Frequently asked questions about expansion vessels and pumps for solar systems

We are still preparing products.

But you can look at other categories.

Frequently asked questions about expansion vessels and pumps for solar systems – a comprehensive technical overview

Expansion vessels and circulation (or filling) pumps are among those components of a solar system about which a lot can be found on the internet – but mostly superficially. Installations I've seen over the years repeat the same mistakes: over- or under-dimensioned expansion vessel, incorrectly set pre-pressure, poorly oriented pump, filling the system without proper air venting. The result? The system shows pressure fluctuations, loses the antifreeze mixture, corrodes from the inside, or simply does not deliver the calculated performance.

This article answers the questions we receive most frequently – from customers, installers, and DIY enthusiasts. It is not a marketing text, but technical answers with numbers, examples, and procedures that work in practice.

1. What does an expansion vessel actually do in a solar system – and why is it irreplaceable?

A solar circuit is a closed pressure system filled with a heat transfer liquid – usually a mixture of water and propylene glycol. When solar radiation heats the collectors, the liquid expands: for every 10 °C temperature increase, its volume increases by approximately 0.3–0.5 % (depending on the mixture composition and glycol concentration). That sounds small, but in a circuit with 15–25 liters of liquid, it means hundreds of milliliters that must go somewhere.

If the system has no space for expansion, the pressure rises sharply. With sufficient overpressure, the safety valve opens and part of the liquid escapes – the liquid is expensive, and worse, it is replaced by air, which causes corrosion and reduces performance. Without an expansion vessel, this cycle repeats every sunny day and the system gradually deteriorates.

An expansion vessel is a pressure tank with a membrane or bladder. On one side of the membrane is air (or nitrogen) filled to a certain pre-pressure, on the other side it is connected to the circuit. When the liquid expands, it compresses the air on the gas side – and when it cools down, the air pushes the liquid back in by its spring-like effect. The system thus maintains pressure within a safe range without losing liquid.

Without a functional expansion vessel, a solar system operates without a safety margin. It's like driving a car without spare tires – most of the time it doesn't matter, but at the first serious incident, the damage is huge.

2. What volume of expansion vessel do I need? Step-by-step calculation

This is the most common question and also the one where the most mistakes are made. People buy the first expansion vessel they find – or follow the rough rule of thumb "12 liters is enough for a household". That may be true, but it doesn't have to be.

The correct procedure for calculating the required expansion vessel volume consists of several steps:

Volume of liquid in the circuit (V_sys): You add up the volume of the collectors (stated in the datasheet, usually 1.2–1.8 l per collector), the volume of the piping (for Cu DN 15 approx. 0.18 l/m, DN 18 approx. 0.25 l/m), and the volume of the storage tank heat exchanger (usually 3–6 l).
Expansion volume (V_dil): For a 50% mixture of water and propylene glycol and a temperature range of 20–120 °C, the expansion coefficient is approx. 10–11 %. V_dil = V_sys × 0.11.
Stagnation volume (V_stag): In stagnation (system without heat withdrawal), the liquid in the collectors can evaporate and the condensate flows back into the circuit. This vapor cushion must also be absorbed – so add the volume of the collectors once more.
Calculation formula: V_exp = (V_dil + V_stag + V_bezp) × ^{p_max + 1}/_{p_max − p₀}

Where p_max is the maximum working pressure of the system (usually 6–8 bar), p₀ is the pre-pressure of the expansion vessel, and V_bezp is a safety reserve (usually 1–3 l depending on the size of the system).

Practical example: A system with 4 flat collectors (total volume 6 l), 18 m of Cu DN 15 pipe (3.24 l), heat exchanger 4 l → total circuit volume ≈ 13.24 l. Expansion 11 % = 1.46 l. Stagnation volume of the collectors ≈ 6 l. Total expanding volume ≈ 7.5 l. With p_max = 6 bar and p₀ = 1.5 bar, the required expansion vessel volume is ≈ 11–13 l. The correct choice is therefore an 18-liter expansion vessel with a reserve, not an 8-liter "standard" one.

More on this topic can be found in the article What expansion vessel volume do I need for my solar circuit, where the procedure is explained in even more detail, including tables for typical system sizes.

3. How to set the pre-charge pressure of an expansion vessel – and why it is often done incorrectly?

Pre-charge pressure is the air or nitrogen pressure on the gas side of the membrane, set before connecting the expansion vessel to the system. This is key: pre-charge pressure is always set on a disconnected, empty expansion vessel, not when the system is under pressure.

Basic rule: the pre-charge pressure p₀ must be equal to the static height of the system above the expansion vessel, expressed in bars (1 bar ≈ 10 meters of water column), plus a small reserve of 0.2–0.3 bar. If the expansion vessel is located in the basement and the collectors are on the roof at a height of 8 m, the static pressure is 0.8 bar. The pre-charge pressure of the expansion vessel should be 1.0–1.1 bar.

