Common faults of solar storage tanks: overheating, corrosion and pressure loss

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Common problems of solar storage tanks: overheating, corrosion and pressure loss

The solar storage tank is the heart of every solar system for the preparation of hot tap water. Unlike a boiler or a circulation pump, most homeowners practically never see it – it stands in the technical room, insulated, quiet, and it is expected to work for decades without problems. The reality, however, is that the tank is subject to several types of faults that develop gradually, over months or even years, and when they become fully apparent, their correction is either expensive or requires the replacement of the entire device.

This article is intended for homeowners with a solar system, but also for installers who want to have a solid basis to explain to their customers why preventive maintenance is cheaper than repair after a breakdown. We will examine three of the most common groups of faults: overheating of the tank and the medium inside it, corrosion of the tank and its heat exchangers, and pressure loss in the system. For each fault, we will explain the causes, symptoms, diagnosis and solutions – with specific values and examples from real practice.

1. Overheating of the solar storage tank

How it arises and why it is dangerous

Solar collectors operate with surprisingly high temperatures. In summer months, when hot water consumption is low (holiday period, empty house) and solar irradiation is maximum, the temperature in the primary circuit – in the fluid connected to the collector – can easily exceed 150 °C, and in a stagnation state even reach 180–200 °C with flat collectors, and even more with vacuum tubes. If the tank does not have sufficient capacity or the regulation is not set correctly, heat is transferred to the tank and the water temperature in it rises well above the safe operating limit of 95 °C.

Overheating does not only represent a safety risk – although this is real (boiling water under pressure, possible opening of the safety valve). It has direct material consequences: it degrades the tank insulation, damages the seals, and in worse cases deforms the inner shell or the heat exchanger. Particularly sensitive are polyurethane insulations, which at a long-term temperature above 90 °C lose their structure and thermal insulation properties. In short: one summer stagnation cycle significantly "ages" the tank.

Most common causes of overheating in practice

From dozens of resolved customer cases, it follows that overheating most often has one of these root causes:

Undersized tank – a family of three has a 120-liter tank, which under full-day sunshine reaches maximum temperature by noon and the rest of the day the collectors only "boil" the fluid without a place to transfer the heat. The solution is a larger volume – when dimensioning, a rough guideline is 70–90 liters of usable volume per person, for solar systems rather 80–100 l. More information can be found in our article What volume of solar storage tank do I need for my home.
Incorrect regulation – the control unit does not send a signal to stop the pump when the maximum temperature in the tank is reached, or the temperature sensor is placed incorrectly (does not represent the real temperature in the upper part of the tank).
Tank without sufficient cooling circuit – some systems have no way of discharging excess heat (cooling elbow, pool circuit, cooling heat exchanger). When the tank is full and hot, the primary pump stops and stagnation occurs in the collector.
Stagnation in the primary circuit – during stagnation, the solar fluid (usually propylene glycol) turns into vapor, which "pushes" the liquid back into the tank and the heat exchanger receives a steam hammer. Repeated stagnation cycles degrade the glycol and clog the heat exchanger and pipe sections.

How to recognize overheating – specific symptoms

In practice, overheating is most often revealed by these symptoms: the tank's safety valve regularly (several times in summer) discharges water – this is a clear sign that the pressure in the tank has exceeded the set value (typically 6 bar), which occurs at high water temperatures. Another symptom is yellowing or browning of the solar fluid – glycol degraded by repeated overheating changes color and pH, becomes acidic and starts to corrode metal parts of the system. Degraded glycol is unfortunately an irreversible change; it must be completely replaced. Sometimes the first symptom is a simple thing: the tank emits a loud sound (cracking, knocking) during heating cycles – this is a consequence of thermal expansion at extreme temperatures.

Preventive measures and solutions

Overheating can be addressed preventively at several levels. At the system design level: proper tank sizing (for solar systems with an area over 6 m², it is reasonable to reach at least 300-liter tanks or larger), or a tank with two heat exchangers, where the second can be used to connect another appliance or a pool circuit. At the regulation level: set the maximum tank temperature to 85–90 °C (not higher), and before that, activate protection via a cooling elbow or night cooling. At the level of regular inspection: check the condition and pH of the solar fluid at least once a year – when the pH is lower than 7 (neutral), the fluid should be replaced. The correct pH value should be 7–8.5.

2. Corrosion of solar storage tanks

Why the tank corrodes despite the fact that it is "stainless" or "enamelled"

Customers often assume that a modern storage tank is immune to corrosion. The reality is more complex. Most tanks on the market have an inner shell made of steel sheet, protected by glazing (a glass-like layer), or are made of stainless steel (stainless steel). Each type has its specific corrosion risks.

