Common faults of expansion vessels and solar pumps – causes and solutions
Common faults of expansion vessels and solar pumps – causes and solutions
The expansion vessel and the circulation pump are two components of a solar system that are often overlooked – as long as they are working, no one notices them. But when they stop functioning properly, the entire solar circuit quickly runs into problems: pressure rises, the safety valve leaks, the collectors stagnate, the system air locks or simply does not heat as it should. From experience, I know that most faults in solar systems are not related to the collector itself, but precisely to the expansion vessel or the pump – and these are faults that could be prevented by regular inspection or correct installation from the beginning.
This article systematically examines the most common faults of both components, their causes, symptoms and – most importantly – specific procedures for identifying and solving them. We will also look at when it makes sense to repair a component, when to replace it, and what to do preventively to avoid the problem recurring.
Expansion vessel faults – a systematic overview
The expansion vessel in a solar system performs a unique and irreplaceable function: it absorbs the increase in fluid volume when heated and keeps the pressure in the circuit within an acceptable range. When this function fails, the system will show it – usually quite dramatically. Let's take a closer look at the individual types of faults.
1. Loss of pre-charge pressure in the gas part
This is by far the most common expansion vessel fault I come across. The gas part (air or nitrogen) is separated from the fluid circuit by a membrane and should be filled to a certain pre-charge pressure – typically equal to the static height of the system in bars, plus a small reserve. If this pre-charge pressure drops or disappears completely, the expansion vessel loses the ability to absorb the fluid's expansion.
Symptoms: Pressure in the system rises sharply when heated (e.g. from 2 bar to 5–6 bar), the safety valve starts to discharge fluid, and after cooling the pressure drops below the starting value. A characteristic feature is this cyclic fluctuation – low pressure in the morning, high pressure in the afternoon.
Causes: The Schrader valve (of the same type as on a car tire) on the gas part of the expansion vessel may slowly leak air. The sealing material degrades due to temperature, oxidation or mechanical damage. In some cases, the cause is incorrect installation – if the expansion vessel was installed on the hot side of the circuit (which is an error), heat accelerates the degradation of the membrane and the valve seal.
Solution: First, it is necessary to determine the current pre-charge pressure: disconnect the expansion vessel from the circuit (close the ball valve if installed), vent the gas part by pressing the Schrader valve and measure the pressure with a pneumatic pressure gauge. If the pre-charge pressure is low but the Schrader valve holds, add nitrogen or dry air to the desired value. If the Schrader valve leaks, replace it (it is a standard garden Schrader valve, readily available). If the pre-charge pressure quickly drops again after refilling, there is a suspicion of a damaged membrane – see below. Correct pre-charge pressure setting is also covered in the article Setting the pre-charge pressure in a solar system expansion vessel in our Knowledge Centre.
2. Ruptured or deformed membrane
The membrane is the heart of the expansion vessel. It is made of elastic elastomer (EPDM for solar applications, or butyl) and must withstand repeated pressure cycles, aggressive media (anti-freeze, inhibitors) and high temperatures. In solar systems, the stress is significantly higher than in standard heating systems – temperatures can reach up to 150 °C during stagnation, which gradually degrades the membrane.
Symptoms: Fluid enters the gas part – when you open the Schrader valve, instead of air, fluid or a mixture of fluid and air comes out. The expansion vessel loses the ability to cushion, the pressure is unstable.
Causes: Aging elastomer, unsuitable membrane type (ordinary rubber instead of EPDM), temperatures exceeding the maximum allowed, aggressive medium (e.g. incorrectly diluted anti-freeze with a higher pH than specified by the manufacturer). Another cause may be cavitation caused by vacuum due to incorrect placement of the expansion vessel on the suction side of the pump – more on that later.
Solution: A ruptured membrane requires replacement. If the expansion vessel is older than 8–10 years, it is usually worth replacing the whole unit. Some manufacturers (e.g. Reflex, Flamco) offer replacement membranes, but their replacement is laborious and requires complete depressurization and disassembly of the expansion vessel. In the case of larger expansion vessels (over 50 liters), membrane replacement is economically justified; for smaller ones, replacing the entire unit is more logical.
