Maintenance of solar storage tank: cleaning, anode and heat exchanger inspection

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Maintenance of Solar Storage Tank: Cleaning, Anode and Heat Exchanger Inspection

A solar storage tank is an investment for 15 to 25 years – provided you take proper care of it. In practice, however, I still often encounter customers who neglect their tank for years after installation, without any attention at all, and then wonder why the system heats water less and less efficiently, why the tank starts to rust from the inside, or why the pressure relief valve starts to drip. Most of these problems have a common denominator: neglected regular inspections and neglected maintenance.

This article will guide you through the complete maintenance of a solar storage tank – from the basic annual visual assessment to the replacement of the magnesium anode, cleaning of the heat exchanger, and testing of safety valves. You will know exactly what to do yourself, what to leave to a professional, and what intervals are relevant for different types of tanks and water conditions.

Why regular tank maintenance is essential

A hot water storage tank essentially operates continuously. Cold water flows through it, is heated, stagnates inside the tank, flows out to the taps, and the whole cycle repeats. This process brings several degradation mechanisms:

Steel corrosion – even though the tank is made of stainless steel or enameled steel, every material is subject to gradual degradation if the anode protection is missing or if the water pH is extreme.
Scale on the heat exchanger – in areas with hard water, a layer of CaCO₃ (calcium carbonate) settles on the heat exchanger tubes, significantly reducing heat transfer. A layer of just 1 mm thickness reduces the heat transfer efficiency by 10–15 %.
Sediments at the bottom of the tank – sludge, iron oxides and lime deposits gradually settle at the bottom of the tank, reducing the effective volume and serving as a breeding ground for bacteria including Legionella.
Anode degradation – the magnesium (magnesium) anode is a sacrificial cathodic protection of the tank. It gradually "eats away" and when it disappears, the tank starts to corrode without protection.
Wear of pressure and safety components – pressure relief valves, expansion vessels and seals wear out over time.

These processes cannot be prevented, but they can be slowed down and controlled. Regular maintenance will ensure that the tank operates efficiently and safely for its entire planned lifespan.

Recommended maintenance intervals – overview by components

In practice, I recommend to customers fixed schedules, not reactive maintenance "when something stops working". Below is a table of intervals that it is advisable to print and stick in the technical room:

Component / task	Interval	Who performs
Visual inspection of the tank, pipes, seals	every 12 months	owner / technician
Inspection of pressure relief valve (functionality)	every 6 months	owner
Inspection of expansion vessel (air pressure)	every 12 months	technician
Inspection and possible replacement of magnesium anode	every 1–2 years	technician
Flushing and descaling of the tank	every 2–3 years	technician
Chemical cleaning of the heat exchanger (descaling)	every 3–5 years (hard water: 2 years)	technician
Inspection of solar fluid (glycol, pH, density)	every 2 years	technician
Full system inspection including pump and regulation	every 5 years	service technician

If you live in an area with hard water (hardness above 20 °dH, which is typical for example for the south and west of Slovakia), shorten the intervals for anode and heat exchanger inspections to every 12 months. On the other hand, soft water accelerates anode consumption for a different reason – it is more aggressive towards metals.

Inspection and Replacement of Magnesium (Magnesium) Anode

The anode is probably the most important element of passive tank protection. Its principle is simple: magnesium is electrochemically more active than steel, so in a galvanic cell (water + two metals), magnesium oxidizes preferentially and "sacrifices" itself instead of the tank wall. As long as the anode is present and functioning, the tank does not rust.

The problem arises when the anode is not inspected for many years. The anode gradually wears out – under normal water hardness and normal operation it lasts 2 to 4 years, and less in soft water, because soft water is more aggressive. When only a metal rod or less than 20% of the original volume remains, the protection disappears and the tank starts to corrode intensively, resulting in rust formation, internal corrosion, and a reduction in lifespan by several years.

