Common solar system faults: overheating, air in the circuit, insufficient water heating

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Common solar system faults: overheating, air in the circuit and insufficient water heating

A solar system is a relatively simple setup – but that's exactly why, when something isn't working properly, the problem is often only discovered after a longer period of time. Owners notice that the hot water isn't as hot as it should be, the system makes strange noises, or on the contrary – the storage tank overheats so much that liquid escapes from the safety valve. In this article we'll look at three of the most common groups of faults: overheating of the collector circuit, air in the system, and insufficient water heating in the storage tank. We'll examine each cause in depth, explain the symptoms, diagnostics and solutions – exactly as we see them in practice on dozens of jobs each year.

1. Overheating of the solar circuit – when the sun causes more harm than good

Overheating is one of the paradoxes of solar technology: a system designed to save energy can cause serious problems in summer if it's not properly sized or set up. Stagnation – the state when the liquid in the collector stops circulating and starts to boil – is a normal operating condition, but a problem arises when it occurs too often or the system isn't prepared for it.

Symptoms of overheating

The safety valve on the collector circuit (typically set to 6 bar) repeatedly releases liquid
The expansion vessel is hardened, and the system pressure drops significantly after cooling
Smell of the heat transfer liquid (glycol mixture degrades when exposed to temperatures above 140 °C for a long time)
The liquid turns brown to black – burnt glycol
The collector circuit makes banging and hissing noises (liquid boiling)
The controller reports an error – high collector temperature, the pump cannot return to normal operation

Causes of overheating – where to look for the problem most often

Insufficient expansion vessel volume: This is by far the most common cause of problems. When properly sized, the expansion vessel must accommodate not only the thermal expansion of the liquid, but also the entire volume of liquid from the collector in case of stagnation. If the vessel is undersized, the pressure during stagnation will escape through the safety valve. As a rough estimate: for a standard two-collector set (approx. 4–5 m² of area), an expansion vessel of at least 18–25 litres is required, with the vessel's pre-charge pressure (air pressure) matching the static height of the circuit.

Incorrect expansion vessel pre-charge setting: The pre-charge pressure in the expansion vessel should be 0.2–0.3 bar higher than the static pressure of the liquid column from the lowest point to the highest (collectors on the roof). If the pre-charge is too low, the membrane will stick to the vessel wall the first time it heats up, and the vessel will stop performing its function. The result is the same as with an undersized vessel.

Poor collector tilt or orientation: Collectors facing directly south with a tilt of 30–45° produce maximum energy in summer, which the storage tank simply cannot absorb. In some cases it helps to tilt the collectors to a greater angle (60–70°), reducing summer output and slightly increasing winter output.

Circulation pump switched off or controller thermostat stuck: If the controller cannot start the pump (fault, stuck contact), the collectors will quickly overheat. The controller should have a protective function – when the tank temperature reaches the maximum set value (e.g. 85 °C), the system stops and the collectors stagnate.

Degraded liquid: Glycol that has gone through several seasons of stagnation loses its corrosion inhibitors and its boiling point drops. Such liquid boils at lower temperatures, produces more vapour, and the system becomes unstable. Recommended replacement: every 4–5 years, or according to pH and density testing.

What to do in case of repeated overheating

The first step is always to check the pressure in the system and the condition of the expansion vessel. If the pressure after the system cools down is below 1 bar, the membrane in the vessel has probably burst or the pre-charge is incorrect. The vessel can be topped up with air (like a bicycle tyre, the valve is the same type) – but be careful, this should always be done with the system cold and depressurised. If the vessel cannot hold the pre-charge, it must be replaced.

Second step: check the condition of the liquid. Take a sample (approx. 50 ml) into a transparent container. The liquid should be clear or slightly coloured – not cloudy, brown, or containing sediment. Check the pH with test paper (optimum 7.5–9.0). Dark, acidic liquid must be replaced, as it continues to damage the collectors, pump and storage tank.

Third step: check the controller settings. The maximum tank temperature should be set to 75–80 °C. Some controllers also allow a so-called "night cooling" function – the system runs the pump at night to transfer excess heat back to the collectors. This is an elegant solution for summer overheating.

2. Air in the solar circuit – the silent efficiency killer

Air in the circuit is a problem that's easy to overlook because it doesn't manifest dramatically – but it significantly reduces the system's efficiency in the long run and can damage the circulation pump. In practice we see this problem after every initial installation where the circuit wasn't thoroughly bled, but also in older systems where a joint has lost its tightness.

