How to choose a regulation for a solar system: what to pay attention to
Why the choice of regulation matters more than most people think
A solar collector system without proper regulation is like an engine without control – it can run, but you won't get anywhere safely. Regulation is not just an "extra accessory," it is the brain of the entire system. It decides when the pump turns on, when it turns off, protects the tank from overheating, prevents reverse circulation, and in modern versions it also records system yields, so you know exactly what you have actually gained.
From practice, I know that the choice of regulation is underestimated especially in smaller installations. A customer buys two collectors, a 200-liter tank, and then tries to save money on regulation. The result? The tank overheats several times during the season, the expansion tank gets damaged, or the tank itself is damaged. Or the opposite extreme – the regulation is set too conservatively and the system uses only 60% of the available solar energy.
This article will show you what to really focus on when choosing regulation – from basic principles, through specific parameters, to typical mistakes that are repeated when choosing.
Basic types of regulation for solar systems
Before we get into the selection criteria, it is good to know what to distinguish. There are basically three categories of devices on the market that ensure the control of a solar system:
1. Differential regulators (simple solar controllers)
This is the simplest type – the device measures the temperature on the collector (sensor T1) and the temperature in the tank (sensor T2) and switches the pump on when the temperature difference is sufficient. Typically, the settable differential ΔT for switching on is 4–10 K, for switching off 2–4 K. Nothing more, nothing less. An example of this approach is Euroster 813 Solar – a compact controller suitable for simple two-collector systems with one tank, where you don't need anything complicated.
Advantage: simplicity, low cost, reliability. Disadvantage: no automatic protection against legionella, no yield monitoring, limited options for multi-circuit systems.
2. Advanced solar regulators with system management
Devices with a larger number of inputs for sensors, multiple relay outputs, functions for protection against overheating, tank cooling at night, pasteurization function (Legionella function), power measurement, and possibly a communication interface. These include Control Station ZPS 6, ZPS 16 and ZPS 28 – complete solutions integrating a regulator, pump group, expansion vessel and manometer into one unit.
3. Integrated control systems and BMS
For large installations – hotel complexes, apartment buildings, industrial dryers – where solar regulation is part of an upper-level building management system. This is a topic for another article; for family homes and smaller businesses, the first two categories are relevant.
Key parameters you must compare before choosing
Number and type of sensors (inputs)
Each sensor that the controller can connect to is one point where it measures temperature or flow. For a basic single-loop system, two sensors are sufficient: one on the collector's absorption surface and one at the bottom of the storage tank. However, as soon as a second storage tank, a pool heat exchanger, or a return loop is added, you will need additional inputs.
Common sensor types: PT1000 (accuracy ±0.5 °C at 20 °C, suitable up to 200 °C), NTC 10k (cheaper, less accurate at higher temperatures). Always verify which type of sensor the controller supports and which one is included in the package – sometimes the manufacturer declares a PT1000 input but only includes an NTC. Swapping is technically possible but requires reconfiguration.
Number of outputs (relays / PWM)
Each pump, three-way valve, or auxiliary device needs its own output. A basic system with one collector field and one storage tank can manage with one output. A system with two storage tanks (e.g., a combination of TÚV + heating), a flushing loop, or a pool requires at least three outputs.
Be careful with PWM control of pumps: some controllers can adjust the pump speed according to the intensity of radiation (so-called variable speed control). This is energy-efficient, but the pump must be prepared for PWM control – not every standard circulation pump supports this function.
Maximum relay load and pump power supply
A controller with an integrated switching relay typically supports a pump with a power consumption of up to 200–250 W at 230 V AC. If you plan a larger system with a more powerful pump (e.g., for a flat collector field of 10+ m²), check the maximum allowable power consumption of the connected device. Control stations with integrated ZPS pumps have the pump physically built-in, so this question is irrelevant.
Temperature measurement range and hysteresis
The standard measurement range of solar controllers is –20 °C to +200 °C, which is sufficient for most flat and tubular collectors. With vacuum tube collectors (ETC), the stagnation temperature may exceed 200 °C – verify that the controller (and sensor) can withstand such temperatures without permanent damage.
Hysteresis (the difference between the on and off thresholds) should be adjustable within at least 2–10 K for turning on and 1–5 K for turning off. Too small hysteresis = the pump "flickers" (switches on/off in short cycles), which shortens its lifespan and reduces efficiency. Too large hysteresis = the system reacts late to weather changes.
Protection functions: what the control must be able to do
This is an area where a cheaper controller, without careful review, can replace expensive repairs to the storage tank or collector. A list of protection functions that the controller should have (or at least those relevant to your system):
- Protection against overheating of the storage tank (max. T of the tank): When the tank reaches the set maximum temperature (typically 80–95 °C), the pump is turned off regardless of ΔT. Without this function, the tank could overheat to dangerous levels in summer.
