Frequently asked questions about solar system control units

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Frequently asked questions about solar system controllers – a comprehensive guide for installers and owners

If you are active in the field of solar technology – whether as a homeowner who wants to understand their system or as an installer who deals with jobs daily – you probably know that the controller is the heart of the entire solar system. At the same time, it is also the component around which the most questions, misunderstandings, and sometimes installation or setup errors revolve. In practice, we see that people invest thousands of euros in collectors and storage tanks, but they look at the controller as a secondary "electronics" – and this is often the source of problems.

This article collects truly common questions that repeat during the sale, installation, and service of controllers. This is not a marketing overview, but rather factual answers with technical depth that a customer or technician really needs. The topics are arranged from basic concepts to more advanced scenarios from practice.

What does the controller actually do – and what doesn't it do?

This is the first question that everyone asks when they encounter the topic for the first time. The solar system controller – technically also called a regulator, differential regulator, or solar automation – basically has one main task: to decide when to start the circulation pump. It does this by comparing temperatures at predefined locations in the system.

The most common principle: the regulator measures the temperature on the collector (using sensor T1) and the temperature in the heat storage tank (sensor T2). If the temperature of the collector is higher by a defined difference (so-called hysteresis – typically 5–8 °C), the regulator turns on the pump and starts pumping the heat transfer medium. When the difference decreases (typically 2–4 °C), the pump is turned off. This is the basic differential algorithm found in every regulator – from the simplest to the most complex ones.

What the regulator does not do: it does not heat the water by itself, it is not a source of heat, it does not replace the backup heating, and it does not control the boiler (unless it is specifically designed for that). This basic definition is important because many customers think that if they buy a "better regulator," they will have more hot water – which is not true. The performance of the system mainly depends on the area and orientation of the collectors, the volume of the storage tank, and the local climatic conditions.

What is the difference between a regulator and a pump unit?

This question is among the most common and at the same time the most important. Many customers confuse these two terms or assume they are the same. They are not.

A standalone regulator (control unit) is an electronic device that measures temperatures, evaluates conditions, and sends control signals to the pump. It does not contain a pump or hydraulics by itself. It is the "brain" of the system.

A solar pump unit is a hydraulic block that integrates a circulation pump, ball valves, check valve, pressure relief valve, flow meter, and – in more complex versions – also control electronics in one compact body. Such a unit is therefore the "brain and heart" combined.

Practical example: if you buy Solar Pump Control Unit ZPS 18e - 01 ECO, you get a compact solution with integrated electronics – you do not need to buy a separate regulator. On the other hand, if you buy only Euroster 813 Solar, you get only the regulator, which you have to connect to an external pump and hydraulic components.

A more detailed comparison can be found in the article Difference between a solar pump unit and a standalone regulator in this Knowledge Center.

How many temperature sensors does the regulator need and where are they mounted?

The number and placement of sensors depends on the system layout and the functions of the regulator. In practice, we encounter several typical scenarios:

Basic system (1 collector, 1 storage tank): 2 sensors are sufficient – T1 on the collector (or on the collector's outlet pipe, so-called flow), T2 in the upper third of the storage tank.
System with two storage tanks: 3 sensors are needed – T1 on the collector, T2 in the first storage tank, T3 in the second storage tank. The regulator then prioritizes filling the first tank and switches to the second one after it is heated.
System with a swimming pool: 3 sensors – T1 on the collector, T2 in the domestic hot water storage tank, T3 in the pool or on the pool heat exchanger.
Two-circuit systems (combined with a boiler): up to 4–6 sensors including the boiler circuit, storage tank, and collector sensors.

It is very important to place the T1 sensor on the collector. It should be inserted into a special sensor hole in the collector (so-called "sensor pocket"), not just placed on the surface. If placed incorrectly, the measured values may be 10–15 °C lower, which can cause the system to not start the pump even when it should.

