Maintenance and service of solar pump unit

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Maintenance and service of a solar pump unit – a complete practical guide

The solar pump unit is the heart of every active solar system. It ensures the circulation of the heat transfer medium between the collectors and the storage tank, controls the pump, monitors temperatures, and protects the entire system from overheating or freezing. When it works reliably, the system operates almost unnoticed. When it starts having problems – the storage tank doesn't heat up, the collectors overheat or the pump runs continuously – the owner often can't even explain where the problem occurred.

Over the years of practice, I have seen dozens of systems where the problem was trivial: a clogged filter, old solar fluid turned into acid, or simply a forgotten annual inspection. This article is intended for anyone who wants to understand what is happening behind the pump unit, how to properly maintain it, when to call a service technician, and what a skilled DIY enthusiast can handle themselves. I will not be focusing on the initial installation – that is covered in a separate article, "Installation of a Solar Pump Unit Step by Step" – nor on the choice of a specific model, for which the article "How to Choose a Control Unit for a Solar System" is available.

What makes up a solar pump unit and why maintenance is essential

Before we get into the service procedure itself, it is important to understand what the unit consists of and where the weak points are. A typical compact pump unit – for example, Solar Pump Unit ZP2-12 ECO – contains the following main functional components:

Circulation pump – most often a wet-running ball or ball-less ECM pump with speed regulation
Ball valves – flow interrupters on the inlet and outlet, allowing the pump to be isolated without draining the entire circuit
Check valve – prevents nighttime thermosyphonic circulation, which would mean heat loss from the storage tank back into the collector
Filter screen (Y-filter or magnetic filter) – captures solid impurities and corrosion products from the piping
Pressure gauges – indicate the operating pressure in the primary circuit
Flow meter – mechanical or electronic, displays the current flow in liters per minute
Pressure relief valve – discharges the medium if the set pressure is exceeded (usually 6 bar)
Controller (control electronics) – the brain of the entire system, processes signals from temperature sensors and controls the pump

Each of these components has a different wear interval and a different method of inspection. For example, an ECM pump with permanent magnets can last 20+ years with proper operation, but a filter can become clogged within a single season if pipe grit was left in the system during installation.

Annual service plan – what and when to check

Experience shows that solar systems most often fail not due to manufacturing defects, but due to neglected maintenance. The good news is that most tasks are relatively simple and do not require special tools. I recommend establishing a two-phase annual service: a spring inspection before the season (April) and an autumn check after the season (October).

Spring inspection (before the solar season)

1. Visual inspection of the entire unit and piping – Check for signs of fluid leakage on the joints, seals, and fittings (white or yellowish coating, rust stains). Hardened seals around pressure gauges and manometers are the first sign of an impending failure. Also check the thermal insulation – after years of operation at high temperatures, it cracks and peels off, increasing heat loss.

2. Pressure check in the system – The primary circuit of the solar system should have an operating pressure of 1.5 – 2.5 bar (cold system, i.e., before heating). If the pressure is below 1 bar, the system was likely de-aerated or the medium has leaked. If it is above 3 bar in the cold state, there may be problems with the expansion tank. Read the pressure from the manometer on the unit – this task will take you literally 30 seconds.

3. Check the expansion tank – The expansion tank is located outside the pump unit, but is closely related to it. Shut down the system and verify the function of the gas bladder in the tank by pressure drop after draining. The pre-charge with nitrogen should be 0.5 bar below the operating filling pressure – for example, if you fill to 1.5 bar, the tank's pre-charge should be 1.0 bar.

4. Visual inspection and cleaning of the filter – Close the ball valves on both sides of the pump unit, slowly unscrew the filter cap, and clean the screen under running water. If the screen looks brown or black due to corrosion, it is time for a laboratory analysis of the fluid (more below).

5. Checking solar fluid – Test the pH using an indicator strip or a refractometer to determine glycol content. The correct pH of the solar fluid is 7–8. Below 6.5, the fluid has degraded and is corrosive – it needs to be replaced. Measure the glycol content with a refractometer: to protect down to –28 °C you need approximately 40 % glycol, down to –35 °C approximately 50 %. In practice, I have seen systems where the owner topped up with distilled water and after a few years, the fluid only provided protection down to –10 °C.

