater that has an element immersed in the water it is heating. This heating method is employed in heating large tanks of water, such as the hot water source for a building, as well as small containers of water, such as a pot for brewing tea. These heaters usually use electric resistance to generate heat.
Portable heaters have an insulated heating coil as the resistor that is placed in the water. When a current is run through the immersed resistor, it passes through the heating coil and gradually raises the temperature of the water. The size of the typical portable immersion water heater varies with the volume of water it is designed to heat, but most are less than 2 pounds (about 1 kg) in weight. Many of these heaters are dual voltage, and they will operate within a wide range of 110V to 240V. As a result, they are frequently used for travel, because they will work with both American and European currents.
Tank heaters are also structured with an immersed, insulated resistor. They frequently contain thermometers and temperature controls, as well as timers to allow users to better regulate the heating process. A typical tank immersion water heater will be placed in a tank shaped like a cylinder, which allows the hottest water to rise and be siphoned off for use.
The three major types of tank heaters include flanged, over-the-side, and screw plug. Flanged heaters are bolted to their container, and are the most permanent form of immersion water heater. Over-the-side heaters are named for the fact that they dangle over the side of the tank, which makes them more easily repositioned and removed. Screw plug heaters are plugged into the side of a tank, and are used for larger volumes of liquid. Both over-the-side and screw plug heaters come in many different designs, and they are also frequently used to heat liquids other than water.
One unique type of immersion water heater does not contain a resistive element or heating coil, but instead uses water itself as the resistor. In these heaters, two electrodes are placed apart from each other, and the current runs through the water to get to the other electrode, generating heat in the process. These types of immersion heaters can be dangerous, because the current does charge the water. This risk means that they are frequently found only in devices such as humidifiers, where the water itself is isolated from potential human contact.