How to connect solar system control with a water heater or TÚV tank

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How to connect solar system control with a water heater or TÚV tank

A solar system without properly set control is like a car without a steering wheel – the collectors do capture solar energy, but without control logic, this energy is either wasted or – worse – causes a failure of the entire system. Connecting the control to a water heater or a TÚV (hot water) tank is the core of the entire solar solution. It is precisely here that it is decided whether the system will work efficiently, safely, and reliably throughout its lifetime.

In practice, we see dozens of different configurations – from a simple family house with one tank and two collectors, to recreational facilities with multi-stage heating, two tanks, and cascade control. This article will guide you through the entire process of connecting the control with a water heater or TÚV tank: from the basic principles of differential control, through the physical connection of sensors and the pump, to specific settings and typical errors that occur during implementation.

What the solar system control actually does

Before we get into specific wiring diagrams, it is important to understand the basic logic that the solar control performs. It is not a standard room thermostat that simply monitors the room temperature and turns on the boiler. Solar control works on the principle of differential temperature comparison – it compares the temperature at the collector outlet (or at the return pipe of the collector) with the temperature of the water in the TÚV tank and, based on this difference, decides whether to start the circulation pump.

When the collector is warmer than the tank by a defined value (typically 5–10 °C), the control starts the pump and begins to circulate the heat transfer medium. When the difference decreases (typically to 2–4 °C), the pump stops. This is not only about saving pump energy – it is mainly about preventing heat from being transferred back from the tank to the cold collector (which would happen if the pump were not turned off in time).

In addition to this basic differential control, modern controls and monitoring stations also monitor other values: maximum tank temperature (protection against overheating), maximum collector temperature (stagnation), minimum collector temperature (frost protection), and various other safety limits. These functions are particularly important when connecting to a water heater or tank.

Types of TÚV tanks and their impact on control wiring

Not all TÚV tanks are wired the same way, and before selecting a control, it is necessary to know which type of tank you are working with. TÚV tanks for solar systems differ mainly in the way heat is transferred:

Indirect heating tank with a coil (solar spiral) – This is the most common type in family homes. The tank has a built-in coiled heat exchanger in the lower part, through which the heat transfer medium from the solar collectors flows. The control manages the circulation pump of the solar loop, and sensor T2 is placed in the tank (usually in the lower third).
Tank with two heat exchangers – The lower heat exchanger is for the solar loop, the upper one for heating from the boiler. The control must be set up so that sensor T2 measures the temperature in the area of the solar heat exchanger (lower third), not in the upper part of the tank.
External plate heat exchanger – In some installations (especially higher performance, legionella requirements), an external plate heat exchanger is used between the solar loop and the tank. In this case, two pumps must be connected, and the control must manage both.
Combined tank (combi-tank) – A tank with an integrated heating element or combined with a heat pump. In this case, the solar system control is only one of several control layers, and the connection is more complex.

For most standard family homes, a tank with one solar spiral and a control of the type differential thermostat or compact monitoring station is sufficient. If you have a tank with a volume up to 300 liters and 2–4 flat collectors, for example, the Euroster 813 Solar will cover your needs. It can directly control one pump and has inputs for two temperature sensors (collector + tank).

Placement of temperature sensors – the most critical step of the entire installation

Correct placement of temperature sensors is probably the most common cause of poor performance of solar systems. We have seen dozens of installations where the system "worked", but delivered 20–30 % less energy than it could – just because the sensor was placed in the wrong location.

Sensor T1 – collector temperature

Sensor T1 (collector temperature) is installed either directly into the sensing housing on the collector (typically at the collector outlet or on the return pipe just behind the collector), or into a special submersion bracket on the collector pipe. Important rules:

The sensor must be thermally insulated from the surroundings to measure the actual medium temperature, not the ambient air.
With a series connection of multiple collectors (e.g., 4 flat collectors in series), T1 should be placed at the outlet of the last collector in the flow direction – where the medium is hottest.
With a parallel connection (collector manifold and distributor), T1 is on the outlet manifold pipe.
The cable route from the collector to the control must be resistant to UV radiation and temperatures up to 180 °C (during stagnation, the temperature in the collector can be very high).
Recommended sensor type: NTC 10 kΩ at 25 °C – this type is used by most standard solar controls including Euroster and ZPS stations.

