Installation of a heating expansion tank – procedure, placement and connection

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Installation of an expansion tank for heating – complete procedure, correct placement and connection

The expansion tank is an unobtrusive but absolutely essential component of every closed heating system. Most homeowners notice it only when something stops working – when the boiler continuously discharges water through the safety valve, when the pressure gauge shows zero or, conversely, excessively high pressure, or when the system behaves unpredictably. Correct installation of the expansion tank is not rocket science, but it does have its rules. Deviations from them can lead to malfunctions, reduced tank lifespan, and in extreme cases, even safety risks.

In this article, I will guide you through the entire process – from choosing the right location, through preparing the tools and pipe route, to the actual connection, setting the pre-charge pressure, and final inspection. If you have not yet selected the tank size or are dealing with pre-charge pressure questions, I recommend first reading the related articles What expansion tank volume do I need for my heating system? and Correct pre-charge pressure for an expansion tank – how to set and check it. Here we will focus exclusively on installation.

Why installation matters more than you might expect

Many people assume that an expansion tank is simply screwed onto a pipe somewhere and that’s it. In practice, however, we see that incorrect placement or faulty connection are the causes of most problems that customers encounter – from the constant discharge of the safety valve to the rapid "death" of the tank’s membrane. A technician who installs expansion tanks over the long term knows this well: an installation error does not show up immediately, but after several heating seasons, when the system is full of rust and the membrane’s elastomeric properties have deteriorated.

What you will need – tools and materials

Before you start, gather everything you need in one place. Additional trips to the store during installation unnecessarily prolong the time and increase the risk of errors.

Tools

Adjustable wrenches (spanners) – at least two, for tightening and loosening
Torque wrench for final tightening of threaded connections
Bicycle pump or compressor with pressure gauge (for setting the pre-charge pressure in the air side of the tank)
Pressure gauge for the system (if not already installed) – range 0–4 bar
Drill and wall anchors (if the tank is mounted on a wall)
PTFE (Teflon) tape or liquid thread sealant for threads
Container for collecting water when draining the system
Test pen or multimeter (for electrical safety near the boiler)

Materials and accessories

Expansion tank of the appropriate volume (e.g., Expansion tank IBO 8l for heating and hot water systems for small apartments, Expansion tank IBO 19l for heating and hot water systems for medium-sized family homes)
Closing ball valve with a drain valve (so-called service valve) – installed between the system and the tank
Sealing/gaskets matching the tank’s thread (standard G 3/4")
Bracket or wall hanger (for larger tanks over 18 l)
Flexible coupling or short section of copper/steel pipe

Selecting the correct location for the expansion tank

This is the step where the most mistakes are made. An expansion tank is not just a bucket to catch excess water – it is a hydraulic damper for the entire system, and its position directly affects how the system "breathes" during temperature changes.

Basic rule: return pipe before the pump

The expansion tank must always be connected to the return pipe (the cooler branch) and ideally to the suction side of the circulation pump – that is, the side where the pump draws in water. Not after the pump, not on the boiler outlet. The reason is purely hydraulic: the pump creates overpressure on the discharge side and underpressure on the suction side during operation. If the tank were connected after the pump, it would be exposed to constant pressure shocks, which shortens its lifespan. In addition, the zero-pressure point (the so-called "neutral point") of the system must be as close as possible to the tank’s inlet – and this is achieved precisely by connecting it to the return pipe before the pump.

Mounting height

For suspended tanks (standard membrane types, which are sold e.g. in IBO 8–36 l series), it is valid that they can be placed in any position – horizontal, vertical, or inclined. Most manufacturers, including IBO, explicitly state this in their documentation. For standing versions (larger than 50 l), different rules apply and they usually have to stand on the floor. In the typical practice of a family house, we work with 8–36 l tanks, where this limitation does not apply.

If you mount the tank on the wall, make sure that access to the filling valve (Schraeder valve) on the air side is free – you will need to access it during every annual inspection.

