Common Solar Collector Faults and How to Fix Them

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Common Solar Collector Faults and How to Fix Them

A solar system runs unnoticed for years – and that's exactly the problem. When something breaks down, owners usually find out only when the water heater temperature starts to noticeably drop, or when the expansion tank bursts under pressure. From experience, I know that most faults can be diagnosed without special equipment, and a significant portion of them can even be fixed on your own – as long as you know what to look for. This article is therefore not just a dry list of faults. It is a practical manual that will guide you from initial diagnosis to specific repair steps, including figures, pressure values, flow rates and temperatures encountered in real-world jobs.

Why early diagnosis of the solar system is so important

From a routine maintenance perspective, solar collectors are relatively low-maintenance devices. They have no moving parts (except the pump and controller), don't require regular fuel replacement, and their service life, if properly installed, reaches 20 to 30 years. Nevertheless, faults do occur, and if left undetected, they tend to chain together: a worn seal leads to loss of coolant, loss of coolant leads to collector overheating, overheating leads to absorber damage, and a minor defect turns into a repair costing hundreds of euros.

Practical experience shows that in an average family house, some kind of solar system fault occurs every 3 to 7 years. Most commonly, this involves degradation of the heat transfer fluid, leaks at connections, or a faulty circulation pump. Less common but more serious are faults in the absorber or the collector glass.

Basic diagnostic principles – what to check first

Before you start looking for a specific fault, a basic system check is needed. Most problems show up in one of these parameters:

System pressure: the correct value for most installations is 1.5 to 3.0 bar in cold fluid. If the pressure gauge shows below 1 bar or above 4 bar, you have a problem.
Collector temperature: on a sunny day, a flat-plate collector should reach 60–90 °C at the outlet. Values below 40 °C in full sun indicate a problem.
Flow through the system: for 1 m² of collector, a standard flow rate of 15–25 liters per hour is calculated. A flat-plate collector with an area of 2.5 m² should have a flow rate of 37–62 l/h.
Condition of the heat transfer fluid: color, pH and glycol concentration are basic parameters. A change of color to dark brown or black is a serious warning sign.
Controller functionality: check whether the controller correctly turns the pump on and off and whether the displayed values match reality.

Fault #1 – Drop in system pressure (leaks)

The most common fault you will encounter. The system gradually loses pressure and the pump starts pumping air. This typically shows up as the system working intermittently – it runs for a while, then stops, the controller reports an error, or the water heater temperature stagnates.

Where to look for leaks

From experience, I know that about 60% of leaks are found at connections in the boiler room or utility room – not on the roof. Check these places:

Threaded pipe connections: typically at ball valves, air vents and filling cocks. PTFE sealing (Teflon tape) or hemp sealing can deteriorate after years, especially at higher temperatures.
Flange connections of the pump station: improper tightening or pump replacement can cause the flange seal to leak.
Collectors on the roof: collector connection points, T-pieces and bypass valves. Checking requires going onto the roof – be careful in wet conditions.
Expansion tank: the rubber seal of a diaphragm expansion tank may be cracked or leaking. If the diaphragm is damaged, oil or fluid penetrates into the gas space and the tank loses its function.

Procedure for finding a leak

The simplest way is to visually inspect connections under increased pressure (3.0–3.5 bar) using leak-detection foam. If you want to be thorough, brush every connection with soapy water solution and watch for bubbles forming. A glycol fluid leak leaves a characteristic brownish-green surface trace – visible on light-colored surfaces around the pipes.

For more extensive leaks, a pressure test helps: pressurize the system to 4 bar (maximum, not more), close all valves and watch the pressure gauge for 30 minutes. A drop of more than 0.2 bar indicates a leak that needs to be found and repaired.

Fault #2 – Degradation of the heat transfer fluid

The heat transfer fluid (typically a mixture of water and propylene glycol in a 40:60 ratio, protecting down to –28 °C) does not last forever. With regular overheating of the system (common in summer), the glycol mixture oxidizes, pH drops, and the fluid becomes corrosive. Instead of protecting the system, it starts damaging it.

How to check the condition of the fluid

Simply take a sample of about 50 ml of fluid using a syringe from the sampling valve and perform these tests:

pH meter or pH paper: the optimal value is pH 8.0–9.5. Below 7.5, the fluid is acidic and corrosive – immediate replacement is needed.
Refractometer: measures glycol concentration. The optimum is 35–45% propylene glycol. Below 25%, there is a risk of the system freezing in winter.
Visual inspection: dark brown or black color = strong oxidation, presence of flakes = corrosion of metal components.

