Expansion vessel vs. pressure tank – what is the difference and when to use which

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Expansion vessel vs. pressure tank – what is the difference and when to use which

When a customer looks at a technical drawing of a solar system or boiler room for the first time, they almost always come across a round tank connected somewhere on the pipe. And immediately they ask: "Is it an expansion vessel or a pressure tank? Isn't it the same?" The answer is no – and this difference has a significant practical impact on the functionality, safety, and lifespan of the entire system. In this article, we will go through both types of tanks from the basics, explain what happens inside, show concrete examples from practice, and help you decide which to use when.

Basic principle: expansion of liquid in a closed circuit

Every liquid – water, antifreeze based on propylene glycol, brine – expands when heated. Water heated from 10 °C to 90 °C increases its volume by approximately 3.6%. In an open system, the excess liquid would simply flow out through the overflow. In a closed circuit – and nowadays, the vast majority of solar and heating systems are closed – this volume has to go somewhere. If it has nowhere to go, the pressure would rise to several times higher than the safety limits of the components, and either the safety valve would open or, in the worst case, damage the pump, collectors, or connections.

Expansion vessels are precisely for solving this situation. But – and here comes the first crucial point – not every expansion vessel is the same. The term "pressure tank" is broader and can include a whole range of devices. An expansion vessel is a specific type of pressure tank with a membrane or bladder, designed precisely for absorbing thermal expansion.

What is an expansion vessel – construction and principle of operation

An expansion vessel (sometimes also called a membrane expansion vessel or bladder expansion vessel) is a hermetically sealed tank divided into two chambers – by a membrane or bladder. One chamber is filled with nitrogen (or air) at a defined pre-pressure, the other is connected to the hydraulic circuit of the system. When the liquid in the circuit expands, it enters the water chamber, compresses the membrane, and compresses the gas chamber. When the circuit cools down, the gas pre-pressure pushes the liquid back into the circuit.

Key structural elements of an expansion vessel:

Steel tank – usually made of carbon or stainless steel, for solar systems with higher temperature resistance (up to 120 °C on the water side)
Membrane or bladder – made of EPDM rubber resistant to propylene glycol and high temperatures; for solar systems, the choice of membrane is crucial, standard SBR rubber is not sufficient for solar circuits
Nitrogen filling valve – type Schrader (like on a bicycle), through which the pre-pressure is set
Connection thread – G ¾" or G 1" for most applications

Membrane vs. bladder expansion vessel

A membrane expansion vessel has a membrane fixed to the walls of the tank – it divides it into two chambers, but the membrane moves only by bending. This type is common in smaller sizes (up to about 35 liters) and is cheaper. A bladder (bag-type) expansion vessel has a free bladder inserted inside the tank – the bladder can expand and contract much more, allowing a higher usable volume and longer lifespan with repeated cycles. For solar systems with intense daily cycling (cold in the morning, hot at noon, cold again in the evening), bladder models are more advantageous.

What is a "pressure vessel" in a broader sense

The term "pressure vessel" (pressure tank) is an overarching category. It includes any enclosed container designed to operate at a pressure different from atmospheric. These include:

Expansion vessels (membrane, bladder) – absorbing fluid expansion
Hydrofor (hydropneumatic tanks) – maintaining pressure in water supply systems, supplying water from wells
Buffer tanks – hot water storage without a membrane, thermal buffer
Water storage heaters (boilers)
Industrial pressure vessels for air, steam, or chemicals

In everyday technical practice with solar and heating systems, three types are commonly confused: expansion vessel, hydrofor, and buffer tank. Let's look at them in detail.

Hydrofor – pressure vessel for water supply applications

A hydrofor looks very similar to an expansion vessel – a round or vertical steel tank with a membrane, one side with air/nitrogen, the other with water. The difference lies in purpose and parameters: a hydrofor is designed for water supply under pressure, typically from a home well or borehole. Operating pressure is usually 1 – 8 bar, water temperature is cold (max. 30 – 40 °C), and the hydrofor must withstand much greater volume changes, as the tank is often completely emptied and refilled multiple times during the day.

The membrane in a hydrofor is adapted for contact with potable water (WRAS or DIN certification), which is irrelevant for a solar circuit. On the contrary, the membrane of a solar expansion vessel must resist propylene glycol and temperatures of 120 °C and higher – which hydrofors typically do not meet.

Conclusion for practice: You CANNOT use a hydrofor as a replacement for a solar expansion vessel and vice versa. Interchange leads to either premature membrane degradation or safety issues.

