Maintenance and service of solar collectors – what and when to check

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Maintenance and service of solar collectors – what and when to check

A solar water heating system is one of those investments where care for the equipment directly translates into energy savings and the system's lifespan. Unlike a boiler or stove, where mandatory service is usually obvious and the customer doesn't question it, solar collectors have a reputation in many households as "maintenance-free" technology – once installed, they work on their own. That is partially true, but only partially. Systems that operate reliably for 15 to 20 years without major problems have regular checks and timely service interventions behind them. Systems that are neglected tend to make dramatic reminders – a burst expansion vessel, degraded heat transfer fluid, or corrosion damage to the heat exchanger.

In this article, we will go through the complete service plan – what to check daily, seasonally, once a year, and once every few years. We will describe specific values to look for during inspections and highlight typical findings we repeatedly encounter in practice.

Why a solar system needs maintenance at all

A solar collector system is not a closed hermetic unit that regenerates itself. It works with a heat transfer fluid (usually a mixture of propylenglycol and water), which heats up to a high temperature and cools down every day. This temperature change – the so-called thermal cycle – has a physical impact on the entire system: the fluid ages, seals wear out, and pressure in the system fluctuates. To this, we must add external influences: UV radiation, frost, dust, wind, bird droppings, moss growth on the collectors.

Moreover, a solar system operates under extreme conditions that a standard heating system does not experience. During stagnation – when the tank is full, there is no heat draw, and the sun continues to shine – the temperature in the collector can rise to 150 to 200 °C. For flat collectors, it is usually 150–180 °C, and for tubular types, even higher. This temperature can destroy degraded fluid in a single summer season. Therefore, checking the quality of the heat transfer fluid is one of the absolutely key service operations.

If you are wondering why your system is not working as it should, also see our article Common faults in solar systems and how to eliminate them, where specific fault scenarios and their diagnostics are described.

Overview of service intervals – table

Interval	What to check	Who does it
Every day (visually)	Tank temperature, pump operation, regulator display	Owner
Every month	System pressure, condition of the expansion vessel, air venting	Owner (after training)
Spring and autumn	Cleaning of collectors, visual inspection of the roof and piping	Owner / technician
Once a year	Comprehensive inspection of the entire system, measuring fluid parameters	Service technician
Every 2–3 years	Replacement of the heat transfer fluid (possibly sooner depending on condition)	Service technician
Every 5–10 years	Replacement of the expansion vessel membrane, inspection of the pump and valves	Service technician

Daily and weekly checks – what the owner can check themselves

Most owners of solar systems have access to the regulator or the display of the solar station. It shows basic operational parameters – collector temperature (T1), tank temperature (T2), and pump status (on/off). These data can be monitored for a few seconds daily, ideally at midday in sunny weather.

What these values should indicate in normal operation:

Collector temperature (T1): On a sunny summer day at midday, it typically ranges from 60–120 °C (depending on the intensity of the sun and whether the pump is running). If T1 is significantly higher than normal and the pump is not running, it may indicate stagnation.
Tank temperature (T2): In summer, it should reach 50–70 °C by the evening. If it remains consistently low even in sunny weather, something is wrong.
Pump status: The pump should run whenever T1 is significantly higher than T2 (typically by 5–10 °C depending on the regulator settings). If the pump does not start at all or runs continuously, it is a signal for inspection.

Monthly inspection – pressure and air in the system

This is the first thing most owners hesitate about, as they fear they might break something. In reality, it is a very simple task that anyone who has ever bled a radiator can handle.

Checking the pressure in the solar circuit

Every solar system has a pressure gauge – manometer – installed on the solar station or directly on the pipe. Reading the pressure takes three seconds. The key is knowing what the correct pressure should be in your system. This number is stated in the documentation from the installation company and usually ranges between 1.5 to 3 bar in a cold system (early in the morning, just before starting up).

Why in the morning? Because in a heated system, the liquid expands and the pressure naturally increases. If the pressure measured when cold is lower than specified, the system is likely losing fluid or has a problem with the expansion tank.

Typical scenarios from practice:

Pressure steadily drops (e.g., from 2.2 bar to 1.4 bar in a month): Look for a leak – check all connections, bleed valves and the safety valve. The safety valve usually does not leak, but if it is neglected or old, it can gradually release fluid into the drain.
Pressure does not drop, but the system does not work properly: The problem may be with the expansion tank (burst membrane, insufficient gas pressure in the chamber).
Pressure rises extremely when heated: The expansion tank is too small or completely non-functional – this is a dangerous condition that leads to the safety valve opening.

System bleeding

Air in the solar circuit is the number one enemy. It causes pump noise, cavitation and uneven fluid distribution. Most systems have an automatic air vent, but it may release air back into the system during stagnation (when the fluid partially turns into vapor).