Replacing the fluid is not complicated, but it requires the correct procedure: the system needs to be bled, flushed with clean water (if the fluid is heavily contaminated) and filled with fresh glycol mixture. For flat-plate collectors such as the Flat-plate AlCu solar collector with structural glass, manufacturers recommend checking the fluid every 2–3 years and replacing it every 5–7 years under normal operation, or sooner if the system regularly stagnates.

Fault #3 – Circulation pump problems

The circulation pump is the only active moving part of the solar system and is responsible for circulating fluid between the collector and the water heater. When it stops working, the system stagnates – and in summer this can lead to dangerous overheating of the collector.

Signs of pump failure

The controller issues a command to start the pump (the pump should be running), but the collector temperature keeps rising while the water heater temperature does not rise.
The pump makes unusual noises – buzzing, squeaking, clicking.
The pump is hot to the touch (during normal operation it is only mildly warm).
The pump runs, but flow is zero or minimal (clogged filter or seized pump).

Solution

The first step is to check whether the pump is receiving power – this can be revealed with a simple voltmeter. If it is, the problem may be a seized rotor (typical after standing idle for a long time in summer, when precipitated salts settle on the rotor). Some pumps have a screw on the side through which the rotor can be manually turned and freed. If this doesn't help, the pump needs to be replaced – it is a consumable part with a service life of 8–15 years.

When replacing it, make sure it is properly sized: the pump's capacity must match the total pressure loss of the circuit. For a typical family house with 2 to 4 collectors and piping up to 20 m long, a pump with a power of 25–40 W and a flow rate of 2–5 m³/h is sufficient.

Fault #4 – Air in the system (cavitation)

Air in the solar system is a chronic problem that manifests as noise, vibration and reduced performance. It arises mainly when filling the system, during a leak and refilling with fresh fluid, or during overheating, when the glycol mixture partially decomposes and releases gases.

Bleeding the system step by step

The procedure depends on where the air is located. A properly designed system has automatic air vents at the highest points of the circuit (typically directly on the collectors or just behind them) and manual air vents on the pump station in the boiler room. Procedure:

Turn on the pump at maximum power.
Open the manual air vent on the pump station until fluid flows out without bubbles (catch the fluid in a container).
Check the pressure – if it has dropped below 1.5 bar, top up the system with fresh fluid.
Repeat the procedure several times – air is released gradually.
If air keeps returning repeatedly, look for the leak point through which air is entering the system.

Fault #5 – Expansion tank failure

The expansion tank compensates for changes in fluid volume caused by temperature fluctuations. In a solar system, these fluctuations are more extreme than in a classic heating circuit – the fluid can reach up to 150 °C during stagnation. That's why solar expansion tanks are sized differently (larger volume, higher pre-charge pressure) than ordinary heating ones.

Signs of expansion tank failure

The safety valve repeatedly discharges fluid (the system "expels" fluid because the tank cannot keep up with compensating for expansion).
System pressure fluctuates extremely – low in the morning (1 bar), jumping to 6 bar in the afternoon sun.
If you open the tank's air valve (a small valve like on a tire), fluid comes out instead of air – the diaphragm is torn.

Solution and prevention

The diaphragm of the expansion tank cannot be repaired – the tank must be replaced. For a typical system with 2 flat-plate collectors (each about 2.5 m²), an expansion tank with a volume of 18–25 liters and a pre-charge pressure of 2.5 bar is sufficient. Before installing a new tank, always check and set the pre-charge pressure according to the height position of the system: pre-charge pressure = 1 bar + 0.1 bar for every meter of height of the collector above the expansion tank. For a collector at a height of 5 meters, this means a pre-charge pressure of 1.5 bar.

Fault #6 – Contamination or damage to the collector glass

The collector glass protects the absorber and also ensures optical transmittance for solar radiation. Dust deposits, bird droppings or mechanical damage can significantly reduce system performance.

Effect of contamination on performance

Measurements show that a thick layer of dust can reduce collector performance by 10–30%. In areas with more dust or near agricultural land, regular cleaning is really important. Recommended frequency: once a year, ideally in spring after winter, when the glass is contaminated with salt residues, moss and dust.

You should clean collectors in the morning or evening – not in full sun, when the glass is hot. Use a soft cloth or sponge with clean water (no abrasive agents). If using a pressure washer, keep a safe distance and do not aim the jet directly at the collector frame seals.

