How to choose regulation for a solar system: differential thermostat vs. control station

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How to choose regulation for a solar system: differential thermostat vs. control station

A solar system without proper regulation is like an engine without control electronics – it may run, but it will never work efficiently, safely, or reliably. Regulation is the brain of the entire solar circuit: it decides when the pump should start, when to stop, when critical temperatures are approaching, and whether the energy from the collectors is usable. Despite this, many customers ask only one question when choosing regulation – the price. This is a mistake that can later prove to be very costly.

In this area, there are essentially two philosophically different solutions: the differential thermostat (also called a solar regulator or solar thermostat) and the control station (solar station), which integrates regulation together with the pump, flow meter, thermometers, and safety components into one compact unit. Both groups have their place in the market, and both types are represented in our category of regulation for solar systems, but they are not interchangeable for every application.

This article will help you understand what these differences really mean – technically, operationally, and economically – and how to make the right decision for your specific case.

What is a differential thermostat and how it works

A differential thermostat is an electronic control unit whose basic function is to measure the temperature difference between two points in the solar circuit – most often between the collector field and the bottom part of the hot water tank. Based on this difference (called the differential temperature), it turns the solar circuit pump on or off.

The principle is simple: if the temperature at the collector is a defined number of degrees higher than the temperature in the tank, the pump is turned on and starts transferring heat. When the difference decreases (the collectors have cooled down, the tank is approaching maximum temperature), the pump stops. This cycle repeats throughout the day according to current conditions.

A differential thermostat is essentially "just" control electronics – it does not include a pump, valves, or an expansion vessel. These are separate components that must be purchased, installed, and hydraulically connected. The advantage is flexibility: you can choose the pump power, pipe diameter, and type of valves yourself. The disadvantage is the more complex installation and higher demands on the installer.

A typical example of a differential thermostat available on the Slovak market is the Euroster 813 Solar – a compact unit with two temperature sensors (NTC), adjustable differential temperature for switching on and off, protection of the tank from overheating, and protection of the collector from frost. It is a basic solution for simple solar systems with one circuit.

What is a control station and how it differs

A control station (also called a solar station) is a compact device that combines control electronics, a circulation pump, a flow control valve, ball valves with a vent, a thermometer, possibly a flow meter (flow meter), and parts needed to connect the expansion vessel into one unit. It is a prefabricated, fully hydraulic solution for a solar circuit.

In practice, this means that when installing a control station, you do not assemble the installation piece by piece – you arrive, mount the compact unit on the wall, connect the collectors and the tank, connect the electrical part, and the system is ready to start. This saves significant time and reduces the risk of incorrect assembly for both the installer and the customer.

On the Slovak market, control stations are available in various performance categories. In our range, you can find the Control Station ZPS 6, Control Station ZPS 16, and Control Station ZPS 28. The numbers in the name correspond to the maximum pump performance in liters per minute – which directly affects the size of the collector field that the station can serve.

When to choose a differential thermostat

A differential thermostat is justified mainly in situations where you need a simple, cost-effective solution for a small system, where you already have some components available, or where you need maximum flexibility in hydraulic configuration.

From practice, I have seen several scenarios where a differential thermostat proved to be the right choice:

Renovation of an existing solar system: The customer has a working pump, valves, expansion tank – only the control thermostat is damaged. Replacing only the electronics is cheaper and logical.
Small solar systems with 1–2 collectors: For heating DHW for 2–3 people with one or two flat collectors, a simple differential thermostat is sufficient – a higher performance pump or complex control is not necessary.
DIY installation by an experienced customer: A technically skilled owner who understands hydraulics will assemble the circuit themselves and only needs the control electronics.
Non-standard hydraulic solutions: For example, two-circuit systems with a custom switching solution, where a standard station does not suit the component layout.
Limited budget for a basic application: If the priority is cost and the system is simple, a differential thermostat significantly reduces the initial investment.

It is important to emphasize, however: a differential thermostat alone is not enough. You must also purchase a circulation pump (dimensioned for the hydraulic losses of the system), ball valves, air vent, pressure gauge, expansion tank, filling and draining valve, and other fittings. The total cost of components can approach the price of a compact control station – but the installation will be more complex.

When to choose a control station

A control station is the right choice whenever the customer wants a reliable, proven, and easily serviceable solution without the need to collect components from multiple sources. In terms of total costs (materials + installer labor), a control station is often cheaper than an equivalent setup built from individual parts.