Buffer tank – tank without a membrane

A buffer tank (thermal storage tank) is simply an insulated fluid tank without a membrane. It is used for thermal energy storage – for example, excess from solar collectors is stored in the buffer and later used for heating or hot water preparation. A buffer tank HAS NO function of compensating for expansion. It is a passive tank. It does not serve as an expansion vessel and cannot replace it.

Comparison of expansion vessels and other types of pressure vessels – table

Property	Solar expansion vessel	Heating expansion vessel	Hydrofor	Buffer tank
Function	Expansion of solar medium	Expansion of heating water	Water supply under pressure	Thermal storage
Membrane / bladder	Yes (EPDM, glycol-resistant)	Yes (EPDM/SBR)	Yes (potable water)	No
Max. medium temperature	120 – 150 °C	70 – 90 °C	30 – 40 °C	90 – 95 °C
Medium	Propylene glycol/water	Water (possibly glycol)	Potable water	Water
Adjustable pre-charge pressure	Yes	Yes	Yes	No
Medium certification	Not potable water	Not potable water	WRAS/DIN potable water	No requirements

When to use what – specific practical scenarios

Scenario 1: Flat solar collectors for hot water heating (2 – 4 collectors)

This is probably the most common case we encounter. A family house, 2 – 4 flat collectors with a total absorber area of 4 – 8 m², a dual tank of 200 – 300 liters, a circulation pump, and a solar station. The volume of the solar medium in the primary circuit is usually 8 – 18 liters (collectors + piping).

Here, a solar expansion vessel with an EPDM membrane is clearly required, sized according to the circuit volume and maximum operating temperature. Pre-charge pressure is set according to the height of the collector installation above the expansion vessel – a rule of thumb: height in meters × 0.1 bar + 0.3 bar. For example, if the collectors are 6 meters above the expansion vessel: pre-charge pressure = 6 × 0.1 + 0.3 = 0.9 bar. The expansion vessel volume for this case will typically be 18 – 25 liters.

Using a hydrofor or a heating expansion vessel with an SBR membrane would be a mistake here – propylene glycol would degrade the membranes within a few years.

Scenario 2: Vacuum tube collectors with risk of stagnation

Vacuum tube collectors can reach temperatures of 180 – 250 °C during stagnation (pump stops, no heat removal). A standard expansion vessel rated for 120 °C is insufficient on the water side – the medium begins to evaporate and pressure rises dramatically. Solutions include:

Solar expansion vessel with increased temperature resistance (150 °C) and larger volume – part of the medium from the collectors must be transferred to the expansion vessel as vapor and there condensed
System with separation of collector and storage circuits via a heat exchanger, where the collector circuit has its own small expansion vessel and safety valve

In this category, it is also important to properly fill and degas the circuit. For this purpose, a Manual pump for filling solar systems is used, which allows pressure testing before start-up and filling the primary circuit with antifreeze mixture. Without proper filling, air in the system will manifest as bubble cavitation and unstable pressure.

Scenario 3: Central heating system with a boiler

A classic condensing boiler, underfloor heating or radiators, a closed water circuit. Here, a heating expansion vessel (red or blue color depending on the manufacturer) with an SBR or EPDM membrane resistant to water up to 90 °C is used. Pre-charge pressure is typically set to 0.5 – 1.5 bar depending on the system height, operating pressure of the system is 1.5 – 2.5 bar. Size is calculated based on the water volume in the system and the temperature difference.

This expansion vessel CANNOT be used for a solar circuit – the medium, temperatures, and cyclic load are different.

Scenario 4: Domestic water well (well, borehole)

A hydrofor is suitable here. The membrane is certified for contact with drinking water and is resistant to common microorganisms in water. The volume of the hydrofor is selected based on the pump performance and the required number of switching cycles per hour – the larger the hydrofor, the fewer switching cycles, and the longer the pump lifespan. Typical domestic hydrofors have 24 – 100 liters, with a pre-charge pressure of 1.5 – 3 bar.

The hydrofor CANNOT replace a solar expansion vessel, because the medium would damage the membrane and the temperatures during stagnation would destroy the entire vessel.

