Common faults of solar regulators: error messages, pump failures and inaccurate sensors

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Common faults of solar controllers: error messages, pump failures and inaccurate sensors

The solar controller is the heart of the entire thermal solar system. When it works properly, you won't even notice it – the pump switches on precisely when needed, the water heater is heated without unnecessary losses and the temperatures in the system are under control. However, when something goes wrong, the system will stop working efficiently or stop completely. Customers usually discover the first problem only when the water in the tank stops getting warm in summer, or when the pump starts in the middle of the night for no obvious reason.

In this article, we will look at the most common faults that we encounter in practice with solar controllers – error codes, non-functional pumps, bad sensors, software errors in the settings and mechanical failures. We will go through each of them in detail so that you know what to look for, how to diagnose the problem and when the situation can be solved by yourself and when you need to call a professional.

If you are still in the selection phase of the controller, we recommend reading the article How to choose a controller for a solar system: what to pay attention to or Euroster 813 Solar vs. ZPS control stations: comparison of functions and use. If you already have a system and are dealing with a specific fault, keep reading.

How a solar controller "knows" that something is not working

Modern solar controllers – whether it is a simple differential thermostat such as Euroster 813 Solar, or full-fledged control stations such as ZPS 6, ZPS 16 or ZPS 28 – continuously monitor the temperatures on the sensors, the state of the pump, and possibly the flow and pressure in the circuit. When they measure a value outside the expected range or detect a logical error (e.g. the collector sensor shows a lower temperature than the tank sensor in a situation where the pump has been running continuously for an hour), they evaluate it as an error and display an error code.

Error messages are therefore not only about the failure of the pump or the controller itself – they usually indicate a problem somewhere in the system, which the controller only detects. Therefore, it is important not to clear the error code without first finding out the cause.

Most common error codes and their meaning

Each manufacturer has its own system of error codes, so the first step is always to reach for the operating manual of the specific controller. Despite this, there are groups of errors that are almost universal:

E1 / Err1 / SC – short circuit or break in the sensor

This is by far the most common error code in practice. The controller displays it when it measures a resistance value on one of the temperature sensor inputs that is outside the valid range for an NTC sensor (usually 10 kΩ at 25 °C). Specific causes:

Broken wire – the sensor cable was mechanically damaged, cut during installation, or the insulation cracked due to thermal stress on the collector. The resistance is infinitely high, the controller evaluates it as "open circuit".
Short circuit in the wire – the two wires are touching, usually at a place where the cable passes through a sharp edge, or moisture has entered the connector. The resistance is close to zero, the controller reports "short circuit".
Loosely inserted sensor into the terminal block – the contact is there, but it has a high transition resistance, which is manifested as random error messages that change with temperature.
Damaged NTC thermistor itself – after years of operation at high temperatures (collectors can reach 180–220 °C in summer stagnation) the thermistor degrades and measures inaccurately, or its internal connection breaks.

Diagnostic procedure: Disconnect the sensor cable from the controller and measure the resistance directly at the connectors. If you have a 10k NTC, at room temperature (approx. 20–25 °C) the meter should show approx. 10–12 kΩ. If you see OL (open loop) or 0 Ω, the problem is clear. Then try the same thing directly at the collector cable connector – if the measurement is correct there, the problem is in the cable between the collector and the controller.

E2 / Err2 – collector temperature too high

The controller has registered a temperature above the maximum configurable limit – typically above 130–150 °C, sometimes up to 200 °C depending on the model. In practice, this occurs:

Long-term system stagnation in summer under full sun and a full tank – the collectors overheat because there is nowhere to transfer the heat.
The pump has stopped (pump failure, power outage) and the collector was in the sun – the temperature rises quickly.
The system lost the antifreeze mixture (leak, incorrect topping up with pure water) and the liquid is evaporating or boiling.

This error is mostly a safety mechanism – the controller stops the pump to prevent cold liquid from entering the hot heat exchanger and causing a thermal shock. After the collector cools down, the error should disappear on its own. If it repeats every morning after a sunny night, the system sizing should be reconsidered or the tank cooling function should be set.

