Common faults of solar systems and how to eliminate them

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Common problems with solar systems and how to fix them

Solar water heating systems are among the reliable devices with a long lifespan – when installed correctly and maintained regularly, they can operate without major problems for 20 or more years. Despite this, every owner of a solar system sooner or later encounters some fault or unexpected behavior of the device. From practice, I know that most problems are recurring and well documented. It is not always necessary to immediately call a service – some faults can be fixed by a resourceful owner themselves, while others require professional intervention. This article will help you systematically diagnose faults, understand their causes, and know when to act on your own and when to call a technician.

Before we get into specific faults, it is important to understand how a solar system works in the basics. The collector on the roof captures solar radiation and heats the heat transfer medium (usually a mixture of water and glycol). The pump station circulates this medium through the circuit to the hot water tank, where it transfers heat to the utility water via a heat exchanger. The regulation ensures that the pump runs only when the collector is warmer than the tank. Each of these components can fail – and we will go through them in detail.

Why the system does not heat water – the most common problem of all

This is by far the most frequent complaint I encounter. The owner notices that the temperature in the tank remains low even on a sunny day. There can be several causes and they need to be verified systematically – from simple to complex.

The pump is not running – electrical and mechanical causes

The first thing to check is whether the pump is running at all. You should hear a soft buzzing or vibrations on the pump station when the system is in operation. If the pump is silent, check the following:

Circuit breaker and fuses: The solar pump has its own circuit breaker in the distribution board. Check if it has tripped. This is responsible for a surprisingly high proportion of service technician visits – the circuit breaker has tripped, the owner does not notice it, and the system is not working.
Regulator: Check the display of the regulator. If the regulator shows a collector temperature higher than the tank temperature (typically a difference of >5–8 °C), but the pump is not running, the regulator or its output relay is likely faulty.
Jammed pump: After a long period of inactivity (e.g., the end of winter, when the system was idle), the pump can jam (calcified rotor). On the pump station, there are usually covers for manually unlocking the rotor with a flat screwdriver. This is a routine maintenance task.
Defective pump winding: If the pump does not respond even after unlocking, the winding may be burned out. Replacing the pump takes about an hour, the price of the pump ranges from 60 to 200 € depending on the type.

The regulator indicates operation, but the heating is weak

If the pump is running and the regulator shows reasonable temperatures, but the tank does not heat up to the expected temperature, there are several possible causes. The first possibility is a mismatch – the system simply does not have enough capacity for your demand or the day is really cloudy. However, if the problem repeats on sunny days, monitor the collector temperature: if, for example, in July on direct sunlight it is only 50–60 °C instead of the usual 80–110 °C, this indicates a drop in the collector's performance or heat loss from the pipe.

Air in the solar circuit – the silent thief of performance

Air pockets in the solar circuit are one of the most common causes of reduced performance or complete stop of the circulation of the heat transfer fluid. Air gets into the system most often during the first filling (incomplete bleeding), after refilling the fluid, or gradually – through micro pores in seals during years of operation. Air pockets cause characteristic gurgling or grumbling in the pipes, the pump may run dry and you may hear irregular sounds from the pump station.

How do you detect air in the circuit? The regulator shows that the pump is on, but the tank temperature does not rise or rises very slowly despite a high collector temperature. The flow meter on the pump station indicates zero or erratic flow. Sometimes you can also hear a characteristic whining in the pipes.

Solution: Solar systems are equipped with automatic air vents (usually at the highest point of the circuit – on the collector or near it). Check whether these valves are functioning and are not clogged or blocked. Manual air venting is done with the pump running – at the venting points, release air until liquid starts to flow. After venting, always check the pressure in the circuit and top up if necessary.

Important warning: when venting the collector in summer, be careful of the very high temperature of the heat transfer fluid (can be up to 150 °C during stagnation) – always vent in the morning or on cloudy days, when the collector is cooled down.

