How to choose a control unit for a solar system

We are still preparing products.

But you can look at other categories.

How to choose a control unit for a solar system – a complete guide to making the right decision

The control unit is the brain of every solar system. Without it, you would have collectors on the roof, a storage tank in the boiler room, and a pump between them – but the whole system would operate blindly, without the ability to react to real conditions. It is precisely the controller that decides when to start the pump, when to stop it, when to open/close valves, when to send an alarm, and when to switch between heat sources. Choosing the right control unit directly determines how much energy your system will actually produce and how many years it will reliably serve you.

In practice, I often encounter people who invest in high-quality collectors and a large storage tank, but approach the control unit with a "cheaper is better" attitude. The result? The system unnecessarily cycles, losses are higher than they need to be, the storage tank overheats or, on the contrary, never reaches the target temperature. This article will help you avoid these mistakes and choose a regulation system that will truly fit your specific system.

What the control unit actually does – basic functions you need to know

Before we move on to the selection, it is important to understand what the control unit actually ensures. The principle is simple, but the details are key to proper dimensioning.

The basic function of any solar controller is differential control: it regularly (usually every 10–60 seconds) measures the temperature on the collector (sensor T1) and the temperature in the storage tank (sensor T2). If the difference between T1 and T2 is greater than the set activation value (typically 6–10 °C), the pump is started. If the difference drops below the deactivation value (usually 2–4 °C), the pump is stopped. This simple rule is the core of the entire solar regulation.

In addition, higher quality controllers handle:

Protection of the storage tank from overheating – when the storage tank reaches the maximum set temperature (e.g. 85 °C), the pump is stopped, or a cooling mode is activated (night cooling through the collector)
Protection of the collector from freezing – at low collector temperatures (below e.g. 4 °C), the pump is briefly started to prevent the antifreeze mixture in the collector from freezing
Protection of the pump from jamming – regular short pump starts during long periods of inactivity
Control of multiple storage tanks or multiple sources – e.g. switching between two storage tanks, or coordination with a backup boiler
Measurement and logging of performance – some units integrate a flow meter and thermal calorimetry, which allows tracking of the system's actual performance
Valve control – three-way or two-way valves for redirecting the flow

Types of control units – what is on the market and how they differ

There are basically three categories of products on the market, although the boundaries between them are not always clear:

1. Standalone controller (controller without a pump)

This is an electronic control unit – typically a plastic box with a display, terminal blocks for sensors, and relay outputs for the pump and valves. It is mounted on the wall in the boiler room, the pump is connected via an external cable and is located directly on the pipe. This concept is more flexible in terms of placement, but installation is more labor-intensive and requires precise dimensioning of the pump.

A typical example is Euroster 813 Solar – a compact digital controller with two-point differential control, protection against overheating of the storage tank and freezing of the collector, with a clear display and simple operation. It is suitable mainly for simpler systems with one collector field and one storage tank, where the pump is already installed or is mounted in a different location than the controller.

2. Pump unit with integrated control

This is the most common solution today for new buildings and complex renovations. Everything is integrated into one compact box: controller, pump, safety valve, air vent, thermometer, manometer, possibly a flow meter. Installation is significantly faster – it is sufficient to connect four pipes (to/from the solar collector, to/from the storage tank) and the electrical wiring. All functional components are pre-set and tuned together.

An example is Solar pump unit ZP2-12 ECO – a compact station with an ECO pump of class A, integrated control and all safety components. Similarly, Solar pump control unit ZPS 18e - 01 ECO offers more advanced control suitable for larger installations with more complex connection schemes.

3. Specialized mini-controllers for simple systems

For small home systems with one collector and a small storage tank, simplified solutions exist, such as Solar pump unit ALEX HX10 for MiniSOL control. These units are compact, cost-effective, and are pre-configured systems – the MiniSOL control is set directly for a specific type of collector and storage tank, so the customer does not have to configure from scratch.

Key parameters when choosing – what to compare and why

Number of temperature sensors and inputs

The simplest controllers work with only two sensors – T1 on the collector, T2 in the storage tank. This is sufficient for a system with 1 collector + 1 storage tank. If you have a more complex system – for example, two storage tanks, a pool heat exchanger, a backup boiler, or two separate collector arrays – you need a controller with three, four, or more sensor inputs.

In practice, I came across a case where the customer bought a cheap two-sensor controller for a system with two storage tanks and a pool. The result? The pool was not heated at all, because the controller simply did not detect it, and the second storage tank only worked occasionally through manual valve switching. The entire system had to be reconfigured with a new controller with four inputs.