If the pump makes bubbling or cracking sounds, it is time to bleed the system – either manually via the bleed valve on the solar station or by calling a technician.

Spring and autumn seasonal inspection

Two inspections per year – in spring before the main solar season and in autumn before winter – should be handled by a skilled owner themselves, or with the help of a technician during the annual service.

Cleaning the collectors

Flat plate collectors and tube collectors gradually get dusty, covered with bird droppings, leaf leachates or moss. Every layer of dirt reduces the transparency of the glass and thus the efficiency of the collector. In practice, we have measured a 5–15 % efficiency drop on collectors that had not been cleaned for three years and were located under deciduous trees.

How to clean properly:

Never clean the collectors when hot or in direct sunlight – you may crack the cold glass with hot water or vice versa, pour cold water on hot glass. The best time is early in the morning or on a cloudy day.
Use a soft cloth or sponge, no wire brushes or aggressive cleaning agents.
Warm water is sufficient, optionally with a mild glass cleaner. Special solar cleaning agents exist, but most installation companies do not consider them necessary.
With tube collectors, be careful – the glass tubes are fragile and can be easily damaged with improper handling. If a tube is cracked or has dark spots inside (condensation), it needs to be replaced.

Visual inspection of the structure and piping

Spring inspection is an ideal time to check the physical condition of the entire installation on the roof. We are looking for:

Mounting condition of the collectors: Whether the structure has slipped, if there are loose screws, or if the consoles are corroded. This happens more often than it seems after a windy winter.
Condition of the pipe thermal insulation: The outer part of the supply and return pipes is exposed to UV radiation. Rubber insulation sleeves (Armaflex and similar) crack and peel after years. A bare metal pipe on the roof will lose thermal insulation and may freeze in winter.
Sealing of roof penetrations: Where the pipe passes through the roof, this area must be reliably insulated and sealed. Roof leakage around the penetration is a common finding in homes where service has been neglected for years.
Condition of the collectors themselves: Cracks in the glass, corrosion on the frame, damaged rubber seals around the frame. With tube collectors: the number of intact tubes (a cracked tube is visually noticeable – it is dull, not shiny).

Annual service – what belongs to the technician

Once a year, the system should be inspected by a trained service technician. Not because the owner has broken something, but because some measurements and tasks require equipment and experience. Annual service usually takes 1 to 2 hours and its cost ranges from 50 to 150 euros depending on the region, system complexity and the technician's travel distance.

Measuring and evaluating the heat transfer fluid

This is the core of the entire annual service. The heat transfer fluid – usually propylene glycol with corrosion inhibitors – has a limited lifespan. With daily thermal stress, the inhibitors gradually deplete, pH drops and the fluid becomes corrosive. At the same time, its concentration (and thus the freezing point) changes, which is critical for winter operation.

The technician measures during the service:

pH of the fluid: The correct value is 7 to 9. Below 7, the fluid is acidic and aggressive to copper and steel components. Such fluid must be immediately replaced.
Density / glycol concentration: Measured with a refractometer. The target concentration of propylene glycol is usually 30–50 % (depending on the climatic zone – higher in mountainous areas). This concentration ensures a freezing point of at least −15 to −30 °C.
Color and clarity: A healthy fluid is clear or slightly yellow. Dark (brown, black), cloudy or smelly fluid is a clear signal for replacement.
Content of corrosion inhibitors: Some technicians have test strips for a quick approximate check.

Expansion tank inspection

The expansion tank is a pressure tank with a membrane that captures volume changes of the fluid during heating and cooling. Its proper function is critical for system safety. The technician checks:

Gas chamber overpressure (nitrogen): Measured using a Schrader valve (the same type as on car tires). The correct overpressure is calculated according to the system height, but typically it is 1.0 to 1.5 bar in a cold system. If it is zero – the membrane is likely broken and the tank is filled with fluid only.
Membrane condition: If liquid flows out of the expansion tank valve during venting (instead of air), the membrane is punctured and the tank must be replaced.
Tank size vs. system volume: Sometimes the tank was undersized from the beginning. Technically correct expansion tank should have a volume of at least 10–15 % of the total fluid volume in the system, taking into account maximum operating temperatures.

Valve, pump and regulator inspection

The solar station (compact unit containing a pump, flow meter, safety valve, air vent and pressure gauge) deserves attention at least once a year. The technician inspects:

Pump: Whether it runs quietly, without unusual sounds. Modern EC pumps have a long lifespan, but mechanical clogging in older models wears out. Typical pump lifespan in a solar circuit is 10–15 years.
Safety valve: Reacts at pressure 6 or 10 bar (according to the setting). It should be functional, without deposits. Do not test it yourself – releasing the safety valve without reason is dangerous (hot fluid).
Check valve: Prevents so-called night circulation (cooling of the storage tank through the collector at night). A clogged check valve is the reason why the tank loses heat during the night.
Controller: The technician checks the settings of differential temperatures, maximum tank temperature, function of pipe sensors and correctness of the wiring.