Pressure faults in the solar circuit

The correct operating pressure of a solar circuit typically ranges between 1.5 – 3 bar (depends on the installation height and specific system – the manufacturer always specifies exact values). The manometer on the pump station should indicate the correct value. Both too low and too high pressure indicate a problem.

Too low pressure – fluid leakage

If the pressure repeatedly drops (e.g., you have to top up every 2–3 months), it is a clear sign of a leak somewhere in the circuit. Leak locations are usually:

Connections and fittings at the collector locations (due to thermal stress, threaded connections can loosen)
Pressure relief valve – if it opens too often (during stagnation), it may be worn and start dripping even at normal pressure
Expansion tank – if the membrane bursts, the pressure fluctuates and the pressure relief valve opens
Seals and O-rings in the pump station or on the valves
Micro-cracks in the pipes – less common, but possible after long operation or freezing

Fluid leakage can be found by visual inspection – characteristic are yellowish to brown stains from the glycol mixture on the pipes, connections, and under the collectors. Sometimes the leak is not visible (e.g., in the roof structure) and must be located by a pressure test with nitrogen.

Too high pressure – problem with the expansion tank

The expansion tank compensates for the expansion of the heat transfer fluid when heated. It has the correct pre-charge pressure (typically 1.0 – 1.5 bar, depending on the system) and a membrane that separates air from the fluid. When the membrane bursts, the air side becomes filled with fluid and the tank stops performing its function. Consequence: during each heating, the pressure rises sharply, the pressure relief valve opens, and fluid escapes. The system thus loses its medium and the pressure drops after cooling down.

Testing the expansion tank is simple: disconnect the tank (close the valve before the tank, release the pressure from the fluid side), then press the nitrogen side valve (as with a tire). If fluid flows out, the membrane is pierced. Solution: replace the expansion tank. Solar expansion tanks are more resistant to higher temperatures than heating ones – always choose a type approved for solar applications.

Degraded heat transfer fluid – a hidden problem with serious consequences

The heat transfer fluid (a mixture of water and propylene glycol or ethylene glycol) is not eternal. During operation, corrosion inhibitors are consumed, glycol can decompose (especially after multiple stagnations at high temperatures), and the pH of the solution changes. Degraded fluid is acidic (pH below 7) and starts to corrode system components – copper, brass, and aluminum in the collector.

Signs of degraded fluid:

Fluid is dark brown to black (correct is light yellow to coppery)
System shows increased resistance to flow (clogged filter in the pump station, deposits in the collector)
Smells burnt (during stagnation, degraded glycol carbonizes)
pH measured with a test strip is below 7 (acidic reaction)
Reduced frost resistance – refractometer measurement shows that the mixture protects only down to -10 °C instead of the declared -28 °C

Fluid replacement is usually necessary every 4–6 years (varies according to the quality of the fluid and the intensity of operation – systems with frequent stagnation require earlier replacement). The replacement procedure includes draining the circuit, flushing with water, filling with fresh solution with the correct concentration, and venting. At this opportunity, always check the filter/screen in the pump station – if it is clogged, the flow decreases and the system performance suffers.

Warning: never add pure water to the solar circuit! It dilutes the glycol, reduces the freezing point, and the circuit can freeze in winter – with disastrous consequences for the collectors and pipes.

Controller and temperature sensor faults

The solar system controller is its "brain" – it compares the temperature of the collector and the storage tank and controls the pump accordingly. Most modern controllers are reliable and last the entire system lifetime. Problems mostly occur with temperature sensors (Pt1000 or Pt500 resistance sensors), which are exposed to temperatures, UV radiation, and humidity.

Broken or shorted sensor

A broken sensor usually displays as a very high or maximum value (e.g., 999 °C or "---") on the controller, a shorted sensor, on the other hand, as a very low value or ambient temperature. The controller will then either constantly turn the pump on or constantly turn it off – neither is desirable.

