Maintenance and inspection of solar regulation: what to check before and after the heating season

We are still preparing products.

But you can look at other categories.

Regular maintenance and inspection of solar regulation: what to check before and after the heating season

A solar system is an investment that pays off only if it works reliably throughout the year. The collectors on the roof are the visible part that most owners are primarily concerned with – they clean them, check the sealing, or clear snow. The regulation, however, is often neglected. And it is precisely here that most problems arise. A differential thermostat or control station manages the entire circulation – it decides when the pump starts and stops, monitors the temperatures in the collector and the storage tank, and in more modern cases, even multiple storage tanks at once. If the regulation works with incorrect values or gets stuck after winter, the entire solar system may operate poorly for the entire summer season without the owner noticing.

In this article, we will go through specific steps – what to check in spring before the sunny season, what to check in autumn before the heating season, what values are normal and where the problem line is. At the same time, we will look at typical situations from practical experience, where neglecting regular inspection led to a significant performance drop or direct failure.

Why regular regulation inspection is more important than it seems

Imagine a common situation: the owner of a family house installs a solar system with two flat collectors and a 300-liter TÜV storage tank. The first year is satisfactory – free hot water during the summer. In the second year, the owner notices that the tank feels slightly cooler in the morning but doesn't pay much attention. In the third year, the system's performance deteriorates further, but the owner attributes it to poor weather. When the technician finally looks at the regulation, he finds a frustrating problem: the collector temperature sensor reports 12 °C higher than the actual value. The regulation therefore believes the collector is hot even when it is not, starts the pump at the wrong time, and the system's efficiency is catastrophic. A simple cleaning of the contact and calibration of the sensor – a half-hour job – would have saved three years of poor performance.

This is not a made-up example. In variations, most technicians who service solar systems older than two to three years encounter it. The problem is systematic: users do not realize that the regulation is not a passive box – it is the active brain of the system, which must be maintained just like the pump, expansion tank, or collector.

Spring inspection: what to check before the summer season

Spring is the ideal time for a preventive inspection. The system was either in hibernation (if you have a summer-winter system with shutdown) or operated under restricted conditions during the winter. In both cases, it is necessary to check several critical points before the start of the solar season.

1. Visual inspection of the regulation and wiring

The first step is trivial, but people often skip it. Open the distribution box or cover where the regulation is installed and look at the condition of the terminal blocks and wiring. After winter, especially in unheated technical rooms or boiler rooms with large temperature and humidity fluctuations, micro-oxidation on contacts can occur. Check the following:

Whether all terminals are tightly tightened – they may loosen after thermal cycles
Whether the sensor wiring has insulation damage (rodents are common guests in boiler rooms)
Whether there are visible signs of moisture or condensation on the regulation's printed circuit board
Whether the LED indicators or display are functional and readable
Whether the display is faded or pixelated characters are unreadable (in older models, this indicates the end of the display's life is approaching)

For regulations such as the Euroster 813 Solar, it is a compact device with a clear display – check whether both temperatures are displayed correctly (collector and storage tank) after turning it on and whether the values are clearly within the range (e.g., –99 °C or 999 °C are classic signs of a broken or shorted sensor).

2. Inspection and calibration of temperature sensors

This is probably the most important point of the entire inspection. Temperature sensors (most commonly NTC or PT1000) are inserted into wells in the collector and the storage tank. Over time, the thermal contact between the sensor and the well may deteriorate, terminals may corrode, or the sensor itself may drift – meaning it systematically reports higher or lower values.

How to verify the correctness of the sensor? The simplest procedure: measure the resistance of the sensor with a multimeter at a known temperature (e.g., at room temperature of about 20–22 °C). For an NTC 10k sensor, the resistance at 20 °C should be about 12 kΩ, and at 25 °C about 10 kΩ. For a PT1000 at 20 °C, it is about 1 078 Ω. If the value differs significantly, the sensor is faulty or has a poor contact. Another option: immerse the sensor in water with a thermometer for a few minutes and compare the displayed temperature in the regulation with the thermometer reading – a deviation of up to ±2 °C is acceptable, while a deviation of more than ±5 °C means the sensor needs to be replaced.

3. Check the setting of the differential temperature

After winter or after a power outage, some regulations may reset to factory settings. Check whether the set values for the differential temperature for turning on and off are the same as those set during installation. Typical recommended values are: turn on at ΔT = 5–8 °C, turn off at ΔT = 2–3 °C. If the differential is set too low, the pump will cycle (short on and off periods), unnecessarily wearing it out and reducing overall efficiency. If the differential is too large, the system starts too late and loses energy. More about this setting can be found in the article Setting the differential temperature in solar regulation: how to correctly configure switching.