Accessibility and space for service

Practical advice from experience: a built-in or hard-to-reach expansion tank is a guarantee of future problems. Every year, someone should check the pre-charge pressure, which requires at least 30–40 cm of space in front of the air valve of the tank (usually located at the bottom near the floor or at the top – depending on the orientation). Never place the tank behind a fixed partition or in a recess where you cannot reach it without demolition.

A boiler room or technical room with a boiler is an ideal location. The tank should not be exposed to freezing – the temperature range of standard IBO tanks is from +4 °C to +99 °C, and the ambient temperature must not drop below the freezing point. Therefore, placement in an unheated garage or in a hallway with a window is problematic.

Preparation before the actual installation

Setting the pre-charge pressure BEFORE installation

This is a step that most DIY masters skip, and then wonder why their tank does not work properly. Before connecting the tank to the system, you must set the air pressure in the gas part of the tank. This value must correspond to the static water pressure at the installation location of the tank.

A simple calculation: static pressure = height of the water column in meters × 0.1 bar. If the highest radiator in the house is 6 meters above the installation location of the tank, the static pressure is 0.6 bar. The pre-charge pressure in the tank is set to the same value, or slightly higher by 0.1–0.2 bar (0.7–0.8 bar in this example). Never lower it.

From the factory, tanks have a pre-charge pressure of 1.5 bar (sometimes even 2 bar). For a typical two-story family house, this may be too much. Release air from the tank via the Schraeder valve (the same as on a bicycle tire) using a pin or a special key, and check the pre-charge pressure with a pump manometer. A more detailed procedure can be found in the article Correct pre-charge pressure of the expansion tank – how to set and check.

Draining or shutting down the system

If you are replacing an existing tank or installing a new one in a functioning system, you must shut down the system and drain the water from the connection point. Turn off the boiler and let it cool down to less than 40 °C – never work on a hot system. Close the water supply to the system (shut-off valve) and open the drain valve at the lowest point of the system, or use a service valve at the expansion tank (if installed).

Self-Installation Step by Step

Step 1: Installation of the Service Valve

Between the system and the expansion tank, there must be a ball valve with the possibility to drain (also known as a service valve or "kugelhahn"). It allows you to isolate the tank from the system without the need to drain the entire circuit—for example, when replacing the tank or checking the pre-charge pressure. This valve must be in the open position during normal system operation.

Screw the service valve into a T-piece or directly into a branch on the return pipe. Use PTFE tape or liquid thread sealant (e.g., Loctite 577) for sealing. Apply the tape in the direction of the thread, 3–4 layers are standard. Too much PTFE tape can cause the valve thread to crack during tightening.

Step 2: Mounting the Tank

For tanks up to 12 liters, such as the Expansion tank IBO 12l for heating, pressure membrane tank, securing by the threaded connection to the pipe is sufficient—the tank is "carried" directly by the pipe. However, I still recommend using a simple bracket or hanger to eliminate mechanical stress on the thread.

For tanks of 19 l and larger (e.g., Expansion tank IBO 24l for heating and TÜV systems or Expansion tank IBO 36l for heating and heating systems), a ceiling or wall hanger is a necessity. These tanks weigh up to 20–35 kg and would impose an unacceptable load on a welded or threaded pipe connection. Anchor the hanger into the wall using chemical anchors or expansion bolts M8—not into drywall or lightweight masonry without chemical anchors.

Orientation of the tank: Most IBO hanging tanks can be hung with the air valve upwards (vertically, neck down) or horizontally. The manufacturer allows this. In practice, the vertical position with the neck down is preferred, as it allows better drainage of any condensation and easier access to the Schrader valve on top.

Step 3: Threaded Connection of the Tank

The standard thread of IBO expansion tanks is G 3/4" (external). Screw the service valve or a reducing piece directly onto this neck. Apply PTFE tape, 3–4 layers, correctly. Tighten the tank by hand and then an additional quarter turn with a wrench—do not over-tighten, the tank thread is made of steel and can crack under excessive tightening.