- Cooling of the tank via the collector (night cooling): In summer, during prolonged overheating, the controller turns on the pump at night and "radiates" the heat through the collector into the atmosphere. This works only with an open loop or with systems equipped with a vent valve.
- Collector freeze protection (frost protection): When the collector temperature drops below the set value (typically 4 °C), the pump is briefly turned on to circulate warmer fluid from the tank into the collector. Important for areas with harsh winters or for recreational properties where the system is not filled with antifreeze.
- Sensor fault detection: If the sensor fails or short-circuits, the controller must recognize that the value is unreliable and safely stop the pump – not run at full speed because "ΔT is 99 K".
- Pasteurization function (anti-Legionella): Once per set interval (typically once a week), the tank is heated to 65–70 °C for 30–60 minutes to eliminate Legionella bacteria. In most installations for single-family homes, this is more of an add-on, but for larger systems (guest houses, hotels), it is a hygiene requirement.
- Stagnation protection (high temperature collector protection): Some controllers start a short pump cycle when the collector temperature exceeds a threshold (e.g., 130–150 °C) to reduce stagnation and fluid oxidation.
System size as a decisive factor
The choice of regulation must correspond to the size of the solar field. A simple controller for a pair of flat collectors (area approx. 4–5 m²) will be absolutely insufficient for a system with six tubular collectors and two storage tanks. On the contrary, installing a controller designed for 28 collectors into a simple system for a family house is an unnecessary expense.
A more detailed analysis of the capacity of individual stations can be found in the article "How many collectors can the control station ZPS 6, ZPS 16 and ZPS 28 handle?" in our Knowledge Center. There you will find specific flows, maximum areas and technical limits of each of these stations.
Practical example: a family house with two storage tanks
The customer had 5 flat collectors installed (total absorption area 10.2 m²), a 300-liter TÚV tank and a 200-liter heating tank. The original simple controller could handle only one tank – it always heated the TÚV and the heating was never heated by solar energy. After replacing it with a controller with two pump outputs and three sensors (collector, bottom of the TÚV tank, bottom of the heating tank), the system started to switch between the tanks depending on where there was a higher potential for gains. The annual solar yield increased by an estimated 35 %.
Type of collector and its impact on the choice of regulation
Flat collectors and tubular (vacuum) collectors have different thermal characteristics, which directly affect the requirements for regulation.
Flat collectors: They react quickly to changes in solar radiation, have a low stagnation temperature (typically 130–170 °C). A basic differential controller is usually sufficient. The sensor can be placed directly on the absorber or in the outlet pipe.
Vacuum tube collectors (ETC): They reach a higher stagnation temperature (200–280 °C), react more slowly to changes in cloud cover, but retain heat even at low outside temperatures. The controller must be able to measure temperatures up to at least 250 °C (sensor and electronics). Protection against overheating and stagnation is critical here. Cheap controllers with PT1000 sensors intended for flat collectors may not be certified for this temperature range when used with ETC collectors.
Concentrated collectors (CPC, parabolic): Extremely high temperatures, special regulation outside the scope of this article.
Monitoring and measuring yield: why it is more important than you think
Without performance measurement, you only know that the system is working – but you don't know if it is working well. An experienced technician can estimate the annual yield from a solar field by calculation (e.g., for Slovakia, approx. 350–450 kWh/m² of collector area per year for flat collectors). But without actual measurement of flow and temperature difference, we cannot say whether your specific system achieves this potential.
Performance measurement (calorimeter) works as follows: the controller measures the temperature at the inlet and outlet of the collector and the flow of the fluid using a flow sensor. From these three values and the density/specific heat capacity of the fluid, it calculates the instantaneous thermal power and integrates it into cumulative energy. Accuracy depends on the quality of the flow sensor – cheap vane sensors can have an error of ±5–15 %, magnetic-inductive sensors ±1–3 %.
For a typical family house, a calorimeter is not mandatory, but I recommend it for practical reasons: if something goes wrong (e.g., the flow drops due to a clogged filter or gas leakage from an expansion vessel), the measured performance will immediately detect it. The system "works" (the pump is running), but the yield is only 20 % of normal. Without measurement, you will only find out when the gas bill remains as high as before.
Compatibility with the existing heating system and TÚV
Solar regulation in most cases communicates with the boiler or storage tank indirectly – via the tank thermostat or input signal of the boiler (typically 0–10 V or digital OpenTherm). If you have a modern condensing boiler with OpenTherm control, it is advantageous to choose a controller with a corresponding interface that can tell the boiler not to start when the tank is already sufficiently heated by solar energy.
