What controller do I need for solar collectors – selection based on the number of collectors and storage tanks

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Introduction: why the choice of controller really matters

Solar collectors without a properly selected control unit are like an engine without control electronics – they can physically function, but they will never operate optimally. The controller decides when to start the pump, when to stop it, how to protect the system from overheating, and in the case of more complex schemes, also which storage tank has priority and when to activate the auxiliary heating. If you choose the wrong controller – the pump runs unnecessarily even at night, the tank overheats, the expansion vessel cannot keep up, and you wonder why your solar system is not paying off. Choose the right one – the system runs quietly, reliably, and with minimal losses.

In this article, we will go through the entire decision-making process from scratch: what the controller actually does, what types exist, how the requirements differ between one collector and one storage tank compared to larger systems, and what to watch out for in specific products. If you are wondering about the difference between a solar pump unit and a separate controller, see the similarly named topic in the Knowledge Centre – here we will focus mainly on practical selection based on the number of collectors and storage tanks.

How a controller works in a solar system – the basis for the correct selection

The controller (control unit) measures temperatures at two or more locations – typically on the collector (sensor T1) and in the storage tank (sensor T2, usually in the lower third of the tank). Based on the temperature difference ΔT, it decides whether the pump should run. A typical logic looks like this:

The pump turns on when the temperature on the collector exceeds the temperature in the storage tank by a set difference – most commonly 5–8 °C (activation hysteresis).
The pump turns off when the difference drops below 2–3 °C (deactivation hysteresis), or when the storage tank reaches the maximum set temperature (typically 60–80 °C).
Protective functions include protection of the collector against frost (circulation at low temperatures), protection of the storage tank against overheating (stopping the pump), and night protection against reverse heat flow.

The more complex the system – more storage tanks, more circuits, a bivalent tank with a solar and boiler heat exchanger – the more inputs and outputs the controller needs, and the more carefully the selection must be considered.

Basic system: 1–3 collectors, 1 storage tank

This is by far the most common case in family homes. The collector area ranges from 4 to 10 m², the storage tank has a volume of 200–400 liters, and the entire system operates on one circuit with one pump. In most cases, you do not need anything sophisticated – a simple two-sensor controller is sufficient.

A typical example is the Euroster 813 Solar. It is a compact, simple controller with two temperature inputs (T1 collector, T2 storage tank) and one relay output for the pump. You set the ΔT for activation and deactivation, the maximum temperature of the storage tank, and that is basically all. A qualified electrician can install it, and the configuration takes about 15 minutes. For a system with 2 collectors and a 300-liter storage tank, this is an optimal choice – you do not need to pay for functions you will not use.

When a simple controller is not enough even for a small system

There are exceptions when even with a single storage tank you need a more sophisticated controller:

Bivalent tank with two heat exchangers – a lower solar one and an upper boiler one. The controller must be able to coordinate solar and auxiliary heating and possibly block the boiler while there is enough solar energy.
Storage tank with electric auxiliary heating – if you want the controller to automatically activate electric heating in the evening when there is not enough solar energy, you need a second relay output.
Legionella protection – regular short-term heating of the storage tank to 60 °C to eliminate Legionella bacteria requires a timer function in the controller.
Systems with flow measurement – if you want to monitor actual performance and measure solar gains (for example, for subsidy purposes), you need a controller with an input for a flow meter.

Medium system: 4–8 collectors, 1 storage tank or 2 storage tanks

Things get more complicated here. A larger collector area brings higher performance, but also a higher risk of overheating – and a storage tank with a volume of 500–800 liters (or two tanks together) requires more thorough control. In practice, we see this in family homes with pool heating, small guesthouses, or combined systems (solar heating + floor heating).

For such applications, pump stations with integrated control electronics are suitable, such as the Solar Pump Unit ZP2-12 ECO. It integrates a circulation pump, ball valves, safety valve, pressure gauges, filling and draining valve, and a controller – in other words, the complete heart of the solar circuit in one compact block. Installation is faster, the hydraulics are airtight from the manufacturer, and the risk of installation errors is significantly reduced.

Two storage tanks: priority and switching of circuits

If you have two storage tanks – for example, one for DHW and another for a swimming pool or floor heating – the controller must handle the priority. In practice, this works so that the DHW tank always has priority: until it reaches the desired temperature, solar heat goes there only. Only after reaching this temperature does the controller switch to the second tank (swimming pool). This switching is ensured by a three-way valve or a second pump controlled by the controller via a second relay output.