Sensor T2 – tank temperature

Sensor T2 (tank temperature) is even more sensitive to correct placement, as it directly determines when the pump starts and stops. Basic rules:

Always in the lower third of the tank – not in the middle, not at the top. The tank is thermally stratified: cold water is at the bottom, hot water at the top. If the sensor were in the middle or upper part, the control would see a higher temperature and the pump would not start even when the lower cold part of the tank could still accept solar heat.
Ideal height: 1/4 to 1/3 of the tank height from the bottom – this is the area just above the solar spiral or at its level.
The tank should have a submersion bracket (well) for the sensor. Never install the sensor on the surface of the tank using tape – the measurement will be inaccurate by 10–15 °C and the system will operate inefficiently.
If the tank has two heat exchangers (solar + boiler), sensor T2 must be at the solar heat exchanger, not at the boiler one.

Hydraulic and electrical connection diagram of the regulation with a storage tank

Let's now look at a specific connection diagram of the regulation with a simple TÚV storage tank. This configuration is by far the most common in family homes in Slovakia and covers about 80% of all installations we encounter.

Hydraulically, the solar circuit consists of: collectors → outlet pipe (hot branch) → circulation pump → expansion tank → check valve → tank (solar coil, inlet) → tank (solar coil, outlet) → back to the collectors. Electrically, the connection is as follows:

The regulation (e.g., Euroster 813 Solar) is powered from 230 V.
Sensor T1 (collector) is connected to the regulation's sensor input 1.
Sensor T2 (tank) is connected to the regulation's sensor input 2.
The circulation pump is connected to the regulation's relay output (230 V, usually max. 3–5 A for a standard circulation pump).
Possible additional functions: output signal for the electric tank heater (heating element) – the regulation can turn it on when the tank temperature is low on unfavorable days.

ZPS control stations as a comprehensive solution for more complex installations

For installations beyond one storage tank and two collectors, or where you want to have the hydraulics, pump, pressure gauge, flow meter, and regulation in one compact unit, control stations are more suitable. There are several variants in the range differing in power and flow:

ZPS 6 control station is intended for smaller installations – typically 2 to 5 flat collectors or 1 to 2 vacuum tube collectors with a flow rate up to 6 l/min. It includes a complete hydraulic block (ball valves, check valve, air vent, pressure gauge, flow meter) and an integrated regulation. It is suitable for tanks up to 300 liters.

ZPS 16 control station covers medium installations with a flow rate up to 16 l/min – ideal for 5 to 12 flat collectors and tanks from 300 to 800 liters. This station is suitable, for example, for larger family homes or small recreational facilities.

ZPS 28 control station is the most powerful in the range, intended for large systems with a flow rate up to 28 l/min. It is used for 12 or more collectors, large tanks (800 liters and more), or in industrial applications. We typically encounter it in hotels, guesthouses, or larger apartment buildings.

The advantage of control stations over individual regulation is that the hydraulic block is already pre-assembled and tested at the factory. The installer simply installs it into the solar pipe and connects the sensor cables. This significantly reduces installation time and reduces the risk of installation errors. For more information on selecting the correct size, see the article "How many collectors can the ZPS 6, ZPS 16 and ZPS 28 control station handle?"

Connection with two storage tanks – two-zone regulation

A specific situation is an installation with two TÚV storage tanks – for example, when we want to preheat water for the boiler storage tank using a solar system. This scenario is common in renovations, where the existing boiler has its own storage tank and the solar system is added as a preheating stage.

The scheme works as follows: the solar circuit heats the solar storage tank (preheating, 200–300 l), from which cold water first goes through this preheating tank and then into the boiler storage tank. The regulation controls only the solar circuit, and the boiler regulation operates separately. Key settings for this connection:

Sensor T2 is still located in the lower third of the solar storage tank (preheating).
Maximum tank temperature (T-max) is set to 70–75 °C – higher temperatures would unnecessarily burden the boiler circuit and reduce efficiency.
If the regulation supports the legionella heating function, set it to at least 60 °C once every 2–3 days (manually or automatically).
The boiler should not "compete" with the solar system – set its tank to heat only if the preheating temperature drops below 40 °C.

Some modern regulations and control stations also allow direct control of two storage tanks – for example, by switching a valve alternately or using a double relay output. In this case, T2 is in the first tank and T3 in the second, with the regulation charging the tank with the lower temperature (priority control).

Setting regulation parameters after connection

The physical connection is only half the work. The regulation must be configured correctly – and it is precisely here that many installations are compromised. Basic parameters that must be set after connection:

Differential temperature for activation (ΔT-on)

This is the temperature difference between the collector (T1) and the tank (T2) at which the pump is started by the regulation. Too low a value (e.g., 3 °C) causes the pump to run even when there is no real energy gain – the medium circulates through the circuit, but in reality, heat is not transferred to the tank because the temperature difference is too small. Too high a value (e.g., 15 °C) causes the pump to start too late and morning gains are lost.

