Common faults of solar regulators and pump units
Common faults of solar regulators and pump units – a complete overview of symptoms, causes and solutions
A solar system runs continuously, day after day, from spring to autumn – and precisely for this reason, sooner or later, something will stop working as it should. From practical experience, I know that most faults of solar regulators and pump units are not mysterious or irreparable. The problem usually lies in a few well-known places: poorly calibrated sensors, neglected maintenance, incorrectly set parameters, or worn mechanical components of the pump. This article goes into depth – we will go through each type of fault systematically, describe the symptoms that reveal them, and show how to diagnose and solve them.
If you are still in the selection or installation stage, I recommend also looking at our other topics in the Knowledge Centre: Installation of a solar pump unit step by step and How to set up a solar system regulator for maximum efficiency. Here we will focus exclusively on faults and their removal.
Why faults occur – a systematic view
A solar system is a closed circuit operating with a heat transfer fluid (usually a mixture of water and propylene glycol), a pump, a regulator, sensors and a storage tank. Each of these components can fail individually, or faults can combine. From customer experience, I know that a problem rarely arises "suddenly" – most of the time there is a warning sign: the system starts to work less efficiently, the pump runs longer, the storage tank heats up later or not at all. Anyone who monitors the system regularly will detect a fault early, before it causes major damage.
For context: a typical solar system for a household has a temperature sensor T1 on the collector (or also a sensor on the return line), a sensor T2 in the storage tank, and the regulator compares their values. When the temperature difference (delta T) is large enough, the pump is started. When the difference decreases or the storage tank reaches the maximum temperature, the pump is turned off. A seemingly simple principle – and yet a lot can go wrong.
1. Temperature sensor faults – the most common source of problems
A temperature sensor is a cheap component, but its failure can effectively "blind" the entire system. The regulator has no way of knowing the actual temperature – and either the pump will not start at all, or it will not stop when it should.
Symptoms of T1 sensor failure (collector)
- The regulator displays extreme collector temperature values – e.g. 999 °C, –99 °C, or a value outside the physical range.
- The regulator starts the pump at night or on a cloudy day (false high temperature).
- The pump does not start at all even on a bright summer day (false low or negative value).
- The error code flashes on the regulator display – with Euroster 813 Solar it is typically "Er1" or a display of a broken connection for the respective sensor.
Symptoms of T2 sensor failure (storage tank)
- The storage tank is overheated – the regulator does not stop it, because it thinks the temperature in the storage tank is still low.
- The system never reaches the set maximum temperature – the pump runs continuously.
- Error code on the display for the sensor.
Diagnosis and solution of sensors
Sensors are mostly NTC thermistors with a value of 10 kΩ at 25 °C. Diagnosis is done with a multimeter: we disconnect the sensor from the regulator and measure the resistance at room temperature (approx. 20–25 °C). The correct value is around 10–12 kΩ. If the multimeter shows OL (break) or 0 Ω (short circuit), the sensor is damaged. We also check the conductor along its entire length – with long runs on the roof, cables are mechanically damaged, and rodent bites are not rare.
Be careful of one subtle phenomenon: the sensor may have the correct resistance at room temperature, but at high temperatures (150–200 °C on the collector during stagnation), its characteristic changes or the sensor burns out. In such a case, it must be replaced. Sensors are standardized, but always check compatibility with the given regulator – some regulators work only with their branded sensors due to the precision of the calibrated curve.
2. Pump does not start or runs continuously
This is the second most common issue I encounter. The pump is the heart of the solar circuit and its failure can have electrical or mechanical causes.
Pump does not start at all
Possible causes can vary: the controller has no power (blown fuse, switched off circuit breaker), the controller evaluates the delta T as too low (see sensor faults), or the pump itself is jammed – so-called "stuck" rotor. This happens after a longer system shutdown, for example after the winter break. The heat transfer fluid partially evaporates, impurities settle, and the rotor becomes blocked.
Solution for a jammed rotor: remove the pump cover (usually a central screw or a cover on the front), and rotate the shaft with a screwdriver until the rotor is freed. For most pumps in pump units – including the one contained in Solar Pump Unit ZP2-12 ECO – this approach is directly provided for by the design.