How air forms and where it accumulates

Air gets into the system primarily during filling – if the circuit isn't filled with a pressure pump from the lowest point but "from the top," air bubbles remain trapped at the highest points of the piping. Typical locations: the collectors themselves (the highest point of the system), pipe bends, or places with insufficient slope. Air can also form chemically – dissolved gases can be released from liquid whose temperature rises. That's why it's important to have an automatic air vent valve at the highest point of the system.

Symptoms of air in the circuit

Noisy pump operation – typical "gurgling" or bubbling sound, the pump appears to be overloaded
The pump is running but the liquid isn't circulating or only partially circulates – flow meter reading is zero or very low
The controller detects no temperature difference between the collector and the tank even though the sun is shining
The tank temperature doesn't rise at all, or only very slowly, in full sunshine
A "gurgling" sound is heard after the pump switches off – air bubbles moving through the system

Diagnosis and venting step by step

Venting a solar circuit isn't as straightforward as with a heating system. The liquid is hot and toxic (ethylene glycol), so we always recommend waiting for the system to cool down – ideally in the morning before the sun comes up. The procedure is as follows:

1. Check the pressure: A cold system should have a pressure of 1.5–2.5 bar (depending on the height of the collectors above the station – 1 bar extra for every 10 metres). If the pressure is below 1 bar, the system has lost liquid or air has escaped from the expansion vessel.

2. Open the automatic air vent valve on the collector circuit (located at the solar station or directly on the collector). It should be open during operation – if it's closed, air cannot escape from the collectors.

3. Start the pump manually (via the controller's service mode) at low speed. Let it run for 10–15 minutes. The air bubbles should pass out through the vent.

4. Increase the pump speed to maximum for 2–3 minutes – the faster flow will "push out" any air pockets from the collectors.

5. Top up the pressure: After venting, the system pressure will drop slightly – top up the liquid with a pressure pump to 1.5–2 bar (cold system).

If venting doesn't help and the pump keeps making noise, the problem could be a damaged circulation pump. Pumps in solar circuits are specifically designed for hot liquid – ordinary heating pumps won't hold up here. In this case we recommend calling a service technician.

3. Insufficient water heating – why the tank isn't hot enough

This is the most common complaint we encounter: "I have solar panels and the water isn't hot." There can be a whole range of causes – from poorly set controls to a clogged heat exchanger. Let's go through them systematically.

Fouled heat exchanger in the storage tank

The storage tank of a solar system has an internal heat exchanger – coiled or tubular – through which the liquid from the collector transfers heat to the potable water. If the water at the installation site is harder (above 15 °dH), limescale gradually builds up on the exchanger. Every millimetre of limescale reduces heat transfer by 10–15%. After 5–7 years, the exchanger can be so fouled that the tank heats water only very slowly – even when the collectors are working at full capacity.

The solution is chemical descaling of the tank – a weak solution of citric or phosphoric acid is dosed into the tank, left to act for a few hours, and then flushed out. We recommend this procedure every 3–5 years in hard water areas. Some modern storage tanks (such as the ones in the Vaillant auroSTEP VSL S 250/2 T set) have an exchanger accessible via an inspection opening, which makes descaling much easier.

Incorrect controller setting – switching temperature difference

The solar system controller works on the principle of temperature difference (ΔT): the pump starts when the collector is X degrees warmer than the tank, and stops when the difference is smaller. Typical setting: start at ΔT = 6–8 °C, stop at ΔT = 2–3 °C.

If the switching difference is set too high (e.g. 12 °C), the pump starts late – on a clear summer day this doesn't matter much, but in spring and autumn you'll lose hours of output. If it's too low (e.g. 2 °C), the pump runs unnecessarily even when the collector cannot heat the liquid to a higher temperature than the tank – effectively cooling it down. Check the setting in the controller's manual and set the values according to the manufacturer's recommendation.