3. Overdimensioning or underdimensioning of the expansion vessel
This is not a fault in the true sense – the expansion vessel works, but an incorrectly chosen volume causes repeated problems. An underdimensioned expansion vessel cannot absorb the full volume of fluid expansion, pressure rises above the limit and the safety valve fails. An overdimensioned expansion vessel is not as problematic, but brings unnecessary costs and may have a problem with excessive pre-charge pressure compared to the system pressure.
Symptoms of underdimensioning: Chronic opening of the safety valve when the system is heated, even if the expansion vessel is working correctly and the pre-charge pressure is in order.
Solution: Correct calculation of the expansion vessel volume – read the article What expansion vessel volume do I need for my solar circuit, where you will find a complete procedure including formulas. In short: for solar systems, the calculation is based on the fluid's expansion (about 7–8 % for a temperature difference of 100 °C), the volume of pipes and collectors, plus a safety margin.
4. Incorrect expansion vessel installation
The expansion vessel must be connected to the correct point in the circuit – on the pressure side after the pump, i.e., on the cold water side (return pipe), not on the pressure side of the pump towards the collector. If it is mounted on the pump's output side (pressure side), the pressure at the connection point increases during pump operation and the expansion vessel works incorrectly: the additional pressure from the pump overloads it. If, on the other hand, it is installed before the pump (on the suction side), the pump may create a vacuum that can damage the membrane or cause cavitation.
Symptoms: Repeated membrane degradation, unstable pressure at pump startup, noisy pump operation (cavitation). Sometimes the problem only becomes apparent after several seasons – the membrane ages faster than it should.
Solution: The expansion vessel should be installed on the return (cold) pipe, behind the collector and before the pump – i.e., on the suction side of the pump. This is the so-called neutral point of the circuit, where the pump has no influence on the pressure at the connection point. Correct installation is described in detail in the article Expansion vessel installation in a solar system – step-by-step guide.
Malfunctions of solar circulation pumps – causes and solutions
A solar circulation pump is different from a standard heating pump: it must handle higher temperatures (up to 130–140 °C in the solar circuit during stagnation), an aggressive medium (antifreeze based on glycols), and must be dimensioned for the correct flow rate for a specific collector and pipe length. Failures are usually the result of mechanical wear, electrical problems, or issues with the medium in the circuit.
1. Air in the circuit – the most common and annoying problem
Air in the circuit is the cause of a huge number of complaints and service calls. It manifests itself as noise (bubbling, gurgling), fluctuating flow, pump overheating (if air prevents fluid flow through the cooled bearing), and in extreme cases, even impeller damage. In solar systems, this problem is even more dangerous because during summer stagnation, the liquid can partially evaporate – and the condensate does not return evenly.
Symptoms: Loud noise from the pump (not mechanical humming, but rather bubbling or splashing), inconsistent flow measured by a flow meter (if installed), unstable pressure, inability of the system to achieve full performance despite the running pump.
Causes:
- Insufficient air venting during the initial filling of the system
- Leaks in the circuit (loose fittings, cracked seals) through which air is drawn in under vacuum
- Low pressure in the system – if pressure drops below the minimum (approx. 1 bar), air can separate from the liquid
- Incorrect placement of air vents (they must be at the highest points of the circuit)
- Stagnation in summer – at high temperatures, partial evaporation of the liquid and formation of steam bubbles can occur
Solution: Proper air venting requires a combination of an automatic air vent at the highest point of the circuit and manual venting at startup. For manual filling and venting of the solar circuit, use the manual pump for filling solar systems, which allows you to fill the circuit with controlled pressure, flush out air bubbles, and at the same time add the correct antifreeze mixture. The venting procedure varies depending on the configuration – whether the collectors are connected in series or in parallel, where the highest points are, etc. More on this can be found in the article Manual pump for filling solar systems – how to correctly fill and vent the circuit.
2. Pump is running, but no fluid is flowing (blocked impeller)
The pump motor starts and makes a typical sound, but the flow meter shows no flow or only minimal flow. The cause may be a blocked impeller – in wet-rotor pumps, the impeller is surrounded by fluid and if sediment, dirt, corrosion, or solidified antifreeze (degraded glycol due to aging) settles in the circuit, the impeller can become jammed.