Another reason for not starting: incorrectly configured delta T start. If it is set too high (e.g. 15 K instead of the usual 6–8 K), the system waits for a difference that simply cannot be achieved in autumn sunshine. Check the settings in the controller menu – I recommend delta T start values of 6 K and delta T stop values of 2–3 K for most installations in Central European conditions.
Pump runs continuously without interruption
If the pump does not stop even at night, it is usually a sensor fault (false high temperature T1) or the controller output is permanently switched on due to an internal error. Check the temperature T1 on the display – if it shows unrealistic values at night, the sensor is faulty. Another possibility: the storage tank is cold and the controller correctly evaluates that heat needs to be "saved", but this is physically impossible at night – in this case, check whether the minimum tank temperature is set too low, to which the system is trying to charge even under unsuitable conditions.
Pump hums, vibrates, but does not rotate
This is a classic sign of a jammed or burned-out pump, or excessive pressure resistance in the circuit. Check whether all shut-off valves in the circuit are fully open – surprisingly often, someone forgets to open a ball valve during service. At the same time, check the pressure in the circuit using the manometer on the pump unit – if the pressure is too high or too low, the system is not functioning properly.
3. Controller faults – electronics and software
The controller is the brain of the entire system. Modern digital controllers are very reliable, but not indestructible. Typical reasons for their failure are overvoltage in the grid, lightning nearby, moisture entering the enclosure, or simply mechanical damage during installation or service.
Controller does not respond to inputs or has a frozen display
First step: disconnect the controller from power for 30 seconds and reconnect it. Most modern controllers (including Euroster 813 Solar) perform a self-test after restart. If the display starts up normally and the system works, it was a software "freeze" – this can happen due to interference in the grid or after a power outage. If the display remains dark or shows nonsensical characters after restart, the electronics are damaged and the controller needs to be replaced.
Controller displays correct values but does not activate the output
Check whether manual or service mode is active. Next, check the output relay – most controllers have an output terminal where you can verify with a multimeter (voltmeter) whether 230 V appears on it when the conditions are met. If not, the relay is damaged. If yes, the problem is in the wiring between the controller and the pump or in the pump itself.
Controller constantly restarts or blinks
Signs of unstable power supply: voltage fluctuations in the grid, poor neutral conductor, or a damaged power supply inside the controller. Check the power supply with a voltmeter directly at the controller terminals – it should be 230 V ±10 %. If the voltage is outside this range, the problem is in the electrical installation, not in the controller.
4. Faults in the pump unit as a whole
The pump unit is a compact device that integrates the pump, safety valve, manometer, ball valves, check valve, and sometimes even the controller. When we talk about faults in the entire unit, we must distinguish which component has failed.
Safety valve leaks or discharges fluid
The safety valve (usually set to 6 bar) opens when the pressure in the system exceeds the set value. If it leaks continuously, it may indicate: too high operating pressure (e.g. due to an under-pressurized expansion tank), a dirty or worn valve, or actual overheating of the system during stagnation. Do not replace the valve with one set to a higher pressure – address the cause, not the symptom. Check the pre-charge pressure of the expansion tank (the correct value is usually 1.0–1.5 bar, depending on the system height) with a pump via the Schrader valve.
Check valve does not function – night cooling of the storage tank
The return valve prevents reverse circulation at night when the collector is cold. If it is worn or dirty, the fluid flows back and the tank transfers heat to the collector. A sign of this is that the tank loses temperature even without consumption – for example, in the morning it is 5–8 °C cooler than in the evening, even though no hot water was consumed. Solution: cleaning or replacing the return valve.
Flow meter shows zero or incorrect value
Mechanical rotameters (floating ball indicator) become clogged with impurities from the heat transfer fluid, especially when glycol breaks down and forms deposits. Electronic flow meters, such as Electronic flow meter for GH 26, provide more accurate values and are less prone to clogging, but their sensor can be affected by the presence of air bubbles or magnetic impurities in the fluid. Always check the flow after every major system maintenance – the correct value depends on the size of the collector and pump (typically 40–70 l/h per m² of collector).