Recommended ΔT-on setting: 6–8 °C for flat collectors, 5–7 °C for vacuum tube collectors. Vacuum collectors have lower thermal losses, so they can operate at a smaller temperature difference.

Differential temperature for deactivation (ΔT-off)

This is the temperature difference at which the regulation stops the pump. It must always be lower than ΔT-on, otherwise continuous oscillating switching and deactivation would occur. Typical setting: 2–4 °C. For example: the pump starts at ΔT = 7 °C and stops at ΔT = 3 °C.

Maximum tank temperature (T-max tank)

When the tank reaches this temperature, the pump is automatically turned off regardless of the differential temperature. This protects the tank from overheating and also protects the heat transfer medium (propylene glycol) from degradation at high temperatures. Recommended setting: 60–65 °C for standard TÚV tanks. For tanks with a legionella function, set to 70 °C and activate legionella heating at least once every 2 weeks.

Maximum collector temperature (T-max collector / stagnation protection)

Some regulations allow you to set the maximum collector temperature at which the system switches to "night cooling" – the pump runs at night when the tank is full to remove excess heat. This function is especially useful in summer when the tank reaches T-max before noon and the collector then stagnates. Recommended value: 130–140 °C (permitted stagnation), activation of night cooling at 120 °C.

Frost protection (T-min collector)

If the collector temperature drops below the set value (typically 3–5 °C), the regulation starts the pump for a short time to heat the medium from the tank heat exchanger and protect the collectors from freezing. This function is important even when using an antifreeze mixture – in extreme cold, the glycol concentration may be insufficient. Set T-min to 4 °C.

For more information on correctly setting all these values, see the article "Setting differential temperature in solar regulation: how to correctly configure switching."

Common errors when connecting the regulation to the tank

From practice, we know several errors that repeat in every second installation. An overview of the most common problems and their solutions:

Sensor T2 too high on the tank – The regulation sees a higher temperature and the pump does not start, even though the lower part of the tank is still cold. Solar gains are lost. Solution: move T2 to the lower third of the tank.
Dipped or poor insulation of the sensor cable – Moisture in the cable jacket causes a shift in the NTC sensor resistance and the regulation measures an incorrect temperature. Typical symptom: the pump runs unexpectedly or not at all. Solution: replace the cable, seal the connection.
Incorrect differential temperature – too low – The pump runs constantly, the tank is not heated well, increased pump consumption. Check ΔT-on and set it to at least 6 °C.
No maximum tank temperature protection – The tank overheats to 90 °C+ in summer, the safety valve drips, the heat transfer medium degrades. Set T-max tank to 65 °C.
Pump with too high power – The flow through the collectors is too high, the medium does not have time to transfer heat and returns to the tank almost as warm as it left. The hydraulic calculation recommends 30–50 liters/hour per m² of collector area. Set the flow rate on the control station using a flow meter.
Absence of expansion vessel or incorrect pre-charge – When the medium is heated, the pressure rises above the safe level, the safety valve opens and the medium leaks from the system. Control stations ZPS have a connection for an expansion vessel; set the pre-charge to 1.5–2 bar depending on the system height.

A detailed analysis of faults and error messages can be found in the article "Common faults in solar regulation: error messages, pump failures and inaccurate sensors".

Specific scenarios from practice

Scenario 1: Reconstruction of an old water heater – adding a solar system

A very common task: the customer has an existing 150-liter electric water heater and wants to connect 2 flat collectors to it. However, the water heater does not have a solar coil – it is a standard electric water heater with a heating rod. Solution: replace the water heater with a storage tank with a solar coil (200–250 l), or connect an external plate heat exchanger between the solar circuit and the existing water heater.

When using an external heat exchanger, it is necessary to connect two pumps (one on the primary solar circuit, the second on the secondary circuit heat exchanger–water heater), and the regulation must have two outputs or a separate regulation for the secondary circuit must be used. For such a scenario, a regulation with the "dual circuit" function or a more compact control station that can be configured for an external heat exchanger is suitable.

Scenario 2: Single-family house with floor heating + DHW

The customer wants to use the solar system not only for heating DHW, but also to support floor heating during the transitional period (spring, autumn). In this case, the connection of the regulation is more complex: the solar circuit charges a combined storage tank with two heat exchangers – a lower one for the solar circuit and an upper one for the boiler. The floor heating regulation is separate (room thermostat + mixer), and the solar regulation controls only the solar circuit. Key setting: DHW priority over heating – set the T-max of the storage tank for solar to 60 °C, and the boiler should only heat the heating if the DHW tank has at least 50 °C.

