2 tanks
The presence
of 2 water tanks in the storage water heater; usually, the total volume is divided equally between these tanks.
This design is noticeably more complex and expensive than the traditional 1-tank arrangement but offers several advantages. Firstly, the tanks can be heated in turn, which significantly speeds up the process. After all, for hot water to be used, it is enough to warm up only half of the total volume to the operating temperature. Secondly, with this format of work, power consumption and the load on the power grid are reduced. And if at a certain moment, the user does not need a lot of water, then the energy consumption turns out to be small (again, because the entire volume does not need to be heated). Thirdly, compared to traditional models of the same volume, such boilers are thinner, which can simplify installation (the price for this advantage is an increase in width, but this moment is not so often critical). Fourth, this arrangement improves thermal insulation and reduces heat loss.
Max. water temperature
The highest water temperature provided by the device. The standard temperature of hot water in the water supply is 60 °C, and this value is actually the minimum for modern water heaters: models with more modest rates (usually from 40 °C) are extremely rare. But higher values can be found much more often: for example,
water heaters of 75 °C and
80 °C are very popular, and in the most powerful models in this regard, the temperature can reach
95 °C and even higher.
On the one hand, strong heating requires appropriate power (which is especially noticeable in the case of instant electric heaters). On the other hand, the higher the temperature of hot water, the less it is needed for a comfortable outlet temperature, after mixing with cold water; this reduces the consumption of heated water, which is especially important for storage boilers. In addition, many models have thermostats (see "Features").
Also, note that heating to operating values may involve different ΔT (degree of temperature change) — depending on the initial temperature of the cold water. The actual performance of the heater directly depends on ΔT; this moment is described in more detail below, in the paragraphs devoted to performance at different ΔT.
Tank lining
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Enamel. Like plastic, enamel is chemically neutral and does not affect the taste and smell of water, while it is considered more durable. Theoretically, this material is prone to the appearance of microcracks, including due to temperature differences (which eventually lead to water contact with metal and corrosion). However, high-quality heat-resistant enamels are most often used in boilers, which have the same coefficient of thermal expansion as the material of the tank and are damaged only in case of violation of operating conditions (or with strong impacts). So the mentioned drawback is typical mainly for the most inexpensive models with appropriate quality materials.
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Stainless steel. Due to its high strength, stainless steel is considered the most reliable and durable material today. Unlike enamelled ones, such tanks are absolutely not afraid of temperature changes, and they also normally withstand hits including pretty strong ones. On the other hand, steel is noticeably more expensive than enamel. At the same time, for such containers, the possibility of corrosion is not ruled out — especially when it comes to cheap devices that use outdated welding technologies, and the material of the seams may differ from the material of the tank. To eliminate this phenomenon, cathodic protection is required, which further affects the cost.
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Glass ceramics.
...Material, in many respects similar to the enamel described above. On the one hand, glass ceramic does not react with water, does not affect its taste and properties, and is also considered quite reliable. On the other hand, this material is more brittle and prone to the appearance of microcracks and the loss of its properties — both as it wears out and due to strong heating. Because of this, such water heaters usually have a recommended temperature limit of 60 °C.
— Plastic. Plastic is chemically resistant, not subject to corrosion and practically does not affect the composition of water, besides it is inexpensive. The main disadvantage of plastic coating is considered to be fragility.
— Copper. Copper coating is used exclusively in instant water heaters (see "Type"); more precisely, in such devices, the entire tank is usually made of copper. This material is not suitable for a storage tank: copper is too heavy, and it has a corrosive effect on some materials (aluminium, cast iron) due to its electrochemical properties, even if these materials are used outside the heater, in other parts of the water supply system. However, in a small tank in an instant water heater, these moments are invisible, while copper perfectly tolerates compression and tension during temperature changes.
— Titanium-cobalt alloy. A special alloy, characterized by the highest strength and resistance to corrosion, but also very expensive. It is extremely rare, only in top-level heaters.Heating time
Time to heat the storage tank (see "Type"), filled with cold water, to operating temperature.
It is worth remembering that this characteristic is not 100% accurate. Manufacturers usually indicate the heating time for certain conditions: a filled tank, maximum heating intensity, and temperature rise (∆T) by a certain number of degrees. In practice, the heating time may differ, both one way and the other. For example, if the heating time for the device is 20 minutes at ∆T = 50 °C, then when the water is heated from 15 °C to 60 °C, the time will be shorter (∆T = 45 °C). Nevertheless, this indicator allows us to evaluate the overall flow rate of the boiler, and with equal ∆T and volumes, different models can be compared in terms of heating time.
Weight
The weight of the device, excluding the water collected (passing) into it.
