Maintenance of immersion heaters – cleaning, descaling, and anode inspection

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Maintenance of immersion heaters in tanks – cleaning, descaling and anode inspection

An immersion heater in a water heater or storage tank is one of those components that is often ignored for years until it causes a problem. However, regular maintenance of the heating element can extend its lifespan by several years, prevent unnecessarily high electricity consumption, and in the worst case, save the entire tank from premature corrosion or cracking. This article will explain what happens to the heater over time, when and how to clean it from limescale, how to properly inspect and replace the magnesium anode, and what to watch out for during handling to avoid unnecessary complications or jeopardizing the tank's warranty conditions.

Why the heater in the tank gets clogged and what it causes

The majority of drinking water in Slovakia has medium to high hardness – it typically ranges between 15 and 35 °dH (German degrees of hardness), while in certain regions, especially in the Danubian Lowland or areas with limestone bedrock, it can exceed 40 °dH. Water hardness is mainly due to the content of calcium and magnesium salts. These are dissolved in water under normal conditions, but when heated above 60 °C, they start to precipitate as calcium carbonate – a white, hard coating commonly referred to as limescale or scale.

The surface of the heating element is precisely the place where this phenomenon occurs most intensely, because the temperature on the surface of the resistance wire is always higher than the temperature of the surrounding water. The layer of limescale has very low thermal conductivity – about 0.5 to 3 W/(m·K) compared to 15–20 W/(m·K) for steel. This means that even 1 mm of limescale can reduce heat transfer by tens of percent, the heater overheats, the resistance wire ages faster, and the power required to heat the same amount of water increases.

Along with the drop in efficiency, there is also a risk of mechanical damage. A thick layer of scale expands and contracts unevenly during heating and cooling cycles, which can lead to microcracks in the surface of the heating element, and eventually to its cracking. This is then followed by corrosive damage to the tank itself, because the loss of protective zinc or anode in combination with damaged surfaces goes hand in hand.

Types of heaters and their different susceptibility to deposits

On the market, we encounter two basic types of heating elements for water heaters and storage tanks – brass-zinc plated and stainless steel AISI 316L. Each of them behaves slightly differently when it comes to limescale deposits, which also affects the cleaning method.

Brass and zinc-plated heaters, such as GRBT heaters for small and medium tanks with brass flange and zinc-plated surface, are cost-effective and very common in standard water heaters with steel tanks. Brass has excellent thermal conductivity, but the zinc-plated surface is more prone to corrosion in more aggressive water (low pH, soft water, water with higher chloride content). Limescale deposits adhere firmly to them, and during improper mechanical cleaning, there is a risk of damaging the protective surface, which accelerates corrosion.

Stainless steel heaters AISI 316L, for example GRBTN heaters for small and medium tanks made of stainless steel AISI316L, are more resistant to aggressive water and soft water with lower pH. Their surface is smoother, which slightly slows down the adhesion of limescale, and during cleaning you can allow for a slightly more intensive chemical process without the risk of damage. The lifespan of a stainless steel heater is generally higher, which is reflected in its price. Both types of heaters are mounted via a 5/4" flange thread – about how to choose the right thread and flange diameter, we talk in a separate article What thread and flange diameter do I need for a heater in a tank.

How to recognize that the heater needs cleaning

The first and most reliable sign is the extension of the heating time. If a water heater that previously heated 100 liters of water in 45 minutes starts taking 70 minutes or more with the same thermostat setting, it is very likely due to the clogging of the heating element. Another sign is a higher electricity bill – the calculation is simple: a 30% drop in heat transfer efficiency increases costs by the same percentage.

Other signs:

Buzzing or crackling sounds during heating – limescale slightly moves due to thermal expansion and emits these sounds.
Increased temperature on the surface of the water heater when touched in the area of the heater – overheated outer wall indicates weak heat transfer into the tank.
Tripping of the circuit breaker – in the case of heavy clogging, the heater overheats, which leads to a short circuit or the operation of the thermal fuse. More about this can be found in the article Common faults of heating rods and heaters – why they don't heat or trip the circuit breaker.
Cloudiness or white sediment in the water – falling pieces of limescale contaminate the drinking water.

