The number of heating elements provided in the design of the water heater. In this case, it is the total number of elements that are taken into account, regardless of whether they belong to the same type or different ones: for example, 2 heat exchangers and 1 heating element are considered as 3 elements.
All gas models (see "Energy source") have only one heating element — this is quite enough for efficient operation. In
combined devices (see ibid.), on the contrary, there are several heating elements by definition (at least two — a heat exchanger and an electric one). In electric and indirect water heaters, the options may be different.
The meaning of several heaters of the same type is primarily to increase the heating efficiency. For example, in an instant (see "Type") electrical water heater, in this way, it is possible to increase the working length — the distance that water passes inside the device from inlet to outlet; by increasing the working length, the water is heated longer. In storage electric models, several heaters provide more uniform heating of the water, and in indirect ones, they allow more heat to be taken away. In addition, in indirect devices, heat exchangers can differ in the source of heating: for example, one can work from a heating boiler, the second from a solar collector.
Also, note that duplication of heating elements can also be used as protection against failures: if one of them
...fails, the heating efficiency decreases, but the device remains operational. However, this possibility is not available in all models with several heaters, its presence should be clarified separately.—
Open coil. The open coil is made from a high-resistance electrical wire enclosed in a thin insulating sheath. The main advantages of such an element are the heating rate, high efficiency and precise temperature control; in addition, scale is almost not formed on the spiral. And of the shortcomings — a low service life.
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Wet heating element. Wet heater is a metal tube with a heating thread laid in the centre; the space between the tube and the thread is filled with an insulating material with good thermal conductivity. Heating elements heat up more slowly than open coils, have lower efficiency and are prone to the formation of scale on them; on the other hand, their service life is much longer, and in instant heaters, heating elements are not so sensitive to air pockets.
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Dry heating element. A kind of heating element with an improved design: the heater tube is enclosed in an additional shell (most often made of metal with an enamel coating on the outside) and does not come into contact with water, hence the name. Thanks to this, the likelihood of scale formation is reduced, which is especially important when working with hard water. Also, the replacement of such elements is significantly easier than conventional ones. Among their shortcomings can be called a rather high cost.
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Infrared heating element. Tu
...bular electric heater of a special design: in the form of a transparent glass tube, in which the incandescent spiral is enclosed. The principle of operation of such an element is somewhat different from a conventional heating element: a significant part of the heating is provided by infrared radiation, which heats not so much the water as the walls of the tank — and heat is already transferred from them to the water. Thus, the water is heated not only at the point of contact with the heating element but also at the point of contact with the walls — which means that the heating is faster and more uniform. Also, note that the IR heater itself is usually "dry"; see above for the advantages of this design. The main disadvantages of such heaters are high cost and relatively short service life.
— Heat exchanger. It is used in gas and indirect heaters (see Water heater type). It is a metal structure heated by burning gas (in gas heaters) or passing inside a heated coolant (in indirect heaters). Usually has a ribbed shape. It is done to ensure the maximum area of contact with the heated water with relatively small dimensions — the larger this area, the more heat is transferred to the water per unit of time and the more efficient the heater is.