Heat transfer fluid replacement – how it works

Heat transfer fluid replacement is a service task that is carried out every 2 to 5 years depending on the condition of the fluid. It is not a complicated task, but it requires proper equipment – a pump for filling the circuit, containers for collecting the old fluid, measuring instruments and the new fluid itself.

Short procedure:

The system is allowed to cool down (never drain a hot system).
The old fluid is drained into a suitable container – propylene glycol is relatively environmentally friendly, but it must not be discharged into the sewer in large quantities without permission.
The circuit is flushed with clean water until the outflow is clear.
The new fluid of the correct concentration is filled using a filling set.
The system is purged of air and the pressure is set to the prescribed value.
The controller function is checked and the replacement date is recorded.

The average price of the fluid (propylene glycol with inhibitors, suitable for solar systems) is 8–15 euros per liter. A typical family two-collector system has a circuit volume of 5–8 liters. Total costs for replacement including the technician's work range from 100 to 250 euros.

Important: Do not use automotive coolant (ethylene glycol) – it is toxic and corrosive for solar systems. Always require propylene glycol with inhibitors specifically designed for solar systems.

Winter operation – what to watch for additionally

Winter is a different kind of load for a solar system than summer. At temperatures below −10 °C, the collectors and pipes must rely entirely on the antifreeze protection of the fluid. If the fluid replacement was neglected and its concentration has dropped (e.g., clean water was added during a leak), freezing and damage to the collectors or pipes is a risk.

Another winter phenomenon – snow load. A layer of snow can remain on flat collectors, which can stop heat production for several days. This is a normal situation – collectors are dimensioned for such a load (typically 150–250 kg/m²). Never remove snow with hard tools – you can break the glass. If snow does not disappear on its own, gentle sprinkling with warm water from the edge can help.

Systems with drain-back function (drain-back systems, where the fluid automatically drains into a tank during stagnation or shutdown) are more resistant to frost, but they also require regular inspection of correct function and tank tightness for the fluid.

If you are interested in how a solar system performs outside the summer season, read our article Is a solar system worth it even in cloudy weather or winter?.

When to call a technician immediately – warning signs

Not every anomaly requires immediate service intervention, but some signs should be taken seriously and not wait for the annual service:

Sharp pressure drop within days: Active fluid leak. Look for wet spots, drops or leakage from the safety valve.
The tank stops heating completely, even in sunny weather: The pump, controller or temperature sensors are likely not working.
Fluid leaks repeatedly from the safety valve: The system is overheated – the cause may be a faulty expansion tank, incorrect controller setting or a clogged circuit.
The pump runs continuously, even at night: The controller is faulty or has faulty sensors.
Visible corrosion on pipes or connections: Immediate inspection to prevent failure.
Broken glass tube in a tube collector: Tube replacement is necessary – an open tube not only loses performance, but moisture enters the collector and degrades the absorber.

Service documentation – why it is important

Experienced owners, or those who plan to sell the house together with the system later, keep the service book for the solar system. It contains:

Date of installation and system type (manufacturer, collector model, storage tank, controller)
Annual service records with measured values
Dates of fluid replacement and type of fluid used
Records of faults and repairs
Contact information for the installation/service company

This documentation also has a practical economic aspect: in the case of component claims, insurance incidents, or when selling real estate, the service book is a verifiable asset. In some cases, insurers may refuse to cover damage caused by neglect in operation.

For more information on the correct start-up and setting of the system, see the article How to set up and start operating a solar system, where the initial parameters that you should record right at the first start-up are described in detail.

Estimated lifetime of individual components

In order to plan service expenses in advance, we provide an approximate lifetime of the main system components:

Component	Typical lifetime	Note
Flat solar collector	20–30 years	With proper maintenance, risk: glass damage, frame corrosion
Tube collector (overall)	15–25 years	Individual tubes: 10–15 years, periodic replacement
Heat transfer fluid	2–5 years	Depends on stagnation temperatures, must be measured annually
Pump (solar station)	10–15 years	EC pumps last longer
Expansion tank (membrane)	5–10 years	Entire tank usually 15–20 years
Safety valve	10 years (check annually)	Preventive replacement is cheaper than failure
Controller / control unit	10–20 years	Problem with availability of spare parts after 15 years
Storage tank with heat exchanger	15–25 years	Depends on water quality, anode protection

Magnesium anode in the storage tank – this must not be forgotten

The hot water storage tank has a magnesium protective anode inside. This anode is electrochemically sacrificed instead of the tank wall – it corrodes, not the tank. It is a simple and brilliant system, but it has a catch: the anode must be regularly checked and replaced.