Checking the sensor: disconnect the sensor from the controller and measure its resistance with a multimeter. A Pt1000 sensor should show 1000 Ω at 0 °C, approximately 1097 Ω at 25 °C, and around 1385 Ω at 100 °C. If the resistance is out of range or infinite (break), the sensor is faulty. Replacing the sensor is a cheap job (10–25 €) and can be done by a skilled owner.

Special attention: the collector sensor is exposed to extreme temperatures (up to 200+ °C during stagnation) and UV radiation. It is common for this sensor to fail before the others. Also check the sensor wiring – on the roof it can be damaged by UV radiation, cold, or birds.

The regulator is not responding or displaying error messages

Modern regulators have a built-in list of error codes – always refer to the regulator's manual. The most common codes relate to faulty sensors (E1, E2 – depends on the manufacturer), overheating of the tank (alarm when maximum temperature is reached – typically 90–95 °C), or pump failure (no feedback from the flow sensor). Do not ignore error messages – the system may be in emergency mode and is not heating or, conversely, is running without control.

Stagnation – a dangerous condition you should be aware of

Stagnation occurs when the collector produces more heat than the system can dissipate – the tank is full, the pump stops, and the collector overheats. Temperatures in the collector can reach 180–250 °C during stagnation, and up to 150–160 °C in the collector loop. Stagnation itself is not a fault – systems are designed for it. The problem arises when stagnation occurs too frequently (e.g., due to an undersized tank or during a long vacation).

Repeated stagnation accelerates the degradation of the heat transfer fluid (glycol breaks down and carbonizes), strains the seals, and causes premature wear of the pump (repeated startups from high temperatures). Solutions for households where stagnation occurs regularly: a larger tank, automatic air vents designed for stagnation, a night cooling system (the pump runs at night to cool the collector), or installation of a shade over the collector during long absences.

Frozen solar loop – a disaster that can be avoided

Frost is the most destructive enemy of an inadequately protected solar loop. Water expands when it freezes and can burst the collector tubes, pipes, or cause the seals to fail. Repairing such damage is expensive – sometimes it requires replacing the entire collector.

Causes of freezing:

Too low glycol concentration (protection only down to -5 °C instead of the required -28 °C for our climatic conditions)
Degraded fluid that has lost its freeze resistance
Plain water instead of a mixture (topping up with water after a leak)
Turned-off system (e.g., a circuit breaker tripped) during frost – the fluid is not stagnant but cooling down from the collector

Preventive measure: Check the freezing point of the heat transfer fluid with a refractometer every year before winter. The correct value for Slovakia is protection down to at least -28 °C (which corresponds to approximately 40% propylene glycol concentration). If the value is higher (weaker protection), top up or replace the fluid. More information on this topic can be found in the article Maintenance and servicing of solar collectors – what and when to check.

Dirty and clogged collectors – performance drop that is easily overlooked

The outer surface of the collector must be clean to maximize solar radiation absorption. In real conditions, dust, dirt, bird droppings, leaves, algae, and moss (especially in shadier positions or at lower slopes) settle on the collector glass. A clogged collector can have its performance reduced by 10–30 %, and even more in extreme cases.

Most collectors clean themselves during rain if the slope is sufficient (over 15–20°). In flat installations or in regions with low rainfall, manual cleaning of the collector is recommended at least once a year. Use soft water (not hard water – leaves limescale), a sponge or soft cloth, and warm water. Never use pressure washers – you may damage the glass, the antireflective layer, or the frame seals.

Internal clogging of the collector (deposits inside the absorber tubes) is a more serious issue. It is usually caused by long-term operation with degraded fluid or with plain water. It manifests as a significant drop in flow despite correct pressure and a functioning pump, and high temperature at the collector inlet with a small temperature difference at the outlet (fluid is not able to extract heat efficiently). The solution is a chemical flush of the loop with a special preparation for solar systems, followed by a rinse with clean water and topping up with fresh fluid. This procedure should be carried out exclusively by a service technician.