In practice, we recommend checking and cleaning the heater every 2 years for moderately hard water (15–25 °dH) and every year for hard water above 25 °dH. In soft water areas (under 10 °dH), an interval of 3–4 years is sufficient, although anode inspection remains on an annual cycle.

Procedure for removing the heater from the tank

Before starting any maintenance, the heater must be physically removed from the water heater. The procedure is the same for brass and stainless steel types. A detailed guide can also be found in the article Replacement of the heating element in a water heater – when and how to do it, here we present a summary with an emphasis on safety.

Disconnect the power supply – turn off the circuit breaker or unplug the appliance. Never work on a device under voltage.
Close the cold water supply to the water heater and open a hot water tap somewhere in the house to relieve pressure.
Drain the tank using the drain valve (if the water heater has one) or via the hose connected to the cold water inlet after closing the supply. A fully filled 80-liter water heater weighs about 80 kg – you won’t be able to open it without draining it first.
Unscrew the electrical connections and disconnect the thermostat from the heater.
Unscrew the flange nut using a wrench (the size depends on the flange diameter – typically a 55 or 65 mm wrench for a 5/4" thread). Use a pipe wrench or a special flange nut wrench.
Carefully pull out the heater – move it carefully to avoid damaging the tank or the ceramic insulation around the resistance wire.

Warning: The gasket under the flange is a one-time use item. Always have a replacement gasket of the same size ready before starting the work. Using an old gasket increases the risk of leakage and unnecessarily prolongs the work, as you may have to drain the tank again.

Descaling methods – mechanical and chemical approaches

There are two basic methods for removing limescale from the heating element: mechanical and chemical. In practice, they are most often combined.

Mechanical cleaning

Mechanical cleaning involves physically removing the layer of limescale using hard-plastic or bakelite scrapers, old toothbrushes, and a soft plastic brush. Never use metal tools (wire brushes, metal spatulas) on brass or galvanized heaters – you will damage the protective surface. For stainless steel heaters AISI 316L, a plastic brush is acceptable, but metal tools are still not suitable.

Steps for mechanical cleaning:

Let the heater dry – dry limescale is more brittle and easier to remove.
Gently tap the scale with a wooden stick or screwdriver handle – thicker layers may come off in whole pieces.
Remove the remaining particles with a plastic brush under running water.
Clean fine deposits on the flange thread and in its grooves with a soft toothbrush.

Mechanical cleaning is sufficient for thin layers of scale (up to 0.5 mm). Thicker layers require chemical assistance.

Chemical descaling

Chemical descaling uses acids that react with calcium carbonate and break it down into CO₂ and soluble calcium salt. The most popular household products are:

Citric acid – the safest, ecological, and available. A 10–20% solution (100–200 g per 1 liter of water) is sufficient. It is not aggressive to brass or stainless steel. Application time: 2–4 hours at room temperature.
Acetic acid (vinegar) – an 8–10% solution is weaker than citric acid, but available in every household. Suitable for thin layers, application time 4–8 hours.
Commercial descaling agents – contain either citric acid, phosphoric acid, or sulfamic acid. Sulfamic acid is effective even on thick deposits (more than 2 mm) and is less aggressive to metal than hydrochloric acid. Always follow the manufacturer’s instructions.
Hydrochloric acid (muriatic acid) – highly effective, but aggressive to brass, zinc, and soldered joints. Use only in extreme cases and exclusively on stainless steel (AISI 316L) heaters at a maximum concentration of 5%. You must have protective equipment (goggles, gloves, apron).

Steps for chemical descaling:

Prepare a container (a plastic bucket or a deep bowl) into which the heater fits entirely.
Pour the descaling solution so that the entire active surface of the heater is submerged.
Let it work according to the type of preparation – citric acid for 2–4 hours, sulfamic acid for 30–60 minutes for a thick deposit. During the process, you will notice bubbling – this is normal, it is CO₂ released during the reaction with carbonate.
After the time has passed, rinse the heater thoroughly with clean water, ideally several times.
Check the surface – if thin residues remain, clean them with a plastic brush.
Let the heater dry thoroughly before reassembly.