The frequency of inspection depends on water hardness (in soft water, the anode wears out faster), but the standard recommendation is to check every 2 years and replace it according to condition. A worn anode (remaining thickness less than 1/3 of the original) must be replaced. A tank without a functional anode will start to corrode from the inside and may crack after a few years.

This inspection is carried out during the annual service of the tank. It is technically simple – the anode is accessible through the top inspection opening of the tank – but it requires disconnecting the tank from the water pressure.

Combination with a backup heat source – service specifics

Most family homes have a solar system connected in combination with a boiler (gas, pellet, wood) or a heat pump. This combined connection brings a few additional service specifics that should be kept in mind.

The tank controller must properly coordinate heating from the solar circuit and the backup source – typically so that the backup source only activates when solar is insufficient. If a regulation error occurs, the backup source may heat the tank even when the solar system is operating at full capacity, which naturally increases energy consumption instead of reducing it.

This topic is described in more detail in the article Solar system in combination with a boiler or heat pump.

Most frequently asked questions (FAQ)

How long can a solar system last without service without visible problems?

In practice, it often happens that systems operate for 5–7 years without service without a dramatic failure. The problem is that they gradually lose efficiency (degraded fluid, air in the circuit, soiled collectors) and long-term neglect shortens the system's lifespan. For example, a storage tank without anode replacement can fail after 8–10 years, whereas with regular maintenance it could last 25 years. Saving on service is therefore not worth it many times over.

Can I perform the service myself, or must a technician always do it?

Visual inspection, pressure reading, monitoring temperatures via the controller, and cleaning the collectors – all of this can be done by a skilled owner themselves. Measuring and replacing the heat transfer fluid, checking the safety valve, diagnosing the controller, and servicing the expansion tank should be done by a technician. Not because it is forbidden for a layperson, but because it is a pressurized system with hot fluid, and an error can cause serious injury or property damage.

How can I tell that the heat transfer fluid needs to be replaced, without a technician?

The simplest indicator is color: a healthy fluid is clear or light yellow. If it is dark brown, gray, or black, or if it smells burnt, it must be replaced without question. Precise pH measurement can be done using either test strips (available in garden or aquarium shops – a pH range of 5–10 is sufficient) or a refractometer for measuring glycol density. Both are relatively cheap tools (5–30 euros), which every skilled owner will appreciate.

What to do if I am on vacation and the system is left unattended during summer?

Systems with regulation, a pump, and a properly planned expansion tank are designed for operation without supervision. Stagnation with a full storage tank is a normal state – the system stops and waits. Problems occur only if the system has a fault (non-functional expansion tank, degraded fluid). Therefore, the best time for annual service is just before summer (right before the vacation season). If you are going away for a long time, you can ask a neighbor to check the controller display once a week.

Does the performance of collectors decrease over the years even with good maintenance?

Yes, but slowly. The absorber layer on flat collectors gradually ages, selective coatings lose some of their efficiency, and glass transmittance slightly decreases after 15–20 years. Realistically, expect a performance drop of 0.5–1% per year in a well-maintained system. After 20 years, you can therefore have a system with performance at 80–90% of the original value – which is still very good. Aggressive degradation occurs with neglected maintenance (damaged fluid, neglected air in the circuit) and can be much faster.

What is the average annual cost of maintaining a solar system?

For a family system with two collectors and one storage tank, expect an average of 80–150 euros per year for annual technician service without fluid replacement. In years when the fluid is replaced, an additional 100–200 euros is added. Replacement of the storage tank's magnesium anode costs 30–60 euros for the anode plus labor. Overall, the annual average including planned replacements is 100–200 euros – which is a very favorable ratio compared to the energy saved (typically 500–1,500 euros/year depending on the system).

Conclusion – service as an investment, not an expense

A solar system is an investment for 20–30 years. This investment pays off only if the system actually works efficiently for all those decades – not just the first three years until the fluid starts to corrode the pipes and the expansion tank ruptures the membrane. Regular service is not a luxury or an unnecessary expense – it is a condition for the solar system to be worthwhile at all.

The good news is that maintenance is not complicated or expensive. Most tasks are simple, some can be done by the owner themselves, and annual technician service costs a fraction of what a replacement of a failed storage tank or a collector with a damaged absorber would cost. In practice, we see that owners who pay for proper installation and regular service from the beginning have systems that are still functional after 20 years – and still saving energy. Those who neglect service end up with extensive repairs or replacement of the entire system after ten years.

For those considering a solar system and wanting to correctly calculate costs and benefits from the start, we also recommend our other articles: How to choose a solar water heating system for a family home, What solar collector capacity do I need for my home, and Flat vs. tubular solar collectors – which type is more profitable.

All solar systems and accessories can be found in our product category, which includes complete sets as well as individual components for servicing existing installations.

Do you have a question about this topic?

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