Failure of the safety valve and leaks through it

The safety valve is a safety component that protects the loop from excessive pressure. It opens when the set pressure is exceeded (typically 6 bar for solar loops) and releases fluid. It normally opens only during stagnation or in case of a malfunction of the expansion tank. If the safety valve is continuously dripping or leaking fluid, it is always a sign of a problem:

Safety valve dripping during normal operation: Expansion tank is faulty or incorrectly pressurized → check and replace the expansion tank
Safety valve dripping only in summer: Stagnation with insufficient expansion capacity → check the size of the expansion tank (must be designed for solar systems with sufficient volume for glycol + vapor)
Safety valve continuously leaking: Valve is worn and not sealing → replace the valve, at the same time investigate the cause of frequent opening

Never block or replace the safety valve with one with a higher pressure setting without consulting a professional – it is a safety component and its removal can lead to tank or collector explosion.

Problems with the hot water tank

The hot water tank has two main vulnerable points: the anode (a protective magnesium or electric sacrificial anode prevents tank corrosion) and the enamel coating inside the tank. A worn or consumed anode leads to tank corrosion, which is manifested by rusty water, a sulfur smell, or the appearance of corrosion at the tank outlets. The anode must be checked every 2–3 years and replaced as needed.

Another problem with tanks: limescale buildup on the solar heat exchanger. In areas with hard water (> 20 °dH), limescale deposits on the inside of the heat exchanger act as thermal insulators. 1 mm of limescale increases thermal resistance by about 20–30 %, which is manifested by longer heating times and higher collector temperatures needed to heat the tank. Solution: chemical cleaning of the heat exchanger with acid (professionally performed), or installation of a water softener at the tank inlet.

When to call a service technician and when you can do it yourself

This is a practical question every owner asks. Summary according to the complexity of the task:

You can handle it yourself:

Check the fuse and circuit breakers
Check and record values on the manometer and regulator
Visual inspection for liquid leaks
Manual air bleeding (if you have experience)
Cleaning the outer surface of the collector
Unblocking the pump rotor
Measuring the sensor resistance and its replacement
Testing the expansion tank

Requires a professional:

Replacement of heat transfer fluid (work with pressure circuits, dosing, air bleeding)
Leak detection and repair (pressure test, work on the roof)
Replacement of the expansion tank, safety valve
Chemical flushing of the circuit or tank
Replacement of the pump or regulator
Repair or replacement of the collector
Any work on the electrical part (230V connection)

If you are deciding whether to install a new system or upgrade components of an existing one, you may find further topics in our Knowledge Center helpful – for example How to choose a solar water heating system for a family home or Solar system in combination with a boiler or heat pump.

Seasonal diagnostics – what to check before each season

An experienced technician will tell you that most problems can be prevented by regular and systematic inspection. I recommend the following annual inspection plan:

In spring (before the season): Visual inspection of collectors (mechanical damage, glass cleanliness), checking the pressure in the circuit, checking the heat transfer fluid (color, refractometer), testing the pump and regulator, checking the air vents.

In summer: Monitor whether the system reacts normally to sunny days, whether the tank temperature reaches the desired level (55–60 °C normally, 65–70 °C on a good sunny day). Record how long it takes to heat the tank – if the time increases compared to the previous season, it may indicate a performance drop.

In autumn: Check the freeze protection of the fluid with a refractometer – this is a key check before winter! Check the pipe insulation (cracked or missing insulation on the roof or in unheated areas). Check the collector mounting (brackets, tightened screws).

In winter: If the system is running (most modern systems operate year-round), monitor the regulator behavior. Collectors are covered with snow in winter – this is normal and does not require intervention. Do not clean the collectors manually in freezing conditions (risk of falling, glass damage).

Learn more about preventive maintenance in the article Maintenance and service of solar collectors – what and when to check, which is part of this Knowledge Center.

Frequently asked questions (FAQ)

The regulator indicates that the pump is running, but the tank is not heating up. What should I do first?