The filling pressure of the system (the pressure at which we fill and at which the system operates cold) must be 0.2–0.3 bar higher than the pre-charge pressure of the expansion vessel. In our example, we fill to 1.2–1.3 bar.

The most common mistake in practice: The installer arrives with a new expansion vessel from the factory – most manufacturers supply it with a pre-charge pressure of 1.5 bar. If the static height is only 6 m (0.6 bar of required pre-charge pressure), the factory pre-charge pressure is too high. The membrane then "blocks" too large a portion of the tank volume and the system later loses pressure, because the expansion vessel has no real capacity to absorb the expansion. Always check and adjust the pre-charge pressure before installation.

A detailed procedure for setting the pressure can be found in the article Setting the pre-charge pressure in a solar system expansion vessel.

4. Expansion vessel for a solar system vs. expansion vessel for heating – what is the difference?

This is a question where many fatal mistakes are made – especially in do-it-yourself installations. The answer is simple, but the consequences are serious: a standard expansion vessel for heating (blue) is not suitable for solar systems.

The reasons are as follows:

Temperature: The solar loop can reach temperatures of 150–200 °C or even higher with vacuum collectors during stagnation. Membranes of standard heating expansion vessels are certified up to a maximum of 70–90 °C. At higher temperatures, the membrane degrades, cracks, and loses elasticity – and the expansion vessel stops performing its function.
Liquid: Propylene glycol is more aggressive to certain types of rubber membranes than pure water with inhibitors. Solar expansion vessels have a membrane made of special EPDM or butyl rubber blends resistant to glycol mixtures.
Pressure: Solar expansion vessels are mostly dimensioned for a maximum operating pressure of 6–10 bar, while heating vessels are only rated for 3–6 bar.

Solar expansion vessels are visually different – they are usually red or clearly marked "solar". Despite this, I recommend always checking the technical data sheet and verifying the certificate for solar applications. More about these differences is covered in the article Expansion vessel vs. pressure tank – what is the difference and when to use which.

5. Where to place the expansion vessel in the system – and what happens if it is connected incorrectly?

The expansion vessel has one correct place of connection: on the cold side of the circuit, i.e., on the suction port of the circulation pump (before the pump, when following the direction of fluid flow). This location is not random – it is the so-called "zero point" of the system, where the pressure is most stable and where the circulation pump "relies" on the expansion vessel.

Why is this so important? When the pump is running, it creates a pressure difference between the suction and discharge sides. The pressure slightly drops on the suction side and rises on the discharge side. If the expansion vessel is connected to the discharge side (a mistake I have seen many times), the pump "pumps" against the pressure of the expansion vessel – the system is unstable, cavitation may occur, and the expansion vessel does not work properly.

Other rules for placement:

The expansion vessel is mounted vertically, with the connection port pointing downward (the membrane is on top). Horizontal mounting shortens the membrane's lifespan.
There must be a shut-off device (ball valve) between the expansion vessel and the circuit, which allows disconnection without draining the entire system when replacing the membrane or the entire expansion vessel.
The safety valve must be placed after the expansion vessel (between the expansion vessel and the collectors), not before it.
The expansion vessel should not be exposed to direct sunlight or temperatures above 50 °C on the outer surface.

The step-by-step installation procedure can be found in the article Installation of an expansion vessel in a solar system – step by step.

6. Circulation pump for a solar system – what distinguishes it from a standard heating pump?

Solar circulation pumps must withstand conditions that standard heating pumps are not capable of enduring. The main differences:

Temperature: The heat transfer fluid in the solar circuit can reach up to 150–180 °C (and even more with vacuum collectors) during stagnation. Solar pumps are designed for fluids up to 140–160 °C at maximum pressure, while standard heating pumps are certified up to 95–110 °C.
Materials: The bodies of solar pumps are made of more durable materials – bronze, stainless steel, or special composites. The shaft and bearings resist higher temperatures and glycol mixtures.
Pressure: Solar pumps operate at a maximum operating pressure of 6–10 bar.
Integrated control: Most solar pump stations include a pump, flow meter, ball valves, safety valve, and sometimes a check valve in one compact unit. This significantly simplifies installation and reduces the number of potential leak points.

If someone recommends that a "classic Grundfos UPS 25-60 is sufficient for the solar circuit" – this is not true. It may work for a year, maybe two, but during the first long stagnation in a hot summer, the inside of the pump will be damaged, and you will be searching for the reason why the solar system suddenly stops working.