Practical scenarios from practice where a control station clearly wins:

New installation of a collector field for a family house: A standard installation of 2–4 collectors + 300–500 liter storage tank. A ZPS 6 or ZPS 16 control station will cover such a system without problems and the installation will take the installer significantly less time.
Larger collector fields (4–12 collectors): At this scale, the flow through the circuits is high and a ZPS 28 control station with a larger pump is essential. Dimensioning the pump for such a hydraulic system would be complex in a separate solution.
Installations in apartment buildings or hotels: A professional environment requires proven, compact solutions with documentation, certificates, and easy serviceability.
A customer who does not want to deal with details: A control station is a plug-and-play solution – the customer receives a working system, not an assembly of parts.
Systems requiring precise yield measurement: Control stations are equipped with flow meters and in combination with temperature sensors allow calculation of the system's real energy yield (kWh).

Dimensioning: what determines the correct type and size

When choosing a specific device, it is not enough to know whether you want a thermostat or a station. You must know the key system parameters that determine the choice. Let's look at them in detail.

Number and type of collectors

Each flat collector has an area of typically 2 to 2.5 m² and can deliver 600–900 W of thermal power under optimal solar irradiation. Vacuum tube collectors are more powerful – they achieve higher yields for the same area, but also produce higher stagnation temperatures. The more collectors, the higher the flow the pump must ensure and the higher the hydraulic losses it must overcome.

As a rough rule of thumb:

1–2 collectors (up to 5 m²): differential thermostat with a suitable pump or ZPS 6
2–4 collectors (5–10 m²): ZPS 6 or ZPS 16 depending on the circuit length
4–8 collectors (10–20 m²): ZPS 16 or ZPS 28
8–12 collectors and more: ZPS 28, or two stations in parallel

You can find more detailed dimensioning in the article How many collectors can the ZPS 6, ZPS 16 and ZPS 28 regulation handle? in our Knowledge Center.

Length and diameter of the pipe loop

Hydraulic losses increase with the length of the pipe and with every valve, elbow and filter in the loop. For shorter routes (up to 15–20 m total length with Cu ½") a smaller pump capacity is sufficient. With longer routes (e.g., a collector on the roof and a storage tank in the basement, total route 30–50 m), losses can increase significantly and you will need a more powerful pump, hence an appropriate station.

The recommended flow for a solar loop is usually 0.5–1 liter per minute for each m² of collector area. For 4 collectors with a total area of 9 m², this is about 4.5–9 l/min. The ZPS 6 station has a maximum flow of 6 l/min – in this application it would not be sufficient, the more suitable option is ZPS 16 or ZPS 28.

Type of storage tank and temperature requirements

A single-shell tank with one solar coil is a standard case – the regulation takes care of the temperature in the lower part of the tank. Some tanks have two coils (one for solar, the other for the boiler) or a bivalent tank with integrated electric heating – the regulation must then be able to coordinate multiple sources. In such cases, a control station with advanced control electronics (with the possibility of connecting additional sensors and outputs) significantly simplifies the configuration.

Temperature limits and safety functions

Modern regulations – whether differential thermostats or control stations – should include several basic safety functions:

Storage tank overheating protection: When the temperature in the tank exceeds the set maximum value (typically 70–90 °C), the pump stops.
Collector frost protection: At low outside temperatures (or low collector temperatures), the pump is briefly started to prevent freezing – important especially in systems with water instead of glycol.
Collector stagnation protection: At extreme summer temperatures, the collector can reach temperatures of 160–200 °C – the device must be able to react to this (pump stop, alarm).
Sensor fault detection: A break or short circuit in the NTC sensor must be detected and indicated as an error.

These functions are also included in the Euroster 813 Solar, making it a reliable choice for basic applications. In more complex systems, it is an advantage if the control station also offers a display with current values, operating hours history and energy statistics.

Differential temperature: a key parameter that affects system performance

One of the most important settings in any solar regulation is the differential (or difference) temperature for turning the pump on (ΔT ON) and off (ΔT OFF). This is the temperature difference between the collector and the tank at which the pump starts or stops.

Typical values:

ΔT ON = 5–10 °C: The pump turns on when the collector is 5–10 °C warmer than the tank. A lower value = earlier activation = more energy captured, but also more unnecessary starts in cloudy weather.
ΔT OFF = 2–4 °C: The pump stops when the difference drops to 2–4 °C. If it were too small, the pump would unnecessarily cycle on and off (so-called "hunting" phenomenon).