4. Pump and circuit flow verification

The controller manages the pump, but if the pump is not working properly, even the best controller will not help. In spring, we recommend manually starting the pump via the controller's service mode (most modern controllers allow this) and monitoring whether:

The pump actually starts rotating – audible sound and vibration are positive signs
The flow is smooth – monitor the value on the flow meter (if installed), a typical flow for 2 collectors is 40–60 l/h
The pressure in the system is normal – a typical pressure for a cold solar loop is 1.5–2.5 bar
There are no visible leaks at the flow meter, ball valves, or pump group connections

Control units such as ZPS 6, ZPS 16, or ZPS 28 integrate the pump, flow meter, safety valve, and controller into one compact unit, simplifying the inspection – everything is in one place and accessible without searching in different corners of the boiler room.

5. Solar fluid Inspection

The controller manages the fluid circulation, but the condition of the fluid itself affects the measured temperatures and the system's long-term performance. After three to five years of operation, we recommend taking a sample and checking:

pH: should be 7–9. Fluid that is too acidic (pH below 7) can damage copper and aluminum infrastructure and may corrode sensor wells.
Glycol content (propylene glycol): a recommended proportion of 30–40 % ensures protection down to –15 to –20 °C. You can measure this with a refractometer in two minutes.
Fluid color: healthy fluid is clear or slightly reddish. Dark brown or black fluid indicates the breakdown of inhibitors and it is time for a replacement.

Degraded fluid can deposit sediments in the flow meter and sensor wells, which directly leads to incorrect readings in the controller. More about this type of fault is described in the article Common faults in solar controllers: error messages, pump failures, and inaccurate sensors.

Autumn inspection: what to check before the heating season

While the spring inspection is about starting up the system after winter, the autumn inspection is about securing the system before winter. Here, the priorities are partially different.

1. Checking the maximum tank temperature and anti-freeze settings

Modern controllers allow you to set the maximum tank temperature at which circulation stops, even if the collector is hot. Typically, this is 60–70 °C for a TÜV tank. Check whether this value is set correctly – if the tank could exceed 90 °C, damage to the safety valve or the tank's thermal insulation could occur.

At the same time, check the anti-freeze function setting. Most controllers can start the pump when the collector temperature approaches the freezing point (e.g., below 5 °C), to protect the pipes from freezing. Verify that this function is active and set to the correct value. In the Euroster 813 Solar, this function is integrated and adjustable directly from the controller menu.

2. Winter and stagnation: preparing the system for high temperatures and frost

Paradoxically – winter preparation also includes protection against overheating. In summer, when the tank is filled with hot water and the sun is shining fully, the collectors can stagnate (the pump stops, the fluid is not moving). In this state, the fluid temperature in the collector can reach 160–200 °C – this is the so-called stagnation temperature. The controller should work to minimize the time of stagnation, or activate night cooling (heat removal at night back into the collector).

Before winter, check whether the expansion tank of the solar loop has the correct gas pressure (typically 1.5 bar in a cold state) and whether the membrane tank is not mechanically damaged. A damaged expansion tank will cause the pressure in the system to rise beyond the limit during stagnation, and the safety valve will release the fluid – which the controller may interpret as a fault (pressure increase without pump activation).

3. Recording operational values

This is a step that most homeowners do once (never). A professional service knows well – recording the system's operational values in a healthy state is extremely valuable for diagnostics. Note down:

The typical collector temperature at 12:00 on a sunny day in the current season
The time when the pump usually starts and stops
The tank temperature in the morning and evening on a solar day
The flow meter reading during normal operation
The working pressure of the solar loop

These values will tell you in spring during the inspection whether something has changed or everything is working as it did a year ago.

Specifications of the inspection according to the type of regulation

Differential thermostat (Euroster 813 Solar)

Euroster 813 Solar is a simple and reliable differential thermostat intended for basic solar systems with one collector field and one storage tank. Its inspection is faster than with more complex units, but nothing should be neglected.

Check the following points in particular:

Display and buttons: all buttons should respond without hesitation. If the settings change on their own or the buttons are stuck, the device should be returned for warranty or replaced.
Maximum tank temperature setting (T-max): typically 60 °C for TÚV, 70–80 °C for a combined tank with heating.
Automatic restart function after power failure: verify that after a simulated power outage (unplug the power cord for 30 seconds), the regulation returns to the correct state and the settings remain unchanged.
ΔT on/off setting: check according to the installation record to ensure the values have not changed.

Control stations ZPS (ZPS 6, ZPS 16, ZPS 28)

ZPS control stations are more complex devices that combine regulation with a pump, flow meter, safety valve and fittings into one compact unit. Inspection is more extensive, but it is worth it all the more.

