Pressure tank in a home water system: what it is used for and how to set it up?

Pressure tank in a home water system: the heart of the entire system

When most people decide on a home water system, they focus mainly on the pump – its power, depth reach, flow rate. The pressure tank stands aside, at best seen as "that big metal barrel" that simply belongs to the water system. Yet it is precisely the pressure tank that determines whether the entire system will operate quietly, reliably, and for a long time – or whether the pump will wake you up every 30 seconds and burn out within a few seasons.

In this article, we will take a thorough look at the pressure tank: what it actually is, how it works physically, what volume to choose, how to correctly set the pre-charge pressure of the air chamber, and what mistakes are most commonly repeated in practice. Whether you are building a new system with a well, borehole, or surface water source, after reading this, you will know what to do with the pressure tank and why.

What is a pressure tank and why does a water system need it at all

A pressure tank (also known as a pressure expansion tank, hydro-pneumatic tank, or pressure accumulator) is a closed steel or stainless steel tank divided by a flexible membrane into two chambers. One chamber is filled with compressed air (or nitrogen in special applications), the other is connected to the water network. The air in the tank acts as a spring – it is compressed during water filling and expands during water withdrawal.

Why does the system need it? A pump – whether submersible or surface-mounted – is not a device that can operate without damage in continuous switching on and off. Every start of an electric motor means a current surge several times higher than normal operation, and at the same time a mechanical shock on bearings and seals. If you did not have a pressure tank, the pump would turn on with every opening of a tap – even with minimal water withdrawal, for example, when filling a glass of water. In a household with 3–4 people, this can mean dozens to hundreds of starts per day. Result: the pump wears out prematurely, the contacts of the pressure switch burn out, and the lifespan of the entire system decreases.

The pressure tank solves this problem simply: the air chamber holds a reservoir of compressed water in the system. For small withdrawals (sink, toilet, glass of water), it supplies water without turning on the pump. The pump starts only when the pressure drops below the set on-pressure (usually 1.5–2.0 bar), refills the supply, and stops when the off-pressure is reached (usually 3.0–3.5 bar).

Membrane or air chamber – types of pressure tanks

On the market today, we almost exclusively encounter membrane pressure tanks. Older "air chamber" tanks without a membrane (where air and water were in direct contact) are no longer used in common household water systems – air gradually dissolved in water, the tank had to be regularly topped up with air, and the entire system was more prone to wear.

Membrane tanks are further divided into two categories:

• With a fixed (clamped) membrane: the membrane is fastened along the edge to the tank wall and divides it into two fixed chambers. The advantage is a simpler construction and longer lifespan. The disadvantage is a limited ability to expand under extreme pressures.
• With a bladder (balloon) membrane: the air chamber contains a rubber bladder into which the air is sealed. The water is located between the bladder and the tank wall. These tanks usually have a higher usable water volume for the same total capacity.

For home water systems with a well or borehole, membrane tanks with a volume of 24, 50, 80, 100, or 150 liters are the most common. Tanks with a volume of 50 liters are the most popular in practice – suitable for typical family homes with 3–5 withdrawal points.

How the pressure tank works together with the pressure switch

The pressure tank always works in tandem with a pressure switch (pressostat). The pressure switch is an electromechanical device connected to the water system, which turns the pump on and off according to the current pressure. Two values are set on it:

• P1 – on-pressure (Einschaltdruck): the pressure at which the pump starts. Typically 1.5 – 2.0 bar.
• P2 – off-pressure (Ausschaltdruck): the pressure at which the pump stops. Typically 3.0 – 3.5 bar. The difference P2 – P1 is called the differential pressure (usually 1.5 bar).

The pre-charge pressure of air in the pressure tank (measured without water, when the system is depressurized) must always be set to a value slightly lower than the on-pressure P1 – typically 0.2 bar lower. For example: if P1 = 1.8 bar, set the air pre-charge to 1.6 bar.

Why exactly this value? If the air chamber's pre-charge pressure were the same or higher than P1, the membrane would be compressed against the outlet flange (water would not enter the tank) and the tank would effectively not perform its function. Conversely, too low a pre-charge pressure causes the membrane to stretch excessively and almost burst.

What capacity of pressure tank do I need?

This is the question customers struggle with most often. The principle is simple: the larger the tank, the longer the time between pump activations and the lower the wear. However, an unnecessarily large tank is expensive and takes up space.