Scenario 5: Combined solar-boiler system with accumulator

In practice, we increasingly encounter combinations: solar collectors + heat pump or condensing boiler + large accumulator tank of 500 – 1000 liters. In this case, you need two separate expansion vessels:

One solar expansion vessel for the primary collector circuit
One heating expansion vessel for the secondary circuit (heating + DHW preparation)

The accumulator tank itself is not an expansion vessel – it only stores heat. The volume of the accumulator is not included in the expansion volume for calculating the expansion vessel, because the internal fluid is the secondary circuit, not the primary solar circuit.

How to dimension an expansion vessel for a solar system – simplified procedure

This is a topic covered in a separate article in our Knowledge Center – What expansion vessel volume do I need for my solar circuit? Here we will go through only the basic logic, to make it clear why it is not enough to just buy "some vessel".

The expansion volume (V_e) consists of three components:

Expansion volume of the liquid – depends on the total volume of the medium in the circuit and the temperature difference (cold vs. maximum operating temperature). Propylene glycol in a 50% mixture expands by about 8.5% from 20 °C to 120 °C.
Security reserve volume – "pocket" volume for unexpected pressure surges
Water cushion volume – minimum volume of water that must remain in the expansion vessel even at maximum pressure, to prevent membrane damage

The resulting technical (gross) volume of the expansion vessel is calculated using a formula that also includes the difference between the pre-charge pressure and the maximum working pressure. In practice, for a family house with 2 – 3 collectors, it usually comes to 18 – 35 liters. For larger installations (6 or more collectors, pool systems), a vessel of 50 – 80 liters or multiple vessels connected in parallel may be required.

The calculation of the pre-charge pressure and the correct installation are described in detail in the topics Setting the pre-charge pressure in a solar system expansion vessel and Installation of an expansion vessel in a solar system – step-by-step procedure.

Where to place the expansion vessel in the system – and where not to place it

The expansion vessel in a solar system must be mounted on the cold (return) branch of the circuit, i.e., before the pump inlet, not after it and not on the hot branch leading from the collectors. The reasons are multiple:

Thermal load: The medium is cooler on the return branch, and the membrane is less stressed by temperature.
Hydraulics: The expansion vessel must be connected to a point with constant pressure (so-called "zero pressure point"), which prevents pressure oscillations during pump operation.
Safety: The safety valve must also be in a suitable location and must not be separated from the expansion vessel by a shut-off valve (except for a valve with automatic shut-off).

A common mistake in practice: the installer mounts the expansion vessel at the most accessible location without considering hydraulics. The result – the pump "sucks" from the expansion vessel, the pressure on the suction side drops below the pre-charge pressure, air bubbles and cavitation appear. The system becomes noisy and performance drops.

Another common mistake: the expansion vessel is installed at an angle or even upside down. Most manufacturers allow installation only vertically (neck down) or horizontally. Inversion (neck up) causes condensate and impurities to accumulate above the membrane, shortening its lifespan.

How to correctly fill the solar circuit – the role of the pump during the first start-up

Filling the solar circuit is not just about "pumping water". It is a precise procedure in which the circuit must be filled with antifreeze under pressure, while removing all air from the entire system. Air in the circuit is an enemy of the expansion vessel and the pump – it causes corrosion, noise problems and inaccurate pressure readings.

For this purpose, the ideal tool is the Manual pump for filling solar systems. This pump allows several important steps at once: pressurizing the circuit to a test pressure (pressure tightness test), slow filling from bottom to top (air escapes through vent valves at the highest points), and checking whether the pressure stabilizes after filling or, on the contrary, drops (which would indicate a leak). More about this procedure can be found in the topic Manual pump for filling solar systems – how to correctly fill and deaerate the circuit.

After filling and deaerating, the operating pressure is set – typically 0.3 – 0.5 bar above the pre-charge pressure of the expansion vessel. This pressure is checked in a cold system (without collector operation) and should remain stable. If the pressure drops after a few days, it indicates either a leak or an incorrectly set pre-charge pressure of the expansion vessel.

Maintenance and common problems

An expansion vessel is not a maintenance-free device. The membrane becomes fatigued over time and loses elasticity, and the pre-charge nitrogen pressure may slightly drop (air/nitrogen diffuses through the membrane). The recommended frequency of checking the pre-charge pressure is once a year, ideally in spring before the season. The procedure and common faults are described in the topics How to check and maintain the expansion vessel in a solar system and Common faults of expansion vessels and solar pumps – causes and solutions.

