Maintenance and servicing of a solar system: what to check every year and when to call a technician
Maintenance and servicing of a solar system: what to check every year and when to call a technician
A solar system is one of the few technologies in a household that owners tend to assume "just works by itself." Collectors are on the roof, there's a tank in the boiler room, the pump hums – and as long as hot water flows, there seems to be no reason to worry about anything. This attitude is one of the most common reasons why systems that should have run smoothly for twenty years start showing serious faults after just seven or eight. Some of these are minor issues an experienced technician could spot in ten minutes. Others are the result of years of neglect and require replacing components worth several hundred euros.
This article is intended for anyone who owns a solar system or is planning to buy one – regardless of whether it's a compact system with a tank integrated directly into the collectors, or a larger setup with a separate tank in the boiler room. We'll go through what you can check yourself, what requires a service technician, typical intervals and symptoms of an approaching fault, and which specific values and conditions to watch for.
Why regular maintenance of a solar system really matters
Unlike a water heater or a gas boiler, a solar system operates under extreme conditions. In summer months, temperatures in the collector without load can exceed 180 to 200 °C. The heat transfer fluid – usually a mixture of water and propylene glycol – is subjected to repeated heating and cooling cycles. Materials age, corrosion inhibitors in the fluid break down, and sealing elements harden and lose flexibility. If these processes are left unchecked, the result is fluid leaks, heat exchanger corrosion, pump station failure, or a burst hose.
From a purely economic point of view: an annual preventive inspection will cost you roughly 60 to 120 euros, including any top-up of the expansion vessel or minor repair. Replacing a flat collector damaged by long-term overheating can cost 400 to 800 euros, while replacing a tank with damaged anodes and a corroded heat exchanger shell can easily run 1,000 euros or more. Investing in an annual inspection therefore pays for itself very quickly – not to mention guaranteed service life, manufacturer warranty conditions, and possible subsidy claims, where documented servicing is a mandatory part of the paperwork.
Overview of the solar system: what's where and what can fail
Before we get into specific inspection steps, it's useful to have a clear picture of the components that make up a solar system and where typical failure points occur. The diagram below shows a typical layout of a drainback or pressurized solar circuit in a family house:
The main components requiring regular attention are: collectors (glass, absorber, frame seal), solar circuit piping (insulation, joints, roof penetrations), expansion vessel (pre-charge pressure, membrane), heat transfer fluid (pH, inhibitor content, freezing point), pump station (pump, valve, flow meter), controller (temperature sensors, settings), and hot water tank (anode, heat exchanger, thermostat). We'll cover each of these in detail.
What you can check yourself – an annual home inspection
1. Visual inspection of the collectors
Once a year – ideally in spring after winter, or in autumn before it – it's worth setting aside half an hour to look up at the roof (or look through an attic window, or use binoculars). Here's what to look for:
- Cracks or scratches on the collector glass. Minor surface scratches aren't critical, but deep cracks or broken glass need to be addressed immediately – moisture inside the collector damages the absorber and insulation.
- Condensation inside the collector. If you see fogging or droplets behind the glass, the frame seal has failed. Moisture inside reduces efficiency by 15–25% and causes long-term damage to the absorber layer.
- Condition of the frame and support structure. Rust, loose screws, a frame deformed after a storm – all of this should be noted and repaired. The support structure must be sturdy, especially for collectors on a pitched roof, where wind loads apply.
- Surface dirt. Bird droppings, smog, moss at the frame edges – not a disaster, but the annual energy loss can be 3–5%. Clean the collectors with a soft cloth and clean water (no chemicals, no pressure washer).
- Absorber coating. This parameter can't be seen without close inspection, but if the collector is older than 12–15 years and output has visibly dropped, have the absorber's optical properties measured during a service inspection.
2. Checking insulation and pipework in accessible areas
In the boiler room or utility room, inspect the solar circuit piping. The insulation (EPDM or mineral wool with a UV-protective jacket) must not be crumbling, cracked, or wet. Wet insulation usually indicates a micro-leak at a joint or connection – a subsequent pressure test will reveal it to the technician. At roof penetrations, check the sealing elements – after years of sun and frost they crack and can cause leaks.