Such schemes are suitable for the Solar Pump Control Unit ZPS 18e - 01 ECO. This is a pump station with integrated electronics that can handle several hydraulic schemes, contains multiple temperature inputs and relay outputs. In practice, we install such a station in family houses with a living area over 150 m², where DHW heating is combined with storage tanks for floor heating circuits or boiler rooms.

Larger system: 8 or more collectors, multiple storage tanks

In such installations – small hotels, guesthouses, apartment buildings, agricultural operations – we enter the area where a simple pump station is not enough and the entire hydraulic circuit must be carefully designed. The controller here must monitor 4–6 temperatures simultaneously, control multiple pumps and valves, manage DHW preparation for a larger number of people, and at the same time prevent stagnation during summer heatwaves.

In these applications, flow measurement is practically essential – not only for the sake of energy balance, but also for diagnostics. If the flow drops below the set value, it may indicate air in the circuit, a worn-out pump or a narrowed heat exchanger. Therefore, it makes sense to install the Electronic Flow Meter for GH 26 – the device accurately measures the volume of the medium flowing through and the controller can calculate the current thermal power of the system based on the flow and temperature difference.

Hydraulic schemes in larger systems

Larger systems usually operate with one of these hydraulic schemes:

Parallel connection of storage tanks – both tanks are heated simultaneously; simpler hydraulically, but less efficient when solar energy is insufficient.
Series connection of storage tanks – the medium first passes through the colder tank, then through the warmer one; higher efficiency, but more complex hydraulics and control.
Scheme with a priority tank and switching valve – the most common compromise in practice; the controller controls the three-way valve according to the temperatures.
Systems with an accumulator tank – with a larger collector area, heat is accumulated in a single large tank (1 000–5 000 liters) and then distributed to consumers.

Special case: MiniSOL systems and compact sets

There are also compact solar sets on the market – typically 1–2 flat or tubular collectors with a 150–200 liter storage tank – intended for smaller houses or recreational buildings. For these systems, dedicated pump stations designed specifically for the set are suitable, such as the Solar Pump Unit ALEX HX10 for MiniSOL control. This unit is shape- and hydraulically optimized for the specific collector setup, has pre-set parameters, and its installation is significantly simpler than with universal stations.

In practice, we see these sets most often in mountain chalets or garden houses, where there is no permanent residence, the DHW volume is low (1–3 people), and the customer wants the simplest possible operation without the need for regular service by a technician. A dedicated station makes perfect sense here – fewer settings, fewer errors, fewer service calls.

Key parameters for comparing controllers

When selecting a controller (whether standalone or integrated into a pump station), keep the following parameters in mind:

Number of temperature inputs: at least 2 (T1 collector, T2 storage tank); for 2 storage tanks you need 3; for complex schemes 4–6.
Number of relay outputs: 1 output = 1 pump or 1 valve; for larger systems at least 2–3.
Maximum allowable temperature at input T1: with vacuum tube collectors, stagnation temperature can reach 200–280 °C; the controller and its sensors must withstand this (standard PT1000 sensors usually up to 130 °C; for vacuum collectors, require sensors with a range up to 200 °C).
Power supply: most controllers operate on 230 V AC; some smaller versions work on 12 V DC (suitable for off-grid applications).
Display and interface: a simple numeric display is sufficient for most domestic installations; a graphical display with curves and history is a convenience, not a necessity (unless you are working for an investor who wants reporting).
Communication interfaces: Modbus, RS-485, Wi-Fi or LAN connection will be appreciated in larger systems or if you want remote monitoring. For standard family homes, it is a nice bonus, but not a requirement.
Hydraulic schemes in the controller memory: some controllers have pre-set hydraulic schemes (e.g. scheme 1 = 1 collector + 1 storage tank, scheme 3 = 2 storage tanks with priority), which greatly simplifies setup.
Protective functions: frost protection, overheating protection of the storage tank, stagnation protection, and protection against reverse heat flow at night – this is standard for any decent controller.

