In the general catalog of storage water heaters, there is a convenient filtering system that allows you to choose based on various parameters. Apart from capacity and shape, you can also specify other significant criteria such as internal coating and tank warranty. Let's discuss each aspect in detail.

Water heater tank requirements: mechanical strength, corrosion protection, hygiene

While it may seem sufficient for the tank of a storage water heater to be simply airtight and free from leaks, in reality, there are additional requirements. The tank operates continuously under difficult conditions, necessitating specific attributes for optimal performance.

One crucial requirement for the tank of a storage water heater is its ability to withstand high pressure without deformation. Additionally, the tank must be designed to accommodate cyclic temperature changes, which cause the tank geometry to fluctuate. These changes are often referred to as "breathing" by engineers. Although the dimensional changes are minimal, they occur repeatedly, and the tank design should ensure that the original operational properties are maintained without compromise.

The use of metal in tank construction offers excellent mechanical resistance, allowing it to withstand repeated cycles of expansion and contraction. However, this introduces the second requirement for the water heater tank, which is corrosion resistance. Continuous contact with water can lead to the formation of rust on the metal surface. Ignoring this factor can result in the tank deteriorating from the inside and developing leaks, not to mention the potential impact on the smell of water.

The main threat to the water heater tank is corrosion processes.

A crucial aspect that users sometimes overlook is the intended use of the water from the heater, primarily for personal hygiene and dishwashing. Consequently, the materials used in the tank construction must be food-safe, ensuring that they do not alter the water's composition or introduce harmful or potentially hazardous substances.

What water heater tanks are made of: most often metal, but exceptions are possible

In the vast majority of cases, storage water heater tanks are made of metal, or rather, steel. Either stainless steel or a combination of conventional (carbon) alloy with an additional protective coating can be used.

In rare cases, plastic may be used as the material for the tank. Plastic is affordable, resistant to corrosion, and generally does not affect the water composition. However, it is important to note that a polymer tank is typically less durable and strong compared to metal tanks. As a result, plastic tanks are becoming less common, and it is advisable to choose water heaters with plastic tanks only for low water pressure systems, such as in private households or in the country.

In tankless water heaters, copper can be used for the internal water supply circuit instead of steel. However, copper is not suitable for the tank itself due to its weight and the potential acceleration of corrosion processes in the water supply system. This can have negative effects on components like aluminum faucets or cast iron pipes.

Before buying a water heater, you will have to choose between stainless steel or metal with additional protective coatings. Everything else is a rare exception to the rule.

Tankless water heaters

Stainless steel tanks: pros, cons and nuances

Stainless steel is highly resistant to corrosion, hygienically safe, and possesses sufficient strength. However, tanks made of this alloy can still have a potential weak point — the welds. Welding is necessary to create a sealed tank, but exposure to high temperatures during the operation of the water heater can cause chromium evaporation ("washout") in the welded areas. Chromium is a vital component that acts as a primary defense against rust. This phenomenon is known as intergranular corrosion and can lead to seal failure and leaks. Budget models of storage water heaters, which generally have lower overall workmanship, are more susceptible to such issues.

To enhance the protection of stainless steel welds, some well-known brands use various advanced technologies. These may include pulsed welding techniques, rigorous quality control throughout the manufacturing process, or the addition of titanium to the steel alloy. Titanium acts as a stabilizer for chromium carbide, effectively preventing intergranular corrosion. However, it's worth noting that water heaterswith these advanced features generally come at a higher price point.

The main thing in a stainless steel tank is the quality of the welds.
Water heaters with stainless steel tank

Steel tanks with a protective coating: pros, cons and nuances

Designing a water heater with a tank made of regular (carbon) steel is a simpler approach. It eliminates the need for specific welding requirements and stringent chemical composition criteria. Carbon steel is durable, hygienic, and, if obtained through cold rolling, exhibits excellent plasticity. It can effectively withstand the expansion and contraction cycles experienced during the normal operation of the water heater, even outperforming stainless steel in this regard.

Regular steel is susceptible to corrosion, which means that it requires the application of protective coatings to prevent direct contact with water. Enamel is one of the most popular solutions for this purpose in water heaters. The techniques for applying enamel layers have been well-established for a long time, and even simple methods such as liquid dyeing followed by heat treatment can be cost-effective. However, there are potential challenges and pitfalls associated with the enamel coating process.

An enameled tank can vary in terms of quality and durability, ranging from budget-friendly options to high-end, more durable ones
Water heaters with enamelled tank

Uneven application of the protective enamel coating can result in thinner areas that are more susceptible to failure and rusting. With liquid dyeing methods, the top of the tank is particularly vulnerable as some of the coating tends to flow down during the drying process. Moreover, the enamel surface is prone to cracking due to the differential thermal expansion between steel and enamel, preventing them from "breathing" in unison. Over time, cracks can form in the coating, leading to corrosion.