Symptoms: The motor heats up, the flow is zero or minimal, the pump makes a monotonous buzzing sound instead of smooth operation, or it may shut off after a short time due to thermal protection.
Causes:
- Degraded antifreeze – after 3–5 years, pH drops and inhibitors begin to precipitate, accumulating on the impeller and in the bearings
- Rust and dirt from steel pipes (if present in the system)
- Clogged mesh filter before the pump
- Longer periods of system inactivity in summer – the liquid may have partially separated and solidified upon cooling
Solution: First, check and clean the mesh filter (if installed). If the filter is clean, shut down the system and bleed the pump (a small screw on the front face of the motor part) – many solar pumps have access to the impeller shaft, where it can be manually turned with a screwdriver. If this helps, restart the pump. If not, disassembly and cleaning are necessary. In the case of degraded fluid, it is necessary to replace the entire fluid – flush the circuit with clean water and refill it with antifreeze according to the article Filling a solar system with antifreeze – what you need to know.
3. Electrical pump faults – motor does not start
The electronics of modern solar pump stations (e.g. Resol, Steca, WATTS Vision and others) are quite sophisticated. The solar controller operates the pump based on the temperature difference between the collector and the storage tank. If the pump does not start, the problem may be electrical or control-related.
Symptoms: The pump is silent even in full sunlight, LED indicators on the controller are unresponsive or flashing with an error code, or the controller is active, but the pump does not physically start.
Causes and solutions:
- Burned fuse or tripped circuit breaker – check the electrical panel, restore power, identify the cause of the short circuit
- Faulty temperature sensor – the collector temperature sensor (Pt1000 or NTC) may be broken or shorted. The controller interprets the nonsensical values and does not start or stops the pump. Measuring the resistance of the sensor at room temperature (Pt1000 = 1000 Ω at 0 °C, NTC depends on the manufacturer) will reveal the fault.
- Burned motor winding – measure the resistance between the motor terminals. If the resistance is infinite (open circuit) or zero (short circuit), the motor must be replaced.
- Controller relay failure – the controller sends a signal, but the relay does not switch. This is evident when there is no voltage at the controller output terminals even though there should be. Solution: replace the controller.
4. Noisy pump operation – where does the noise come from
Not every pump noise is alarming, but each one is worth diagnosing. Different types of noise indicate different causes:
- Bubbling, splashing – air in the circuit. Solution: bleed air (see above).
- High-pitched tone (whistling) – cavitation: the pump is operating at too low an inlet pressure or at a temperature close to the boiling point of the liquid. Solution: increase the system pressure, check the position of the expansion vessel, check the pump inlet temperature.
- Mechanical knocking or vibrations – worn bearings or foreign object in the impeller. Solution: disassemble and inspect, or replace the pump.
- Buzzing at start-up – the pump is trying to start but cannot (blocked impeller or too low supply voltage). Solution: see the previous section on blocked impeller.
5. Pump and pump station leaks
Leaks most often occur at the pump inlet connection to the pipe (on threaded or flanged joints), at the bleed valves, and at the safety valve. Solar fluid (propylene glycol) is slightly more viscous than water, and thermal cycling causes sealing materials to shrink.
Symptoms: Wet spots or deposits of solar fluid (brownish, sticky) near fittings or under the pump. A pressure drop in the system without any obvious other cause.
Solution: Seal threaded connections (retighten, replace Teflon tape or hemp gasket with glycol-resistant sealing compound). Replace rubber gaskets with new ones – EPDM for glycol media. At the safety valve: if the valve leaks even at rest pressure, it likely has dirt under the seat or is damaged. The valve must be replaced (it cannot be reliably repaired).
Maintenance and prevention – how to extend the life of the expansion vessel and pump
Most of the faults we have described are predictable and preventable. A solar system is not maintenance-free – annual inspection can prevent problems that would otherwise require costly service calls or component replacement. A detailed procedure for annual inspection is described in the article How to check and maintain the expansion vessel in a solar system. Here is a brief overview:
- Every year: Check the system pressure when cold (should be 1.5–2 bar, or according to the project). Check the pre-charge pressure of the expansion vessel (ideally when the system is bled). Visually inspect the pump and pipe connections – look for signs of dripping.