Controller settings after installation: what not to forget

Even the best controller will not work properly if it is set up incorrectly. From practical experience, we know that up to one-third of service calls to solar systems are caused by incorrect parameter settings – not by hardware failure. Here are the most important points:

ΔT activation: we recommend 5–7 °C for flat collectors, 7–10 °C for vacuum tube collectors (they have lower heat losses and the pump could be unnecessarily activated under low solar conditions).
ΔT deactivation: typically 2–3 °C; with too low a value, the pump "toggles" (starts and stops quickly), which shortens its lifespan.
Maximum tank temperature T-MAX: for drinking water 60–65 °C (a compromise between hygiene and safety), for a DHW tank or pool 70–80 °C.
Frost protection function: activate when the collector temperature drops below 3–5 °C; circulation temperature is usually 10 °C.
Anti-stagnation function: during summer heatwaves, the collector reaches stagnation temperature; the controller should stop the pump in time to prevent the medium from boiling in the circuit.
Sensor calibration: new sensors occasionally show a deviation of ±1–2 °C; compare with a reference thermometer and adjust the offset in the controller menu.

A detailed setup procedure can be found in the Knowledge Center article How to set up a solar system controller for maximum efficiency – we recommend reading it after installation, before the first heating season.

Pump station vs. standalone controller: what to choose

This is a question we face with every larger order. Short answer: for family homes and small businesses, a compact pump station with an integrated controller is almost always the better choice. It is long gone when you had to buy a pump separately, a controller separately, pressure gauges separately, a safety valve separately, and then assemble everything at the installation. Integrated solutions are hydraulically balanced, dimensioned for efficiency, and their warranty claims are simpler (one manufacturer = one responsible party).

A standalone controller makes sense when you have a non-standard hydraulic system (for example, an existing pump from another manufacturer that you want to keep), or when you are doing a major renovation and need to adapt the control to a specific scheme. More on this topic can be found in the article Difference between a solar pump unit and a standalone controller in the Knowledge Center.

Flow meter in the system: when it is essential and when it is just nice to have

A flow meter is not mandatory for every system, but its value increases with the size of the installation. In a small system (2 collectors, 1 storage tank), it mainly serves as a diagnostic tool – you can see how many liters per hour pass through the circuit and adjust the flow accordingly (the optimum is 40–60 l/h per 1 m² of collector at unit flow, or about 700–900 l/h·m² at High-Flow concept). In larger systems, the flow meter is part of thermal energy measurement (calorimeter) – it combines flow with temperature difference and calculates kWh. You will appreciate this especially with subsidies, where performance measurement is a condition for billing.

Information on connecting a flow meter and its integration into the controller can be found in the article Electronic flow meter in a solar system – what it is for and how to connect it.

Typical errors in controller selection and their consequences

Over the years of sales and installation practice, we have seen recurring mistakes that unnecessarily complicated our customers' lives:

Undersized controller for a complex system: the customer bought a simple two-sensor controller, but the system had two storage tanks. Result: both tanks were heated incoordinately, TÚV was cold in summer because the heat went to the pool.
Sensors with insufficient temperature range: vacuum collectors reached 220 °C in summer and damaged the plastic housing of sensor T1. The controller then reported an error and the pump did not work for three months – the customer found out only in autumn.
Inappropriate ΔT setting: ΔT deactivation set to 1 °C – the pump turned on and off every 30 seconds in transitional weather, the bearing was worn out in a year.
Lack of frost protection: without setting the antifreeze function, the circulation did not start during the first frosts and the old solution without sufficient glycol concentration froze in the collectors.
Purchase of a controller without checking compatibility with the pump: some ECO pumps with electronically regulated speeds require a specific signal output from the controller (0–10 V or PWM); if the controller has only a contact output, the pump runs at full speed only.

Further faults and their diagnostics can be found in the article Common faults of solar regulators and pump units.

ECO versions of regulators and pump stations: is the extra cost worth it?

We also offer ECO versions of pump stations in our range. The difference lies in the type of pump used: standard versions have asynchronous pumps with constant speed (and therefore constant power), while ECO versions have synchronous or EC pumps with electronically regulated speed. The advantage? Electricity consumption drops by 50–70 % – instead of 50–80 W, an ECO pump consumes only 8–20 W at partial load. In a solar system where the pump runs for 1 500–2 000 hours per year, this results in a saving of 50–120 kWh per year – which at an electricity price of 0.20–0.25 €/kWh means a saving of 10–30 € per year. The extra cost for the ECO version is therefore paid back in 3–7 years, but the pump lasts 15–20 years, so the economics make sense. More details can be found in the article Solar pump unit ECO vs. standard – what is the difference and when is it worth it.