6. Functional test of the controller – Start the system and verify that the controller reacts correctly to temperature differences: when the difference is 5–8 °C between the collector and the bottom sensor of the storage tank, the pump must start, and when the difference drops below 2–4 °C, it must stop. These threshold values can be verified in the controller menu. For systems with Solar Pump Control Unit ZPS 18e - 01 ECO, you will find the settings in the main menu under the item "Delta T".

Autumn inspection (after the season)

The autumn inspection is shorter, but no less important. The season leaves its mark: higher temperatures accelerate fluid degradation, a coating may form in the collectors due to long-term stagnation at maximum temperatures, and the pump has been operating for several hundred hours.

Repeat the pH test of the fluid – summer is much more demanding for the fluid than winter
Check whether the safety valve shows any signs of leakage (a drop of fluid or a coating under the valve indicates that it was active)
Check whether the electronic flow meter indicates values corresponding to the pump setting – if the system is operating with Electronic Flow Meter for GH 26, record the measured value and compare it with the spring value; a drop in flow of more than 20 % indicates partial clogging or a problem with the pump
Check the wiring of the temperature sensors – autumn rodents are a common nuisance in boiler rooms
Verify the setting of the anti-freeze function of the controller

Flushing and replacing solar fluid – when and how

Solar fluid (propylene glycol or ethylene glycol with corrosion inhibitors) is not eternal. Inhibitors are gradually depleted, glycol oxidizes, and the result is an acidic fluid that attacks copper, aluminum, and steel. Replacing the fluid is the most important and most underestimated maintenance task.

Replacement interval: Manufacturers of most solar fluids guarantee a lifespan of 5–7 years at operating temperatures up to 120 °C. In practice – if the system is properly regulated and does not overheat (which also depends on the controller) – the fluid will last closer to the upper limit. Systems where the controller failed and the fluid repeatedly boiled must be tested every year.

How to replace the fluid:

Cool the system down to at least 40 °C – never work on a hot system under pressure
Close the primary circuit from the storage tank, connect the drain hose to the valve on the unit
Drain the old fluid into a suitable container – it is chemical waste, do not discharge it into the sewer
Rinse the circuit with distilled water (2–3 rinses)
Fill the new fluid using a filling pump – always draw from the container of new fluid, not from the pipe
Deaerate the system – for units with a screw-type deaerator, proceed from the highest point of the system downward
Set the operating pressure (usually 1.5–2.0 bar in the cold position)
Start the system and check the seals and flow

Practical tip: Never top up the fluid with distilled water without measuring the glycol content. I have seen systems where the owner topped up with "that blue water" for five years – in the end, the circuit had only 15 % glycol and the frost protection was almost non-existent. A refractometer costs less than 15 € and is basic equipment for every technician.

Deaerating the solar circuit – why it is critical

Air in the primary circuit is a silent enemy. It causes pump cavitation, noise, reduces flow, and in extreme cases can completely stop circulation (airlock). Solar systems are more susceptible to air than classic heating circuits, because at stagnation temperatures (when the storage tank reaches maximum temperature and the pump stops), the fluid in the collector can boil and release gases.

Deaerating should be a standard part of every spring inspection. The procedure is simple:

Let the system run at low speed (1st pump stage or minimum speed at ECM pump)
Slowly open the air vent valve on the unit or on the automatic air vent
Listen and observe – air release is audible hissing or bubbling
Close the valve immediately when liquid starts to flow without air bubbles
Top up if pressure drops

Automatic air vents on the unit are practical, but have one drawback: at higher temperatures, they can release steam and steam bubbles, which over time lead to their contamination. Every 2 years, disassemble, clean and check the float mechanism function.

Service and pump replacement in the pump unit

The circulation pump is a mechanical component with moving parts – thus the only part where wear is inevitable. Modern ECM class wet-running pumps (such as those found in ZP2-12 ECO) have significantly longer lifespans than older constant-speed wet-running pumps, but even these are not eternal.

Signs of a worn pump:

Increased noise – buzzing, knocking, vibrations
Drop in flow at the same speed setting
Higher power consumption (measured at pump power supply)
Pump occasionally jams after a period of stagnation
Excessive heating of the pump body

What a DIY enthusiast can do: If the pump does not start after a period of stagnation in summer (a common occurrence with bearing pumps), simply locate the bleed screw on the side of the motor, unscrew it and gently turn the pump shaft with a flat screwdriver. This usually unjams the pump without the need for disassembly.