5. Air in the solar circuit
Air in the circuit is an especially tricky problem because it manifests in various ways and many installers diagnose it as the last option, although it should be the first. Symptoms of an air lock include a noisy pump (gurgling, bubbling sounds), fluctuating flow on the rotameter, low system performance under good sunlight, and pump overheating.
Deairing is done using an automatic air vent, which is part of most pump units. During commissioning, we must ensure that the air vent valve is open. If the automatic air vent is insufficient (in large systems), the circuit must be manually flushed – the pump is operated at a higher speed for several minutes until the sounds stop. More on this topic can be found in the article Maintenance and service of solar pump unit in our Knowledge Center.
6. Degradation of the heat transfer fluid
The heat transfer fluid based on propylene glycol has a lifespan of 4–6 years under normal conditions. When repeatedly overheated (stagnation), it degrades faster – its pH may change, the fluid darkens in color, and a characteristic unpleasant odor appears. Degraded fluid corrodes metal parts of the circuit, clogs the flow meter and valves, and reduces heat transfer efficiency.
Diagnosis: the pH of the fluid should be in the range of 7–9. Acidic fluid (pH below 7) is dangerous for copper and brass. Using a refractometer, we check the glycol content – frost protection should be at least down to –28 °C for Central European conditions, which corresponds to a concentration of approximately 35–40% glycol. If the refractometer shows a lower value or the fluid visually shows signs of degradation, it is time to replace the entire content of the circuit.
7. Specific faults in combined and multi-circuit systems
Modern solar systems often cover multiple storage tanks, combine solar heat with a boiler or heat pump, and have controllers with multiple outputs. Diagnostics become more complicated in these cases.
The unit Solar pump control unit ZPS 18e - 01 ECO is a typical example of a compact solution for more demanding configurations – it integrates the controller, pump, and other components into one unit. In case of faults in such systems, it is necessary to systematically go through each circuit: which tank is being heated and which is not, what are the temperatures at individual sensors, in what order the controller switches the outputs.
A typical error in two-tank systems: the priority tank (usually DHW) is set to a too high maximum temperature, so the solar system never switches to heating water – the secondary tank remains cold all summer. The solution is to correctly set the priorities and maximum temperatures in the controller menu.
MiniSOL and simpler systems
For smaller and simpler installations, there is a compact solution – Solar pump unit ALEX HX10 for MiniSOL control – where the control and pump are directly integrated and the settings are minimal. The same principles still apply here: sensors, pump, air in the circuit. The advantage is that fewer parameters mean fewer possible sources of incorrect settings.
8. Faults related to controller settings
Not every "fault" is a physical failure – very often the system is in good condition but incorrectly set. From experience, I know that this accounts for at least a third of service calls. The symptoms are identical to real faults: the system does not heat, runs unnecessarily, the tank overheats.
Most common incorrect settings:
- Delta T start too high (e.g. 15 K) – the system waits for a difference that is not achievable in autumn or spring conditions, even though it would be energy-efficient to start the pump. I recommend 6–8 K for most situations.
- Storage tank maximum temperature set too low – the system stops the pump at 55 °C, even though the tank could handle 80 °C and the collector has excess energy. A suitable maximum temperature for the TÚV storage tank is 70–75 °C.
- Collector maximum temperature set too low – the controller stops the pump using a protective function before it is actually necessary. Check whether the anti-stagnation function is activated correctly.
- Pump speed set to minimum – when manually setting the pump speed to level 1, the flow may be too low, leading to overheating of the heat transfer fluid in the supply pipe and premature degradation of the glycol.
- Backup heater function is active continuously – in combined systems, the backup heating by boiler is sometimes mistakenly turned on even on days when the solar system would be sufficient on its own. This means extra costs in gas or electricity.
More detailed settings are covered in our article How to set up a solar system controller for maximum efficiency – I recommend going through it again after every major system maintenance and verifying the parameters.
9. Seasonal and long-term phenomena – what to check annually
Some faults are not sudden, but develop over years. These include: gradual degradation of pipe insulation (especially on the roof, where UV radiation and temperature cycles damage the insulation cover), aging of seals in valves and pump, corrosion inside the collector due to insufficient fluid pH, and wear of the pump bearings.