Step-by-step procedure for selection

To conclude, we give you a simple methodical procedure, according to which we have prepared dozens of customer orders:

Determine the number of collectors and their type (flat / vacuum tube).
Determine the number of storage tanks and their function (TÚV, pool, heating, accumulation).
Decide whether you want flow measurement (for subsidies, monitoring, larger systems).
Find out whether the storage tank has electric or boiler auxiliary heating – this determines the required number of relay outputs.
Check the compatibility of sensors with the maximum temperature of the collector (vacuum = minimum 200 °C range).
If you have an ECO pump with speed control, check the type of control signal.
Select the product category: simple regulator / basic pump station / pump station with complex regulation.
Check that the selected product has a hydraulic diagram in memory corresponding to your installation – you will save hours of setup.

Most frequently asked questions (FAQ)

Can I use a regulator from another manufacturer than my pump station?

Technically yes, provided the electrical parameters are compatible (supply voltage, type of relay output, sensor impedance – typically PT1000 or NTC 10kΩ). In practice, we recommend this only for experienced installers. If the pump station manufacturer guarantees compatibility with a specific regulator, stick to that recommendation – it will save you trouble with warranty and service.

How many sensors do I need for a system with two storage tanks?

At least 3: T1 on the collector, T2 in the first tank (usually TÚV, lower third), T3 in the second tank (again lower third). If you also want to measure the temperature at the collector outlet and return (for ΔT and heat output calculation), you need 4–5 sensors. Regulators for larger systems usually support 4–6 temperature inputs by default.

How long will a solar system regulator last without failure?

From practice, we speak of 10–15 years with proper installation and minimal maintenance. The most common failing component is the capacitor in the power supply – after 8–12 years, the display may start to drift or the regulator may randomly reset. PT1000 sensors can practically remain unchanged for 20+ years, provided they are not mechanically damaged. Regular annual service (checking the tightness of the circuit, pressure, and glycol mixture condition) significantly extends the life of the entire system – more in the article Maintenance and service of solar pump units.

Can I control the solar system via mobile or smartphone?

It depends on the specific regulator. Common simple regulators (such as Euroster 813 Solar) do not have Wi-Fi or LAN; they serve purely as local control. If you want remote access, monitoring, alarm notifications by email or push notifications in an app, look for regulators with Wi-Fi or Ethernet interface. For larger systems, Modbus/RS-485 is standard, which connects to a central Home Automation system or BMS. For most family homes, this is not necessary – a simple annual check of the values on the display during a service inspection is fully sufficient.

What happens if the regulator fails in summer – will the system be damaged?

It depends on the scenario. If the regulator completely loses power, the pump stops and the collector reaches stagnation temperature – the glycol medium may start to boil and evaporate into the expansion vessel. A properly dimensioned expansion vessel (usually 10–18 liters for a typical family system) can handle this without damage. Problems arise if the regulator "freezes" in the "pump on" state at extremely high temperatures – this is rare, but can cause the mixture to shift into the storage tank. Therefore, a good installation includes a safety valve set to 6 bar and a properly pre-charged expansion vessel. More in the article Common faults of solar regulators and pump units.

What is the price of a regulator and where is it worth saving (and where not)?

A simple two-sensor regulator (Euroster 813 Solar and similar) usually costs 40–80 €. A compact pump station with a regulator (ZP2-12 ECO) is in the range of 250–450 €, more advanced solutions for larger systems (ZPS 18e-01 ECO) range around 400–700 €. Do not recommend saving on the regulator itself beyond the technical need – if your system technically requires 3 sensors, buy a regulator with 3 sensors. Where to save? On extra-standard functions you will not use: a graphical display with touch control, Wi-Fi, integrated calorimeter – these are nice features, but if they are not required by the project, they add costs without real utility.

Conclusion: the right regulator is the basis of a functional solar system

Selecting a regulator for solar collectors is not rocket science, but it requires a systematic approach. Start from the number of collectors and storage tanks, determine which functions you really need, and then choose a product with the corresponding number of inputs, outputs, and protective functions. For a simple home setup of 2–3 flat collectors and one TÚV storage tank, a compact and affordable regulator is fully sufficient. For a larger and more complex system, an investment in a more advanced pump station with an integrated regulator pays off in the form of higher solar gains, lower component wear, and fewer service calls.

If you are unsure which product is suitable for your specific system, take a look at the entire category of control units or other topics in the Knowledge Center – for example, How to choose a control unit for a solar system or Step-by-step installation of a solar pump unit. Every system is a bit different, but the basic principles always apply the same.

Do you have a question about this topic?

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