The following additional inspection points apply to ZPS 6, ZPS 16 and ZPS 28:

Flow meter: clean the flow meter screen if it is accessible. A clogged screen causes a drop in flow and the regulation may record incorrect energy values or the pump may appear "weak".
Ball valves and thermostatic valves: move them to full position and back – they may stick after winter. A loose valve causes leaks.
Safety valve: briefly turn the lever of the safety valve (if it has manual control) to verify that it is not stuck or hardened by deposits.
Energy counter (if installed): record the amount of energy extracted and compare it with the previous year. A significant drop under similar weather conditions indicates a system fault.
Flow setting: on the balancing valve, check whether the flow is set to the correct value according to the number of collectors (typically 20–25 l/h per collector).

The choice between a differential thermostat and a control station is discussed in detail in the article Euroster 813 Solar vs. ZPS control stations: comparison of functions and use and in the article How to choose regulation for a solar system: differential thermostat vs. control station.

Common errors from practice and how to avoid them

Forgotten settings after battery replacement or power failure

Many regulators store settings in EEPROM memory, which does not require power. However, older models or cheaper devices may lose settings after a longer power outage and revert to factory settings. This is problematic – the system works technically, but with a factory ΔT setting of 10 °C, it starts too late and unnecessarily loses energy. I recommend checking the regulator after any power outage lasting longer than an hour and verifying the settings.

Collector sensor placed in the shade

This often happens during renovations – the collector is moved or new shading appears, but the sensor remains in the original position. A shaded sensor measures a lower temperature than the actual collector, and the regulation does not start the pump even when it should. The solution is to physically move the sensor to the correct position – ideally into the collector's outlet pipe or into the pump pipe at the outlet. For details on the correct sensor wiring, see the article Installation of solar system regulation: pump, sensor and expansion tank wiring.

Storage tank sensor too close to an electric heating element

If the tank also has an electric heating element and the regulation sensor is placed close to the element, the heated water from the element may distort the tank temperature measurement. The regulation assumes the tank is warmer than it actually is and delays or completely skips solar circulation. The tank sensor should be placed in the lower third of the tank, away from the electric heating element – typically 30–50 cm from the element downward.

Pumping cycle as a sign of incorrect settings

A pump that turns on and off every 30–60 seconds is a classic sign of too small a temperature difference or a weak thermal contact of the collector sensor. In practice, it looks like this: the sun is shining, the collector slowly heats up, reaches the on ΔT (e.g. 5 °C), the pump starts, the fluid cools the collector, ΔT drops below the minimum off threshold, the pump stops, the collector heats up again – and the cycle repeats. Solution: increase the on ΔT to 7–8 °C, set the off ΔT to 3 °C and check the sensor contact.

When to call a service and when you can do it yourself

Most routine inspections can be handled by a skilled owner themselves, provided they have the regulator manual and a multimeter. Call a service technician in these situations:

The regulator displays a permanent error message (e.g. "E1", "Err", blinking display) and a reset does not help
The pump does not start even during a manual test from the regulator menu – possible pump or regulator failure
The pressure in the system rises by more than 1–1.5 bar above the resting state after each heating – expansion tank failure
The fluid is dark or contains solid particles – the fluid needs to be replaced and the circuit flushed
The measured temperatures of the collector and tank do not match expectations even after sensor calibration
The safety valve repeatedly discharges fluid – the system has a serious pressure problem

Replacing a sensor or calibrating settings, on the other hand, are tasks that an average technically skilled owner can handle after a quick review of the manual. Always remember to write down the original settings before making any changes – it takes just five seconds and can save you hours of searching for "where it went wrong."

Regular inspection checklist: what to record

Professional service technicians use standardized inspection forms. For home use, a simple table in a text editor or a paper notebook in the boiler room will suffice. Minimum content of an inspection record:

Parameter checked	Spring (date)	Autumn (date)
Collector temperature (displayed)	_____ °C	_____ °C
Storage tank temperature (displayed)	_____ °C	_____ °C
Working pressure of the solar circuit	_____ bar	_____ bar
Flow rate of the solar circuit	_____ l/h	_____ l/h
ΔT activation (set)	_____ °C	_____ °C
ΔT deactivation (set)	_____ °C	_____ °C
Max. storage tank temperature (set)	_____ °C	_____ °C
Fluid condition (color, pH)	________	________
Error messages / notes	________	________

Store the completed record with the system documentation. If you ever sell the system along with the house, a complete service history increases the credibility of the installation and can be an interesting selling point.

What to check on the regulation connected to a TÚV tank

Solar systems primarily used for hot water preparation have a few specific features that are worth knowing. First and foremost is the setting of the Legionella protection function – protection against legionella. This function periodically (most often once a week) heats the tank to 60–70 °C to destroy any legionella bacteria. If you have this function enabled, verify that it is correctly configured – incorrect settings can cause the tank to be unnecessarily overheated during the day (wasting energy) or, conversely, the function may not be active at all (health risk).