Number and type of outputs (relays)

Each relay controls one device – a pump or a valve. A basic controller has one relay for the pump. More complex systems require two, three, or four relay outputs: for example, a primary circuit pump, a secondary circuit pump (storage tank 1 vs. storage tank 2), a three-way valve for the pool. Check how many relays the controller has and what power each of them can handle – typically 1 A or 5 A at 230 V.

Pump power (for pump units)

For integrated pump units, it is crucial that the pump power matches your installation. For a system with 2–4 flat collectors (area approx. 4–8 m²), a pump with a flow rate of approx. 2–4 l/min is sufficient. For larger systems with vacuum collectors or multiple rows, a pump with a higher flow rate is needed. The available pressure (pump head) is also important, as it must overcome the hydraulic resistance of the entire route – collectors, piping, storage tank, fittings.

ECO pumps vs. standard pumps

Modern solar pump units use ECO pumps with electronically controlled speed (energy efficiency class A). These pumps consume significantly less electrical energy – typically 5–25 W compared to 60–80 W for older wet-rotor pumps. For a solar system where the pump runs hundreds of hours per year, the savings can be 20–30 EUR per year on electricity – and this adds up over the 10-year lifespan of the system. This topic is covered in a separate article "Solar pump unit ECO vs. standard – what is the difference and when is it worth it".

Flow meter – yes or no?

A flow meter is not a necessity, but it is a very useful tool. It allows for precise measurement of the volume of solar fluid flowing through the system, which in combination with the temperature difference ΔT allows the controller to calculate the current thermal power and total energy balance (kWh). This is valuable for optimizing settings, diagnosing problems, and for subsidy applications, where proof of actual performance is sometimes required.

For example, Electronic flow meter for GH 26 is designed for a specific type of pump unit and allows the integration of flow measurement directly into the system without the need for a separate meter. More on the topic can be found in the article "Electronic flow meter in a solar system – what it is used for and how to connect it".

Selection according to the size and complexity of the system

Small family house – 1 to 3 collectors, one storage tank

This is the most common scenario in Slovak practice. You have 2–3 flat collectors with a total area of 4–6 m², a 200–300 liter storage tank, and a simple installation. For such installations, a simple controller with two sensor inputs, one relay output, and basic protective functions is sufficient. An ideal choice is either a compact pump unit with integrated control, or a separate controller in case the pump is mounted at a separate location.

There is no point in paying for advanced features in this category – multi-sensor inputs, logging, or valve control are unnecessary here. What matters is reliability, ease of operation, and the availability of spare parts.

Medium family house – 4 to 8 collectors, two storage tanks or a swimming pool

Here, the requirements for control are significantly higher. Typically, this is a system where the solar circuit supplies both the TÚV storage tank and, in summer, a swimming pool heat exchanger or a second storage tank for floor heating. In such a case, you need a controller with at least three sensor inputs (T1 collector, T2 TÚV storage tank, T3 pool or second storage tank) and two relay outputs (pump + valve or two pumps).

The control must be able to define priorities – for example, first charge the TÚV storage tank to the target temperature, and only then redirect the excess to the pool. Without this logic, you risk having a hot pool but cold showers in summer.

Larger installations and commercial buildings

For apartment buildings, hotels, sports facilities, or agricultural operations, where the collector area exceeds 20 m² and the storage tanks have thousands of liters, the requirements for control are completely different. Here, a fully parameterizable control with many inputs, the possibility of communication via Modbus or other protocols, and usually also remote monitoring are necessary. At the same time, it controls multi-stage pumps or frequency converters, and the system must be integrated into the BMS (Building Management System). This is a topic for specialists and direct system design.

Practical scenarios from practice – what works and what doesn't

Scenario 1: The controller is too sensitive – the pump cycles every minute

The customer had a new system with flat collectors, and the controller was set to a 5 °C activation difference and a 2 °C deactivation difference. During partly cloudy weather, the pump started and stopped every 30–90 seconds. Result: increased pump wear, noise in the boiler room, and paradoxically lower energy yield, because the cold solar fluid was re-cooling the collector. Solution: increase the activation difference to 8–10 °C and the deactivation difference to 3–4 °C. The system then ran in long stable cycles and the yield increased.

Scenario 2: The storage tank overheats, but the protection does not work

A customer with an older controller found that every summer the storage tank overheated to 92 °C, even though the maximum temperature was set to 80 °C. Cause: the controller had the maximum temperature set, but the tank sensor was placed too low (in the cooler part of the tank), so the controller only saw 72 °C. The actual temperature in the upper part of the tank was much higher. Solution: move the sensor to the upper third of the tank. After the move, the controller stopped the pump at 80 °C correctly, and the safety valve stopped opening.