One specific problem from practice: a customer calls to say that the safety valve regularly discharges the medium. A technician arrives, measures the pressure – in the cold system, the pressure is 2.8 bar, the safety valve is set to 3 bar. Problem? The expansion vessel has a pre-charge pressure of 0.5 bar, but the system operating pressure is too low, so the pressure quickly reaches the limit when heated. Solution: set the expansion vessel pre-charge pressure to 1.2 bar and the system operating pressure to 1.5 bar. Discharge stops and the system works correctly. This is a typical situation that arises when the expansion vessel pre-charge pressure is set "by eye" without calculation.

Most frequently asked questions (FAQ)

Can I use a standard heating expansion vessel (red) for a solar system?

No, and for two reasons. First, the membrane of a standard heating expansion vessel is not certified for contact with propylene glycol-water mixture – glycol gradually degrades the membrane and it cracks. Second, heating expansion vessels are dimensioned for a maximum medium temperature of 70 – 90 °C, while solar circuits can reach 120 – 150 °C. A correct solar expansion vessel has a membrane made of EPDM or butyl rubber resistant to glycol and high temperatures.

Can I connect two smaller expansion vessels instead of one larger one?

Yes, two expansion vessels connected in parallel (each on its own T-piece valve) function the same as one larger one – their volumes add up. This is a common practice in larger installations, where a large tank would be problematic due to space constraints. It is important that both have the same pre-charge pressure and both are connected to the cold branch of the circuit.

How do I know that the membrane in the expansion vessel is damaged?

Classic symptoms: After filling the circuit, liquid leaks from the nitrogen charging valve on the expansion vessel (the membrane is pierced and the liquid has entered the gas chamber). Another symptom – when you manually press the charging valve, liquid comes out instead of air. A damaged membrane cannot be repaired; the vessel must be either completely replaced or – in the case of a bladder type with a removable cover – only the bladder. Always check that the replacement bladder corresponds to the original material (EPDM for glycol).

Can I use a hydrofor from a water installation as an expansion vessel for heating?

Technically, it would work for a short time, but it is incorrect and potentially dangerous. A hydrofor for drinking water is not dimensioned for a temperature of 90 °C or for the chemical composition of heating water. The membrane of drinking water can degrade when in contact with corrosion inhibitors. In addition, a hydrofor usually does not have the correct pressure range for heating. Always use the correct type of vessel for the specific application.

What is the difference between an expansion vessel with a pre-charge pressure of 1.5 bar and 3 bar when purchasing – should I choose based on the pre-charge pressure?

The pre-charge pressure with which you receive the expansion vessel from the factory is only the initial setting – before installation, you must always adjust it according to your specific installation (collector height, system operating pressure). A more important parameter when choosing is the maximum allowable operating pressure (PS) of the vessel – it must not be lower than the setting of the safety valve in your system. Common solar expansion vessels have PS 6 – 10 bar. The manufacturer always specifies the adjustable range of the pre-charge pressure – typically 1 – 6 bar.

Do I have to drain the entire solar circuit when replacing the expansion vessel?

It depends on the connection design. If there is a shut-off valve before the expansion vessel (which is good practice during installation), it is sufficient to close the valve, equalize the pressure via the vent and replace the vessel without draining the entire circuit. If the shut-off valve is missing, the circuit must be drained – ideally, the medium should be collected in a container and refilled after replacement (propylene glycol-based liquid is not cheap and is not environmentally neutral). Refilling can again be assisted by the manual pump for filling solar systems, which allows controlled pressurization without air bubbles.

Conclusion – choosing the correct device

An expansion vessel and a pressure vessel are not synonyms – each type has its specific design, parameter range and area of use. In solar technology, it is crucial to distinguish between a solar expansion vessel (EPDM membrane, resistance to glycol and high temperatures) and a heating expansion vessel, a hydrofor or an accumulator tank. Confusing them leads, at best, to premature failure, and at worst, to safety risks and damage to the entire installation.

The correct selection involves several steps: calculating the required volume of the expansion vessel, setting the pre-charge pressure according to the height of the installation and the system operating pressure, placing it correctly on the cold branch of the circuit, and regular annual checks of the membrane and pre-charge pressure. These topics are detailed in further articles in our Knowledge Center – specifically in the topics How to choose an expansion vessel for a solar system – key criteria, Filling a solar system with antifreeze – what you need to know and Common questions about expansion vessels and pumps for solar systems.

If you are unsure about the calculation or selection, it is always better to choose a larger expansion vessel than a smaller one – in practice, an overdimensioning of 20 – 30 % is common and safe, while underdimensioning leads to constant discharge of the safety valve and degradation of the entire system.

Do you have a question about this topic?

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