Scenario 3: DHW storage tank in the basement, collectors on the roof – long pipe

When the distance between the storage tank and the collector is more than 10–12 meters, hydraulic resistance and pressure losses in the pipe increase. The pump must be dimensioned for higher pressure (e.g. Wilo Star-ST 15/6 or equivalent), and the flow meter on the control station will indicate this. Another problem is heat loss from the long pipe layout – an uninsulated pipe in a cold room can absorb 15–20 % of solar gains. All pipes in the solar circuit must be insulated with thermal insulation resistant to temperatures up to 200 °C (e.g. Armaflex HT).

Most frequently asked questions (FAQ)

Can I connect a solar regulation to an existing water heater without a solar coil?

This is not possible directly without modification. Solar regulation controls the heat transfer medium pump, which must transfer heat via an exchanger – either an internal solar coil directly in the tank, or an external plate heat exchanger. If your water heater does not have a solar coil, you have two options: replace the water heater with a storage tank with a solar coil (the cleanest solution), or connect an external plate heat exchanger with two pumps (technically more complex, more expensive, but feasible).

To what temperature should I set the T-max of the storage tank?

We recommend 60–65 °C for normal DHW use. This temperature is sufficient from a hygiene point of view (elimination of legionella), the tank will not overheat and the heat transfer medium (glycol) will remain in good condition. In summer with intense sunlight, the tank heats up quickly and T-max is reached before noon – this is normal. If you set T-max too high (80–90 °C), you risk the safety valve, degradation of glycol and excessive pressure in the system.

What is the difference in connection between Euroster 813 Solar and the control station ZPS?

Euroster 813 Solar is the control unit itself – it can be compared to the brain of the system. You must separately obtain and install a separate pump, ball valves, check valve, air vent, pressure gauge and expansion vessel. The ZPS control station contains all these hydraulic components integrated in one block together with the regulation. For an installer, the ZPS station is therefore easier to install, but more expensive. For a do-it-yourself solution, the Euroster 813 together with ordered components is sometimes more practical, as you have greater flexibility in placement. More about this comparison can be found in the article "Euroster 813 Solar vs. ZPS control stations: comparison of functions and use".

How many sensors do I need for a tank with two heat exchangers (solar + boiler)?

The basic configuration still requires 2 sensors: T1 on the collector and T2 in the tank (at the solar heat exchanger in the lower third). The boiler has its own regulation and its own tank temperature sensor – these systems work in parallel and do not interfere with each other. If you want to also have the function of legionella heating or night cooling via the tank, a third sensor T3 in the upper part of the tank may also be needed – it depends on the specific model of the regulation and its functions.

What to do if the pump is running, but the tank is not heating up?

This is one of the most common service problems. First thing: check the flow – if the flow meter is at zero despite the pump running, the system is probably airlocked or clogged. Second thing: check the temperatures T1 and T2 on the regulation display – if T1 is not significantly higher than T2, either the T1 sensor is faulty or the collectors are not getting enough sunlight. Third thing: check the flow direction – if the system was incorrectly connected (the medium is flowing in the opposite direction), the heat exchanger in the tank is not transferring heat properly. A detailed diagnostic procedure is described in the article "Common faults in solar regulation: error messages, pump failures and inaccurate sensors".

Is regular inspection of the regulation connection necessary?

Yes, at least once a year before the start of the season, we recommend a visual inspection of the sensor cables (cracking of the insulation on the roof from UV radiation), checking the T1 and T2 values on the display, checking the pressure in the system (it should be 1.5–2.5 bar in the cold state) and checking the flow (flow meter on the ZPS station). A detailed procedure can be found in the article "Maintenance and inspection of solar regulation: what to check before and after the heating season".

Conclusion: the correct regulation is an investment, not just an add-on

Connecting a solar system regulation to a water heater or DHW storage tank is not a complicated operation if you understand the basic principles and follow proven procedures. The key is the correct placement of sensors, correct configuration of differential temperatures and the selection of regulation appropriate to the size of the system.

For small systems (2–4 collectors, tank up to 300 l), the excellent choice is Euroster 813 Solar in combination with hydraulic components according to the project. For medium and larger systems, where you want to have everything in one compact unit and shorten the installation time, there are ZPS 6, ZPS 16 and ZPS 28 according to the system performance.

A well-designed and properly set up solar system can cover 55–70 % of the annual energy consumption for DHW heating in Slovak conditions. This figure, however, decreases to 30–40 % if the regulation is incorrectly connected or improperly configured. Time spent on proper regulation setup is returned many times over – whether on the energy bill or on the lifespan of the entire system.

Do you have a question about this topic?

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