Cracked or broken glass

Glass damage can be caused by a hailstorm, a falling branch, or vandalism. The glass of a flat-plate collector is not ordinary window glass – it is tempered safety or structural glass with specific optical properties. For example, the Flat-plate AlCu solar collector with structural anti-reflective glass uses an anti-reflective treatment that increases light transmittance to more than 91% – you cannot achieve this property with ordinary glass. Replacing the glass therefore requires an original or certified replacement part, otherwise you will reduce the collector's performance.

Replacing the glass of a flat-plate collector is possible – the collector is removed from the roof, the frame profile is unscrewed, the glass is removed and replaced with a new one. This is work that takes several hours and requires increased caution when handling the heavy collector (the weight of a flat-plate collector is usually 30–45 kg).

Fault #7 – Controller or temperature sensor failure

The differential controller is the brain of the solar system. It compares the collector temperature and the water heater temperature and switches the pump on or off accordingly. If the controller or one of the temperature sensors is not working correctly, the system may behave unpredictably.

Typical controller failure scenarios

Pump does not run even in full sun: likely a fault of the collector sensor or the differential set too high (e.g. delta T = 15 °C, but the collector is only 12 °C warmer than the water heater). Check the differential value – 6–8 °C is recommended for switch-on.
Pump runs constantly, even at night: fault of the collector sensor (reporting a higher temperature than actual) or a shorted sensor circuit. Sensors are mostly NTC thermistors with a resistance of 10 kΩ at 25 °C – measuring resistance with a multimeter will reveal the fault.
Controller shows an error code: check the controller manual. Most modern controllers display codes E1 (collector sensor fault), E2 (water heater sensor fault), or E3 (communication interruption).

Replacing the temperature sensor

Collector sensors must withstand high temperatures – typically up to 200 °C. Not every NTC sensor is suitable. Use only sensors recommended by the controller manufacturer or certified for solar systems. The water heater sensor is usually inserted into an immersion sleeve – replacement is easy and does not require draining the system.

Fault #8 – System stagnation and its consequences

Stagnation occurs when the collector is fully illuminated by the sun, but the heat has nowhere to be transferred – the water heater is full, the system is switched off. The temperature in the collector can spike to 180–200 °C, the fluid evaporates, and the vapor pushes the fluid back into the piping and expansion tank. This condition is very stressful for the system.

Stagnation itself is not a fault – it is a physical phenomenon that solar systems must be designed to withstand. A problem arises when the system is oversized (too large a collector area for a small water heater), when the expansion tank is not properly sized, or when stagnation repeats every day.

Measures against harmful stagnation

Proper system sizing – the ratio of collector area to water heater volume should be 50–70 liters per 1 m² of collector.
Shading the collectors during periods of prolonged stagnation (e.g. when leaving for vacation) using UV-resistant covering sheets.
Solar fluid with a higher boiling point – quality propylene glycols withstand 180 °C without permanent degradation during short-term exposure.
Regular fluid replacement, because degraded fluid handles stagnation much worse.

If you're interested in how to properly design a system in terms of sizing, also read our article What Collector Output Do I Need for My House and Collector Dimensions and Area – How Many Units Do I Need in the Knowledge Center.

Fault #9 – Reduced absorber performance

The absorber is the heart of the collector. It is a metal panel (most often aluminum with copper piping, or an aluminum-copper composite – AlCu) covered with a selective coating that absorbs solar radiation and minimizes heat losses. Degradation of the selective coating leads to a significant drop in performance.

Causes of absorber degradation:

Corrosion: caused by acidic heat transfer fluid (pH below 7), moisture penetrating through a damaged frame seal or cracked glass.
Coating delamination: with long-term overheating (repeated stagnation), the selective coating may peel off the absorber, losing its optical properties.
Mechanical damage: due to improper handling during installation or repair.

Visual signs of a damaged absorber include a color change from deep black to gray or brown, visible coating peeling, or signs of corrosion visible through the glass. In the case of significant absorber damage, replacing the entire collector is more economical than repair – for example, AlCu collectors have readily available spare parts and a long warranty period.

Preventive maintenance – how to avoid most faults

A preventive annual inspection of the solar system takes 1–2 hours and will prevent 80% of faults. Recommended schedule:

Every year (spring): visual inspection of the collector and frame, glass check, collector cleaning, pressure check, fluid color and pH check, system bleeding.
Every 2–3 years: measuring glycol concentration with a refractometer, checking the expansion tank (pressure and diaphragm condition), checking the safety valve (manual release and closing).
Every 5–7 years: replacing the heat transfer fluid, checking seals at connections, checking pipe insulation (especially on the roof, where UV radiation degrades insulating materials).