E3 / Err3 – tank temperature too high (boiler protection)

If the tank reaches the set maximum temperature (usually 70–80 °C, sometimes adjustable up to 95 °C), the controller stops the pump regardless of the temperature difference. This is normal and no real fault. A problem arises when:

The temperature is set too low (e.g. 55 °C) and the tank is "filling" quickly, the pump runs for a minimum of time.
The tank sensor is placed at the wrong height – too low or too high, it does not represent the actual average water temperature.

E4 / Flow error – flow error

This error occurs only in controllers that have an input for a flow sensor (flowmeter). The controller decided to start the pump, but the flow did not appear within the expected time. Causes: clogged filter in the solar circuit, air in the system (air pocket blocks the flow), pump failure, closed shut-off valve in the circuit.

Pump failures – causes and diagnostics

Solar pump failure is the second most common reason why the system stops working. The pump does not belong to the controller itself, but the controller controls it – and that is why many customers mistakenly diagnose the problem on the controller side, while the actual issue is directly in the pump or its power supply.

The pump does not run at all – not even during testing

Most controllers (including ZPS stations) have a manual pump test function – it can be started directly from the menu. If the pump does not run even during manual start, the problem is almost certainly:

No power to the controller – check if the display is lit. If not, check the fuses, circuit breaker, and 230 V supply.
Controller output failure for the pump – the controller has an internal relay or triac that switches the pump. These components can fail, especially if the system has been exposed to long-term voltage spikes. To measure: use a voltmeter to check the voltage at the pump output terminals during a manual test – it should be 230 V. If yes, the problem is in the pump. If not, the problem is in the controller.
The pump is mechanically blocked – after a winter break or after a long period of inactivity, the pump rotor can seize due to corrosion or deposits. Most circulation pumps have a bleed screw on the front side, behind which is the shaft – turn it with a screwdriver (carefully, the medium is hot).
Air in the pump – the pump runs but does not pump, as it is filled with air instead of liquid. Bleed it – on Grundfos, Wilo and similar pumps, the bleed screw is directly on the body.

The pump runs, but the system does not work

This is a more subtle case – the controller reports "OK", the pump hums, but the tank temperature does not rise. Possible causes:

The pump runs "dry" – there is an air lock in the circuit, it spins, but the actual flow is zero or minimal.
The solar fluid has leaked and the circuit is filled with air.
The check valve is blocked in the closed position – common after years of operation.
The solar circuit filter is clogged with sediment (especially if the system has not been regularly inspected and the antifreeze mixture has decomposed into acidic compounds that attack copper and aluminum).
The expansion tank has lost pressure – the system does not have the correct working pressure, the pump cannot overcome the hydrostatic resistance of the circuit.

The pump switches unnecessarily – too short cycles or night operation

Customers occasionally report that the pump turns on for a few seconds and immediately stops, and this repeats in rapid cycles. The cause is almost always an incorrectly set differential temperature (ΔT for turning on and ΔT for turning off) – the collector and tank sensors are too close in values, the controller jumps around the hysteresis. This setting topic is discussed in detail in the article Setting the differential temperature in a solar controller: how to correctly configure switching.

Night operation of the pump is a different issue – usually caused by the fact that the sensor on the collector is incorrectly placed (not on the absorber, but on the pipe connection) and at night it transfers heat from the pipe, misleading the controller into thinking the collectors are warm.

Inaccurate or faulty sensors – how to identify them

Temperature sensors for solar regulation are almost exclusively NTC thermistors with a resistance of 10 kΩ at 25 °C. They are cheap, reliable and durable, but not immortal. In practice, there are three scenarios: the sensor is completely damaged (the regulator reports an error), the sensor measures inaccurately (the regulator works, but inefficiently), or the sensor is correct, but improperly placed.