3. Checking the controller readings
Modern controllers (e.g. in kits such as the Vaillant auroSTEP VSL S 250/2 T or Protherm HelioSet FES2 250 BM) display current temperatures, operating hours, and sometimes energy statistics. It's worth checking these once a month:
- Collector temperature – on a sunny summer day it should be in the 60–120 °C range with active pumping; the stagnation temperature without load can easily reach 150–200 °C.
- Tank temperature – the upper zone should reach the set target (usually 55–65 °C); the lower zone doesn't need to be as hot, which is normal.
- Error messages or warnings – if the controller shows an error code, don't just clear it with "reset" without noting the code. The code is important information for the technician.
- Pump operating hours – if in summer the pump barely runs (few hours), it could indicate a fault in the controller, pump, or temperature sensor.
4. Checking the pressure in the solar circuit
The pump station has a pressure gauge – usually a small round dial. In a cold system (in the morning before startup), the pressure should correspond to the expansion vessel's set pre-charge pressure plus the height difference between the collector and the expansion vessel. For a typical family house, this comes to 1.5 to 2.5 bar in a cold system, rising to 3–4 bar at operating temperature.
If the pressure repeatedly drops and you have to top up the system, you either have a faulty expansion vessel membrane or a leak somewhere in the circuit. You won't be able to fix either yourself without professional equipment – but do note the values and how often you need to top up. This is valuable information for the technician.
Heat transfer fluid: the most important and most neglected component
This is a topic where even technically savvy owners often go wrong. Propylene glycol in a solar system does not age indefinitely. Manufacturers (e.g. Vaillant, Protherm, Resol) recommend checking the fluid every 2 years and replacing it every 5–8 years – depending on the temperatures it has been exposed to. In systems where repeated stagnation occurs (the collector overheats without load), the fluid degrades faster.
What happens during degradation: corrosion inhibitors get used up, the fluid's pH drops below 7 (it becomes acidic), and the acidic fluid starts corroding the copper and aluminum in the tank's heat exchanger. We've seen cases where a tank had to be replaced simply because the owner "saved money" by skipping a fluid change worth 30–50 euros. Replacing the tank on a 10-year-old system ended up costing 1,200 euros.
Parameters a technician measures during a service inspection:
- Fluid pH – the optimal range is 7.5 to 8.5. Below 7.0, the fluid is aggressive and must be replaced immediately.
- Freezing point – for Slovakia, the minimum requirement is −28 °C to −35 °C. If the freezing point has risen above −20 °C, the mixture is too diluted and there's a risk of freezing. You can read more about this in the article Solar systems in winter: how they work at low temperatures and how to prevent the circuit from freezing.
- Visual clarity – the fluid should be clear or slightly pink (if dye has been added). Dark, cloudy, or containing sediment – these are warning signs of corrosion or biological contamination.
- Inhibitor content – measured with test strips or titration. Most technicians do this on site.
Expansion vessel: the silent hero and silent threat
The expansion vessel in a solar circuit has a different design from the one in a heating system – it must withstand higher temperatures (over 130 °C during stagnation) and is sized for a larger expansion volume. The air chamber, separated by a membrane, has a set pre-charge pressure that must correspond to the height of the system.
The membrane ages and hardens. If the pre-charge pressure has dropped (the membrane has lost elasticity or is perforated), the vessel loses its buffering function. The system pressure then fluctuates dramatically – and when overheated, the system releases fluid through the safety valve. The result is not only loss of fluid but also air ingestion during cooling – leading to problems discussed in the article Common solar system faults: overheating, air in the circuit, insufficient water heating.
Checking the pre-charge pressure is simple – all you need is a tire pressure gauge and the air valve on the expansion vessel. But: always check with a cold system and with the supply to the system shut off, otherwise you'll get incorrect readings. If you're unsure of the procedure, leave it to a technician.