Low flow rate through the collector circuit

The optimal flow rate through a solar circuit is 30–50 litres per hour per square metre of collector. A standard two-collector set (5 m²) should have a flow rate of 150–250 l/h. If the flow meter at the solar station shows less, the causes may be:

A partially closed shut-off valve (gate or ball valve) – this happens after servicing when someone forgets to open it
A clogged filter before the pump – the mesh filter should be cleaned every 1–2 years
A worn pump – after 10–15 years of operation, the circulation pump's output decreases
An air pocket in the piping (see previous chapter)
A supply pipe diameter that's too small (15 mm instead of 22 mm for larger sets)

Improperly sized system

Sometimes the problem isn't a fault but basic undersizing of the system. For a 4-person household, the standard calculation is 4–6 m² of collector area and a storage tank of 250–300 litres. If the family has only 1 collector (2 m²) and a 120-litre tank, there will be a shortage of hot water even if the system works flawlessly. The article How to choose a solar system for a family house: output, number of collectors and tank volume contains detailed sizing with examples.

A well-sized and proven system for a family house is, for example, the Protherm HelioSet FES2 250 BM with a 250-litre tank – suitable for a 3–4-person household with two flat collectors, or the Protherm HelioSet 2.250C HT with a combined tank for connection to a boiler. If you're dealing with combining a heat source, we also recommend reading topics from the Knowledge Centre such as Combining a solar system with a boiler or heat pump: how to correctly connect the systems.

4. Combination of faults – when problems overlap

In practice, we quite often encounter systems with multiple problems at the same time. For example: old glycol caused corrosion in the collector, the corrosion released deposits that clogged the filter and reduced the flow, low flow caused the collector to overheat, and the glycol degraded even further. It's a vicious circle that can only be broken with a thorough service.

An example from practice: a customer had an 8-year-old system on a family house and complained that the water hadn't been hot since spring. During inspection we found: a flow rate of 60 l/h instead of 180 l/h (clogged filter + partially closed valve after a previous boiler service, where the technician accidentally also partly closed the valve on the solar circuit), glycol pH 6.2 (strongly acidic, should be at least 7.5), and a switching ΔT set to 10 °C instead of 6 °C. After a complete service intervention – liquid replacement, filter cleaning, opening the valve, reprogramming the controller – the tank reached 55–60 °C by 10:00 on a sunny day. The customer had previously been considering replacing the entire system. The whole service cost a fraction of that amount.

5. Faults specific to particular types of systems

Compact systems with integrated storage tank

Some systems – for example the Solar System No. I S – have the storage tank and collector formed as a single unit or a tightly connected system. The advantage is simpler installation, the disadvantage is that service access to the exchanger may be limited. For these systems, descaling is done using a special procedure according to the manual – we recommend following the manufacturer's documentation.

Vaillant auroSTEP systems

The Vaillant auroSTEP VSL S 250/2 F (for flat roofs) and Vaillant auroSTEP VSL S 250/2 T (for pitched roofs) systems are equipped with their own intelligent auroMATIC 620 controller, which automatically reports error codes. If the controller displays an F.XX or E.XX code, before calling for service, check the manual to see what the code means – most codes are informational, and some (such as F.10 – low pressure) can be resolved by the owner simply by topping up the pressure. Note: after every liquid top-up, check whether there's a leak at any of the joints – a common reason for a pressure drop is a small leak, not just venting.

6. Preventive maintenance – how to avoid 80% of faults

Most of the problems described can be prevented with regular annual inspection. Here's an overview of what to check – ideally every spring before the season:

Pressure in the collector circuit: Cold system = 1.5–2.5 bar. If lower, top up the liquid and check for leaks.
Condition of the expansion vessel: Check the air pre-charge (via the valve, with the system cold) – it must match the static height of the system.
Liquid (glycol): Take a sample, check the colour, measure the pH and density (with a densitometer or refractometer). If the pH is below 7.0 or the density doesn't match the measured concentration – replace it.
Filter (dirt trap): Clean the mesh filter before the pump – shut off the circuit, close the valves, remove the mesh, rinse with water.
Flow rate: Check the value on the flow meter with the pump running – compare with the design value.
Collectors: Visual inspection – cracks in the glass, loose seals, corrosion on the frame. Rinse with water (no pressure washers – they could damage the absorber's surface layer).
Controller: Check the ΔT setting, the maximum tank temperature, and the anti-legionella protection function (the tank should be heated to at least 60 °C once a week).
Safety valve: Check that it's not clogged or permanently open. A safety valve that repeatedly fails must be replaced.

A detailed annual inspection procedure can also be found in the article Maintenance and servicing of a solar system: what to check every year and when to call a technician in the Knowledge Centre.

7. When to call a technician and when you can handle it yourself

Not every intervention requires a service technician. Here's a rough breakdown:

You can handle yourself: topping up the circuit pressure (if you know where and have the liquid), checking and cleaning the filter, venting (if you know where the valves are and the system is cold), setting the controller, visual inspection of the collectors.