How to Inspect the Anode Step by Step

The procedure is the same for most tanks including models such as 250-liter tank or 300-liter tank. The anode is usually located at the top of the tank and is accessible via an inspection opening or directly through a threaded plug.

Turn off the heating – turn off the solar circuit, electric heating elements and the boiler connected to the tank. Let the tank cool down to at least 40 °C, ideally completely cold.
Close the cold water supply – close the ball valve on the cold water inlet to the tank.
Depressurize – open the nearest hot water tap so the tank loses pressure and air can enter through the tap during anode removal.
Unscrew the anode – most anodes are fastened using a larger hex key (usually 32 mm or 1 1/4" thread). Use a pipe wrench or a larger box-end wrench. Be prepared for some water to flow out – a tray under the inspection opening is essential.
Visual assessment – a new anode has a diameter of 20–26 mm and a length of 200–500 mm depending on the tank model. If the diameter is reduced to less than 10 mm or if the anode is shorter than half the original length, you must replace it immediately. If it is covered with a white-gray layer of scale, this is normal – it can continue to work, but the layer can be gently removed with a plastic brush.
Thread and seal inspection – check the condition of the thread at the anode mounting location. Remove any deposits. Replace the Teflon tape or hemp sealing string on the thread of the new anode.
Mounting the new anode – tighten with a wrench, but do not over-tighten – plastic or enameled threaded sleeves can be damaged by excessive tightening. Recommended torque 60–80 Nm.
Refilling and checking the seal – open the cold water supply, let the tank fill, and check the seal at the anode thread.

In practice, I have encountered a case where in a 10-year-old tank we could not find the anode at all – it was completely consumed and the customer had not even noticed, because the system "was still heating water". The tank, however, was internally covered with red rust corrosion and would have had to be replaced prematurely. A 30-minute inspection every two years would have saved it.

Tank Cleaning – Descaling and Flushing

Sediment composed of iron oxides (rust from pipes), lime scale and mechanical impurities from the water supply gradually settles inside the tank. This sediment:

reduces the effective volume of the tank,
insulates the bottom of the tank from the heat exchanger (worsens heat transfer),
represents a risk of growth of the bacterium Legionella pneumophila, which multiplies at temperatures of 25–50 °C and in biofilm on deposits,
accelerates corrosion of the tank bottom.

Tank Descaling Procedure

Descaling is a simpler task than anode replacement, but it requires some patience. Procedure:

Let the tank cool down to a temperature below 40 °C.
Close the cold water supply, turn off the heating.
Connect a hose to the drain valve (usually at the bottom of the tank, DN 15 or 20, ball valve) and lead it to a drain or outside.
Open the drain valve and let the tank drain. The first few liters will be brown or gray – this is the sediment. Monitor the color.
After draining the tank, briefly flush it with fresh water – open the cold water supply, let 20–30 liters flow through and then drain again. Repeat until the water runs clear.
Fill the tank, check the seal of the drain valve, and turn on the heating.

In larger tanks, for example in a 400-liter tank, the amount of sediment can be surprising – during the first descaling after 5 years of operation without maintenance, I saw a tank from which the first 50 liters looked like a thick rust soup. Since then, the customer has been descaling every two years and the water is clean almost immediately.

Chemical Cleaning – Tank Disinfection

If you suspect biological contamination (sulfur smell in water, yellowish-gray color, or the tank has not been operating for a long time), thermal or chemical disinfection is recommended.

Thermal disinfection (Legionella program): The tank heats the water to 70–75 °C and this temperature is maintained for at least 3 minutes. Legionella bacteria die at this temperature almost immediately. Most solar controllers or boiler controllers have a "anti-Legionella" or "thermal disinfection" function that activates automatically once a week at a set time. Make sure this function is active.