- Every 2–3 years: Take a sample of the solar fluid and have it tested (pH should be 7–8.5, protection down to -28 °C or according to local conditions). Fluid with a bad pH quickly damages membranes, seals and metal.
- Every 5–8 years: Replace the entire solar fluid – glycols degrade even with proper operation. When replacing, also flush the circuit with clean water using a manual pump for filling solar systems and refill with a fresh mixture.
- As needed: Replace the expansion vessel membrane or the entire expansion vessel if it shows faults that cannot be repaired by recharging the pre-charge. Replace the pump seal if there are signs of leakage.
Doubtful component selection as a source of faults – what to avoid
Experience has shown that a significant portion of faults has its roots even before installation – it is about incorrect component selection. A few specific examples:
Expansion vessel for potable water standard (red membrane, butyl) in a solar system: Standard expansion vessels for potable water circuits are not designed for solar applications. The membrane degrades faster; EPDM membranes are mandatory for solar systems. This applies to purchases as well – always request confirmation from the manufacturer regarding use in solar circuits.
Standard heating pump instead of a solar pump: Standard heating pumps (e.g., Grundfos Alpha, Wilo Yonos, etc.) have a maximum medium temperature of 110–120 °C and are not certified for solar glycol mixtures. They operate outside their specifications in a solar circuit, and mechanical seals degrade quickly. Solar pumps (e.g., Grundfos Solar, Wilo-Star-ST, LAING D5-Solar) are built for conditions up to 140 °C and glycol resistance.
Pipe diameter too small and high flow velocity: If the pipe diameter is insufficient (e.g., Cu 12 instead of Cu 18 in a larger circuit), the flow velocity increases, hydraulic losses grow, and the pump operates at the edge of its capabilities – shortening its lifespan and increasing noise. The optimal flow velocity in a solar pipe is 0.4–0.8 m/s.
Diagnostic table – quick overview of symptoms and solutions
| Symptom | Most likely cause | First solution |
|---|---|---|
| Pressure rises when heated, safety valve drips | Low pre-charge pressure or burst membrane | Check and top up pre-charge pressure; if it helps, OK; otherwise, replace |
| Cold pressure too low (<1 bar) | Leak or air in the system | Top up fluid, check for leaks, bleed air |
| Pump is running, but the system is not heating up | Air in the circuit, blocked impeller, clogged filter | Bleed air, check the filter, unblock the impeller |
| Pump noise – gurgling | Air in the circuit | Bleed air via automatic or manual bleed valve |
| Pump whining or cavitation | Low inlet pressure, medium too hot at the inlet | Increase system pressure, check expansion vessel position |
| Pump does not start | Electrical fault, faulty sensor, fuse | Check power supply, sensors, controller |
| Leak at the pump or fittings | Leaky seal or thread | Tighten, or replace the seal if necessary |
| Fluid leaks from the expansion vessel Schrader valve | Burst membrane | Replace the membrane or the entire expansion vessel |
Practical scenarios from installation practice
Scenario 1 – Customer calls saying the safety valve drips every morning: A classic case. We arrived, measured the pre-charge pressure of the expansion vessel with the system depressurized: 0.3 bar instead of the required 1.5 bar. The Schrader valve was leaking minimally but consistently over time. We topped up the nitrogen to 1.5 bar and replaced the valve. The issue was resolved within an hour without replacing the expansion vessel.
Scenario 2 – New system, pump buzzes but the solar storage tank does not heat up: The installation was new, the collector had a temperature of 90 °C, but the storage tank remained cold. Cause: during filling, the system was not properly bled, and an air pocket remained at the top of the collector. We used a manual pump for solar system filling to flush the circuit under pressure, opened the bleed valve on the collector, and the air was released. After bleeding, the system worked properly.
Scenario 3 – 7-year-old system, pump runs but flow is minimal: The customer stated the system "seemed to have lost its power". A sample of the fluid showed pH 5.8 (too acidic), dark brown color and turbidity. The inhibitor had long been exhausted, and the glycol had deteriorated. Deposits blocked the pump impeller and also clogged the filter. A complete flush, fluid replacement, and pump cleaning were carried out – after the intervention, the system worked properly again.