With simple solar controllers without this function, the boiler is controlled by its own thermostat and may start even when the tank is at 60 °C from solar energy – which is unnecessary waste. This is a topic we write about in more detail in the article "Connecting solar regulation with an existing heating system or TÚV tank".
Practical scenario: a customer with a Vaillant ecoTEC boiler and a TÚV tank connected the solar regulation via the "AUX" input of the tank. The boiler received the instruction: if the tank temperature is above 45 °C (which the solar system achieved every day by 11:00 in summer), do not ignite the burners. Annual gas savings increased by another 8 % compared to the situation when the solar system was running, but the boiler was "ignoring" it.
Control stations ZPS: integrated solution versus separate controller
A separate controller (e.g., Euroster 813 Solar) provides only intelligence – pump control and protection. The pump, valves, flow meter, expansion vessel and safety valve must be dimensioned and obtained separately. This is not bad if you have an installer who has experience with it and selects the right components. But it is more steps, more decisions and more room for error.
Control stations, such as ZPS 6, ZPS 16 and ZPS 28, integrate into one compact device a circulation pump, controller, expansion vessel, safety valve, manometer, thermometers and usually also a drain valve. The dimensioning is designed by the manufacturer for the corresponding range of the collector field. This significantly simplifies installation and reduces the risk of incorrect dimensioning.
A detailed comparison of both approaches can be found in the article "Euroster 813 Solar vs. control stations ZPS: which solution is better for your system" in the Knowledge Center.
Where and how to install the regulation: practical considerations
The regulator or control station is installed in the boiler room (or technical room) near the storage tank. The ambient temperature should be in the range of 0–40 °C (most manufacturers specify a maximum of 50 °C), and the regulator must not be exposed to direct sunlight or splashing water. Wall mounting must be secure – control stations with an integrated pump can weigh 8–15 kg and vibrate during operation.
Sensor cables must not be run together with 230 V AC power – induction can cause false measurements. Most manufacturers recommend a minimum distance of 15–20 cm or the use of shielded cables. The length of the sensor cable can be up to 50 m without the need for an amplifier (for PT1000), while for NTC sensors it depends on the specific manufacturer – usually 30 m is sufficient without problems.
A detailed installation procedure including the most common mistakes (incorrect sensor placement on the collector, reverse polarity supply, incorrect sensor immersion depth) can be found in the article "Installation of solar system regulation: procedure and most common mistakes".
Setting the differential and optimizing yield
Correctly setting the switching and switching-off differential belongs to the most frequently overlooked steps. Most regulators come with pre-set values (e.g. ΔT on = 8 K, ΔT off = 4 K), which is a reasonable compromise, but not necessarily the optimum for your specific system.
Too high a switching differential: the system turns on later, and you miss out on morning and evening hours with weaker radiation. With flat collectors, a 10 K on setting can cost you 5–10 % of annual yield compared to a 6 K setting. Too low a switching differential: the pump runs even when ΔT is so small that the thermal output is lower than the pump input. In such moments, the system actually transfers energy from the storage tank to the environment through the collector – i.e., it cools rather than heats.
In practice, for a flat collector with a stainless steel storage tank, a setting of ΔT on 5–7 K and ΔT off 3–4 K has proven effective. For vacuum collectors, due to their slower response: ΔT on 8–10 K and ΔT off 5 K. A more detailed analysis with specific measurement examples can be found in the article "How to set a differential regulator for optimal solar system yield".
Price versus performance: where it is worth investing
A basic differential regulator typically costs 40–100 €. An advanced regulator with monitoring costs 150–350 €. A complete control station for a smaller system (ZPS 6) ranges from 400 to 700 € including pump and valves. ZPS 16 and ZPS 28 are more expensive, but still significantly cheaper than assembling the system from individual components.
Where is it worth investing in a higher quality regulator? When:
- you have an expensive storage tank (stainless steel, with a double coil) – protective functions protect it from damage
- you have vacuum collectors – high temperatures require a more robust regulation
- you plan to expand the system – buy a regulator with extra capacity and save on future replacement
- you want to monitor yields and optimize – a calorimeter has real value
- it is a larger object (guest house, apartment block) – reliability and serviceability are key
Where can you save? On a simple system for a holiday cottage with two flat collectors and a 200-liter storage tank, where the water is not health-risky and the system is not in operation all year round – a simple regulator is fully sufficient there.
What to watch out for when buying: red flags and quality verification
In the market for electronics for solar systems, a large number of cheap products from China without relevant certification have appeared in recent years. Not all of them are bad, but you need to know what to look for:
- CE mark and real certification: CE is mandatory for sale in the EU, but on its own it does not mean independent testing. Manufacturers such as Euroster, Resol, Viessmann, Grundfos have a long history and products that are genuinely verified.