Chemical disinfection: The tank is filled with a sodium hypochlorite solution (common bleach, 200–300 ml per 100 liters of water). The solution remains in the tank for 2–4 hours, after which the tank is flushed several times with clean water until the chlorine smell disappears. This method is more suitable for serious contamination or when starting the tank after a long period of inactivity.

Heat exchanger inspection and cleaning

The heat exchanger is the heart of the solar tank. Solar fluid (propylene glycol mixed with water) flows through the heat exchanger and transfers heat to the water in the tank. The efficiency of this process directly depends on the cleanliness of the heat exchanger.

We usually encounter two types of heat exchangers in Slovakian tanks:

Plain (coiled) tube – a classic meandering heat exchanger located in the lower part of the tank for the solar circuit and possibly in the upper part for the boiler. This type is more compact, but has a smaller heat transfer surface.
Finned heat exchanger – a tube with fins, larger surface area, higher heat transfer efficiency, but also faster scale formation in the fin space.

If you have a solar tank with two heat exchangers, maintain both – the lower solar and the upper boiler – each according to its own load and the hardness of the medium flowing through it.

Chemical cleaning of the heat exchanger – descaling

The heat exchanger in the solar circuit is not directly accessible – solar fluid (glycol) flows through it, not potable water. Descaling is done by flushing through the solar collector circuit. Procedure:

Turn off the solar circuit and let it cool down.
Disconnect the solar fluid – drain it and collect it (glycol must be disposed of as hazardous waste, not through the sewer).
Pump a citric acid solution (5–10 % solution) or a special descaling agent intended for solar systems into the circuit. Hydrochloric acid is not used for solar circuits – it could damage aluminum fittings or copper heat exchangers in some types of tanks.
Pump the solution through the heat exchanger for 1–3 hours. Monitor the output: when CO₂ bubbles (released during the reaction with CaCO₃) stop, the cleaning is complete.
Flush the circuit with clean water at least twice, checking the pH of the outflow – it must be neutral (6.5–7.5).
Fill the circuit with fresh solar fluid in the prescribed ratio (usually 40 % propylene glycol / 60 % demineralized water, frost resistance up to –25 to –30 °C).

In areas with water hardness above 25 °dH (e.g. Záhorie, part of the Danubian Lowland), I recommend chemical descaling of the heat exchanger every two years. For medium hardness (10–20 °dH), three years is sufficient, and for soft water, five years.

Inspection of safety and pressure components

The safety of the solar tank depends on the proper function of the pressure relief valve (TPV) and the expansion tank. These components are legally required, and their failure poses a serious risk.

Pressure relief valve (TPV)

The TPV is set to a specific pressure according to the tank – usually 6 bar or 8 bar. Every six months, the owner should manually test the valve:

Turn the handle or lift the lever on the valve (depending on the type) – water or steam must flow out.
After releasing the lever, the valve must seal again – if it continues to drip, it is damaged and must be replaced.
Under the TPV, there should always be an installed drain pan (tray) leading to a drain, not a closed plug – if the TPV opens under pressure, the water must have a place to drain.

Expansion tank

The expansion tank compensates for the increase in water volume when heated. If the pre-charge pressure in the expansion tank is incorrect (significantly lower or higher than prescribed), the tank will enter pressure instability and the TPV will unnecessarily drip or, conversely, the tank may reach dangerous pressure peaks.

The pre-charge pressure of the expansion tank is measured with a pneumatic pressure gauge on the valve (air valve similar to a car tire valve), when the tank is cold and depressurized.
Standard pre-charge pressure: 1.5–2 bar (depending on the installation height and water pressure in the water supply).
If the membrane in the expansion tank is perforated, water will flow out of the valve when pressed instead of air – the tank is non-functional and must be replaced.

Inspection of solar fluid and shut-off valves

Solar fluid (propylene glycol + corrosion inhibitors + water) has a limited lifespan. Glycol gradually degrades, its corrosion inhibitors are depleted, and the pH drops – an acidic fluid then corrodes the heat exchanger, pump, and collectors much faster.