After tightening the thread, check that the tank is in the correct position (not twisted, air valve accessible). If you need to adjust the position, use a transition piece with a sealing gasket instead of PTFE tape—this allows rotation without the need to unwind the tape.

Step 4: Connecting to the Service Valve and Piping

Connect the service valve to the tank neck using a short pipe section or a flexible braided hose (minimum pressure rating 6 bar). A flexible hose is more practical in sometimes awkward pipe geometries in the boiler room. The length of the connection should not exceed 50–70 cm—longer connections increase hydraulic resistance and hinder the proper function of the tank.

Visually check the connections—no strained hoses, no sharp bends, and sufficient slack for thermal expansion of the piping.

Filling the System and First Start-up

Filling the System

Open the service valve (open position = handle parallel to the pipe). Open the filling valve to fill the system with water from the mains. The manometer should rise slowly—the ideal filling pressure for a cold system in a typical two-story house is 1.2–1.5 bar. This pressure must be 0.2 bar higher than your pre-charge pressure in the tank.

For example: tank pre-charge pressure set to 0.8 bar → fill the system to 1.0–1.2 bar (cold). If the filling pressure is the same or lower than the tank's pre-charge pressure, the membrane will remain compressed against the tank wall and water will not enter the tank at all—the tank will not perform its function.

Depressurization

After filling, bleed air from each radiator and all air vents in the system. Air in the system causes noise, poor heating of some radiators, and accelerated corrosion (air contains oxygen, which attacks steel sections and pipes). Use an air bleed key and a container to collect drips.

First Boiler Start-up and Inspection

Turn on the boiler and let the system reach operating temperature (60–75 °C for standard heating). Monitor the manometer: when heated, the pressure should rise by 0.5–0.8 bar compared to the cold state. If you filled to 1.2 bar, the hot system should show 1.7–2.0 bar. Above 2.5 bar, the safety valve is active at standard settings—if this occurs, something is wrong (undersized tank, too low pre-charge pressure, or the system was overfilled).

If the pressure during operation does not drop below the minimum and does not exceed 2.3 bar, everything is correct. Also check the threaded connections—no signs of moisture, no dripping.

Common installation errors and how to avoid them

From practice, I know that the same mistakes repeat themselves over and over. Here are the most common ones:

Incorrect pre-charge pressure before installation: The tank is installed with a factory pre-charge of 1.5 bar into a system where the static pressure is only 0.6 bar. Result: the diaphragm remains compressed for a long time, does not expand properly, and the safety valve discharges with every heating cycle.
Missing service valve: Without it, it is not possible to replace the tank without draining the entire system. A small time-saving during installation will cause complications during every service.
Too short connection between pipe and tank: If the connection is too rigid (short pipe without a flexible part), thermal expansion of the pipe may cause mechanical damage to the tank thread or dislodging of fittings.
Installation on a hot system: Risky for the sealing (PTFE expands differently when tightened hot than when cold) and for handling (risk of burns).
Ignoring the support for large tanks: A 36-liter tank full of water weighs about 40 kg. No pipe with a G 3/4" thread can withstand this weight for a long time without cracking.
Too long PTFE tape or incorrect winding direction: Tape wound in the wrong direction unwinds during tightening and does not press, the thread leaks and drips after filling the system.
Checking tightness only when cold: Some connections only become apparent at operating temperature – always check the sealing after the first heating of the system.

Special situations and different systems

Floor heating

In floor heating systems, the water volume in the piping is significantly higher than in classic radiators. For a 100 m² floor area with a 16 mm pipe diameter, the water volume in the piping alone is 12–18 liters for the floor loop, plus the volume in the manifold, collector and boiler. This has a decisive impact on the required expansion tank volume. I do not recommend solving floor heating with an 8 or 12-liter tank unless you also have classic radiators in the system that would compensate for the smaller floor volume. For details on the calculation, see the article What expansion tank volume do I need for my heating system?.