In practice, the following general rules apply:

• Small cabin, garden irrigation, 1–2 water outlets: 24 liters is sufficient, but 50 liters is more comfortable.
• Family house, 3–5 water outlets, regular use: 50 liters is standard and recommended.
• Larger family house, 5+ water outlets, frequent simultaneous use: 80–100 liters.
• Farm, horticulture, irrigation system with higher consumption: 150+ liters, or two tanks in parallel.

It is also important to calculate the usable water volume in the tank (the amount that is actually drawn off between P1 and P2). This volume is always significantly smaller than the total tank volume. With a typical setting (pre-pressure 1.5 bar, P1 = 1.8 bar, P2 = 3.5 bar), the usable volume in a 50-liter tank is only about 15–18 liters. So if a water pump manufacturer boasts of a "50-liter tank," they do not mean 50 liters of water storage – it is the total tank volume including the air chamber.

There is also a mathematical formula for calculating the usable volume: V_util = V_total × (P2 – P1) / (P2 + 1) × (P1 + 1) / (P_air + 1). For common applications, however, the approximate tables provided by the manufacturer are more reliable and practical.

Practical example: a family house in the village of Záborské

A customer who had installed a water pump system with a 24-liter tank at an old well near the house complained that the pump was switching on every 20–30 seconds even during regular showering. After measurement, we found that the pre-pressure and pressure switch were correctly set – the problem was simply that the tank was too small. Replacing it with a home water pump 3 SQIBO 0.55 / 50l with a 50-liter tank completely solved the issue – the pump now switched on every 3–4 minutes during normal use, which is an excellent result.

Step-by-step procedure for setting the pre-pressure

Setting the pre-pressure is a simple task that any DIY enthusiast can perform with standard tools. You will need: a pressure gauge for measuring air pressure (accurate, with a range of 0–6 bar and readings in 0.1 bar increments), a pump (manual or compressor with a regulator), and possibly a needle for releasing air.

Detailed procedure for setting the pre-pressure:

1. Turn off the pump – disconnect it from the power supply or switch off the circuit breaker. If possible, close the ball valve on the discharge pipe between the pump and the tank.
2. Depressurize the system – open any faucet in the house (e.g., the lowest one) and let the water drain until the pressure in the system drops to zero. Check the pressure gauge on the valve assembly. If you cannot depressurize the system this way, at least depressurize the pressure tank – release pressure at the hose valve point on the tank.
3. Find the air valve – it is usually a Schrader valve (the same as on car tires) located on the top or side of the tank under a plastic protective cap.
4. Measure the current pre-pressure – use the connected pressure gauge to determine the current air pressure in the tank. If the tank is new from the factory, the pre-pressure is usually set to 1.5 bar (check the documentation).
5. Set the desired pre-pressure – if the pressure is too high, release air using a needle; if too low, add air with a manual pump or compressor. Target value: P1 minus 0.2 bar. For example, if P1 = 2.0 bar, set the pre-pressure to 1.8 bar.
6. Verification and start-up – after setting, tighten the valve cap, turn on the pump, and observe whether the system pressurizes normally and the pump turns off at P2. Check the first few switching cycles.

Pressure switch: how to set it and why it matters

The pressure switch (pressostat) is usually adjusted directly on the valve block of the water pump. Most standard switches (types PM/5, Condor MDR-F, Italtecnica PM-5 and similar) have two adjustment screws under the cover:

• Large screw (marked "ΔP" or "main spring"): adjusts the overall pressure levels – turning it clockwise increases both values (P1 and P2) at the same time.
• Small screw (marked "Δ" or "diff spring"): adjusts the difference (differential pressure) between P1 and P2. Turning it increases or decreases the difference. A larger difference = the pump runs longer, but switches less often. A small difference = shorter run time, but more frequent starts.

Standard factory setting (usually P1 = 1.5 bar, P2 = 3.0 bar) is suitable for most home installations. If you have a pump with a higher discharge pressure or a long pipe with higher losses, it may be appropriate to increase P1 to 2.0 bar and P2 to 3.5 bar.

Warning: P2 must never exceed the maximum operating pressure of the pump or the pressure tank. Most common membrane tanks have a maximum pressure of 8–10 bar, and home water pumps usually have a discharge pressure of 5–6 bar. The recommended P2 for standard family homes is 3.0–3.5 bar.

Positioning of the pressure tank in the installation

The pressure tank should be placed as close as possible to the point where the pipe branches off into the home network – that is, near the pressure switch and pressure gauge, usually in the technical room, basement or home pump room.