The sensor measures inaccurately – a sneaky problem

A deviation of 5–10 °C on the collector sensor may mean that the pump starts too late (loss of solar yield) or too early (night cooling). A deviation on the tank sensor leads to the system heating the tank to different values than expected.

Signs of an inaccurate sensor without an error code:

The tank is warm in summer even after a long sunny day.
The regulator shows a collector temperature of 40 °C in the morning, although it is clear outside and the sun has been shining for 2 hours – this is an unrealistically low value.
The regulator shows a tank temperature of 68 °C, but the water from the tap is barely warm – the sensor is in the wrong position, measuring a dry layer of insulation, not the water.

Checking the accuracy of the sensor: Measure the resistance of the sensor at a known temperature. One of the simplest methods – place the sensor in a glass of water together with a thermometer. If the thermometer shows 40 °C, an NTC 10k should have a resistance of approximately 5.3 kΩ (at 50 °C approximately 3.6 kΩ, at 60 °C approximately 2.5 kΩ). A deviation of up to ±3 % is normal; a larger deviation means the sensor needs to be replaced.

Characteristic value table for NTC 10k

Temperature (°C)	Resistance NTC 10k (kΩ)
0	32.7
10	19.9
20	12.5
25	10.0
40	5.3
60	2.5
80	1.3
100	0.68
120	0.38

Sensor placement – source of many mysteries

In practice, we often see situations where the entire regulation works technically correctly, but the system performs poorly because the sensor is not where it should be. The collector sensor should be placed on the collector absorber (directly on the metal, not on the connecting pipe outside), and the tank sensor should be placed in the lower third of the tank (measuring the temperature to which it needs to be heated, not the temperature of the top hot layer). If the tank has two inlets (for solar and for an additional source), the solar sensor must be placed below the solar heat exchanger, not above it.

More about correct placement and wiring can be found in the article Installation of solar system regulation: pump, sensor and expansion tank wiring.

Software and configuration errors in regulator settings

Not every problem is mechanical or electrical – a significant portion of solar system faults in practice is caused by incorrect regulator parameter settings. A list of the most common configuration errors:

Too small or too large differential temperature ΔT

Differential temperature ΔT is the difference between the collector temperature and the tank temperature at which the pump starts. If ΔT is set too low (e.g. 3 °C), the pump starts even when the collector has brought minimal heat and the system runs almost without gain. If it is set too high (e.g. 15 °C), the pump starts too late and you lose morning and evening solar gains.

Recommended values for most systems: ΔT on 6–8 °C, ΔT off 3–4 °C. Hysteresis between on and off should be at least 2–3 °C, otherwise the pump will rapidly cycle.

Maximum tank temperature set too low

If you limit the maximum tank temperature to 55 °C due to legionella (which is a correct idea), but forget to activate the thermal disinfection cycle function, the system works safely, but solar yields in summer are limited – the tank fills up quickly and the system stagnates. The solution is to set the max temperature to 70–75 °C and activate the anti-legionella cycle at 60 °C with weekly frequency.

Incorrect sensor assignment in the menu

With regulators that have multiple inputs for sensors (which is the case for all ZPS control units), it can happen that during configuration the tank sensor was mistakenly assigned to the collector input and vice versa. Result: the regulator switches the pump exactly the opposite of what it should. Symptoms: the tank does not cool down, the pump runs at night, the system is stopped during the day. Solution: check the input configuration in the menu.

Tank cooling function (night cooling) incorrectly enabled

Some regulators, including ZPS control units, have a tank cooling function at night – the pump transfers heat from the tank to the cooler collector at night. This function is useful in summer when the tank has reached maximum temperature and you want to cool it to a safer temperature for handling. If it is mistakenly enabled in a system where you do not want it, the tank will cool down at night and you will lose the heat collected during the day.

Electrical faults and interference – what you can't see, but still damages

Solar regulators are mostly electronic devices powered from the 230 V grid, but they switch low-current sensor signals in the range of mV to units of V. Sensor cabling should therefore never be run in the same bundle as power cables (230 V, phase shifter, pump cable). In practice, however, installations are not always ideal, and the result is interference – the regulator displays fluctuating temperatures, random error codes, or restarts.