Glazed tanks: glaze is a brittle ceramic layer. If the tank is transported improperly, if it is hit during installation, or if a microcrack appears during the manufacturing process, the glaze cracks and the steel sheet underneath is exposed to water. Without protection, the surface will rust within a few years and the tank will start to leak. Therefore, glazed tanks have active protection – a magnesium anode (or an electric titanium anode) that "sacrifices" itself instead of the base material. But only if the anode is functional. A worn-out anode, untouched for years, does not provide protection.

Stainless steel tanks: stainless steel (typically AISI 304 or AISI 316) is much more resistant, but not indestructible. It reacts to aggressive water with a high chloride content (most tap water in Slovakia has chlorides below the problematic threshold, but with hard water and higher salt content, pitting corrosion occurs on the stainless steel surface – so-called pitting corrosion). Pits are dangerous because they are almost invisible from the outside, but they spread inside. The first visible sign is a rusty stain in the water or on the drain – and that is sometimes too late.

Types of corrosion and where they start

Corrosion in a solar storage tank can start in several places at once:

Bottom of the tank – sediments (sludge, scale) create an anaerobic environment under the sludge layer, where microbiologically influenced corrosion (MIC) occurs. Sulfate-reducing bacteria (SRB) produce hydrogen sulfide, which is aggressive to steel. A tank that has not been flushed for years has a layer of sludge at the bottom and corrosion attack on the shell underneath.
Welded joints and connections – every opening in the shell (connection, anode supply, heat exchanger) is a potential weak spot. A galvanic cell between different metals (e.g., a copper heat exchanger in a steel shell) accelerates corrosion around the joint.
Heat exchanger – copper heat exchangers in steel tanks are relatively resistant, but with hard water and pH below 7, surface acidification and copper dissolution into water occur (greenish water from the tap – a characteristic sign). Laminate heat exchangers are chemically inert, but are prone to mechanical damage under excessive pressure or vibration.
Inner glaze – under thermal shocks (sudden filling of cold water into a hot tank), the glaze can crack. The resistance of glaze is given in temperature change cycles; quality glazes withstand thousands of cycles, cheaper products less.

Corrosion diagnostics – what home signals reveal

Corrosion manifests in several ways that a typical homeowner can detect. The first and most important signal is discolored water: brownish-orange color (rust) at the first morning draw indicates corrosion of the shell or heat exchanger; greenish color indicates copper corrosion. The second signal is odor – a sulfur-like smell (rotten eggs) from hot water is a sign of sulfate-reducing bacteria in the sediment. The third signal: reduced heating efficiency – corrosion and scale deposits on the heat exchanger act as thermal insulation; the system has to work longer to achieve the same temperature.

The anode should be physically checked at least once every two years. Access is through a service opening at the top of the tank. A new anode has a thickness of about 25–30 mm; when it is reduced to a diameter below 10 mm or when the remaining part is shorter than 30 % of the original length, it should be replaced. In practice, in soft water (northern Slovakia, mountainous areas), anodes last 4–6 years, in hard water (Danubian Basin, Nitra, Trnava areas) sometimes only 2–3 years. A more detailed procedure can be found in the article Maintenance of a solar storage tank: cleaning, anode and heat exchanger check.

Why a two-heat exchanger tank is more resistant to corrosion

An interesting practical detail: tanks with two heat exchangers have a primary solar circuit (glycol) closed, without contact with drinking water. The secondary circuit (e.g., boiler) is also closed. Drinking water is therefore in contact only with the inner shell and the secondary heat exchanger. This minimizes the risk of contamination and overall corrosion load is better distributed. If you are interested in comparing construction solutions, also read the article Storage tank with one or two heat exchangers: which is more suitable. For households with a solar system and a boiler, we recommend a solar storage tank with two heat exchangers including insulation, where each circuit is separated and the risks of galvanic corrosion are minimized.

3. Pressure loss in the solar system

Why pressure in the system is important and what are the normal values

A solar system is a closed hydraulic circuit. The primary circuit (collectors – pump – heat exchanger of the tank – back to the collector) is filled with solar fluid under pressure, typically 1.5–3 bar in cold operation. An expansion tank (pressure expansion tank) serves to absorb pressure fluctuations due to thermal expansion of the fluid. When the pressure drops below the minimum value (typically 1–1.2 bar), the pump draws in air, loses hydraulic power, and in the worst case stops, and the collectors operate at no load.