You can extend the life of the protective layer in different ways. The simplest way for the user is to limit the water heating to 60 °C. However, the risk of developing pathogens in the tank will also increase.

Water heaters manufacturers use their expertise to address the issue of uneven expansion. For instance, the Atlantic brand uses a more expensive technology known as powder electrostatic enameling, followed by high-temperature firing at over 800 °C. This advanced process helps align the coefficients of thermal expansion, resulting in a more durable protective coating that extends the lifespan of the tank.

Many brands are experimenting with the composition of enamel to make it more reliable and durable. Usually various additional components are introduced here, but company secrets often remain a trade secret and are not disclosed. The addition of titanium powder is considered a pinnacle in the technological advancements for enamel coatings. It effectively equalizes the coefficient of thermal expansion between the steel tank and the enamel coating, providing an extended lifespan for the protective layer. However, water heaters incorporating this technology are priced similarly to top-quality stainless steel models.

Glass ceramic is ideal in terms of inertness, but not the best in terms of strength.

Glass-ceramic is gaining popularity as an alternative protective coating to enamel. It involves the addition of silicon dioxide and a high-temperature process, providing a glass-like layer that is hygienic and inert to water. However, glass-ceramic coatings have lower mechanical strength, making the tank susceptible to shocks, and constant exposure to hardness salts in water can degrade the coating. Moreover, subjecting glass-ceramic to high heating temperatures (over 60°C) is not recommended.

Other components can also be added to glass ceramic, such as aluminium and/or lithium oxides. They make the protective layer stronger and more resistant to temperature extremes. But the price of such devices is also higher.

Water heaters with glass ceramic tank

Water heaters can also feature other branded enhancements. One common addition is silver, which is believed to inhibit the growth of pathogens. However, the effectiveness of such antibacterial coatings remains uncertain, as it depends on the silver content, which manufacturers often do not disclose for conventional 60-liter tanks. Therefore, for improved water tank hygiene, it is more advisable to rely on proven methods such as setting higher heating temperatures, choosing models with anti-legionella functions and using main water filters.

Usually, the higher the heating temperature, the greater the load on the protective coating of the tank.

How to choose a really durable tank: focus on the manufacturer and warranty

At first glance, the options available to buyers when choosing a water heater may not seem very diverse. There are three main options for the tank:

  • stainless steel;
  • steel with enamel;
  • glass-ceramic coating.

But there are plenty of nuances in any of these three groups, and there is no universal answer to the question of which is better. For stainless steel, the role of welding seams is critical, for enamel — the method of its application, for glass-ceramic — the presence of additional components and / or the significance of strength.

Within each category, there are both budget-friendly and high-end options available. Therefore, it is important to consider the budget when making comparisons. In some cases, a water heater with a tank made of regular steel but featuring modern enamel coating can prove to be more durable in practice than a stainless steel device from the lower price range, where the quality of welds may be questionable.

Finding the optimal water heater solution can be difficult due to manufacturers' limited disclosure of tank manufacturing details. They often provide little information about the steel grades used, welding methods and the composition of protective coatings. Marketing tactics often amplify certain qualities, and one such example is the addition of titanium to stainless steel or enamel, which is presented as a favorable factor for tank durability and reliability. However, titanium is an expensive metal that significantly impacts product pricing. Therefore, if the cost is suspiciously low and the manufacturer's credibility is questionable, blindly assuming the benefits of titanium may not be advisable.

If you want to choose a cool tank, first of all, start from the manufacturer's reputation. Proven brands tend to use advanced technology and quality materials. In addition, in their lineup there are often devices that have already proven themselves in terms of durability and reliability.

Water heaters on the market now often come with specified warranty periods for the tank, ranging from a few years to more than seven years. For example, the Gorenje GBF 80 UA model with an enamel coating offers a minimum tank lifespan of ten years, while the Electrolux EWH 80 Maximus with stainless steel guarantees a minimum of eight years. These warranty durations can serve as useful indicators when making a decision, alongside considering the materials used. However, it is essential to research whether there are official service centers nearby and understand how the warranty obligations are implemented in practice before making a purchase.

In conclusion, it should be said that in a modern water heater, even with the most reliable and technologically advanced tank, there must be a magnesium anode inside. The anode is a crucial component that helps protect the metal parts of the water heater from corrosion by sacrificing itself. It absorbs rust, extending the lifespan of the tank. It is important to follow the manufacturer's recommendations and replace the anode as advised. By doing this, the water heater can continue to function without leaks for many years.