The T2 sensor in the storage tank should be placed in the lower third of the tank (not at the bottom, not at the top), so the regulator "sees" the actual temperature of the cold inlet and correctly evaluates whether it makes sense to pump heat from the collector. If the sensor is placed too high (in the upper, hottest part), the system will almost shut off because the warm layer rises upwards – the tank may appear to be "heated," even though the water at the bottom is still cold.

What does hysteresis mean and how to set it up?

Hysteresis is a parameter that directly affects the efficiency and lifespan of a solar system. It refers to the temperature difference between the conditions for turning the pump on and off.

Concrete example: you set ΔT on = 8 °C and ΔT off = 3 °C. This means the pump will turn on when the collector is 8 °C warmer than the tank, and it will turn off when this difference decreases to 3 °C. The difference between these two values is exactly the hysteresis (5 °C in this case).

Why is this important? If you set too small a hysteresis (e.g., ΔT on = 5 °C, ΔT off = 4 °C), the pump will switch on and off very frequently – so-called "cycling." Each pump start is a strain on the electronics and motor, and also causes heat losses with each new cycle. On the other hand, too large a hysteresis (e.g., ΔT on = 15 °C) causes the system to react late and lose usable solar heat during early morning and evening hours.

From practical experience, we recommend for standard systems: ΔT ON = 6–8 °C, ΔT OFF = 2–4 °C. For long pipe runs (more than 15 m), increase ΔT ON to 9–10 °C, because the cold medium in the pipe "cools" the collector at start-up, and the controller would otherwise immediately turn off the pump.

Why does the controller not turn on the pump even though the sun is shining?

This question is at the top of the service call list. A customer calls in frustrated – "the sun is shining, the collector is hot, but the pump is not running." There can be several causes, which need to be systematically ruled out:

The tank has reached the maximum temperature: Most controllers have a maximum tank temperature (TMax) set, after which the pump stops even if the collector is hot. Typically, this is 60–75 °C. Check the TMax setting and the actual tank temperature.
Collector sensor T1 failure: If the sensor has come loose from the collector, it measures ambient temperature instead of the collector temperature. We recommend physically checking whether the sensor is properly inserted into the socket and secured.
Broken terminal or damaged sensor cable: A digital controller will display an error code (e.g., "E1" or "---") when the sensor is broken. Check the sensor resistance – for a standard NTC 10K, it should be approximately 10,000 ohms at 20 °C.
The pump is jammed after a summer break: Circulation pumps that have been idle for several months can have a jammed rotor. Unscrew the access plug and turn the rotor with a screwdriver – the problem usually disappears immediately.
Incorrectly set ΔT: If you have ΔT ON = 20 °C and the tank is at 55 °C, the pump will only start when the collector reaches 75 °C – which may not happen on a cloudy day.

What is the function of tank overheating protection (cooling function)?

Solar systems can reach temperatures above 90 °C in the tank during summer, which is dangerous in terms of shower comfort (discoloration), as well as for the tank technology and safety valves. The "cooling" function (tank cooling) is a special operating mode in which the controller starts the pump at night in the opposite direction – heat from the tank is removed back to the collector, where it is radiated into the environment.

It works like this: when the tank temperature exceeds the set safety value (e.g., 85 °C), the controller at night (when the collector is cooler than the tank) reverses the logic – the pump starts working and heat is removed from the hot tank through the collector, which at night functions as a radiator. In the morning, the tank is 5–10 °C cooler and the system can again normally collect solar energy.

This function is standard in most modern controllers, but it must be properly activated and set. If it is not enabled and the tank overheats in summer, the safety valve may activate and cause a leak of the heat transfer medium.

What is the difference between ECO and standard pump units?

The ECO designation for solar pump units mainly refers to the type of circulation pump. Standard pumps have fixed speeds (or a few manually selectable levels), while ECO pumps – technically known as electronically commutated motors (EC motors) – regulate the speed smoothly and automatically according to the system's needs.

