Which type of boiler does not allow electric current to travel through the boiler water?

The type of boiler that does not allow electric current to travel through the boiler water is the immersion heater boiler. This type of boiler utilizes electrical heating elements that are directly immersed in the water. Because the design is focused on heating the water without allowing it to conduct electricity through the water itself, this prevents any current from traveling through the boiler water.

In contrast, firetube and watertube boilers are designed to transfer heat from combustion gases through tubes that are surrounded by water. These systems can produce electric currents under certain conditions, especially when there are issues related to water quality or maintenance.

Steam trap boilers, on the other hand, are not a distinct type of boiler like the others listed. They are components used in steam systems to remove condensate and air, but do not define a boiler type and are not applicable in this context regarding current flow through boiler water.

Understanding these distinctions and the operating principles of each type of boiler helps clarify why the immersion heater boiler is specifically designed to prevent electric current from traveling through the boiler water.