7. Manual filling pump – when and how to use it?

Filling the solar circuit is a task that is done once (at the first start-up) and then possibly after a major repair or fluid replacement (recommended every 4–5 years, depending on the condition of the inhibitors in the glycol). Proper filling is much more important than most people assume – an improperly filled and not de-aerated system operates noisily, has reduced performance, and accelerates corrosion.

An ideal tool for filling solar circuits is a special manual filling pump for solar systems. It is a simple but effective tool: a hand pump with a tank (usually 15–20 liters) and a hose connection to the system's filling and venting valve. It works on the principle of overpressure filling – it pushes the fluid into the circuit from the bottom (from the lowest point), while air escapes from the top through the venting valve or open automatic air vents.

Short filling procedure:

Fill the tank of the manual pump with prepared glycol mixture (usually 40–50 % propylene glycol for our conditions, which corresponds to protection down to -25 to -28 °C).
Connect the filling hose to the system's filling valve (usually on the pump station).
Open the venting valve at the highest point of the system.
Pump the fluid into the system until clean fluid without air bubbles flows out of the venting valve.
Set the operating pressure to the prescribed value (usually 1.2–2.5 bar cold, depending on the height and pre-charge pressure of the expansion vessel).
Start the pump and let the system run for 15–20 minutes until the remaining air escapes through the automatic air vents.

A detailed procedure with tips for solving problems can be found in the article Manual filling pump for solar systems – how to properly fill and de-aerate the circuit. More about the glycol mixture itself and its preparation is covered in the article Filling a solar system with antifreeze – what you need to know.

8. How to recognize that the expansion vessel has stopped working?

A failure of the expansion vessel does not announce itself with a loud bang or an alarm. The system simply begins to show symptoms that an inexperienced eye is likely to attribute to other causes. Here are typical symptoms in practice:

The safety valve regularly vents: If the safety valve (set to 6 or 8 bar) regularly reacts – typically during the day in sunny weather – the expansion vessel either has insufficient capacity or the membrane has burst and the vessel is filled with fluid instead of air.
The pressure in the system fluctuates significantly: In the morning when cold, the pressure is fine – say 1.5 bar. After heating the system, it rises to 5–6 bar or higher. A normal increase is only 1.5–2.5 bar. A larger difference indicates a problem with the expansion vessel.
The pressure drops continuously when cold: If you have to refill the system every two to three weeks, the fluid is escaping somewhere – either through the safety valve (a consequence of a burst membrane) or through a leak. Check the expansion vessel and all connections.
Liquid flows out of the Schrader valve of the expansion vessel: Unscrew the plastic cap and press on the valve. If liquid (not air) flows out, the membrane is burst – the liquid has penetrated to the gas side.

The diagnostic procedure and solutions are discussed in detail in the article Common expansion vessel and solar pump failures – causes and solutions, as well as in the article How to check and maintain an expansion vessel in a solar system.

9. How often and how to check the expansion vessel?

An expansion vessel is not a "set and forget" component. The recommended inspection interval is at least once a year, ideally in spring before the season. The inspection takes 10 minutes and costs nothing – it allows you to detect a problem before it causes damage.

What to do during the inspection:

Disconnect the system from the expansion vessel using a shut-off valve (hence always install a shut-off valve!)
Unscrew the cap on the Schrader valve. Use a pressure gauge (a tire pressure gauge works well). Measure the current pre-charge pressure.
If the pre-charge pressure is lower than the set value, air has escaped. Add nitrogen or dry compressed air to the correct value.
If liquid flows out when pressing the valve: the membrane is damaged. The expansion vessel needs to be replaced (or only the membrane, if the design allows it).
After the inspection, open the shut-off valve again, check the operating pressure of the system, and add fluid if necessary.

10. Can I replace the expansion vessel for a solar system myself, or do I need a professional?

It depends on the specific situation. Replacing an expansion vessel is a relatively simple task that a skilled DIYer can manage – provided that:

The system has a shut-off valve before the expansion vessel, allowing it to be disconnected without draining the entire circuit.
The new expansion vessel has the same or compatible thread (most commonly 3/4" or 1" external thread).
You correctly set the pre-charge pressure of the new expansion vessel before connecting it.

If there is no shut-off valve, you will have to drain the entire circuit – which means refilling and bleeding the system again. In such a case, it is advisable to call in someone more experienced, or at least get a manual pump for filling solar systems and carefully study the procedure.

In any case, it is always better to call a professional if you are unsure. Damage caused by incorrect filling or wrong pressure settings can easily exceed the cost of a service call.