Every two years, a sample of the solar fluid must be taken and checked for:

pH – must be between 7.5 and 9.0. Below 7, the fluid is acidic and actively corrodes the system.
Density – check the glycol content with a refractometer. At 40 % propylene glycol, the density is 1.040–1.045 g/cm³. If the value drops, the fluid has been diluted or degraded.
Color and turbidity – clean solar fluid is transparent or slightly bluish. Brown or cloudy fluid is degraded and must be completely replaced.

Solar fluid must be replaced with a propylene glycol mixture – never use ethylene glycol (toxic) or non-freezing automotive chemicals, which are not intended for solar systems and do not contain suitable inhibitors.

When replacing the solar fluid during service, also check the ball valves on the solar distribution. For professional installations, high-quality industrial distributors and collectors are recommended – for example, industrial stainless steel distributor/collector with ball valves 6/4"×1", 2-way, which allows convenient disconnection of individual circuits during service without the need to drain the entire system.

Documentation and service book of the tank

Each tank should have its own service book – whether physical or digital. Record the following in it:

date of each inspection and the technician's name,
anode condition at each inspection (diameter, length, surface condition) and replacement date,
results of pH and solar fluid density measurements,
date of descaling and a visual description of the sludge condition,
all replaced parts with type and production batch,
regulation settings (temperatures, anti-Legionella program times).

This documentation is not only important from the perspective of the tank warranty (the manufacturer may require proof of regular maintenance), but also from the perspective of property sale or insurance incident – a clear service history of the tank is a significant added value.

If you are planning the installation of a new tank, also read other practical topics in our Knowledge Centre: Installation of a solar storage tank: procedure, placement and installation requirements, Connecting a solar storage tank to a boiler and circuit distributor or Common faults of solar storage tanks: overheating, corrosion and pressure loss, where you will find a detailed description of faults that you can avoid with regular maintenance.

Typical mistakes in tank maintenance – from customer practice

Over the years of work on field services, I have seen many recurring mistakes. Here are the most common ones to avoid:

„I will replace the anode when it stops heating" – by that time, the tank is likely already damaged internally. Anode must be replaced preventively, not reactively.
Wrong replacement anode – anodes have different thread sizes (1/2", 3/4", 1 1/4", G6/4") and different lengths. Always replace the anode with one of the same dimensions as the original. A too short anode does not protect the entire tank.
Sealing the drain valve „for good" – some tank owners wrap the drain valve so tightly with Teflon that it cannot be opened during the first descaling without damaging the tank. The valve must remain functional.
Adding the wrong fluid – ethylene glycol, non-freezing automotive mixtures, or ordinary tap water – all these „cost-saving" solutions accelerate corrosion and devalue the inhibitors in the original fluid.
Ignoring TPV dripping – if the safety valve occasionally drips, some owners „solve" the problem by placing a bucket under the valve. Dripping from the TPV is a sign of a problem (overpressure, non-functional expansion tank, incorrect pre-pressure), not normal operation.
Tank without anti-Legionella function – if the regulation does not allow thermal disinfection, a tank operating below 60 °C is a potential risk. Check the regulation settings.

Specifics of maintenance of solar storage tanks with two heat exchangers

Tanks with two heat exchangers, such as solar storage tank with two heat exchangers including insulation, have a specific layout: the lower heat exchanger is connected to the solar circuit and the upper heat exchanger to a backup boiler or heat pump. Each heat exchanger operates with a different medium and at different temperatures, which means different levels of limescale deposition.

The lower solar heat exchanger operates at higher temperatures (solar system stagnation can reach up to 150–180 °C if the tank is full and the solar controller does not switch off the pump), so it is more prone to deposit formation. The upper boiler heat exchanger operates at more constant temperatures and with modified boiler water (usually demineralized), so deposits form more slowly on it.