An annual spring inspection should include:
- Visual inspection of all visible pipes, connections and insulation
- Pressure measurement in the cold phase of the system (early morning before the sun) – it should be 1.5–2 bar
- Check of the expansion tank pre-charge pressure (disconnect it from the system before measuring)
- Sampling of the heat transfer fluid for pH and glycol concentration check
- Check of sensor readings (compare T1 and T2 with a reference thermometer)
- Test pump start and check for noise, vibrations, and flow
- Verification of the controller settings
Frequently asked questions (FAQ)
Why does my solar system not heat the tank even on a bright summer day?
There may be several reasons: a faulty T1 sensor (the controller thinks the collector is cold), a jammed pump, air in the circuit preventing flow, or the tank is set to a too low maximum temperature and the controller considers it "done". I recommend starting diagnostics by checking the values on the controller display – T1 should show at least 60–80 °C on a summer midday, and significantly more in stagnation conditions.
The controller starts the pump at night – is this normal?
No, under normal conditions the pump should not run at night (except in the case of an active anti-frost function, if the system is configured that way). Nighttime pump operation typically indicates a faulty T1 sensor with a falsely high reading, or a stuck relay in the controller due to an internal fault. Check the T1 value on the display at night – if it shows, for example, 40–90 °C, the sensor is faulty.
The pump does not start after winter – what should I do?
This is almost always a jammed pump rotor. Procedure: turn off the power, unscrew the pump cover (front side, usually a plastic cover with a slot for a flathead screwdriver), and turn the shaft until the rotor is freed. Then close the cover and turn the system back on. If the pump still does not work after freeing the rotor, check the power output of the controller and the pump windings with a multimeter.
The safety valve drips every afternoon – is this a problem?
Yes, this is not a normal condition. The safety valve should only open in real emergency situations (pressure over 6 bar). Regular dripping means that the operating pressure at the maximum fluid temperature exceeds the valve's set value. Most common cause: an undercharged or damaged expansion tank. Check the pre-charge pressure of the expansion tank – if it is below 1 bar, add air to the correct value (usually 1.0–1.5 bar). If the expansion tank maintains the pre-charge pressure but the problem persists, the tank is likely mechanically damaged (cracked membrane) and needs to be replaced.
Sensors are working, the pump is running, but the tank is not heating – what could be the cause?
If everything is working electronically but the performance is zero or minimal, we are looking for mechanical problems: a closed ball valve somewhere in the line (check every valve), air in the circuit (check flow on the rotameter – if it shows zero or fluctuates, there is air), or a clogged filter in the pump unit. In older systems, the problem could also be in the tank heat exchanger – scaling on the hot water side rapidly reduces heat transfer. In this case, the heat exchanger needs to be chemically cleaned.
How long does the heat transfer fluid last and when should it be replaced?
A standard solar heat transfer fluid based on propylene glycol has a lifespan of 4–6 years under normal operating conditions. If the system repeatedly stagnates (the collector reaches temperatures above 150 °C without circulation), the fluid degrades much faster – sometimes already after 2 years. Indicators for replacement: dark color of the fluid, pH below 7 (measured with test strips or a pH meter), reduced refractometric value (glycol concentration below 30 %), or visible deposits in the system. Never delay the replacement – degraded fluid corrodes copper, brass and steel parts in the circuit and regeneration is then much more expensive.
Conclusion – systematic diagnostics saves time and money
Failures of solar controllers and pump units are mostly solvable by oneself if approached systematically. The key is the order: first verify the power and values on the display, then check the sensors, then the pump and its physical condition, and finally the valves, pressure and fluid. Most of the really tricky faults I have encountered had a simple cause – a damaged sensor, a jammed rotor or a wrongly set delta T. More complicated diagnostics only come into play when these basic points are in order and the system still does not work properly.
For more information on choosing the right controller and pump unit, I recommend our articles How to choose a control unit for a solar system and Difference between a solar pump unit and a separate controller. If you are dealing with a specific system and are unsure, I can also help with the parameters from the article Common questions about solar system control units.
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