Scenario 3: ECO pump saves energy, but is noisy at low speeds

Modern ECO pumps are capable of running at very low speeds, which is excellent in terms of consumption. Some models, however, produce a faint hissing or buzzing sound at minimum speed, which is audible in a quiet boiler room. Solution: set the minimum pump speed to a slightly higher value (if the controller allows it), or simply ensure better acoustic insulation of the inlet pipes.

Scenario 4: Combination of solar system with heat pump

Increasingly common scenario: the customer has solar collectors and a heat pump (HP), both sources charge a common storage tank. The control must be able to coordinate both sources – prioritize solar whenever it is available, and only activate the HP in case of insufficient solar gain. This requires a controller that can communicate with the HP control (e.g., via a digital "permission" input or via bus communication). A cheap controller cannot do this, and the customer ends up with a scenario where the HP and solar run simultaneously and interfere with each other.

Installation and mounting – what influences the choice

Selecting a controller is not only about functions, but also about the mounting method. Answer these questions in advance:

Where will the controller be mounted? In the boiler room near the storage tank, or elsewhere? If the pump cannot be placed near the controller, choose a standalone controller with an external relay output.
What type of piping do you have? Pump units are standardly equipped for threaded connections of 3/4" or 1". Check compatibility with your piping.
Length of sensor cables – standard sensors have a cable of 1.5 m or 3 m. If the collector sensor is far away (flat roof of a panel house, sloped roof with a long route), you may need an extension or a sensor with a long cable.
Type of sensor – most modern controllers work with NTC 10 kΩ sensors (at 25 °C). Not all sensors are interchangeable between brands!

Detailed instructions for installation can be found in the article "Installation of a solar pump unit step by step" in this Knowledge Center.

Settings and parameterization after installation

A good controller must not only be chosen correctly, but also set up correctly. The most important parameters to verify and set for each installation:

On differential (ΔT on) – recommended value 6–10 °C. Too low a value causes undesirable cycling, too high reduces energy yield.
Off differential (ΔT off) – recommended value 2–4 °C. It must always be lower than the on differential, otherwise the controller will never stop.
Maximum tank temperature (T max) – usually 60–85 °C depending on the tank material and safety requirements. For TÚV (hot domestic water), the minimum is 60 °C due to hygiene standards (Legionella).
Maximum collector temperature (T kol max) – when this temperature is reached, the controller stops the pump regardless of the tank, to protect the collector from overheating (typically 130–150 °C for flat collectors).
Frost protection (T frost) – temperature at which the controller briefly starts the pump to mix warm fluid from the tank with cold fluid in the collector (typically 4–6 °C).

More detailed settings are covered in the article "How to set up a solar system controller for maximum efficiency".

Price vs. value – where to save and where not to

On the market there are controllers from a few tens of euros (basic digital) up to several hundreds of euros (fully parameterizable multi-functional). Where is the reasonable limit?

Where not to save:

Quality of sensors and measurement accuracy – a cheap sensor with a deviation of ±3 °C can completely disrupt the system
Number of inputs/outputs – buying a controller "just right" without a reserve is risky for every future system change
ECO pump in the pump unit – electricity savings will show up every season
Protection and sealing of electronics – a controller in the boiler room must withstand humidity and heat

Where you can save:

Advanced communication interfaces (Wi-Fi, Ethernet) – for a standard family house they are unnecessary
Integrated datalogger with SD card – a nice feature, but not a necessity for proper operation
Color display vs. monochrome – it is irrelevant for system function

Safety functions – what every controller must have

A solar system operates under much more extreme conditions than standard heating. A collector can reach temperatures of 180–220 °C during stagnation (when the pump is not working and the system is stuck). Solar fluid (propylene glycol) is under pressure at such temperatures and causes corrosion products. Every controller must contain or allow the following protections:

Protection against tank overheating (T max of the tank) – stopping the pump when the maximum temperature is reached
Collector protection (stagnation protection) – some controllers briefly turn on the pump at night and "vent" heat from the collector through the tank
Frost protection – starting the pump at low collector temperature
Pump jam protection – weekly test run during long breaks
Alarm / fault indication – sensor failure, short circuit, high temperature – visual or audible indication

If the controller does not include one of these protections, you must replace it with an external solution (thermostatic safety valve, bimetal switch, etc.). Otherwise, you risk system failure or – in the worst case – a safety incident.

