Installation of solar system regulation: pump, sensors and expansion tank connection

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Installation of solar system regulation: connecting the pump, sensors and expansion vessel

The installation of a solar system is not only about collectors and the storage tank. In fact, the regulation, pump and expansion vessel form the "nervous system" of the entire device – and their correct connection determines whether the system will work reliably for ten years or will trouble you from the first season. In practice, we see that most of the problems when commissioning solar systems stem precisely from errors in the installation of the electrical part of the regulation, from incorrect placement of sensors or from an undersized expansion vessel. This article walks through the entire process step by step – from the placement of temperature sensors, through connecting the circulation pump to the regulator, to the calculation and installation of the expansion vessel.

If you are still deciding on the choice of regulation, I recommend reading the article "How to choose a regulation for a solar system: what to pay attention to" and "Euroster 813 Solar vs. ZPS control stations: comparison of functions and use" – there you will find an overview of which type of regulator is suitable for which system. Here we will focus purely on installation and connection.

Basic topology of the solar circuit: what the regulation must "see"

Before you reach for the screwdriver and terminal block, it is good to understand the logical basis of the regulation of a solar system. The regulator works on the principle of a differential thermostat: it compares the temperature at the collector (sensor S1) with the temperature in the storage tank (sensor S2) and switches or turns off the circulation pump according to the set difference. When the collector is sufficiently warmer than the tank, the pump runs – energy is transferred. When the difference is small or negative, the pump stops.

In addition, the regulation can monitor other values: the maximum temperature of the tank (protection against overheating), the return temperature, or possibly control multiple circuits at once. That is why there are products of different complexity – from a simple Euroster 813 Solar to a compact ZPS 28 control station, which has an integrated pump, safety valve and drain valve in one unit.

The diagram above captures the basic logic: the regulation reads two temperatures, controls one pump and the entire circuit is protected by an expansion vessel. In more complex systems (two circuits, a tank with two heat exchangers), the topology is expanded, but the principle remains the same.

Temperature sensors: where exactly to place them and how to connect them

Sensor on the collector (S1)

This is the thermometer that the regulation uses as a reference value "is there something to transfer?". The correct position is in a special housing (bore) directly on the output pipe of the collector – ideally as close as possible to the output of the absorber, maximum 10–15 cm from the collector. The further the sensor is from the absorber, the greater the delay – the regulation reacts later and during short sunny periods it may miss part of the yield.

An important point that people underestimate: the sensor on the collector must be in contact with the pipe, not just close to it. A proven practice is to use thermal paste (same principle as with processors) and to press the sensor firmly with a clamp or spring terminal. On roof pipes that are exposed to frost, wind and UV radiation, additionally insulate the sensor with mineral wool or PE insulation – otherwise it will measure the temperature of the surrounding air, not the pipe.

Run the cable from the sensor to the regulation separately from the power cables of the pump. The NTC thermistor signal is in the millivolt range and interference from the 230V cable can cause jumps in the measurement. In practice, it is sufficient to maintain a minimum distance of 5 cm or to run the cable in a separate installation pipe.

Sensor in the tank (S2)

The tank sensor determines when the tank is sufficiently charged and when the system should stop. The placement depends on the construction of the tank:

Tank with an immersion housing (bore): insert the sensor into the bore, which is usually located at the height of the lower third of the tank – that is, where the water is the coldest and where energy can still be stored. If the sensor were placed higher, the regulation would prematurely turn off the pump, because the upper layer is always warmer.
Tank without a bore: the sensor is glued or clamped to the outer shell of the tank and insulated with heat-insulating insulation. The measurement accuracy is 2–4 °C worse, but it is sufficient for basic regulation. Additional insulation around the sensor is mandatory – otherwise you are measuring the temperature of the air in the boiler room.
Tank with two heat exchangers (bivalent): the sensor is placed in the lower heat exchanger, i.e. in the area of the solar heat exchanger, not in the area of the boiler heat exchanger. The height depends on the model – usually the bore is marked "solar sensor" or similar.

Types of sensors and compatibility

The vast majority of solar controllers on the market use NTC thermistors with a resistance value of 10 kΩ at 25 °C (so-called NTC 10k). This is the de facto standard. Always check the resistance characteristic before purchasing a replacement or additional sensor – some manufacturers use NTC 6k8 or Pt1000. Mixing types will cause a systematic measurement error of 5–20 °C, which will completely disable the control.

Euroster 813 Solar works with two NTC 10k sensors, which are usually included in the packaging. With Control Station ZPS 6 and other models from the ZPS series, the sensors are also part of the delivery – check this before ordering, so you don't surprise the customer without sensors on the site.