Recommendation: run sensor cables separately from power cables, ideally in a separate conduit or pipe. For long runs (over 10 meters), use shielded cable and ground the shielding only on one side (at the regulator side). At points where the sensor cable runs next to a frequency inverter (e.g., at solar pumps with speed control), shielding is absolutely necessary.

Overvoltage spikes from the electrical grid are another source of damage – especially during thunderstorms. The regulator should be connected via overvoltage protection class 3. If the regulator repeatedly fails without an obvious mechanical cause, always check the condition of the grid and consider installing overvoltage protection.

Concrete examples of faults from practice

Case 1: Cold tank in August – collector shows 22 °C

The customer reported that the solar system seemed to be "asleep" – in summer, the tank temperature was always around 35 °C, and the collector showed only 22–28 °C even at noon. Diagnosis: the collector sensor cable was run in a box on the roof without protection during installation, and in summer, heat accumulated there and the insulation cracked – a partial short occurred via moisture. Resistance measurement showed 18 kΩ at an external temperature of 28 °C, which corresponds to 8 °C instead of the actual 100 °C of the collector. Solution: a new cable in a thermally insulated strip with the correct temperature resistance (min. 150 °C).

Case 2: Pump runs all night – electricity bill is double

The customer noticed that the pump was running continuously – even at night and on rainy days. The regulator showed no error. Cause: the collector sensor was not inserted into the housing on the absorber, but a careless installer had just glued it to the outer pipe of the connecting pipe running through the insulated roof. At night, that pipe was much warmer (about 35 °C) than the tank (about 25 °C), so the regulator detected an active temperature difference and started the pump. Solution: correct placement of the sensor into the housing on the collector absorber.

Case 3: Control station ZPS 16 reports E1 after every rain

This case was tricky – the error code E1 appeared every time after rain and disappeared after drying. Cause: moisture penetrated the cable connector on the roof during rain (unprotected screw terminal without sealing). Moisture changed the sensor resistance and after drying, the cable returned to normal. Solution: replacement of the connector with heat-shrink tubing with adhesive and sealing the entire area with self-adhesive tape.

Case 4: The system works, but the yield is 30 % lower than the neighbor's

The customer had the same system as the neighbor, but the tank temperature was always lower every day. After inspection, it turned out that the ΔT shutdown was set to 8 °C (same as ΔT activation) – in reality, there was no hysteresis. The pump was switching on for a few seconds and immediately stopping, the real flow was minimal. Lowering the ΔT shutdown to 3 °C immediately improved the situation.

When to fix the fault yourself and when to call a technician

Some tasks are within the competence of an ordinary homeowner, while others require professional skills – especially work with 230 V power lines, refilling the solar circuit, and setting the pressure in the expansion tank.

You can fix it yourself: checking and resetting the error code, measuring the sensor resistance with a multimeter, checking the cabling and connectors, bleeding the pump, checking the parameter settings in the regulator menu, and manually testing the pump.

Call a technician: if error code E1 persists after replacing the sensor (there may be an internal regulator fault), if the pump does not start even with 230 V at the terminals (faulty motor), if the system pressure repeatedly drops (leak), if the antifreeze has a bad color (oxidation) or pH below 7 (glycol breakdown – the entire filling must be replaced and the system flushed).

If you are considering replacing the regulator with a newer model, take a look at the comparison of Euroster 813 Solar and control stations – depending on the number of collectors and system complexity, you can choose ZPS 6 for smaller installations, ZPS 16 for medium-sized, or ZPS 28 for larger and commercial systems. A detailed comparison of performance and the number of collectors can be found in the article How many collectors can the ZPS 6, ZPS 16, and ZPS 28 control stations handle?.