Scenario 4 – Expansion vessel burst after the first winter: The installer used a standard expansion vessel for the potable water circuit (blue, butyl membrane, max. 70 °C). The collector reached a stagnation temperature of about 140 °C in summer, and the membrane physically melted. Replacement with a solar expansion vessel with an EPDM membrane and temperature resistance up to 130–150 °C was necessary. Lesson: always specify for solar applications.
Most frequently asked questions (FAQ)
How can I determine if the expansion vessel membrane is burst without disassembling it?
The simplest test: depressurize the system (close the valve before the expansion vessel if installed, or turn off the system and let it cool), then press the Schrader valve on the gas side of the expansion vessel. If instead of air, liquid or a mixture comes out, the membrane is damaged and liquid has entered the gas chamber. If only air comes out, the membrane is likely intact and it is sufficient to top up the pre-charge pressure.
Can I use regular air instead of nitrogen in the expansion vessel?
Technically yes, dry air can be added using a regular pump (e.g., a bicycle pump). It is important that the air is dry – moist air causes corrosion of the internal tank surface. In practice, a small amount of regular air is tolerated for one-time topping up. For larger refills (e.g., after a burst membrane), technical nitrogen is recommended, as it is inert and completely dry.
What is the lifespan of a solar circulation pump and when should it be replaced?
High-quality solar pumps (Grundfos, Wilo, Laing) have a lifespan of 15–20 years with proper operation and clean medium. Bearings and seals wear out faster with frequent stagnation (pump is off, fluid is hot) and degraded medium. Replace the pump if: it makes mechanical noise (knocking, rough vibrations), has a constant leak at the shaft, cannot be unblocked due to a jammed impeller, or the winding is burned out. Replacement is more cost-effective than repeated servicing.
Why does the safety valve release fluid only in summer and not in winter?
This is a typical sign of an undersized expansion vessel or low pre-charge pressure. In winter, the fluid temperature is lower, the volume is smaller – the system can handle the pressure. In summer, with full solar performance, the fluid temperature rises to 80–100 °C (or more in stagnation), the volume increases significantly, and the undersized expansion vessel cannot absorb it. Solution: check the pre-charge pressure, or replace the expansion vessel with a larger one. The calculation can be found in the article What expansion vessel size do I need for my solar circuit?.
Do I need to flush the pump in any special way when replacing the fluid?
Yes, we recommend flushing the circuit with clean water (deminished or at least soft) before filling it with a new mixture. The purpose is to remove remnants of degraded fluid, deposits, and impurities that could contaminate the new medium and reduce its lifespan. Use the manual pump for filling solar systems for this – press clean water into the circuit, drain it, and repeat at least twice. After that, fill the system with a fresh antifreeze mixture in the correct ratio.
Can the expansion vessel be installed vertically or must it be horizontal?
Most membrane-type expansion vessels can be installed in any position – manufacturers typically state this in the technical documentation. The preferred position is usually with the connection pointing downward (the expansion vessel hangs above the pipe) – this way, the membrane is less strained by the weight of the fluid and air remains more easily on the correct side. In any case, always follow the recommendations of the specific manufacturer, as some constructions (e.g., diaphragm vessels) are more sensitive to orientation.
Conclusion – Faults That Can Be Prevented
The expansion vessel and circulation pump are components that most solar system owners notice only when something stops working. However, most of the faults we have described here have one common characteristic: they are predictable and preventable. Choosing the right components during installation, placing them correctly in the circuit, and performing regular annual checks are enough to ensure that a solar system operates reliably for 15 or more years without major service interventions.
If you are unsure about selecting an expansion vessel for your specific system, we recommend reading the articles How to Choose an Expansion Vessel for a Solar System – Key Criteria and Expansion Vessel vs. Pressure Vessel – What Is the Difference and When to Use Each. For all questions regarding expansion vessels and pumps, you will also find comprehensive answers in the article Common Questions About Expansion Vessels and Pumps for Solar Systems. A solar system is not a black box – with a bit of knowledge and regular maintenance, it belongs to the most reliable heat sources available today.
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