The secondary circuit (tank – hot water distribution) is typically pressure-connected to the cold water supply and operates under a network pressure of 2–6 bar. Problems may arise in the event of a tank or heat exchanger rupture, leakage at seals, or malfunction of the safety and expansion valve.

Causes of pressure drop in the primary circuit

Pressure loss in the primary circuit may have several possible sources:

Leak at fittings, hoses or heat exchanger – the most common cause. In practice, this is often an oil-glycerol seal that has lost its elasticity after repeated overheating. If you see a faint haze or dripping glycol under the tank or near the collector, this is a clear diagnosis. Glycol has a characteristic sweet smell and oily texture – it is easily recognizable.
Worn expansion tank – the membrane in the expansion tank cracks with age. When the membrane is damaged, the expansion tank no longer performs its function and pressure in the system rises sharply during heating, causing the safety valve to open and part of the fluid to escape. After cooling, the pressure drops below the minimum. Symptom: the safety valve discharges during every heating cycle.
Air in the system – due to improper filling, very low pressure (below 1 bar), or after stagnation, air bubbles appear in the system, which hinder circulation. The pump emits a characteristic gurgling sound when started.
Micro-cracks in the tank heat exchanger – in the event of a failure, water from the high-pressure primary circuit may leak into the tank or vice versa. The danger lies in the fact that the pressure difference is small and contamination may not immediately become apparent. Detection: if the pressure in the primary circuit drops faster than would be expected from a normal leak, and at the same time the pressure in the tank increases, you likely have a problem with the heat exchanger.

Pressure loss in the tank secondary circuit

The secondary circuit (drinking water) has different issues. The most common cause here is an incorrectly set or worn expansion valve and the tank’s safety valve. The tank’s safety valve (typically 6 bar) must have a free discharge – if the discharge is clogged or blocked, a dangerous pressure situation may occur during overheating. Conversely, if the safety valve leaks and constantly drips, it is either set to too low a pressure or it is worn and needs to be replaced.

The pressure in the secondary circuit is also affected by the condition of the household water supply. In areas where the public water pressure fluctuates (higher in the morning, lower in summer), the tank may operate under unstable conditions. This is resolved by installing a pressure-reducing valve before the tank, set to 3–4 bar.

Hydraulic balance and circuit distributors

An important, yet often neglected aspect: a solar system connected to the boiler circuit via a distributor and collector must have properly set hydraulic balancing. If one circuit dominates (e.g., floor heating with low resistance), the pressure in the solar primary circuit may drop due to improper flow direction. Therefore, in the installation of larger systems where the solar storage tank is connected to the distribution group, we recommend using an industrial stainless steel distributor – for example, industrial stainless steel distributor/collector assembly with ball valves 6/4"×1" 2-way, which allows precise adjustment and servicing of each circuit separately. For more information on connecting the storage tank with the boiler and distributor, see the article Connecting the solar storage tank with the boiler and circuit distributor.

4. Connection between faults and tank size

An undersized tank is almost guaranteed to lead to faults. A small tank heats up more quickly, exposes the heat exchanger to extreme temperature cycles, accelerates the degradation of the solar medium, and shortens the system's lifespan. On the contrary, a properly dimensioned tank evenly absorbs heat, maintains more favorable temperature gradients, and the entire system operates within an optimal range.

For families of 3–4 people with a solar collector area of 5–8 m², a standard solution is a tank with a volume of 250–300 liters. If the collector area is larger or the tank is also used to support heating, we recommend going for a 400-liter tank. For smaller households or cottages with a smaller collector area, a 250-liter tank is sufficient, or in the case of a 3-person family, a 300-liter tank, which provides enough buffer volume to cover peak consumption even on less sunny days. A complete overview of selection criteria can be found in the article How to choose a solar storage tank: volume, heat exchangers and system type.

5. Long-term preventive maintenance – consolidated overview

The three described groups of faults (overheating, corrosion, pressure loss) are closely related. Overheating accelerates the degradation of glycol, degraded glycol becomes acidic and corrodes metal parts, corroded parts create leaks, and leaks cause pressure loss. It is a chain that can only be broken by regular preventive checks.

Recommended maintenance intervals for the solar storage tank and primary circuit:

Every year: visual inspection of the tank and pipes for leaks, check the pressure in the primary circuit (should be 1.5–2 bar in cold operation), check the tank temperature and regulation function, visual inspection of the color of the glycol (should not be brown/yellow).
Every 2 years: check the pH and density of the solar medium (pH 7–8.5; density corresponding to a mixture with a freezing point of at least −25 °C), check the condition of the anode, check the pre-charge of the expansion vessel (disconnect it from the system and check the nitrogen pressure – usually 0.5–1 bar).
Every 4–6 years: replacement of the solar medium (regardless of condition – glycol has a lifespan of 5–8 years), possible replacement of the anode, overall pressure test of the system.
As needed: replacement of the safety valve, worn gasket, or membrane of the expansion vessel.