Hot water tank: anode and heat exchanger
The tank in a solar system is most often a combi tank or a solar tank with two heat exchangers (solar + boiler/heat pump). Unlike a standard pressurized tank, it must withstand higher temperature cycles and occasional temperatures above 90 °C during thermal disinfection against Legionella.
The magnesium anode is a piece of active metal that sacrifices itself instead of the tank's steel shell. Checking and replacing it is a mandatory part of every tank service inspection. Most manufacturers recommend checking it every 2 years, with replacement depending on condition – in hard water (above 300 mg/l CaCO3), the anode wears out faster and may need replacing every year. An anode 350 mm long, worn down to less than 30 mm in diameter, must be replaced immediately.
Limescale on the heat exchanger is a problem in hard-water areas. A 3 mm scale layer reduces heat transfer by more than 20%. Chemical descaling of the tank is a service task, not a DIY job – it requires neutralizing the acid before draining it into the sewer and inspecting the heat exchanger afterward.
Tanks from systems such as the Vaillant auroSTEP VSL S 250/2 F or Protherm HelioSet 2.250C HT are designed for a long service life, but only if servicing intervals – including anode inspection – are observed. Their manufacturers strictly include this condition in the warranty terms.
Pump station and controller: electronics and mechanics
The pump station includes the circulation pump, check valve, flow meter, safety valve, and shut-off fittings. The pump runs thousands of hours per year, and its bearings have a limited service life. Modern EC pumps (e.g. Grundfos Solar, Wilo Stratos) last 15–20 years, older types 7–12 years.
During a service inspection, the technician will check:
- Pump noise – cavitation, unusual vibration, or bearing hum are signs of an impending failure.
- Flow rate setting – flow in the solar circuit should be in the range of 40–60 liters per hour per m² of collector (for flat collectors with forced circulation). A properly set flow rate is important both for system efficiency and to prevent erosion in the collector.
- Safety valve – check for signs of recent discharge (limescale, corrosion at the outlet). If it has discharged, find out why.
- Controller – firmware update (if available), checking temperature sensors (a deviation greater than ±3 °C is reason to replace the sensor).
Service schedule: what and when
In practice, it looks like this:
| Interval | Task | Who |
|---|---|---|
| Every year | Visual inspection of collectors and insulation, circuit pressure, controller and sensor check, collector cleaning | Owner (visual), technician (measurements) |
| Every 2 years | Check and possibly replace tank anode, check fluid pH and freezing point, expansion vessel pre-charge pressure | Service technician |
| Every 4–6 years | Replace heat transfer fluid, check pump, replace seals in pump station | Service technician |
| Every 8–12 years | Replace expansion vessel (or membrane), check heat exchanger, possible descaling | Service technician |
| As needed | Pump replacement, collector replacement, tank replacement | Service technician |
When to call a technician immediately – symptoms you shouldn't ignore
There are several situations where you shouldn't wait for a scheduled annual service. As soon as you notice any of the following symptoms, call an authorized service technician:
- The safety valve repeatedly discharges fluid. Once every few years, during extreme stagnation overheating, this is possible and not necessarily an emergency. If it happens several times in a season, something is seriously wrong – overheating, incorrect pre-charge pressure, or a faulty pump.
- Hot water doesn't smell right or has a brown color. This may indicate tank corrosion, a depleted anode, or biological contamination (Legionella pneumophila). This is a health risk, not just a technical issue.
- The controller reports a sensor error or a persistent pump fault. This could be a broken cable, but also a burnt-out pump or a faulty control board.
- You see signs of a leak on pipes or joints – droplets, a white crust of glycol crystals. Glycol leaves a characteristic white or yellowish crust when it dries. Every leak must be repaired and the system topped up and vented.
- The tank doesn't pre-heat the water even on long sunny days. If this persists and the controller reports no fault, it may be a clogged heat exchanger or incorrect flow rate setting. You can't diagnose this without measurements.