Compatibility with the sensor and with the pump

This is an area where customers most often make mistakes when ordering online. Before purchasing a controller, check:

Type of sensor the controller accepts – NTC 10 kΩ is the dominant standard today, but some older or special controllers work with PT1000, PT100 or K-type thermocouples. Sensors are not interchangeable!
Power supply voltage of the pump output – most controllers operate a pump on 230 V AC, but there are also 24 V DC systems (e.g. for off-grid applications)
Maximum current of the relay output – for pumps with higher starting current, the relay limit must be respected (typically 5 A = approx. 1150 W at 230 V)
Adjustability of pump speed – for ECO pumps with PWM input or 0–10 V control, you need a compatible controller

Maintenance and long-term reliability

Control units are relatively reliable – electronics itself can last 15–20 years under proper operating conditions. The main causes of failure are burnt relays (due to a too powerful pump or too frequent switching), damaged sensor cables (UV radiation on the roof, mechanical damage), or moisture infiltration into the controller housing.

Annual inspection should include: visual inspection of sensor cables, verification of displayed temperatures against a reference thermometer, checking the system pressure and functional test of protections (e.g. manually heating the tank to maximum temperature and verifying pump stop). More on this topic can be found in the article "Maintenance and service of a solar pump unit".

Most frequently asked questions (FAQ)

Can I use any controller with any collector?

The controller is essentially independent of the type of collector – it works only with temperatures from external sensors. The type of collector (flat, vacuum) does not directly influence the choice of controller. What is more important is the system layout (number of tanks, valves), not the type of collector itself. The only exception are special collectors with built-in sensors, where you must verify the compatibility of this sensor with the selected controller.

How many sensors does a solar controller need at minimum?

At least two: T1 on the collector (usually on the return pipe from the collector or directly on the absorber) and T2 in the tank (in the lower third, where the solar heat exchanger is located). If you have more tanks, a pool or a backup boiler, you need additional sensors (T3, T4…). Rule: one sensor for each point where a control decision is made based on temperature.

What if the controller displays an error code on the display?

It is usually a broken sensor cable (the controller indicates a sensor input error – typically E1, E2, etc.) or a short circuit in the sensor. Turn off the power, check the sensor contacts in the terminal block, measure the resistance of the sensor with a multimeter (at 25 °C it should be approx. 10 kΩ for NTC 10k). If the resistance is correct, the problem may be in the cable route (damaged insulation, moisture). A more detailed diagnostic procedure is described in the article "Common faults of solar controllers and pump units".

Is it worth paying extra for a controller with Wi-Fi and remote monitoring?

For a typical family house, usually not – monitoring via a mobile app is a pleasant feature, but the system works just as well without it. The added value is in rented properties (guest house, cottage), where you want to monitor the system status remotely, or if you are applying for a subsidy and want to document the actual energy balance. If you operate the system yourself and regularly check the boiler room, the display on the controller is completely sufficient.

Is a pump unit necessary, or is a controller with an external pump enough?

Both solutions are technically fully functional. A pump unit is more advantageous in new builds and complete renovations, where the entire solar circuit is built from scratch – installation is faster, everything is hydraulically matched and tested. A standalone controller with an external pump makes sense in renovations, where the pump is already installed (e.g. after replacing an old controller), or where it is necessary to place the pump in a different location than the controller. To compare both concepts, read the article "Difference between a solar pump unit and a standalone controller".

How long will the control unit last and when to replace it?

A quality control unit, with proper installation and maintenance, lasts 15–20 years. Signs that it needs to be replaced are: frequent error messages without an obvious cause, unstable switching (the pump turns on/off without logic), damaged display, incorrect temperature readings even after replacing sensors, or simply the inability to find spare parts (sensors). In practice, controllers last longer than pumps, which need to be replaced sooner due to mechanical wear.

Conclusion – how to decide in 5 steps

Selecting a control unit doesn't have to be complicated if you proceed systematically. I summarize it into five practical steps:

Draw a system diagram – storage tanks, collectors, valves, heat sources. From this, you will immediately see how many sensors and relay outputs you need.
Decide on the type – a pump unit with control (for new installations), or a standalone controller (for renovations or atypical placements).
Check the compatibility of sensors and pump – sensor type (NTC 10k), maximum relay current, method of speed control.
Consider an ECO pump – if the pump unit runs more than 500 hours a year (a typical solar system in Central Europe), the ECO version is almost always worth it.
Check the availability of spare parts and service network – a cheap unit from an unknown brand without support can be a problem in 5 years.

All products in the control unit category are selected with these criteria in mind – you will find there solutions for simple and complex systems, always with available spare parts and technical documentation in Slovak.

Do you have a question on this topic?

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