The advantage of ECO pumps is significantly lower electricity consumption. A standard pump with fixed speeds consumes 40–80 W, while an EC pump consumes 5–25 W at the same hydraulic performance. Over a year of operation (approximately 2,000–2,500 hours of annual operation), this can be a difference of 50–100 kWh, which at an electricity price of 0.20 €/kWh represents a saving of 10–20 € per year. Not an astronomical amount, but over 10 years of operation it adds up.

Moreover, ECO pumps are quieter and have a longer lifespan due to lower thermal stress on the electronics. For example, the Solar pump unit ZP2-12 ECO achieves only a few watts of power consumption during normal operation thanks to this solution. A detailed comparison can also be found in the article Solar pump unit ECO vs. standard – what is the difference and when is it worth it.

How does the flow meter in the control unit work and why do I need it?

The flow meter measures how many liters of heat transfer medium flow through the solar circuit per hour (or per minute). Together with the temperatures at the collector inlet and outlet, it allows the controller to calculate the current thermal power of the system in kW and the total energy gained in kWh since installation.

This is not just a "decorative" function. Monitoring flow and power allows you to:

Verify the correct system setup – the optimal flow for flat collectors is usually 30–50 l/h per m² of collector (lower flow means higher thermal efficiency, but worse heat transfer capacity).
Detect partial clogging of the filter or system – if the flow drops significantly compared to usual values, it indicates a problem in the hydraulics.
Calculate the energy balance and verify the investment return.

Modern units such as the Electronic flow meter for GH 26 digitally integrate the measured values and communicate with the controller. Analog flow meters (rotameter with a floating ball) are cheaper, but do not allow data recording. More about connecting the flow meter can be found in the article Electronic flow meter in a solar system – what it is for and how to connect it.

Can I connect multiple collectors or multiple tanks to the controller?

This depends on the specific controller model and its features. In practice, we encounter several situations:

Multiple collectors in parallel: If you connect multiple collectors into one field (in parallel), the controller "sees" it as one heat source – there is only one sensor T1 on the collector (usually in the last collector in the series, where the highest temperature is). The controller does not need to know how many collectors are connected, it only needs the temperature at the field's outlet. This is problem-free for all controllers.

Two storage tanks (priority charging): For this, you need a controller with the function "2 storage tanks" or "dual-storage system". The controller controls a switching valve (three-way motor valve), which alternately directs the flow of the medium to the first or second storage tank. The priority can be set – typically, the TÚV (domestic hot water) tank is heated first, and when it reaches the desired temperature, it switches to the floor heating or pool tank.

Multi-loop systems: For more complex schemes (e.g., collectors + storage tank + pool + solar floor heating), a more advanced controller with multiple outputs and sensors is required. We recommend consulting with a system designer, as the hydraulic scheme must be properly designed.

A detailed guide on selecting a controller based on the number of collectors and storage tanks can be found in the article What controller do I need for solar collectors – selection based on the number of collectors and storage tanks in this Knowledge Center.

The controller shows an error code – what does it mean?

Each manufacturer has its own set of error codes, but some types of faults are common to all systems. The most common error conditions and their causes:

E1 / Sensor open (open circuit): Broken cable or disconnected sensor. Check the terminals and the integrity of the cable. Measure the resistance of the sensor – a broken sensor will show OL (out of range).
E2 / Sensor shorted (short circuit): The sensor cable has a short (e.g., gnawed by a rodent, crushed in a door). A shorted NTC sensor will have almost 0 ohms resistance.
Pump is running, but no flow: Jammed pump, closed valve, air in the system, or clogged filter. Check the manual air vent.
Fluctuating temperatures on the display: Unstable sensor contact or electromagnetic interference (e.g., proximity of high-current cables to the sensor).
System turns on and immediately off: Too small a hysteresis ΔT OFF, or air in the system causing temperature fluctuations.