Checking and replacing the magnesium anode

The anode may be more important than the heater itself, yet most water heater owners don’t even know about it. It is a rod made of magnesium (in some cases zinc or aluminum) inserted through the flange or a separate thread into the tank. It works on the principle of a galvanic cell: magnesium has a more negative electrochemical potential than steel, so corrosion "attacks" the anode first and not the tank walls. That is why it is also called a sacrificial anode.

Without a functioning anode, the tank itself starts to corrode – first in the form of rusty water discoloration, later in the form of holes in the wall, which means the end of the water heater’s life. Repairing a steel tank after corrosion is not feasible – the entire tank must be replaced.

The lifespan of a magnesium anode depends on several factors:

Water hardness – in soft water (under 8 °dH), the anode paradoxically wears out faster, as soft water is more electrically conductive and the galvanic process is more intense.
Tank volume – smaller volume = higher concentration of corrosion products = faster wear of the anode.
Water temperature – higher set temperature (over 65 °C) accelerates the galvanic process and shortens the anode’s lifespan.
Water quality – increased content of chlorides, sulfates, or low pH makes water more aggressive and the anode wears out faster.

The typical lifespan of a magnesium anode is 2–4 years under standard use and average water hardness. We recommend checking it every year during heater disassembly.

How to Recognize a Worn Anode

A new magnesium anode has a diameter of 26 mm and a length according to the water heater (usually 400–700 mm). A used anode, which you encounter during an inspection, can look different:

Significantly thinned – the diameter has dropped to less than 10–12 mm, the rod is uneven, cracked in places.
Fully consumed – only the steel wire frame remains, the magnesium has completely disappeared.
Covered with a white-gray substance – these are corrosion products of magnesium (magnesium hydroxide), which is normal, but if the layer is thick and firm, the anode has lost a large part of its active surface.

Practical rule: if the remaining diameter of the anode drops below 50% of the original diameter, it needs to be replaced. Better to replace it a year earlier than a year later.

Selection and Installation of a Replacement Anode

When selecting a new anode, check the thread (most commonly 3/4" or 1") and the length, which must match the internal height of the tank. Before installation, wrap the thread with Teflon tape (2–3 layers) and tighten with a wrench to the recommended torque – usually 20–30 Nm. Do not overtighten, to avoid damaging the thread in the tank.

Note: On some water heaters, the anode is integrated into the heater flange, which means that when replacing the heater, you automatically also replace the anode. Check this during disassembly – this information will save you unnecessary searching.

Cleaning the Inside of the Tank and the Area Around the Flange

When removing the heater, you have a unique opportunity to look inside the tank and check its condition. Use a flashlight or LED light – make sure the light is battery-powered and not connected to the grid, as you are working in a wet environment.

What to look for inside the tank:

Layer of sediment at the bottom – white or gray fine dust (limescale, corrosion products of the anode). We recommend rinsing it out with a water jet through the heater opening.
Rust spots on the walls – local rusting may indicate the beginning of corrosion under anode protection. If the spots are widespread, consider replacing the anode in advance and shortening the inspection intervals.
Cracked glass enamel coating – steel tanks have an enamel coating inside. Microcracks in the enamel are normal and the anode protects them. Larger cracks or pieces of enamel that have come off are a sign that the tank is in an advanced age and you should consider replacing it.
Thickness of sediment at the bottom – if it is more than 20–30 mm of deposits, it is necessary to rinse the tank more thoroughly or consider installing a water softener.

To rinse the inside of the tank, insert a hose nozzle through the heater opening and use a water jet to loosen the sediment towards the drain valve. Repeat the process 2–3 times until the draining water is clear.