A few practical rules:

• The tank must be accessible for inspection and pressure measurement – do not place it in an inaccessible corner behind other equipment.
• The room temperature should be above freezing point – membrane tanks must not freeze, as frost would damage the membrane and the tank body.
• Larger tanks (80–150 liters) must be properly secured or mounted on a solid base to prevent movement during pressure surges.
• When installing, consider access for membrane replacement – most tanks have a removable lid, but you need space for disassembly.
• The tank should be installed in a vertical position (air valve upwards) – in horizontal position, uneven stress on the membrane may occur.

Most common problems with pressure tanks and their causes

Over the years of practice, problems with pressure tanks tend to repeat in several patterns. Most of them can be diagnosed without special tools.

The pump turns on too often (short cycles)

This is the most common complaint. Possible causes include:

• Burst membrane – if the pressure gauge shows a rapid pressure drop immediately after the pump stops, the membrane is likely damaged. You can confirm this with a simple test: drain the system and try to press the core of the air valve on the tank – if water flows out of it, the membrane is burst.
• Lost pre-charge pressure – air slowly escapes from the tank (through the valve or a leaky screw connection). Regular annual checks will detect this in time.
• Too small tank volume – see the example mentioned above.
• Pre-charge pressure set too high – the membrane is constantly pressed and the tank has no usable volume.

Pressure in the system constantly fluctuates and hammering in the pipes

Hydraulic shock (water hammer) is caused by sudden pressure changes. A properly functioning pressure tank dampens this phenomenon – if it occurs, check the pre-charge pressure and the condition of the membrane.

Water from the tap comes in waves (alternating strong and weak flow)

A classic sign of a burst membrane or completely depleted pre-charge pressure – water and air mix in the tank and flow together into the network. Sometimes air also flows out of the tap.

The tank is covered with condensation or rust from the outside

Condensation on the surface is normal with temperature differences, but rust is a problem. If the tank body is rusting, check for internal corrosion and consider replacing the tank. Some cheap tanks without coatings or with thin sheet metal corrode already after 3–5 years in damp basements.

Practical tips from years of customer experience

Here are specific recommendations we have given to hundreds of customers and that have proven themselves in practice:

• Buy a tank with a higher capacity than you think is necessary. The price difference between 50 and 80-liter tanks is usually less than 30 euros, but the comfort and pump lifespan are significantly higher.
• Note the installation date and pre-pressure value on a piece of paper that you stick directly to the tank. It will save you hours of searching a year later.
• Check the pre-pressure every year – ideally in spring before the season. A drop of 0.2–0.3 bar per year is normal; a larger drop indicates a problem with the valve or the tank shell.
• When buying a water pump as a set always check what pre-pressure the tank is factory set to and how the pressure switch is set – these values may not be mutually aligned, especially in cheaper models from unknown brands.
• Never use an air compressor without a regulator to fill air into the tank – you can easily overinflate the pre-pressure, which causes the membrane to press against the tank wall and can be damaged during the next pump operation.
• If you are installing a system for drinking water, make sure that the membrane and internal parts of the tank are certified for contact with drinking water (EN 1717, ACS or WRAS certificate). Some cheap tanks have a membrane made of non-certified rubber, which can affect the taste and quality of the water.

If you are looking for a complete set, where the pressure tank and pump are mutually matched, we recommend looking at home water pump 3 SQIBO 0.75 / 50l for well and borehole water or at the model 3Ti-20 / 50l RTS for drinking water, where the pressure switch, pressure gauge and 50-liter tank are part of the delivery and are factory set to mutually compatible values.

Membrane replacement: when and how

The membrane is a consumable part with a typical lifespan of 5–10 years with proper pre-pressure setting and normal use. Its lifespan is shortened by insufficient pre-pressure (the membrane stretches too much), too high pre-pressure (the membrane is constantly compressed to the limit) and aggressive water (acidic pH, chlorinated water in higher concentrations).

Membrane replacement is possible in most standard tanks – the tank is disassembled into two parts (flange + bolts), the old membrane is pulled out, the new one is inserted in exactly the same direction (the shape of the membrane opening must match the shape of the flange). After replacement, do not forget to reset the pre-pressure according to the procedure described above.

The price of a replacement membrane ranges from 15 to 50 euros depending on the tank volume and manufacturer. It is a much cheaper option than replacing the entire tank – it makes sense if the tank shell is in good condition.

Pressure tank in automatic systems (RTS, frequency converter)

Modern home water pumps increasingly use a pressure control system RTS (pressure switch with dry run protection and electronic control) or a frequency converter, which regulates the pump speed according to current consumption. An example is home water pump with IBO 3 SDM24 / 50l pump – RTS, where the electronic regulator with integrated protection is an integral part of the system.