More detailed troubleshooting solutions can be found in the article Common faults of solar controllers and pump units directly in this Knowledge Center.

How to set the maximum collector temperature (Tmax collector function)?

Most controllers allow you to set the maximum collector temperature – when this temperature is reached, the controller turns off the pump to prevent overheating of the heat transfer medium and damage to the collector. It is typically set to 110–130 °C for vacuum collectors, while for flat collectors, 95–110 °C is sufficient.

Why not leave it at maximum? At high collector temperatures (over 150 °C, which can occur in summer during stagnation), there is a risk of glycol medium decomposition, pressure loss in the system, and in the worst case, damage to seals. The controller should be set so that the system is turned off before a critical condition occurs.

Systems with vacuum tube collectors are more sensitive to overheating than flat ones, as during stagnation they can reach temperatures over 200 °C inside the tube. Therefore, for vacuum collectors, we also recommend mechanical measures – shading in summer, dimensioning the expansion tank with a larger reserve, and using a solar medium with a higher boiling point (e.g., propylene glycol 50 %).

Is it necessary to set the time and date on the controller?

For basic differential controllers (only comparing T1 and T2), time is not needed – the system is controlled exclusively by temperatures and not by time. For controllers with more advanced functions, such as night cooling time blocking, backup heating source activation during specific hours, or data logging with time stamps, setting the time and date is important and affects the correct function.

In practice, we often encounter situations where the customer activates the "cooling" function (night heat dissipation through the collector), but has an incorrect time setting – and the controller starts cooling during the day, when the collector is illuminated, which is exactly the opposite of what it should do. Therefore, after every replacement of the backup battery or after a long power outage, always check the time setting.

Do I need to calibrate the temperature sensors after installation?

Common NTC sensors (thermistors) used in solar controllers have a production tolerance of ±1–2 °C. For most applications, this is sufficient and calibration is not necessary. Some advanced controllers allow an offset – manual correction of the displayed value by a defined number of degrees, which you can use if you have a reference thermometer and detect a systematic deviation.

More important than calibration is the correct installation of the sensor: secure mechanical mounting in the well, thermal insulation of the cable at the contact point (to prevent ambient temperature from being transferred inside), and for external sensors (on the collector), a cable design resistant to UV radiation and heat.

What is the lifespan of the control unit and when to replace it?

The electronics of solar controllers have a lifespan of 10–20 years under normal operation. The most common reason for replacement is not a fault in the controller itself, but:

End of production of spare parts or sensors – after 15+ years, it may be difficult to find a compatible sensor or display.
Need for modernization – an older controller may not support EC pumps, WiFi monitoring, or other new features.
Physical damage – most commonly moisture in the distribution box or overvoltage during a storm (we recommend protecting controllers with surge protection).

Circulation pumps typically have a service life of 8–15 years under solar conditions (high medium temperature, glycol mixtures). A characteristic sign of a failing pump is increased noise or a drop in flow at the same settings. Regular annual inspection and maintenance of the system (flushing, pressure check, visual inspection) significantly extends its lifespan – for more on this topic, see the article Maintenance and service of a solar pump unit.

How to optimally set up the system for maximum energy savings?

This is a topic we elaborate on in detail in the article How to set up a solar system controller for maximum efficiency, but briefly: the most important parameters are the correct ΔT ON/OFF, the correct position of sensors, the correct flow (pump power setting), and activation of protective functions.

A practical tip: many installers set the pump to maximum power (3rd stage) – they believe that "the faster the medium flows, the more heat it transfers." This is not always true. With excessively high flow, the temperatures at the collector inlet and outlet are almost the same (small ΔT), which means low thermal efficiency of the collector. The optimal flow for most flat plate collectors is 30–50 l/h per m² – for a 4 m² collector, this is 120–200 l/h. Set the pump so that the ΔT between the collector outlet and inlet is 8–12 °C on a typical sunny day.