Pump replacement: When replacing the entire pump, it is crucial to maintain the same hydraulic resistance (discharge head Hv in meters and flow Q in m³/h). Replacing it with a different type without comparing curves can lead to undercooling of the collectors or, conversely, unnecessarily high flow and pressure losses. With modern ECM pumps with performance regulation, replacement is usually simple – connect the same hydraulic connector and reconfigure the controller.

Inspection and maintenance of the controller and temperature sensors

The controller is an electronic component that does not require mechanical maintenance during normal operation. However, this does not mean you can ignore it. Experience from customer service shows that up to 40% of solar system faults originate from a malfunction or incorrect setting of the controller or temperature sensors.

Checking temperature sensors (Pt1000 or Pt100)

Temperature sensors are mostly Pt1000 resistance sensors. Their verification is simple: disconnect the sensor from the controller and measure the resistance with a multimeter. At 25 °C, the Pt1000 sensor should show approximately 1 097 Ω. At 100 °C, it should be approximately 1 385 Ω. If you measure significantly different values (e.g., infinite resistance = broken wire, or zero = short circuit), the sensor needs to be replaced.

The collector sensor is exposed to extreme temperature cycles (from –25 °C in winter to +200 °C during stagnation in summer). The sensor wiring becomes brittle and insulation cracks over time. Visually inspect the wiring from the collector to the controller – especially at points where it passes through the roof and in areas with alternating heat/cold.

Controller settings backup

This is a detail that most owners underestimate. More advanced controllers (e.g., Euroster 813 Solar) allow memory backup mode or export of settings. If the controller fails and you need to replace it, without backed-up settings you will have to set everything up from scratch. Spend 10 minutes recording all settings in the service sheet – Delta T activation, Delta T deactivation, maximum tank temperature, anti-freeze temperature, pump stage.

More about controller settings can be found in the article How to set up a solar system controller for maximum efficiency.

Fault diagnosis – steps for solving common problems

When the system is not working properly, systematic diagnosis saves time and money. Below is an overview of the most common situations from practice:

Further specific error conditions and their causes are discussed in a separate article Common faults of solar regulators and pump units – I recommend reading it before the first diagnosis.

Service specific to different types of units

Not all pump units are the same. Simpler units without an integrated regulator (where the regulator stands next to it as a separate box) have different service requirements than compact units with integrated electronics. More about the differences between these concepts can be found in the article Difference between a solar pump unit and a separate regulator.

For systems with Solar pump unit ALEX HX10 for MiniSOL control, which is intended for smaller installations and MiniSOL controls, it is especially important to check the hydraulic connection to the regulator – this combination works with an external control unit, so it is necessary to verify the correctness of the connection and calibration of the sensors at both locations.

Compact units with integrated control electronics (e.g., ZPS 18e) have the advantage of centralized servicing, but if the electronics fail, the whole unit or just the control module must be replaced – it depends on the design. Always keep the purchase receipt and warranty card, as electronics may have a warranty period of 3–5 years.

Service documentation and service log

Keeping a service log is a practice I value more and more from my experience in the field. When a technician comes to solve a fault and the owner can tell them when the fluid was last changed, when the filter was replaced, and what the pressure values were a year ago – diagnosis is twice as fast. A service log does not have to be anything complicated: a notebook or a simple table is enough.

Always record the following in the service log:

Date of inspection
Operating pressure (cold system)
Flow rate in liters per minute (if you have a flow meter)
pH of the fluid and measured protection temperature (refractometer)
Performed tasks (filter cleaning, fluid replacement, regulator adjustment...)
Condition of sensors and any notes on noise or other anomalies

When to call a service technician – limits of DIY maintenance

DIY maintenance makes sense and saves money. However, there are situations where it is worth calling a professional:

Fluid leakage from a seal or pipe connection – if the leak is active and not just old corrosion, it is a service case. Using glue for sealing at solar temperatures does not work.
Pressure relief valve repeatedly leaking – this indicates either a problem with the expansion tank, incorrect regulator setting, or a faulty check valve. In any case, it is a diagnosis for a technician.
Pump failure with mechanical damage – if the pump makes metallic sounds or vibrates, turn off the system and call – further operation may damage the unit's hydraulics.
Complete fluid replacement in large systems – for systems with more than 4 collectors and a primary circuit volume over 20 liters, a filling pump is essential, which is equipment for service.
Error codes from the regulator that you do not understand – every regulator has a table of error codes in the manual. If the code is not in the manual or repeats after solving the apparent cause, call a technician or contact the distributor.