These are not recommendations from manufacturer brochures tucked away in drawers – they are real intervals based on how customers come in with faults. Most of the problems we encounter are the result of neglected basic checks. A customer who came in with a corroding tank after 12 years without service had a tank from a quality brand – the problem was not the product quality, but the complete lack of any maintenance.

6. Frequently asked questions (FAQ)

My tank is discharging water through the safety valve every summer. Is this normal?

No, it is not normal and should definitely not be ignored. The safety valve is a safety component, not a regulating valve – it should only open in an emergency. If it discharges regularly, the most common causes are an undersized tank (heat has nowhere to go), a worn expansion vessel (membrane is no longer functioning), or faulty regulation (the system does not stop heating in time). All three causes are fixable, but they need to be diagnosed separately.

How can I tell if the anode in my tank is worn out?

The most reliable method is a physical inspection – unscrew the anode from the service opening in the tank (usually M40–M60 threaded connection on the top) and inspect its condition. A new anode has a thickness of about 25 mm and a length according to the manufacturer. If it has a diameter of less than 10 mm or is disintegrating, it needs to be replaced. An indirect sign is brownish water from the tap during the first morning draw, or a rotten egg smell from hot water. However, these symptoms appear only after the anode has been non-functional for a long time and corrosion is already in progress.

Why is the pressure in my solar primary circuit dropping every few months?

If the pressure drops again after refilling (e.g., from 2 bar to 1 bar) after a few months, there are three likely causes: a small leak at one of the connections (check the fittings, Tichelmann group, or safety valve), a damaged membrane in the expansion vessel (valve discharges during heating and releases fluid), or micro-cracks in the tank's heat exchanger. We recommend a systematic diagnosis in steps: first the expansion vessel (disconnect and measure the nitrogen pre-charge pressure), then a visual inspection of all connections, and finally a pressure test of the heat exchanger.

Can I use regular antifreeze from my car instead of solar glycol?

No, absolutely not. Automotive ethylene glycol has a different additive composition than solar fluid (propylene glycol). Ethylene glycol is toxic (a problem if it leaks into drinking water), has a different viscosity, different boiling point, and different corrosion inhibitors. Solar fluid is specially formulated for a temperature range of −30 °C to +180 °C, is food-grade safe (propylene glycol), and contains inhibitors that protect metal from oxidation at high temperatures. Using the wrong fluid leads to rapid degradation, corrosion, and voiding of the tank's warranty.

The tank is 15 years old and has started to leak. Is it worth repairing or replacing it?

It depends on the location and extent of the damage. If it is a gasket or anode replacement at the connection, the repair is cheap and simple. If the leak is coming from the tank's shell or the heat exchanger (internal cracking), the repair is usually not economical – weld repairs on pressure vessels require certification and verification, which is expensive. In addition, a 15-year-old tank likely has degraded insulation and a worn heat exchanger. In such a case, replacing it with a new tank is more economically advantageous and efficient – a new tank comes with a warranty and modern thermal insulation properties.

Must my tank have only one heat exchanger if I have only a solar system?

Technically, no, one heat exchanger is sufficient for a simple system with only solar collectors. However, a two-exchanger tank provides greater flexibility – the second heat exchanger can be connected to a boiler, heat pump, or another heat source in the future. When purchasing a tank for the long term, we therefore recommend considering a two-exchanger solution, even if you are not using it to full capacity at the moment. A detailed comparison of both concepts can be found in the article Storage tank with one or two heat exchangers: which is more suitable.

Conclusion: faults are predictable, damage does not have to be

Overheating, corrosion, and pressure loss are among the predictable faults that can either be completely avoided (proper sizing, quality installation, regular maintenance) or detected at an early stage, when the remedy is cheap and quick. The key preventive tool is a regular annual inspection with simple measurements – pressure, pH, fluid color, and anode condition. A tank that is carefully maintained can last 20 years or more without major expenses. A tank that has been ignored since installation usually ends up in need of replacement after 8–12 years – unnecessarily early and unnecessarily expensive.

If you are considering the selection, placement, or specific technical parameters of a tank before purchase, we recommend reading our articles Dimensions and connections of solar storage tanks: what to check before purchasing and Installation of a solar storage tank: procedure, placement, and installation requirements, where you will find specific practical information on preparing the technical room and the correct connection of all circuits.

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