Incorrect ΔT settings are one of the most common causes of poor system performance. Too high a ΔT ON value means the pump starts too late and the early low-power sunny hours are not used. Too low a value, on the other hand, causes frequent switching, pump wear and unnecessary electricity consumption. We will discuss this topic in more detail in the article Setting the differential temperature in solar regulation: how to correctly configure switching.

Placement of temperature sensors: where millimeters matter

Even the best regulation cannot compensate for poorly placed temperature sensors. From practice, I know that about 30–40 % of problems with solar regulation (too late activation, premature shutdown, false alarms) originate precisely from incorrect sensor placement.

The sensor on the collector (T1) must be placed directly in the absorber chamber or in a designated opening in the collector's outlet pipe – not on the external pipe, not on the return line. The tank sensor (T2) must be inserted into a flange or well in the lower third of the tank – not at the level of hot water in the upper part, where the measurement would be distorted.

A detailed procedure for connecting sensors, pump and expansion vessel can be found in the article Installation of solar system regulation: connecting the pump, sensors and expansion vessel.

Differences in practice: real installation experiences

Over the years of practice in the sale and installation of solar systems, certain types of customer orders and associated decisions keep recurring. Here are a few typical situations that will help you get oriented.

Example 1 – Family house, new build, 3 collectors: The customer is building a new house and wants solar heating for domestic hot water for a family of four, 300-liter tank, collectors on the roof, boiler room in the ground floor. Loop length approx. 18 m. Ideal solution: control station ZPS 16 – compact mounting, proven hydraulics, pump dimensioned precisely for this range, flow meter allows system performance monitoring. Installation takes 4–5 hours including sensor wiring.

Example 2 – Recreational cabin, 1 collector: The customer has a cabin with a simple solar water heating system – one collector, 150-liter tank, short circuit. The old differential thermostat failed. Replacement: Euroster 813 Solar – affordable, simple, reliable. The pump remains the original one, as it is still functional. The customer saves money and the system continues to operate.

Example 3 – Apartment building, 8 collectors: The apartment owners' association wants a solar water heating system for the building, 8 vacuum tube collectors, 800-liter tank. Circuit length approximately 40 m. Solution: control station ZPS 28 with a powerful pump, capable of overcoming significant hydraulic losses in a long circuit at high flow rate. The system features energy yield measurement, which is important for the building manager when documenting savings.

Example 4 – Non-standard hydraulics, technical customer: An experienced customer is building a combination of solar and heat pump systems with a custom switching valve, prioritization of tanks, and an atypical layout of components. In this case, a differential thermostat makes sense – the customer knows what they are doing and needs only the control logic, not a pre-fabricated hydraulic system.

Connecting the control with other systems

Modern control stations and more advanced differential thermostats allow connection with other systems in the house. The most common scenarios:

Tank prioritization: The system charges the primary TÚV tank first, and switches to the heating system tank (e.g., underfloor heating) only after it is full. This requires a controller with multiple outputs or support for a 3-way valve.
Connection to the boiler: The controller signals the boiler that the tank is sufficiently heated and the boiler does not need to provide additional heating – saving fuel and energy. A prerequisite is a compatible connection (potentially dry contacts, OpenTherm, etc.).
Remote monitoring: Some stations allow connection to a WiFi module or a BUS line and sending data to a mobile device or cloud application.

For more information on how to correctly connect the control with a tank or water heater, see the article How to connect a solar system control with a water heater or TÚV tank.

Maintenance and long-term reliability

Solar system control is not a "install and forget" device. Like the collectors and pump, it requires regular inspection. The most common problems I have seen in practice:

Deterioration of the heat transfer fluid (glycol solution): After 5–7 years, the pH level drops and corrosion inhibitors are depleted. This can damage the pump in the control station and the sensors. The result is incorrect temperature measurement and inaccurate control.
Loose or wet sensor: An NTC sensor not properly inserted into the tank well may measure the ambient air temperature instead of the water – the system then behaves erratically.
Pump failure in the control station: The pump may jam after a long period of inactivity (e.g., after winter). The solution is simple – a manual turning element on the pump allows manual rotation.
Incorrect pressure in the expansion tank: The control does not ensure this – periodic pressure checks (every 1–2 years) are necessary.

A systematic inspection before and after the season is described in the article Maintenance and inspection of solar control: what to check before and after the heating season. Common error messages and their solutions can be found in Common faults in solar controls: error messages, pump failures, and inaccurate sensors.