Maintenance economics – how much does neglected service cost

Let’s compare two real-life situations from practice. The first owner implements an annual inspection: costs about 50 € for fluid replacement once every 6 years, 10 € for pH test strips once a year, and 2 hours of personal time annually. Over 10 years, the investment in service is roughly 150–200 € and the system works without major breakdowns.

The second owner does no maintenance. After 7–8 years, degraded fluid damages the storage tank heat exchanger (standard price of a heat exchanger or tank 800–2 500 €), clogged pipes and filter damage the pump (200–400 €), and degraded fluid contaminates the entire circuit so much that a complete flush with special cleaner is needed (technician, 4 hours of work, chemicals – another 300–500 €). Total costs: 1 300 – 3 400 €, most of which are not covered by warranty, as it is due to neglected maintenance.

The numbers speak clearly. Investment in regular maintenance is economically absolutely justified.

Most frequently asked questions (FAQ)

How often should solar fluid be changed?

The recommended replacement interval is 5–7 years under normal operation. However, you should check the pH with an indicator strip every year – if the pH is below 6.5, replace the fluid immediately regardless of its age. Systems where the regulator repeatedly failed and the fluid boiled should be tested every year. Propylene glycol is safer from an environmental perspective, but its inhibitors deplete faster than with ethylene glycol.

What pressure should the solar primary circuit have?

A cold system (before heating) should have an operating pressure of 1.5 – 2.0 bar. The maximum operating pressure when the system is hot must not exceed the opening value of the safety valve, which is usually set to 6 bar. If the pressure in the cold state drops below 1 bar or rises above 3 bar, check the expansion tank and the tightness of the circuit.

What should I do if the safety valve is discharging fluid?

First, verify: is it a single discharge or does it happen regularly? Single discharges can occur during extreme summer overheating (long stagnations). If the valve discharges regularly, it indicates a problem with the expansion tank (lost pre-charge or burst membrane), incorrect regulator setting, or too low a maximum tank temperature setting – the system stagnates too early and the expansion tank cannot catch the expanding fluid volume.

Why is the pump running, but the storage tank is not heating up?

There are several possible causes: an air lock in the circuit (flow is zero or very low, although the pump is running), a clogged filter, a damaged check valve (allows reverse circulation and heat loss), degraded fluid with low thermal capacity, or a dirty tank heat exchanger. Checking the flow on an electronic or mechanical flow meter is the first step in diagnosis – if you see zero or very low flow with the pump running, the problem is hydraulic (air, blockage, filter).

Can I use a standard heating pump in a solar circuit?

No, this mistake has serious consequences. Solar pumps must be able to operate at temperatures up to 130–140 °C and with aggressive solar fluids based on glycol. Standard heating pumps are designed for a maximum of 95–110 °C and their seals and materials are not resistant to glycol and temperature shocks typical of solar primary circuits. Technically, it is definitely the wrong choice.

Is it necessary to shut down the solar system in winter?

No, a properly dimensioned system with sufficient glycol content operates all year round. The regulator has an anti-freeze function that, when the collector temperature drops below a set value (typically +5 °C), briefly starts the pump and heats the collector with thermal energy from the storage tank. It is important to verify the fluid's protection temperature before winter and ensure the regulator's anti-freeze function is correctly set – more in the article How to set the solar system regulator for maximum efficiency.

Conclusion – summary of key principles

A solar pump unit is a robust device that, with proper handling, can serve for 20 or more years without major problems. The key to long life lies in a few simple principles: regular spring and autumn inspections, annual fluid testing, filter cleaning, pressure checks, and correct regulator settings. Most service cases I have dealt with over the years were caused not by manufacturing defects, but by neglected routine maintenance or incorrect settings during installation.

If you are in the selection phase or planning an upgrade, also see the articles What regulator do I need for solar collectors – selection based on the number of collectors and storage tanks and Solar pump unit ECO vs. standard – what is the difference and when is it worth it. And if you have any specific questions about service intervals for a specific product from our range of control units, you will find answers in the article Common questions about control units for solar systems.

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