If you want to know more about the proper operation of the system during the colder period, read the article Winter Operation of Solar Collectors – What You Need to Know. For those still considering a purchase, the guide How to Choose a Solar Collector – What to Watch Out for Before Buying is suitable.

When to call an expert and when you can manage on your own

Not every fault requires calling a service technician. Here is a practical breakdown:

Do it yourself (after reading the manual): bleeding the system, checking and topping up pressure, cleaning the collector, checking and replacing the temperature sensor, setting the controller, replacing the safety valve or drain cock.
We recommend an expert: replacing the entire collector or absorber, replacing the circulation pump (if you have no experience with plumbing work), replacing the expansion tank, complete fluid replacement (requires flushing and correct filling), repairing or replacing the collector glass.
Expert necessary: a burst pipe, repeated leaks without an obvious cause, a controller fault connected to a smart home control system, any work on the electrical part of the system (if you do not hold an electrician's license).

Frequently Asked Questions (FAQ)

Why does my solar system work normally in the morning but stop in the afternoon?

The most common cause is reaching the maximum set water heater temperature (e.g. 65 °C). The controller turns off the pump because further heating is not needed. If the water heater temperature is lower, the cause may be a power failure of the controller, pump overheating, or air in the system preventing flow. Check the controller display and compare the displayed temperature values with the actual state.

The fluid in the system is dark brown – do I need to replace it right away?

Yes, dark brown fluid is a clear sign of strong oxidation. Such fluid usually has a pH below 7 and actively corrodes the metal parts of the system (copper, aluminum, steel). Don't wait – the longer such fluid circulates, the more damage it causes to the absorber, pump and connections. Before filling with new fluid, we recommend flushing the system with clean water.

Can I use ethylene glycol instead of propylene glycol in a solar system?

Technically yes, they are similar in performance, but ethylene glycol is toxic. In case of any leak, it contaminates soil and water, which is an environmental and legal problem. Moreover, in the event of a water heater leak, it can contaminate drinking water. For family houses, propylene glycol is clearly recommended – it is harmless to people and the environment, and its use is standard in solar systems.

My safety valve regularly discharges fluid. Is that normal?

No, it is not normal and needs to be addressed. There are three causes: the safety valve is worn and opens at a lower pressure than it should (valve replacement), the expansion tank is undersized or damaged (check the pre-charge pressure, possibly replace the tank), or the system stagnates too intensely and produces more expansion than the tank can handle (consider shading the collectors or a larger tank). Ignoring this problem leads to fluid loss, pressure drop, and the introduction of air into the system.

My collector produces much less heat than in the past, but looks fine visually. What should I look for?

Check in this order: 1) glass contamination (cleaning helps within 30 minutes), 2) system flow rate – a clogged filter or seized pump reduces flow without visible symptoms, 3) fluid degradation – even visually clean fluid can have a pH of 6.5 and low heat capacity, 4) absorber condition – look through the glass to see if the color has changed or if there are signs of moisture inside the collector (condensation = broken frame seal).

Can I leave the solar system unattended during a three-week summer vacation?

You can, but with precautions. Three weeks of stagnation in summer is a strain on the system, but if it is properly sized and the fluid is in good condition, it is manageable. I recommend: before leaving, check the pressure and fluid condition, set the controller to summer (stagnation) mode, or cover the collectors with a reflective sheet if the system is not designed for intense stagnation. After returning, check the pressure and bleed the system.

Conclusion – a systematic approach saves both time and money

Solar collector faults are mostly predictable and repairable. The key is a systematic annual check and knowing what a normal system does and does not do. If you know that your system should have a pressure of 2 bar in the boiler room, the pump should run from 8:00 to 16:00 on a sunny day, and the water heater should reach 60 °C by noon, you will immediately notice a deviation. And a deviation is exactly what leads you to a fault before it becomes an expensive problem.

Investment in a solar system pays back within 8–12 years under normal operation. Neglected systems that lose 20–30% of their performance due to degraded fluid or a clogged filter may have a payback period 3–5 years longer. Regular maintenance is therefore not just a technical obligation – it is economic logic. Learn more about optimizing the position and performance of solar collectors in the articles Collector Tilt and Orientation – How to Maximize Energy Yield and Structured vs. Anti-Reflective Collector Glass – What's the Difference.

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