Connecting the circulation pump to the controller

Electrical connection: terminals, protection and grounding

The circulation pump of the solar circuit is a standard single-phase consumer powered from the grid 230 V / 50 Hz. The control switches it on and off via a relay contact. The connection procedure is usually as follows:

Phase (L): comes from the grid to the controller's input terminal "L" or "LINE"
Neutral conductor (N): comes from the grid and goes in parallel to the controller and directly to the pump
Control phase: from the controller's output terminal "PUMP" or "OUT" to the pump
Ground (PE): mandatory to each metal – controller, pump, frame of the control station

The maximum power of the pump that you can connect directly to the controller's relay is usually 300–600 W (depends on the model – always check the technical data sheet). Modern solar circulation pumps have a power consumption of 20–100 W, so this is not a problem. If you want to connect a larger consumer (e.g. a motor valve with higher current draw), it is necessary to install an auxiliary relay.

One thing we often encounter in practice: with control stations of the ZPS series, the pump is integrated directly in the station. ZPS 6, ZPS 16 and ZPS 28 have a pump, thermometer, safety valve and drain cock in one compact unit. This significantly simplifies installation and eliminates frequent errors when connecting the pump separately. On site, it is sufficient to connect the pipes, sensors and 230 V – and the pump is already wired by the manufacturer.

Pump speed and hydraulic setting

Most solar circulation pumps have 3 speed levels (I, II, III). The correct choice affects the efficiency of the entire system. At too low a speed, the flow is small, the heat transfer medium overheats in the collector and the collector loses efficiency. At too high a speed, the hydraulic losses are unnecessarily large and the pump unnecessarily consumes electricity.

Approximate recommendation: for most home installations with 2–4 collectors and a pipe length of up to 20 m, speed level II is sufficient. Precise setting requires a flow calculation according to the collector area (typically 40–50 l/h per 1 m² of absorber area). For a system with two 2 m² collectors (total area 4 m²), the target flow is therefore 160–200 l/h.

Connection when using control unit ZPS

When you install the control unit instead of a separate controller and pump, the work is significantly easier. The control unit has an electrical connection on the front or side – usually a cover with a terminal block. You only connect: the supply cable 230 V (L, N, PE) and the signal cables from sensors S1 and S2. The pump is internal and connected to the controller by the manufacturer. You save installation time and eliminate the risk of mixing up the wires.

Difference between models: ZPS 6 is suitable for smaller systems (1–2 collectors, tank up to 200 l), ZPS 16 handles medium systems and ZPS 28 is intended for larger installations. A more detailed comparison can be found in the article "How many collectors can the control unit ZPS 6, ZPS 16 and ZPS 28 handle?"

Expansion tank in the solar circuit: calculation and installation

Why the expansion tank is critical in a solar system

A solar system operates under much harsher conditions than standard heating systems. The heat transfer medium (glycol-water mixture) can heat up to 120–180 °C in summer, and even up to 200 °C or more in stagnation (pump not working, sun shining). This causes enormous thermal expansion of the liquid. If there is no place for the expanded liquid to "hide", the pressure in the system rises catastrophically and the safety valve discharges the medium – this not only pollutes the surroundings, but also drains the system and dries out the pump.

The expansion tank therefore serves a dual function: it absorbs the thermal expansion of the medium and stabilizes the operating pressure in the system. For solar systems, special membranes made of EPDM rubber, resistant to glycol-water mixtures and high temperatures, are used – a standard heating expansion tank with a red membrane must not be used in a solar circuit (the membrane degrades within 1–3 years).

Calculation of the expansion tank volume

Simplified formula for dimensioning the expansion tank of a solar circuit:

V_EN = (V_sys × n + V_kolekt) × SF

where:

V_sys – total volume of the medium in the circuit (pipes + heat exchanger of the storage tank, in liters)
n – expansion coefficient of the medium (for 40% glycol at ΔT = 140 °C ≈ 0.12, i.e. 12%)
V_kolekt – volume of the medium in the collectors (stagnation volume – in case of evaporation, this medium must go somewhere)
SF – safety factor 1.2–1.3

Practical example: a system with two flat collectors (volume in the collector 2 × 1.5 l = 3 l), pipe 10 m (DN 18, approx. 2.5 l) and heat exchanger of the storage tank (approx. 1.5 l). Total volume V_sys = 7 l. Expansion 12% = 0.84 l, stagnation volume of the collectors 3 l. Result ≈ (0.84 + 3) × 1.25 = approx. 4.8 l → we choose a standard expansion tank of 8 liters. It is better to dimension a slightly larger tank than a smaller one – an oversizing of 20–40% is completely acceptable and will extend the life of the membrane.