You need a technician for: replacing the liquid (disposing of the old glycol solution), descaling the tank, replacing the pump, replacing the expansion vessel, repairing a leak at a joint under insulation, servicing the control unit, any work on the collectors at height.

Never work on the collector during a sunny day – the absorber can reach a temperature of 200 °C even with an empty circuit. Burns are serious and do happen. Recommended time for work: early morning or late evening, or during cloudy weather.

If you're interested in topics related to operating solar systems under special conditions – for example during freezing months – we also recommend reading the article Solar system in winter: how it works at low temperatures and how to prevent the circuit from freezing.

Frequently Asked Questions (FAQ)

Why doesn't my solar system heat water in summer even though the collectors are hot?

The most common cause is air in the circuit – the liquid isn't circulating even though the pump is running. Another cause could be that the tank is already fully heated (above 80 °C) and the controller has correctly switched off the pump. Check the tank temperature – if it's above 70 °C, the system is working correctly and simply has nowhere to transfer the heat. If the tank is cold and the collectors are hot, the problem lies in circulation.

Liquid is leaking from the safety valve on the solar circuit – what should I do?

The safety valve opens when the pressure exceeds its setting (usually 6 bar). Causes: system overheating (stagnation), an undersized or faulty expansion vessel, or the valve itself is worn and opens prematurely. First step: let the system cool down, check the pressure in the cold expansion vessel. If the pre-charge is within norm and the valve keeps releasing liquid, the safety valve is probably worn and needs replacing. This is a job for a professional, as it involves working with hot liquid under pressure.

My solar system makes loud gurgling noises – is that normal?

No, it isn't. Quiet rustling of the liquid while the pump is running is normal, but pronounced bubbling, gurgling or banging indicate air in the circuit or local boiling of the liquid (during stagnation). If the system bubbles even during normal operation (not during stagnation), the circuit needs venting. If the banging always occurs around 12:00–14:00 and the pump stops at that time, it's probably the controller switching off the pump when the tank reaches its maximum – and you're hearing stagnation in the collector. We recommend checking that the maximum tank temperature is set correctly.

When should the glycol in the solar circuit be replaced?

The recommended interval is every 4–5 years, but in practice it depends on how many stagnation cycles the system has experienced. Glycol degrades with every stagnation event – in systems where stagnation occurs every summer (e.g. due to a long holiday or excessive output), the glycol may become unsuitable for use after just 2–3 years. Indicators: pH below 7.0, dark or cloudy liquid, a pungent smell. Never add plain water to a glycol circuit – it lowers the freezing point and dilutes the corrosion inhibitors.

My system overheats the tank to 90 °C in summer – is that dangerous?

For the tank itself, this usually isn't immediately dangerous (most tanks are tested for higher temperatures), but for people's safety it is – water above 60 °C causes serious burns on contact with skin. In addition, long-term overheating shortens the life of the tank's anode, damages rubber seals and accelerates corrosion. The solution is to set the maximum tank temperature to 75–80 °C in the controller and possibly add a valve to mix hot water with cold (thermostatic mixing valve).

After years of normal operation, the system suddenly stopped heating – what could have gone wrong?

This is a classic scenario of combined faults after a long period without servicing. The most common reason: a clogged filter (the pump runs but flow is minimal) + degraded glycol (acidic, poor heat transfer) + an exchanger clogged with limescale. Another possibility: the automatic air vent got stuck in the closed position and the circuit is full of air. We recommend a comprehensive service – liquid replacement, filter cleaning, venting, checking all valves. More in the article Installing a solar system step by step: what you can do yourself and what a professional must do.

Conclusion: prevention is cheaper than repair

A solar system is a well-designed technology with a minimal number of moving parts – essentially just one pump, a controller and a few valves. Most of the faults described here don't arise suddenly – they're a gradual deterioration that the owner doesn't notice because the system still "somehow works." Regular annual inspection (ideally professional every other year, DIY every year) catches 80% of problems before they cause more serious damage or a leak of liquid into the potable water tank.

If you're still just planning the installation and want to avoid problems from the very start, read the article What solar system output do I need: a calculator based on number of people and hot water consumption and Solar system on a pitched vs. flat roof: which to choose and what it means for installation – correct sizing and installation are the foundation of trouble-free, long-term operation.

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