Immersion heating rods consist of a coil, metal loop or rod that heats up when an electric current is passed through it. The heating element is designed primarily to be submerged in a liquid, such as water, oil or solvent, but can also be passed through molten materials or gas.
As of 2015, there are many different kinds of immersion heaters. Over-the-side immersion heaters consist of a unit that is installed from the top with the heated rods along the side or at the bottom of the tank.
Flanged immersion heaters are through-the-side heaters that are energy efficient, easy to install and maintain, and one of the most widely used designs in industrial applications. They consist of bent tubular metal elements that are welded into a flanged frame. They are installed using a matching flange welded into the tank.
Threaded immersion heaters, also referred to as screw plug heaters, consist of a similar bent tubular metal element that is screwed directly into a threaded opening or plug in the side of the tank.
Immersion heating rods are also used in hot water heaters. In this event, an immersion heater usually consists of a metal loop or coil and uses a through-the-side installation. They have an on-off switch and are supplied with their own power through a cable. They are sometimes installed in hot water tanks with other primary heating systems as a backup heating source. They are not connected to boilers, the primary heating element in a standard hot water heater system. Therefore, if the boiler breaks down, hot water can still be generated for the home. Heating rods retain heat, keeping the water hot for several hours after they have turned off if they are well insulated and this reduces or negates the need to heat water constantly in the hot water cylinder.