Cleaning and Checking the Thermostat and Thermostatic Safety Device

The heater and thermostat form a functional pair. The thermostat regulates the water temperature in the tank, while the thermostatic safety device (thermal fuse, STB) stops the heating when the maximum allowed temperature is exceeded (usually 90–95 °C). If the safety device has activated and you had to reset it manually, it is a sign that something is wrong – either the heater is too dirty or the thermostat is not working properly.

Check the following during each maintenance:

Condition of the thermostat seal and its housing – no cracks or deformations.
Cleanliness of the contact surface of the thermostat, which rests against the heater body – it must be in close contact, otherwise the measurement is inaccurate.
Function of the reset button of the thermostatic safety device – press it (with the electrical installation disconnected) and check whether it can be pressed easily and clicks in. If it cannot be reset, the safety device is likely faulty and needs to be replaced.

If your water heater uses a heating rod with an integrated thermostat for a radiator fin, for example Heating rod with thermostat for radiator fin (300 W, 300 mm - white), the principle of thermostat checking is similar, only the geometry and mounting method differ. More about the differences between thermostatic and non-thermostatic types can be found in the article Heating rod with thermostat vs. without thermostat – difference and cost-effectiveness.

Prevention – How to Slow Down the Fouling of the Heater

The best maintenance is the one you don't have to do. There are several proven measures that can significantly extend the intervals between cleanings:

Lowering the Temperature Setting

Every degree above 60 °C significantly accelerates the formation of limescale. A temperature of 55–60 °C is a compromise between the hygienic minimum (killing Legionella bacteria) and slower fouling. Below 55 °C is not recommended from a hygiene perspective. If your water heater is set to 75–80 °C, lowering it to 60 °C can extend the cleaning interval of the heater from 1 year to 2–3 years.

Water Softener or Anti-lime Filter

Whole-house water softeners using ion exchange (via salt) can reduce water hardness to 5–10 °dH. This is the most effective solution for households with very hard water. A cheaper alternative are flow-through anti-lime devices (magnets, electrolytic devices, polyphosphates) – their effectiveness is questionable and depends on the specific product and conditions.

Regular Flushing of the Tank

Every 3–6 months (even without removing the heater), briefly open the drain valve at the bottom of the water heater and let 2–3 liters of water drain out. This removes loose sediment before it has time to harden and stick to the heater.

When it is Time to Replace the Heater, Not Just Clean It

Even with regular maintenance, there will come a moment when cleaning is not enough and the heater needs to be replaced. Signs that it is time for a replacement and not just cleaning:

Visible cracks or broken ceramic insulation on the heater body – there is a risk of short circuit or electric shock.
Measurable resistance from phase to neutral conductor is zero or extremely low – a multimeter practically measures a short circuit, which means a fault in the insulation of the resistance wire.
Corrosion of the flange – if the brass flange is corroded to a depth, the thread is damaged and the seal will not be sufficient.
Heater is deformed – physical deformation of the heater loop may be due to corrosion or mechanical damage.
Repeated tripping of the circuit breaker despite cleaning – it indicates a fault in the internal insulation of the heater.

More about this decision can be found in the article Replacing the heating element in a water heater – when and how to do it. When choosing the right replacement heater – whether to go for a brass-zinc or stainless steel type – the article Water heater heating element: brass, zinc or stainless steel AISI316L? will help you decide.

Working safely with the heater – what to remember

Working with an electric heater in combination with water requires following basic safety rules. It's not paranoia – caution here quite literally saves lives.

Never switch on the heater outside the tank or in an empty tank – in a dry environment, the heater will overheat immediately and be damaged within seconds.
Always physically disconnect the appliance from the electrical supply before any manipulation (not just via the thermostat, but by turning off the circuit breaker or unplugging the plug).
Always work with an emptied tank – any remaining water in the tank must not be under voltage.
Use chemical descaling agents in well-ventilated areas, and wear rubber gloves and safety goggles.
When installing a new heater, first fill the tank with water, check for leaks, and only then connect the electrical power.

Most frequently asked questions (FAQ)

How often should I clean the heater in the water heater?