Check whether the pump is actually running (place your hand on the pump station – it should vibrate). Check the flow meter on the pump station – if it shows zero despite the pump running, there is likely air in the circuit. Bleed the system according to the manufacturer's procedure. If the flow is fine, check the condition of the heat transfer fluid and the filter screen in the pump station – a clogged screen significantly restricts flow. If the problem persists, the issue may be a clogged heat exchanger in the tank or a clogged collector.

Water is dripping from the safety valve of the solar circuit every summer. Is this normal?

No, it is not normal, although it is common. The cause is almost always the expansion tank – either it has a cracked membrane, it is too small for your system, or it has an incorrect pre-charge pressure. Summer stagnation causes a significant increase in fluid volume (and partial evaporation), which the expansion tank must safely compensate for. Have the expansion tank checked and possibly replaced by a professional – this issue should not be underestimated, as repeated opening of the safety valve damages the valve itself and the system loses heat transfer fluid.

The solar collector is gray and cloudy from inside the glass. What does this mean?

Cloudiness from inside the glass (fog, condensation, gray discoloration) usually means a leak in the collector frame seal – moisture is entering the collector. In flat collectors, it can also be a crack in the glass. Condensation inside rapidly reduces optical transparency and thus the collector's performance. If it is only temporary condensation (in cold mornings, when the collector is cold and the outside air is humid), it may evaporate on its own – this is sometimes normal. Permanent cloudiness requires repair or replacement of the collector, as moisture inside long-term damages the absorber.

The temperature on the collector regulator shows -99 °C or a similarly unrealistic value. Is the regulator broken?

This value typically indicates a fault in the collector temperature sensor – either a broken cable, poor contact, or a faulty Pt1000/Pt500 sensor itself. Check the sensor wiring from the collector to the regulator – especially on the roof, where the cable may be damaged by UV radiation or mechanically. Disconnect the sensor and measure its resistance with a multimeter (at room temperature, a Pt1000 sensor should show about 1085–1100 Ω). If the resistance is infinite or zero, replace the sensor. The regulator itself is rarely the cause of such a fault.

Can I leave the solar system running during a long vacation (3–4 weeks)?

Yes, you can leave the system running – it is designed for that. The tank will heat up to maximum, the regulator will stop the pump, and stagnation will occur. Modern systems can handle this. The problem arises if stagnation occurs repeatedly every day for several weeks – it accelerates the degradation of the fluid. I recommend checking the fluid condition and pressure before the vacation, or informing someone in the household or a neighbor to check the manometer. Some owners install remote monitoring via smart regulators, which is a very practical solution. More about combined systems and stagnation prevention can be found in the article How to set up and start a solar system.

The solar system worked for 10 years without problems, but now the performance has suddenly dropped by half. What happened?

After 10 years of operation, several types of wear can manifest at once. The most likely causes: degraded heat transfer fluid (should be replaced every 4–6 years), clogged tank heat exchanger (limescale), worn tank anode, clogged filter screen in the pump station, or reduced performance of the collector's absorber coating (after 10–15 years, the selective coating may lose 10–15 % of its original absorptivity). I recommend a comprehensive service – fluid replacement, anode inspection, and a professional inspection of the collector and tank. This comprehensive service is economically worthwhile after 10 years – it will extend the system's lifespan by several more years.

Conclusion

A solar system is an investment for decades – and like any technical system, it requires attention and regular care. The good news is that most problems can be prevented by regular inspection (pressure, fluid, sensors), and catching a problem early costs a fraction of what neglected damage would. If you encounter a fault, proceed systematically from simple to complex causes – most problems have simple solutions. Don't be afraid to diagnose yourself, but know where the line is, beyond which it makes more sense to call a professional.

For a deeper understanding of how the entire system works from the ground up, we recommend also looking at other topics in the Knowledge Center – for example Flat vs. tubular solar collectors – which type is more cost-effective, Is a solar system worth it in cloudy weather or winter, or Common questions about solar water heating systems. The complete range of solar systems and accessories can be found directly in the category Solar systems.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your household? Write to us – we are happy to help.