Preventive maintenance – preventing faults

The vast majority of the faults we described would not occur at all if the system underwent regular annual inspection. At least once a year – ideally in spring before the season – you should:

Check the pressure in the solar circuit (should be 1.5–3 bar cold, depending on the height of the collectors). If it has dropped below 1 bar, top up the antifreeze and find the cause of the leak.
Check the pH and density of the antifreeze (using a refractometer or pH paper). Glycol should have a pH of 7–8.5; below 7 is acidic and damages the system.
Verify the accuracy of the sensors (resistance measurement).
Check the physical condition of the sensor cables on the roof and in the boiler room.
Test the manual pump operation from the regulator menu.
Clean the solar circuit filter (if installed).
Check the pressure in the expansion tank (should be the same as the static pressure of the cold filled system, usually 1–1.2 bar).

More about the overall inspection and maintenance can be found in the article Maintenance and inspection of solar regulators: what to check before and after the heating season.

Most frequently asked questions (FAQ)

What does it mean when the regulator displays "– –" or "????" instead of temperature?

This display usually means that the regulator has no valid data from the sensor – the sensor is either disconnected or measuring out of range (short or broken). Check the cabling and measure the sensor resistance with a multimeter. If the cable and sensor are in good condition, the problem may be in the input module of the regulator (internal fault).

Can I replace the collector sensor with any NTC 10k, or must it be original?

For most common solar regulators, a standard NTC 10k with a B-constant around 3977 K is compatible with all common regulators. More important than the brand is the temperature resistance: the collector sensor must withstand at least 200–220 °C (stagnation). Common NTC sensors in plastic housings are not sufficient – use a type with a metal body and silicone-insulated cable with a minimum resistance of 180 °C.

The pump switches on every minute for 5 seconds – is it a regulator fault?

Probably not. This symptom (so-called short cycling) is almost always the result of incorrect ΔT settings – the hysteresis between the activation and deactivation values is too small. Check the settings: the ΔT activation must be at least 2–3 °C higher than the ΔT deactivation. If ΔT activation = 5 °C and ΔT deactivation = 5 °C, the pump will cycle. Set ΔT deactivation to 2–3 °C.

The regulator shows a collector temperature of 180 °C – is that possible, or is the sensor faulty?

It is realistically possible during system stagnation – collectors without flow in summer reach stagnation temperatures of 180–230 °C. If this is combined with an error code (e.g., E2), the regulator correctly stops the pump for safety reasons. If the temperature of 180 °C is displayed at night or during rainy weather without sunlight, the sensor is likely damaged (shorted) or mixed with another input.

The regulator works normally, but the system slowly cools down during the night in winter – is that normal?

Yes, partially normal – at night, the collector radiates heat to the cold sky (radiative cooling). If the regulator does not have a function for protection against nighttime backflow (so-called anti-freeze circulation or legionella lock), some tank cooling is normal. If the loss exceeds 5–8 °C per night, check if you have the tank cooling function activated – it should be turned off in winter.

Can I connect two storage tanks to one controller?

It depends on the controller model. A simple differential thermostat (Euroster 813 Solar) controls one solar circuit and one storage tank. ZPS control stations have multiple outputs and can control a switching valve between two storage tanks – this depends on the specific model and its software functions. More details can be found in the article How to connect a solar system controller with a water heater or TÚV tank.

Conclusion

A solar controller is a relatively simple system – but if something stops working, the causes can be various and interwoven. The most important lesson from practice: never reset an error code without first understanding its cause. Resetting the code will clear the symbols from the display, but the cause remains and usually triggers the same error again – or worse.

Systematic diagnostics – from checking the power supply through measuring sensors to inspecting the pump mechanics and parameter settings – usually reveals the problem within an hour, even without special tools. A multimeter, a temperature sensor (digital thermometer), and the operating manual for the specific controller are all you need.

If after reading this article you are still unsure about what you are dealing with, take a look at other topics in the Knowledge Centre – for example, Setting the differential temperature in a solar controller: how to correctly configure switching or How to choose a controller for a solar system: differential thermostat vs. control station. A properly configured and correctly set controller ensures that your solar system will produce the maximum usable heat throughout its entire lifetime.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.