Pre-charge pressure of the expansion tank

The expansion tank has an air cushion (or nitrogen in premium models) pre-charged to a certain value. This pre-charge must correspond to the installation height of the system – specifically the height from the expansion tank to the highest point of the circuit (collectors on the roof).

Formula: p_pred = h / 10 + 0.3 bar

Where h is in meters. For example, at a height of 6 m: 6/10 + 0.3 = 0.9 bar. In practice, the pre-charge is set to 0.8–1.2 bar for most single-family homes. The operating pressure (cold system) should be 0.2–0.3 bar higher than the tank's pre-charge. The safety valve is set to a value the system must not exceed – standard is 6 bar for solar systems.

Where to install the expansion tank

The expansion tank is connected to the cold (return) circuit – never to the hot side of the collector outlet. The reasons are two: hot medium degrades the membrane faster and, in case of stagnation, the medium from the hot side is pushed into the tank, which is not desirable. The correct position is therefore on the return pipe, near the pump (before the pump, if we look in the direction of flow).

The tank must be accessible for checking the pre-charge pressure (manometer, Schrader valve) and must not be exposed to direct sunlight or freezing. Vertical mounting (air cushion up) is preferred, but most tanks also work horizontally. Check the mounting position in the manual of the specific tank.

First filling and air venting of the solar circuit

After assembling the entire circuit and connecting the controller electrically, the critical phase comes – filling the system with a glycol-water mixture and venting air. Air in the circuit is enemy number 1: it causes pump cavitation, corrosion processes, and inaccurate flow measurement.

Filling procedure:

Check the pre-charge pressure of the expansion tank (recommended value according to system height, usually 0.8–1.2 bar) – do this dry, before filling
Fill from bottom to top – this pushes air upwards toward the venting valve
Use a filling pump (hand pump or electric) with a pressure gauge – fill to operating pressure 1.5–2 bar
Turn the pump wheel manually (if this function is available) or let the pump run briefly (5–10 seconds in cycles) and observe whether air is escaping through the vent
After venting, top up the medium to the required pressure and close the filling valve
Check all connections for tightness – at 2 bar, leave the system under pressure for 15–30 minutes and monitor the pressure drop on the pressure gauge

For glycol-water medium: the standard concentration for SR is 30–40% glycol, which ensures protection down to -15 to -20 °C. A higher concentration, although it protects against more severe frost, reduces thermal capacity and increases viscosity – unnecessarily overloading the pump.

Controller setup after installation

After filling and venting, you set up the controller. The basic parameters you must set:

ΔT-ON (differential temperature for turning on): standard 6–10 °C – that is, the pump turns on when the collector is 6–10 °C warmer than the tank. A smaller value = higher yield, but also more pump starts. A larger value = the pump runs less, but less efficiently.
ΔT-OFF (differential temperature for turning off): standard 3–4 °C – the pump turns off when the difference drops below this value. It must be smaller than ΔT-ON, otherwise the pump would cycle continuously.
T-MAX of the tank: maximum tank temperature at which the control stops the pump regardless of ΔT. Standard 60–65 °C for a TÚV tank.
T-MAX of the collector (cooling): with some controllers – when the tank is at maximum and the collectors are too hot, the controller may turn on the pump at night to cool the collectors.

A detailed guide on setting up differential temperatures can be found in the article "Setting up differential temperature in solar control: how to correctly configure switching". If you are connecting the control to a tank or water heater, the article "How to connect solar system control with a water heater or TÚV tank" will help you.

Typical installation errors and how to avoid them

From our experience with customer orders, we know that most problems with solar systems are not caused by product defects, but by installation errors. Here are the most common ones:

Mixed sensors S1 and S2: the control then works in reverse – the pump runs at night and circulates cold medium at night. Symptom: the tank does not heat up in summer, but cools down at night. Solution: check the temperatures displayed on the controller and compare them with the real situation.
Sensor S1 located far from the collector: delayed reaction by 5–15 minutes. Solar yield drops by 5–15%. Solution: place the sensor as close as possible to the absorber outlet, max 10 cm from the collector.
Standard heating expansion tank (red membrane): the membrane degrades within 2–3 years, the tank stops functioning, and the system regularly "discharges" medium through the safety valve. Solution: always use a solar expansion tank with an EPDM membrane.
Undersized expansion tank: during stagnation, the medium has nowhere to expand, and the safety valve opens. Solution: calculate according to the formula, dimension with a reserve.
Air in the circuit: the pump "hums", the flow is unstable, and temperatures fluctuate. Solution: thorough venting during filling, installation of an automatic vent at the highest point of the circuit.
Signal interference from sensors: sensor cables run next to power cables – temperature "jumps" by 5–20 °C. Solution: separate cable routing, minimum distance of 5 cm.