Another specific feature with TÚV tanks is the setting of prioritization: if the tank is connected to a boiler as well, the regulation must know who to prioritize for solar heating. Some systems prefer solar heating and the boiler only supplements it, others do the opposite. Check this setting in the documentation of your installation. More about connecting solar regulation with tanks can be found in the article How to connect solar system regulation with a water heater or TÚV tank.

Lifespan of regulators and when to consider replacement

Solar regulators are reliable devices with an average lifespan of 10–15 years with proper maintenance. Older electronic components, however, are more sensitive to power fluctuations, humidity, and heat. Consider replacing the regulator in these cases:

The regulator is older than 12–15 years and is starting to show random outages or resets
The display is unreadable or partially non-functional even after checking the power supply
Replacement sensors or accessories are no longer available for the regulator (manufacturer has discontinued production)
A new regulator offers features that significantly improve system efficiency (e.g., energy meter, multi-circuit regulation, logging)
The regulator does not match the system performance – for example, a simple thermostat originally used when the system is expanded with an additional tank or collector array

Replacing a regulator is usually a simpler task than replacing a pump or collector – it involves electrical disconnection, physically removing the unit, mounting the new one, and setting the parameters. On atria.sk's solar regulator page, you will find the current range of devices in various performance classes, including ZPS control stations and differential thermostats.

Most frequently asked questions (FAQ)

How often should I check the solar regulator?

At least twice a year – in spring before the summer season and in autumn before winter. If the system runs continuously even in winter (a combined system), we recommend extending the spring check to include measuring the resistance of the sensors and verifying the pressure. Every three years, have a complete service inspection done, including checking the solar fluid and the expansion tank.

What does it mean if the regulator displays a temperature of –99 °C or 999 °C?

This is a classic sign of a sensor failure. A value of –99 °C (or a similar negative extreme value) typically indicates a broken sensor circuit – either the cable is broken, the connector is loose, or the sensor itself is defective (internal resistance element is broken). A value of 999 °C or a very high positive number, on the other hand, indicates a short circuit in the sensor. In both cases, you should disconnect the sensor cable, measure its resistance, and compare it with the values in the table for the given sensor type (NTC or PT1000).

Can I replace a temperature sensor myself, or should I call a professional?

In most cases, replacing a temperature sensor is a simple task that an average homeowner can handle. The sensor is simply pulled out of the well, the cable is disconnected from the regulator's terminal block, the new sensor is inserted and connected. It is important to: use a sensor of the same type (NTC 10k or PT1000 according to the regulator), apply heat transfer paste to the well for good thermal contact, and securely tighten the terminal. If the sensor is integrated in a non-commercial well (poured installation), we recommend a service technician.

Why is the pump running at night even when the collector is cold?

The most common reason is the active nocturnal cooling function, which cools the tank by pumping hot water into the collector, where it is radiated into the air. If you do not want this function, disable it in the regulator's menu. Another possible cause may be a faulty collector sensor – if the sensor indicates a higher temperature than actual (e.g., due to a disconnected cable with incorrect polarity), the regulator may mistakenly believe that circulation is necessary. Check the collector sensor.

How can I determine whether my system is producing enough energy?

The most reliable way is to have an energy meter (heat meter) installed, which measures the total delivered energy in kWh. Modern ZPS control units can be equipped with a flow sensor, which in combination with temperature readings allows this value to be calculated. As a rough estimate: a well-functioning system with two flat collectors (5 m²) should produce 800–1,200 kWh annually in Slovakia, depending on the location and roof slope. A significantly lower value indicates a problem in the system or in the regulator settings.

Do I need to shut down the system before winter, or can it run year-round?

This depends on the system configuration. If your solar circuit is filled with frost-resistant fluid with sufficient glycol concentration (protection down to –20 °C or lower) and the regulator has an active anti-freeze function, the system can run year-round. The benefit in winter months is low, but still positive. Systems using water as the heat transfer fluid (rare, but they exist) must be drained before freezing weather. Check your installation documentation or consult the installation company.

Conclusion: A small investment in maintenance, a big saving in operation

Solar regulation is one of the least expensive components of the entire system, yet it is the one that determines the efficiency of everything else. Checking sensors, settings, and operating values twice a year will take you a maximum of two hours per year. In return, you will have the assurance that your solar system is operating at full capacity and your investment is being repaid as calculated during the design phase. Neglect, on the other hand, can mean a performance drop of 30–50 % without any visible warning signs – the system is working, but very poorly.

If you are looking for a new or replacement regulator, take a look at

Do you have a question about this topic?

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