During a service inspection of a tank with two heat exchangers, check the flow through both circuits (measuring flow or temperature differences Δt during operation). If Δt on the heat exchanger increases by more than 5–8 °C compared to the original design, the heat exchanger is likely clogged.

If you are deciding on a tank for a specific installation, the topics Storage tank with one or two heat exchangers: which is better or How to choose a solar storage tank: volume, heat exchangers and system type in our Knowledge Centre may also help you.

Frequently asked questions (FAQ)

How often should the magnesium anode in a solar tank be replaced?

With normal water hardness (10–20 °dH) and normal tank operation, we recommend checking it every 12 months and replacing it according to its condition – usually every 2 years. In the case of soft water (under 8 °dH), the anode can be consumed in 12–18 months, as soft water is more aggressive towards metals. In the case of hard water (over 20 °dH), it may last longer, but thicker lime deposits often form on it, which must be mechanically removed before evaluation. Never forget: the anode is consumed not by deposits, but it must be physically measured – diameter under 10 mm = replacement.

Can I descale the tank myself, or do I need a technician?

Basic descaling – draining the tank via the drain valve and rinsing with clean water – can be done by a capable owner themselves, if they know where the drain valve is and how to close the water supply. Replacing the anode requires a bit more skill (a torque wrench, knowledge of sealing threads) and at least basic experience with plumbing. Chemical cleaning of the heat exchanger and checking the solar fluid are tasks for a professional – they require the right tools, preparation of the fluids, and working with a pressurized tank.

The tank is dripping from the safety valve – what does that mean?

Occasional dripping of the TPV during heating is a sign that the tank is reaching or exceeding the set pressure. The most common cause is a non-functional or incorrectly set expansion vessel – the membrane is perforated or the pre-pressure is too low. A second cause may be excessive water pressure (more than 5 bar), which must be reduced with a pressure-reducing valve. If you ignore the dripping and just drain the water, the TPV will gradually wear out and stop sealing permanently.

What happens if I do not replace the solar fluid for a long time?

Propylene glycol itself does not age dramatically, but its corrosion inhibitors (usually silicates or phosphates) are depleted. After 4–6 years without replacement, the fluid pH drops (below 7), which means it actively corrodes the inside of the heat exchanger, pump body, collectors, and piping. The result is brown corrosion deposits in the circuit, a clogged filter before the pump, reduced circulation, and in the worst case, a perforated heat exchanger, through which the solar fluid mixes with potable water. Professional replacement of the solar fluid every 2–4 years is much cheaper than repairing the heat exchanger.

What is the difference in maintenance between enamelled and stainless steel tanks?

An enamelled tank (steel covered with glass enamel) requires a mandatory magnesium anode – without it, the steel under the enamel corrodes quickly at every crack in the enamel layer. A stainless steel tank (austenitic stainless steel AISI 316L) theoretically does not require an anode, but many models still include one for greater certainty, especially if the water is more aggressive (low pH or high chloride content). For stainless steel tanks, chloride content in the water is key – if the chloride content exceeds 200 mg/l, even stainless steel can be subject to pitting corrosion.

Can I leave the tank unattended during summer vacation?

A solar tank can reach high stagnation temperatures during summer absence if no water is drawn from it. Before a longer absence (more than 2 weeks), I recommend: 1) setting the solar controller to the cooling function of the collector (circulation at night to release heat), 2) checking the pressure and condition of the TPV, 3) switching the tank to a summer mode with a reduced set temperature. Tanks, for example, 300-liter tank, tend to enter boiling of the fluid during summer stagnation, and repeated steam cycles stress the seals and membranes.

Conclusion: regular maintenance is always worth it

A solar tank is not a device that functions trouble-free for decades without attention. It is a technological unit that requires systematic care – not expensive or technically demanding, but regular and thorough. Replacing an anode for 15 euros and 30 minutes of work every two years is a negligible investment compared to premature tank replacement, which can

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