In frequency converter systems, the function of the pressure tank partially changes: since the pump regulates power smoothly and maintains a constant pressure, the pressure tank no longer primarily serves to reduce the number of switchings (which are minimal in a frequency converter system), but rather as a hydraulic shock absorber and a reserve for occasional small withdrawals. In such systems, a smaller tank volume (24–50 liters) is sufficient even in a larger house.

The pre-pressure setting remains the same – the air pressure must be set to the minimum pressure maintained by the system minus 0.2 bar. In systems maintaining a constant pressure of 3.0 bar, you set the pre-pressure to 2.8 bar.

Related topics in the Knowledge Center

The pressure tank is only one part of a functioning system. If you are still planning the installation, I recommend reading other articles as well: Installation of a home water pump with a submersible pump step by step will show you the entire installation from the well to the tap, What pump power do I need for a home water pump? will help you choose the right pump and Home water pump not pumping or losing pressure: causes and solutions covers problem diagnostics including those related to the pressure tank. For those dealing with drinking water, the article Drinking water from a well through a water pump: what must the system and pump meet? is also important.

Most frequently asked questions (FAQ)

What pre-pressure should I set in the pressure tank?

The air pre-pressure must always be 0.2 bar lower than the pressure switch turn-on pressure (P1). If P1 is set to 1.8 bar, set the pre-pressure to 1.6 bar. If P1 = 2.0 bar, pre-pressure = 1.8 bar. Important: always measure the pre-pressure on a depressurized tank (without water), otherwise the measured value will not be correct.

Why does the pump turn on every 30 seconds, even though the pressure tank is new?

The most common cause is an incorrectly set pre-pressure – either it is too high (the membrane is pressed and the tank has no usable volume) or too low (the membrane stretches to the shell and is damaged). Less often, it is due to an insufficient tank volume for the given system. Check the pre-pressure according to the procedure in the article and compare it with the P1 value on the pressure switch.

How do I find out that the membrane in the pressure tank has burst?

The simplest test: depressurize the system (close the supply and open some tap), then press the air valve core on the tank. If water (not just air) comes out of the valve, the membrane is broken. Another symptom is a rapid pressure drop immediately after the pump stops and air escaping from the taps. In the case of a broken membrane, the solution is to replace the membrane or the entire tank.

Can I use a larger pressure tank than the water pump manufacturer recommends?

Yes, a larger tank will not harm the system – on the contrary, the pump will switch less often and its lifespan will be longer. Just make sure that the pre-pressure and pressure switch setting remain in the correct relationship. The only disadvantage is a larger footprint and higher tank price, but in terms of functionality it holds: a larger tank = better performance.

How long will a pressure tank last?

The tank shell will last 15–25 years with proper maintenance and protection against corrosion. The membrane is a consumable part with a lifespan of usually 5–10 years, depending on water quality, correct pre-pressure setting and number of cycles. Tanks with certified butyl or EPDM membranes will last longer than those with cheap rubber. Regular annual pre-pressure checks are the most important preventive measure that will extend the membrane's lifespan.

Do I need to drain the pressure tank before winter if the pump room is not heated?

Yes, if freezing is a risk, the entire system including the pressure tank must be drained. The membrane tank must not be exposed to freezing with water inside – water will freeze, expand and can damage the membrane, tank shell and fittings. Draining is done via the drain tap on the valve assembly. The air in the tank part will remain, which is not at risk of freezing.

Conclusion: pressure tank as an investment in the system's long life

The pressure tank is apparently a passive component of the home water pump – it has no motor, does not need electricity, and does not require complex programming. Despite that, it is one of the most important components of the entire system. A properly chosen and correctly set tank will extend the pump's lifespan, ensure even pressure throughout the house and protect the pipes from hydraulic shocks.

Three things are key: the correct tank volume for the given system, correctly set air pre-pressure (always P1 – 0.2 bar) and regular annual checks. The rest is physics – air is compressed and expanded, water flows, the pump switches contentedly and rarely. That is how a properly functioning home water pump should look like.

If you're still looking for a complete system, take a look at the range of complete systems in the home water supply category – each of them includes a pressure tank and a pump mutually adjusted to the correct values, which will save you time and worries about calibrating from scratch. For example, the water supply system with pump SCR-0,50 / 50l is suitable for garden irrigation or a well, where you don't need drinking water, but a reliable pressure for higher consumption.

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