For more advanced settings, such as the system Solar pump unit ALEX HX10 for MiniSOL regulation, where flow and temperature regulation is automated, the optimization parameters are usually pre-installed by the manufacturer and only require basic input of the collector type and storage tank volume.

Frequently asked questions (FAQ)

Can I connect the solar system controller myself without an electrician?

This depends on the electrical supply and legislation in Slovakia. The controller itself is powered from a socket (230 V) or from a fixed electrical circuit. If it is connected to a fixed circuit (without a socket), an electrician with the appropriate qualification is required by law (Decree No. 508/2009 Coll.). The hydraulic part – the pump unit, pipe connections, and sensors – can be installed by a heating or plumbing installer. The complete installation procedure can be found in the article Installation of a solar pump unit step by step.

What heat transfer medium should I use in the solar circuit and does it affect the controller?

Propylene glycol (non-freezing solar mixture) in a concentration of 30–50 % is used as standard, depending on the local minimum temperature. The controller must know which medium is in the system, as the specific heat capacity of propylene glycol differs from pure water – the controller uses this parameter in its power calculation. Some controllers allow you to set "water" or "glycol 30/40/50 %" directly in the menu. Do not use ethylene glycol (automotive antifreeze) – it is toxic and in the case of a leak, it would contaminate the water supply.

The controller displays real values, but the storage tank is not heating – why?

The most common cause: air in the solar circuit. Air reduces effective flow and can cause the pump to pump only air without heat transfer – the flow meter shows zero or very low values. The solution is thorough system degassing. Another possible cause is a clogged filter (strainer) in the pump unit, which needs to be cleaned. Less common, but still real in practice: frozen medium in the pipes during the first start-up in winter, if the non-freezing mixture was not properly mixed.

Is every controller compatible with every pump?

Not always. Standard controllers control pumps via relays – simple on/off. With EC pumps (PWM or 0–10V control), you need a controller that supports these protocols. If you buy an ECO pump but an old controller, the EC motor will either run at full power (no speed control) or not at all – depending on the specific wiring. Always check the compatibility of the controller and pump before purchase, ideally in consultation with the supplier.

How can I tell if my solar system is working properly if I have no technical experience?

The simplest way: compare the temperatures on the controller display on a sunny summer day (11–14 h). The collector should have a temperature of 60–90 °C (higher in stagnation), and the storage tank should have a temperature at least 5–10 °C higher than in the morning before sunrise. The pump should be running. If these conditions are met, the system is working. If the tank is not heating despite the collector being hot and the pump running – there is likely a problem in the hydraulics (air, clogged valve). An annual visual inspection and tracking of energy consumption (if the controller has a kWh counter) will give you a good overview of the system's health.

What happens if the controller fails – is it dangerous for the system?

In the event of a controller failure, the pump stops – the system goes into stagnation. Collectors heat up to maximum temperatures (up to 200 °C for flat plate, up to 250 °C for vacuum), and the heat transfer medium evaporates into the expansion tank. The system does not burn, as the safety valve releases steam when the maximum pressure is exceeded (usually 6 bar). After the controller is repaired, the medium will return. Risk: frequent stagnation degrades glycol and seals – after a long stagnation (several days), it is advisable to have the medium composition checked. Therefore, it is advisable to have a backup power supply for the controller on a UPS or at least a fast fault signal (e-mail alert with modern WiFi controllers).

Conclusion

The control unit of a solar system is not just an "appendix" – it is a component that determines the overall energy balance of the installation. The correct choice, installation, setting, and regular inspection of the controller or pump unit directly affect the amount of solar heat gained during the season and the lifespan of the entire system.

If you are unsure which type of control unit is suitable for your specific system, we recommend reading the article How to choose a control unit for a solar system, where you will find a step-by-step selection process based on the system layout. An overview of all available controllers and pump units can be found in the control units category on atria.sk.

Do you have a question about this topic?

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