Inspection after startup and handing over to the customer

After successful filling and controller setup, perform a functional test of the system. Best on a sunny day – monitor whether the collector temperature (S1) rises, the pump starts at sufficient ΔT, the tank temperature gradually increases, and the pump stops when the T-MAX of the tank is reached. Document the entire cycle (screenshots from the controller display, pressure in the system, set parameters).

Hand over a written report to the customer containing: set controller parameters, type and volume of the medium, operating pressure, pre-charge pressure of the expansion tank, volume of the expansion tank, location and types of sensors. People often call after 3 years when a problem occurs, and you will be glad you have the records.

For regular maintenance and inspection points during the season, see the article "Maintenance and inspection of solar control: what to check before and after the heating season".

Most frequently asked questions (FAQ)

Can I use a standard heating expansion tank (red) for a solar circuit?

No. Standard heating expansion tanks have a membrane made of butyl or SBR rubber, which is not resistant to glycol-water mixtures or temperatures above 80–90 °C. In a solar circuit, it would last 1–3 years, after which the membrane would degrade, the tank would lose its function, and the system would regularly discharge the medium through the safety valve. Always use a solar expansion tank with an EPDM membrane, marked as "solar" or with a sun symbol.

What is the correct pre-charge pressure of the expansion tank for a family house?

It depends on the height of the system – specifically on the height difference between the expansion tank and the highest point of the circuit (collectors on the roof). Formula: pre-charge pressure [bar] = height [m] / 10 + 0.3. For a typical family house with a height of 6–8 m, the pre-charge pressure is 0.9–1.1 bar. The operating pressure (cold system) should be 0.2–0.3 bar higher. Always check the pre-charge pressure before filling, when the tank is empty (dry).

Sensor S1 on the collector shows strange values – 15 °C in summer even when the sun is shining. What happened?

The most common cause: the sensor is not in contact with the pipe or is insulated from the pipe and measures ambient air temperature. Check the mechanical mounting of the sensor, apply thermal paste, and thoroughly wrap the sensor location with insulation. Another possibility: broken or shorted sensor cable (NTC thermistor shows very low temperature when shorted). Measure the resistance of the sensor at room temperature – it should be approximately 10 kΩ.

The pump runs continuously even at night. Is this a fault?

Not always – some controllers have a night cooling function (protection of collectors from overheating) or a frost protection function (circulation at low outside temperatures). If these functions are not enabled, the problem is in the wiring or setup: either the sensors S1 and S2 are swapped (the controller thinks the collector is always warmer than the tank), or ΔT-OFF is set too low (close to 0 °C). Check the displayed temperatures S1 and S2 on the controller display and compare them with the physical reality.

How many meters can the cable from the sensor to the controller be?

For NTC thermistors, long cables increase the line resistance and introduce systematic measurement errors. With a standard twin-core cable 0.5 mm², a length of up to 30 m is practically problem-free (error under 0.5 °C). For lengths of 30–50 m, use a cable with a larger cross-section (0.75 mm²). Above 50 m, it is advisable to use a shielded cable or check whether the controller supports Pt1000 sensors, which are more suitable for long distances. Always route the sensor cable separately from the pump power cables.

Do I need to buy a special pump for the solar system, or is a standard circulation pump for heating sufficient?

It depends on the system. For flat-plate collectors with propylene glycol up to 120 °C, a quality circulation pump with a cast iron housing and a durable shaft sealing assembly is sufficient. For vacuum tube collectors, where the medium can reach 150 °C and more, it is recommended to use a pump certified for solar applications – with a bronze or stainless steel body and temperature-resistant seals. If you install a control station from the ZPS series, the pump is part of the station and is correctly dimensioned for solar operation.

Conclusion: installation that pays off when done correctly

A solar system can operate for years with little attention – provided it is assembled correctly from the beginning. The key is precise placement of the sensors (as close as possible to the absorber and in the lower third of the storage tank), correct electrical connection of the pump via the controller (with emphasis on separating power and signal cables), a properly oversized and pre-pressurized solar expansion vessel, and thorough air removal from the circuit during filling.

When choosing a controller, compact control units such as ZPS 6, ZPS 16, and ZPS 28 significantly simplify installation by integrating the pump, safety valve, and controller into one unit. For simpler or cost-optimized systems, where the customer selects components separately, a solid choice is Euroster 813 Solar with an external pump. In both cases, the same rule applies: time invested in correct installation is many times returned in trouble-free operation for years to come.

Do you have a question on this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.