It depends on the hardness of the water in your area. For hard water (over 25 °dH), we recommend cleaning once a year. For medium hardness (15–25 °dH), cleaning every two years is sufficient. For soft water (under 10 °dH), you can extend the cleaning interval to 3–4 years, but still check the anode annually, as soft water wears it out faster. If you're unsure about the hardness of your water, most water utility companies publish this information on their websites or can provide it upon request.

Can I clean the heater myself or should I call an electrician?

A more experienced DIYer can handle the disassembly, cleaning, and reinstallation of the heater, provided you follow the safety procedures – especially disconnecting from electricity and draining the tank. In Slovakia, connecting the electrical wiring (supply to the heater's terminal block) is technically acceptable to do yourself as part of regular household maintenance. However, if you're unsure or the water heater is old with outdated electrical installation, we recommend calling an electrician at least for the first inspection.

What happens if I don't use a gasket during reinstallation or use an old gasket?

Using an old or missing gasket is one of the most common reasons people call for a "water heater repair" – in reality, it's a leak at the flange. An old gasket has lost its elasticity and deforms unevenly when tightened, causing a leak. The result is a slow water leak that you won't notice for a long time until mold appears or the ceiling under the water heater starts to crack. A gasket is a cheap consumable – always replace it with a new one.

Does a stainless steel heater AISI 316L require less cleaning than a brass one?

Mildly yes. The surface of stainless steel AISI 316L is smoother and less prone to limescale buildup compared to zinc-coated steel. The difference in practice is not dramatic – with the same water hardness and temperature setting, you can extend the cleaning interval by 20–30%. A bigger difference is in resistance to more aggressive water – the stainless steel heater GRBTN causes fewer problems in soft and acidic water, where the zinc-coated surface corrodes quickly.

What does it mean if the water from the shower or tap smells like hydrogen sulfide (rotten eggs)?

This characteristic odor is a sign of activity by sulfate-reducing bacteria, which react with the magnesium anode and produce hydrogen sulfide. This typically happens when the water heater has been idle for a long time (e.g., during a vacation), the water stagnates, and the temperature drops. The solution is to temporarily replace the magnesium anode with a zinc or aluminum one (less reactive, less hospitable environment for these bacteria), or to heat the water to 75 °C for 1–2 hours for disinfection. At the same time, check whether the anode is excessively worn.

Can I leave the heater in the water and pour the descaling agent from the outside into the tank?

This method (so-called in-situ descaling) is possible and some service technicians use it for lighter deposits. The process involves sealing the water heater, filling it with a citric acid solution (5–8% concentration for the entire tank volume), letting it sit for 4–6 hours, and then thoroughly rinsing. Disadvantage: it is not as effective as submerging the removed heater in the solution and is not sufficient for thick deposits. In addition, it should not be used in water heaters with aluminum interiors – the acid would damage them. The method is safe only for enameled steel tanks and stainless steel tanks.

Conclusion – investment in maintenance pays off

Regular maintenance of the heating element in a water heater is not just a technical obligation – it is an economic investment with a quick return. A water heater with a clean heater and a functional anode consumes 15–40% less electricity than a neglected one, lasts several years longer, and does not contaminate drinking water with sediment. The total cost of citric acid, a new gasket, and a replacement anode is in the range of a few euros – compared to the cost of replacing the entire water heater or repairing a flooded bathroom due to a leaking flange, it is a negligible amount.

If you plan to replace the heater in an old water heater with a more modern one, or are looking for the right type for a new project, take a look at our range of GRBT heaters with brass flanges and zinc-coated surfaces for standard applications, or premium GRBTN heaters made of stainless steel AISI 316L for more demanding conditions – both types with 1,400 W power and 5/4" flange thread for small and medium tanks. And if you're dealing with a bathroom ladder, all the necessary information about heating rods can be found in the articles How to choose a heating rod for a bathroom ladder – what to focus on and What power of heating rod do I need – 150, 300 or 600 W?

Do you have a question about this topic?

Not sure what to choose or dealing with a specific situation in your home? Write to us – we're happy to help.