Combined system with TÚV tank

If you have indirect hot water storage (TÚV) connected to the boiler circuit in addition to heating, you must either take its volume into account when calculating the expansion tank for heating or install a separate expansion tank for the TÚV circuit. Membrane tanks for heating and for TÚV are not interchangeable – TÚV tanks must be made of materials certified for potable water. More on this topic can be found in the article Expansion tank for TÚV systems – what it must meet and how to choose it.

Older system after renovation

Many older houses had an open expansion system (expansion tank in the basement) and during renovation to a closed system with a condensing boiler, a new closed circuit was added. In such cases, it is especially important to thoroughly flush the entire system before installing a new expansion tank – the old system is often full of rust, sediment and sludge, which quickly damage the diaphragm. Membrane tanks are sensitive to abrasive impurities and chemicals.

Correct choice of tank size for standard houses – approximate overview

Type of building	System volume (water)	Recommended tank	IBO model
Small apartment, 1–2 radiators	up to 60 l	8 l	IBO 8l
Apartment/small house, 4–6 radiators	60–120 l	12–18 l	IBO 12l
Family house, 8–12 radiators	120–220 l	19–24 l	IBO 19l / IBO 24l
Larger house or floor heating	220–350 l	36 l	IBO 36l

Maintenance and regular inspection after installation

Installation itself is not the end of care for the expansion tank. Manufacturers and standards recommend checking the air pressure in the tank once a year (ideally before the heating season). Procedure: close the service valve, attach a pressure gauge to the Schraeder valve and read the pressure. If the pressure is lower than the original setting, add air with a pump. If water (not air) comes out of the valve, the diaphragm is likely cracked – the tank must be replaced.

The topic of common faults and their diagnostics is covered in more detail in the article Common expansion tank faults – symptoms, causes and solutions, where you will also find a detailed guide on how to recognize the end of the diaphragm's life. Information on overall lifespan and suitable replacement time is summarized in the article How long does an expansion tank last and when to replace it.

Most frequently asked questions (FAQ)

Can I connect the expansion tank directly to the boiler outlet instead of the return?

No, this is one of the most common mistakes. At the boiler outlet, the water temperature and pressure are highest, which loads the diaphragm and shortens its lifespan. In addition, from a hydraulic point of view, the correct connection point is on the return before the pump – only then is the neutral point of the system where it should be, and the pump does not negatively affect the pressure in the tank.

Is a service (shut-off) valve really necessary, or can I do without it?

Theoretically, the system works without it – standards do not explicitly require it for all types of installations. In practice, however, every experienced plumber includes it, because without it, every tank service (pressure check, replacement) means draining the entire heating system. This takes hours and often costs more than the valve itself. In addition, if you forget to close the service valve during tank replacement, the system will only leak into the room – without it, you will drain the entire house.

Why does my safety valve still discharge after installing a new expansion tank?

The most common causes are three: the tank pre-charge pressure was set too high (the diaphragm does not compensate for water expansion), the tank is undersized (too small volume for the system), or the system was filled with too high a filling pressure. Check: the air pressure in the tank (after closing the service valve), the system filling pressure and compare the tank volume with the water volume in the system. If you are unsure about the calculation, refer to the article How to choose an expansion tank for heating – volume, pressure and system type.

Can an expansion tank be installed outdoors or in an unheated space?

It is not recommended. The expansion tank contains water from the system (above and below the membrane in case of failure) and at temperatures below freezing point, freezing and cracking of the tank is possible, just like with any other water-containing component. The tank should therefore always be installed in a heated or at least temperature-controlled space – boiler room, technical room or another space protected from freezing. If it is absolutely necessary to place it in an unheated space (e.g. in a garage), it is essential to ensure at least minimal heating or thermal insulation of the tank's surroundings and connecting piping.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we will be happy to help.