- You hear loud cracking or banging from the pipes when the pump starts. This is water hammer or cavitation – it can damage seals and joints; the flow rate needs adjusting and the circuit checked for trapped air.
What "authorized service technician" means and why it matters
Many owners assume that the plumber or heating engineer who does routine boiler maintenance can also handle a solar system. Not always. Solar systems – especially those from Vaillant or Protherm – require technically trained staff with access to diagnostic tools and original spare parts for warranty service. For the Solar System No. I S or other sets in the portfolio, the documentation always specifies the warranty conditions and the certifications required of the service technician.
Outside the warranty period, you have more freedom in choosing a technician, but we recommend someone with direct experience in solar technology, not just conventional heating systems. The solar circuit operates with different pressures, temperatures, and fluids than a heating system, and mistakes in setup or repair can have a much greater impact.
Seasonal preparation: spring and autumn
In addition to the annual service inspection, it's worth having two short seasonal checkpoints:
Spring (March–April): Check whether winter has left any traces – frost can damage seals at roof penetrations, snow may have loosened the collector's mounting brackets, and frozen water in the circuit (if the fluid's freezing point was too high) could have cracked fittings. Turn the system on manually and observe whether the pump starts up normally and whether the pressure stays stable during the first 30 minutes of operation.
Autumn (September–October): This is the ideal time for a service inspection – the solar season is behind you, technicians are less busy than in summer, and you have enough time to fix any issues found before winter. Check the fluid's freezing point – if you had an active season with high stagnation temperatures, the glycol may have degraded. Check the outdoor pipe insulation (on the roof) – UV radiation destroys the insulation jacket, and cracks should be sealed with self-adhesive insulation tape or a replacement cover.
Real-world examples: what happens when servicing is neglected
From real service work, we can identify several typical recurring scenarios:
Scenario 1 – Black fluid and a corroding tank. A customer had a 9-year-old system and never changed the fluid. During the service inspection (the first in 9 years), the technician found the fluid nearly black, with a pH of 5.8 (strongly acidic), and visible corrosion on the tank's internal heat exchanger. The tank had to be replaced, the fluid changed, and the circuit flushed. Total damage: about 1,400 euros. Five fluid changes over 9 years would have cost about 250 euros.
Scenario 2 – Anode down to zero. During an inspection of a Protherm tank in its 6th year of operation, the technician pulled out an anode that had literally crumbled to pieces – remaining diameter under 8 mm from an original 26 mm. The tank had been without cathodic protection for probably 3–4 years. The interior of the tank showed early-stage corrosion. The anode was replaced in time, and the tank was saved. Cost: 45 euros for the anode plus labor. Had it waited another year, the tank would have had to be replaced.
Scenario 3 – Air in the circuit and a non-functioning pump in summer. A customer called in July: "The collectors are hot, but the tank is cold." Diagnosis: the pump was running, but there was zero flow – a large air pocket in the circuit was blocking flow. Cause: a year earlier the safety valve had discharged fluid (unreported), the system was topped up unplanned with plain water without glycol, and air got into the circuit. Repair: venting, refilling with the correct fluid mixture, and checking pressure. Repair time: 2 hours, cost about 80 euros. Without the repair, the collector would have been exposed to extreme stagnation temperatures all summer.
Combination with a backup heat source: maintaining the whole system
Most solar systems in family houses work in combination with a boiler, heat pump, or electric backup heating. When servicing the solar system, you also need to consider how the interconnection works. The solar system's controller must be set up to properly cooperate with the backup source – if both sources have incorrectly set priorities, the boiler may end up heating the tank even when the collector is producing free energy. You can read more about this topic in the article Combining a solar system with a boiler or heat pump: how to properly interconnect the systems.
Documentation and records of service inspections
One last thing that often gets forgotten in maintenance: keeping service records. Every inspection, every measured parameter, every replaced component should be logged. Why? First, manufacturers require it to maintain the warranty. Second, when selling the property, a documented service history is a huge added value and proof that the system is in good condition. Third, the technician who comes next year will know what was done last time and what still needs attention.