11. What are typical pump failures in a solar circuit?

A solar pump operates seasonally, mostly from April to September under maximum load. Despite this, failures do occur and tend to follow similar patterns:

The pump is running, but no fluid is flowing: The most common cause is an airlock. Solution: bleed the system, or temporarily increase the pump speed to maximum and let it run for 10–15 minutes.
The pump is making noise (knocking, rumbling): Air in the system or cavitation (the suction side pressure is too low – possible causes include an under-pressurized expansion vessel or a clogged filter). Check the filling pressure and the pre-charge pressure of the expansion vessel.
The pump does not start: Electronic failure, an overheated motor (common after long periods of stagnation), or rotor blockage due to deposits. Manually turning the shaft with a screwdriver through the bleed valve sometimes helps.
High energy consumption with lower performance: Clogged filters, cavitation, or incorrectly set pump speed. Check the flow meter – the flow should be 0.5–1.5 l/min per collector, depending on the system.

Frequently Asked Questions (FAQ)

Can I use a standard red heating expansion vessel for a solar system if it has the same volume?

No. Heating expansion vessels are not designed for temperatures above 90–110 °C or for the aggressiveness of glycol mixtures. During solar system stagnation, temperatures can reach 150–200 °C – the membrane of a standard heating expansion vessel cannot withstand these conditions and will crack after one or two seasons. Always use expansion vessels certified for solar applications (usually red with the label "solar" and a temperature range up to 130 °C or 160 °C).

What pre-charge pressure should I set if I don't know the exact height of the system?

Measure the height from the point of expansion vessel installation to the highest point of the system (usually the top edge of the collector). Every 10 meters of height corresponds to 1 bar of static pressure. Add 0.2–0.3 bar as a reserve. For example, if the height is 7 meters, set the pre-charge pressure to 0.7 + 0.3 = 1.0 bar. The filling pressure of the system will then be 1.2–1.3 bar. This procedure applies to most standard single-family homes.

Why does the safety valve regularly drip during a sunny day?

Regular dripping (or a more significant discharge) from the safety valve during a sunny day is a clear sign that the system cannot absorb the expansion. The causes are threefold: (1) the expansion vessel is too small, (2) the membrane is damaged and the entire vessel is flooded with fluid without pressure reserve, (3) the pre-charge pressure was set too high, resulting in a small usable volume. Check the condition of the membrane and the pre-charge pressure. If the membrane is in good condition and the pre-charge pressure is correct, consider a larger expansion vessel.

How long does the membrane in an expansion vessel last?

Under normal conditions, 8–15 years. The lifespan is reduced by: high temperatures (stagnation under full sun without heat extraction), incorrect composition of the heat transfer fluid (too acidic or alkaline mixtures damage the rubber), air in the system (air in contact with the membrane oxidizes the material), and an unsuitable membrane type (not solar-certified). I recommend replacing the membrane every 8–10 years regardless of its condition as a preventive measure.

Do I need to use a special pump when filling the system, or is a garden hose from the tap enough?

Technically, it is possible to fill the system using water pressure from the tap (about 3–4 bar), but it is very cumbersome and impractical – it does not allow for easy dosing of the glycol mixture at the correct concentration. In addition, tap water fills the system from the top, which worsens the bleeding process. A proper manual pump for filling solar systems fills from the bottom, allows direct use of the prepared mixture, and gives the installer an accurate overview of the amount of fluid drawn into the system. It is a small investment with a significant benefit for the quality of filling.

Is it necessary to replace the charge in the expansion vessel when changing the fluid?

Air or nitrogen in the expansion vessel is not replaced during normal operation – the membrane is sealed and the gas remains in the vessel. When changing the fluid in the circuit, it is sufficient to check the pre-charge pressure and possibly adjust it. If the expansion vessel has been in operation for a long time and the pressure has dropped significantly (air has leaked through a micro-leak in the valve), add fresh dry air or nitrogen to the correct value. Technical nitrogen (N₂) is preferable to air – it does not contain moisture or oxygen, which extends the life of the membrane.

Conclusion: expansion vessel and pump – the heart and lungs of the solar circuit

A solar collector system can be very reliable and long-lasting – if its "quiet helpers" are properly sized, set up, and maintained. The expansion vessel protects the system from pressure surges, the pump ensures circulation, and the manual pump ensures proper filling and bleeding during the first start-up and during service interventions.

Problems that customers bring to us are 80% the result of three mistakes: incorrectly sized expansion vessel, incorrectly set pre-charge pressure, and underestimating the filling and bleeding process. If you give these three points your due attention, your solar system will reliably serve you for 20 or more years without major problems.

If you want to go deeper into individual topics, we recommend a series of articles in this Knowledge Center: How to choose an expansion vessel for a solar system – key criteria, What expansion vessel volume do I need for my solar circuit, Setting the pre-charge pressure in a solar system expansion vessel, and How to check and maintain an expansion vessel in a solar system. Each of them covers its topic in such depth as is simply not possible with short answers.

Do you have a question about this topic?

Having trouble making a decision or dealing with a specific situation in your home? Write to us – we are happy to help.