- Availability of replacement sensors: Sensors can be damaged after years of operation (moisture, corrosion). If replacement sensors for your regulator are not available in 5 years, the whole regulator will end up in the trash.
- Documentation in Slovak/Czech: Clear documentation is key when programming and setting up the regulator. A manual only in Chinese is a real problem.
- Supported temperature range of sensors: Check not only the declared range of the regulator, but also the actual range of the supplied sensors. It is not uncommon for a regulator to declare 200 °C, but the attached sensor has a PVC insulated cable that melts at 130 °C.
- Warranty service in Slovakia: Where will you file a complaint if the device stops measuring correctly after 8 months?
Planned system expansion: think ahead
One of the most expensive mistakes in practice: the customer buys a regulator precisely dimensioned for the current number of collectors and storage tanks. A year later, they add a swimming pool heat exchanger or a second storage tank for heating. Result: the regulator is insufficient, a new one must be bought (and the old one discarded), and the cost of a new installation is paid again.
Recommendation: if you have at least a 20 % chance of expanding the collector field or adding a storage tank in the future when planning your system, choose a regulator (or control station) one step higher. The price difference between ZPS 6 and ZPS 16 is relatively small compared to the cost of a later upgrade.
Most frequently asked questions (FAQ)
Can I use any differential regulator for tubular (vacuum) collectors?
Not necessarily. Tubular vacuum collectors (ETC) can reach temperatures of 200–280 °C during stagnation. The regulator and sensor must be certified for this temperature range. Common sensors with PVC cables intended for flat collectors can fail at these temperatures. Always check the maximum temperature resistance of the sensor (usually a specification in the technical data) and choose a sensor with a cable jacket made of silicone or teflon.
How many sensors do I actually need for a basic system?
For a single-loop system (one collector field, one storage tank), at least two sensors are required: T1 on the collector (or in the collector's outlet pipe) and T2 at the bottom of the storage tank. Adding a third sensor (e.g., at the top of the storage tank) allows monitoring of stratification and more precise control. For two-loop systems, you need at least three sensors, ideally four.
What is "stagnation" and how does regulation handle it?
Stagnation occurs when the storage tank has reached maximum temperature and the pump has turned off, but solar energy continues to hit the collectors – the temperature in the collector continues to rise and the fluid can boil. Proper regulation prevents this by activating night cooling (the pump runs at night and radiates heat through the collector), or by using a hydraulic bypass. It is important that the expansion tank is correctly sized for the stagnation scenario – this, however, is not the task of the regulation, but the overall system design.
Can I connect the regulation to a smart home system (Home Assistant, KNX)?
It depends on the model. Some advanced regulators have Modbus RTU, RS-485 or even Wi-Fi with cloud monitoring. ZPS control stations in the basic version are not directly connectable to smart home systems via standard protocols, but there are external modular solutions (e.g., Modbus to MQTT converters for Home Assistant). If smart home integration is important to you, consider this before buying a regulator and check compatibility.
What is the lifespan of a regulator and when should it be replaced?
A quality regulator can last 15–20 years if properly installed and not exposed to unsuitable conditions (moisture, vibration, overheating). Sensors are often the weaker link – typical lifespan is 8–12 years, and even shorter with vacuum collectors and high stagnation temperatures. Regular inspection and steps for diagnostics of faults are described in the articles "Common faults in solar regulators and how to fix them" and "Maintenance and servicing of solar system regulation: what and when to check".
Do I have to drain the entire solar loop when replacing the regulator?
Replacement of the regulator (electronics) alone usually does not require draining the circuit, provided the pump and valves are retained. If you are replacing the entire control station or pump unit, draining or at least isolating the circuit with closing valves is necessary. When working with solar fluid (glycol), use protective gloves – glycol solutions are harmless during normal contact, but vapors at stagnation temperatures may be unpleasant.
Conclusion: an investment that pays for itself many times over
Selecting the regulation for a solar system is not just a technical formality – it is a decision that will influence the performance, safety, and lifespan of the entire installation for the next two decades. The correct regulation for the right system increases solar yield, extends the life of the storage tank and collectors, simplifies service and diagnostics, and ultimately results in lower heating and domestic hot water costs.
If you are unsure which solution is optimal for your specific system, carefully review all products in the regulation for solar systems category on atria.sk and compare their technical specifications with the requirements of your system. Further expert articles in the Knowledge Center – for example, "Control station for a solar system: what is included and what you need to buy separately" or "Frequently asked questions about regulations for solar systems" – will help you make an informed decision and avoid the most common mistakes when selecting and installing.
Do you have a question about this topic?
Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.