If your original installer didn't give you a service log, simply create a spreadsheet or notebook: date, tasks performed, measured values (pressure, pH, freezing point, anode condition), technician's name, notes. Neglecting this formality means that after a few years, no one knows when the fluid was last changed or whether the anode was ever checked at all.
Frequently Asked Questions (FAQ)
Do I need annual servicing even if the collectors are new and everything works?
Yes, for several reasons. First, most manufacturers (Vaillant, Protherm, and others) make warranty validity conditional on regular servicing by an authorized technician – without service records, you could lose your warranty claims. Second, some parameters (fluid pH, expansion vessel pre-charge pressure, anode condition) change slowly, but neglecting them leads to costly damage. These are routine preventive costs, similar to changing the oil in a car.
Can I top up the heat transfer fluid myself if the pressure has dropped?
Technically yes, if you have the correct fluid, a manual pump, and know how to shut off and vent the system. But: never top up with plain water – you'll dilute the glycol and reduce both frost protection and inhibitor content. Always use a pre-mixed solution with the correct concentration (typically 35–40% propylene glycol), or leave the top-up to a technician who can also check the cause of the pressure drop. A pressure drop by itself is not normal – it signals a problem.
What happens if the collector stagnates for a long time without load (e.g. during a summer vacation)?
Short-term stagnation (1–2 weeks) isn't a problem for modern flat collectors if the heat transfer fluid is in good condition and the expansion vessel is properly pre-charged. With repeated long stagnation periods (several weeks every summer), the fluid degrades faster and needs checking sooner – after 2–3 years instead of 4–5. Some systems allow reducing collector output during vacations via the controller (a "Holiday" or "Vacation" function).
How do I know if the tank anode is depleted?
The simplest way: pull it out and measure its diameter. A new anode is typically 22–26 mm. Below 10 mm diameter, it must be replaced immediately. Another sign is a sulfurous smell (a rotten-egg or burnt smell) from the hot water – this indicates the anode is too small or completely used up and anaerobic bacteria are developing in the tank. This is a serious health issue.
How much does an annual service inspection of a solar system cost?
Depending on scope and region, it typically costs between 60 and 150 euros for the visit and basic measurements. If spare parts are needed (anode, fluid top-up, seal replacement), you'll pay extra for materials. The total average annual servicing cost for a typical family system (2 collectors, 200–300 l tank) is around 80–120 euros per year over a 5-year horizon, including fluid and anode replacement.
Do collectors need cleaning? Isn't rainwater enough?
In many parts of Slovakia, rainwater really does keep collectors relatively clean – the glass surface is designed with this self-cleaning effect in mind. But in industrial areas, areas with heavy bird activity, or areas with agricultural dust, contamination can cause a 5–10% output loss. In such cases, a spring cleaning of the collectors with gentle water and a soft cloth (no pressure washer!) is well justified. Always inspect from the ground or from a secured ladder – climbing onto the roof without safety equipment is an unnecessary risk.
Conclusion: servicing isn't a cost, it's protecting your investment
A solar system is an investment with a payback period of 8 to 14 years, depending on the system, consumption, and location – you can read more about this in the article Frequently asked questions about solar systems: return on investment, subsidies, permits, and connection. But this payback only holds true if the system operates close to its designed output the whole time. A neglected system can lose 20–30% of its output without the owner noticing at first glance – the hot water is just a bit less hot, the boiler kicks in a bit more, and the bills are a bit higher. Without measurements and historical comparison, this goes unnoticed.
If you're looking for a reliable system that, with proper servicing, will last for decades, take a look at our range of solar systems with flat collectors – from compact systems for a two- to three-person household to larger sets for four or more people. And when choosing your system, also think right away about who will service it – ideally the same authorized partner who installs it.
Do you have a question on this topic?
Can't decide, or dealing with a specific situation in your household? Write to us - we're happy to help.