Economic comparison: total installation costs

Many customers compare only the price of the control itself, which is a mistake. Real costs include the entire group of components and the installer's work. Let's look at a model example for a system with 3 collectors:

Variant A – differential thermostat + separate components:

Differential thermostat: ~60–100 €
Circulation pump (e.g., Grundfos UPS 25-40): ~80–120 €
Ball valves with thermometer (2 pcs): ~40–60 €
Drain valve + manometer: ~20–30 €
Make-up valve / drain valve: ~15–25 €
Fittings, clamps, insulation: ~30–50 €
Installer's work (2–3 extra hours): ~80–120 €
Total: ~325–505 €

Variant B – control station ZPS 16:

Control station ZPS 16: ~200–280 €
Minimum additional valves: ~20–30 €
Installer's work (shorter installation): ~60–80 €
Total: ~280–390 €

Thus, in this comparison, the control station is cheaper even though the base price of the device is higher, because it eliminates the need to buy additional components and reduces installation time. We have not yet included the long-term advantage of integrated hydraulics – no leaks from many connections, easier service.

Frequently asked questions (FAQ)

Can I connect a differential thermostat to any pump?

Most differential thermostats, including the Euroster 813 Solar, have an output for a pump with a maximum current of 1–3 A at 230 V. This covers standard circulation pumps with a power consumption of up to 100–150 W. For more powerful pumps (rare in domestic solar systems), it is necessary to install a relay. Always check the technical parameters of the controller and pump before combining them.

What is the difference between ZPS 6, ZPS 16, and ZPS 28?

The main difference is in the maximum flow rate of the pump and thus in the range of collector arrays that the station can handle. ZPS 6 is intended for small systems (1–2 collectors), ZPS 16 for medium (2–5 collectors), and ZPS 28 for larger installations (5–12 collectors). They also differ in the hydraulic losses they can overcome – for long circuits with smaller pipe diameters, it is important to choose a sufficiently powerful station.

What should I pay attention to when selecting temperature sensors for solar control?

NTC sensors for solar systems must be resistant to high temperatures – collectors can reach 200 °C and more. Standard NTC sensors for heating (with a range up to 80–100 °C) are unsuitable. Always use sensors certified and supplied by the specific controller manufacturer. Interchangeability (different manufacturers, different NTC characteristics) is problematic – the controller must be calibrated for a specific type of sensor.

Do I have to replace the entire control station if only the pump fails?

No. The pump in the control station is usually a standard component (e.g., Grundfos, Wilo, or OEM equivalent) with standard connection dimensions. It can be replaced separately without the need to replace the entire station. When ordering a replacement pump, it is important to know the type and power of the original pump – this information is listed in the technical documentation of the station.

Can I use a solar control for other applications, for example, to control a swimming pool pump?

A differential thermostat (including the Euroster 813 Solar) can in principle control any pump based on a temperature difference – thus also for pool heating. However, the temperature range of the sensors and the maximum output current must be verified. Control stations are specifically designed for solar circuits with glycol – using another liquid (e.g., clean pool water) may be in conflict with the manufacturer's warranty conditions.

Is it worth investing in a more advanced control with WiFi and energy statistics?

From a long-term perspective, yes. Access to system operation data allows problems to be detected before they cause damage – for example, a drop in flow, collector stagnation, or incorrect sensor readings. Energy statistics will show the real yield of the system in kWh, which is valuable for evaluating the return on investment or for resolving warranty claims. For standard households without the need for performance monitoring, a basic controller is sufficient.

Conclusion: the right control is always worth the investment

Choosing between a differential thermostat and a control station is not only a technical, but also an economic and operational question. If you are building a new system and want simple installation and reliable operation with minimal hassle, the ZPS 16 or ZPS 28 control station is the right choice in most cases. If you are only replacing the control electronics of an existing system, or you have a truly small and simple circuit, a differential thermostat like the Euroster 813 Solar will perform its job well at a significantly lower price.

The key is always correct dimensioning – do you know the number of collectors, the length of the circuit, the type of storage tank, and whether you need only basic switching or also performance monitoring and connection to other heat sources? Based on these data, a clear decision can be made. If you are unsure, the article Euroster 813 Solar vs. ZPS control stations: comparison of functions and use provides an even more detailed comparison of specific devices.

Investing in the right regulation pays off not only in energy savings, but mainly in trouble-free system operation throughout its entire lifetime – and with a well-designed solar installation, this can reach 20–25 years.

Do you have a question on this topic?

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