What Thread and Diameter to Choose for the Expansion Tank Connection
Which thread and pipe size to choose for the expansion tank
Seemingly trivial question – and yet one of those where surprisingly many mistakes are made in practice. An incorrect thread or pipe size of the expansion tank connection can complicate the entire installation so much that the installer spends an hour searching for a reducer that could have been with them from the beginning. Or – and that is worse – the connection lasts a year and then starts to slowly leak. In this article, we will look at the whole issue systematically: from the basics of thread systems through pipe sizing to specific recommendations for common installations in single-family homes and apartments.
Why choosing the right connection is more important than it seems
An expansion tank is not just a passive component of the system – it is a safety component that protects the entire circuit from pressure shocks and overheating. Its connection must not only be geometrically correct (so that it can be connected at all), but it must also seal reliably for the entire lifetime of the device, which typically ranges between 8 and 15 years. During that time, the system will experience thousands of temperature cycles, pressure fluctuations, and possibly also service interventions. Each of these factors places demands precisely on the connection between the tank and the piping.
From practice, I know that the most common cause of a leak at an expansion tank is not the membrane itself or the tank, but an inappropriately chosen or improperly tightened connection. In the article Why the connection at the expansion tank leaks: causes and solutions, we will address this topic in more detail; here we will focus on prevention – that is, on the correct selection even before installation.
Basics: pipe threads vs. metric threads
Before we move on to recommendations, it is important to clarify the basic terminology. In heating and sanitation installations, we encounter two completely different families of threads that must not be confused:
Pipe threads according to the BSP standard (British Standard Pipe)
These are the threads that dominate the entire sanitary and heating technology. They are marked with inch values – 1/2", 3/4", 1", etc. – but note: this number is not the actual thread diameter. It is a historical reference to the internal diameter of the pipe for which the thread was originally intended. The actual dimensions are larger. For example, a 3/4" thread has an external diameter of 26.44 mm and a pitch of 14 threads per inch.
Pipe threads are further divided into:
- G (BSP parallel, cylindrical) – both parts have parallel thread flanks. Sealing is achieved by a flange, an O-ring, or hemp/tape sealing. Designation e.g. G 3/4".
- R/Rc (BSP tapered) – one or both parts have tapered flanks (slope 1:16). Sealing is achieved by the threads themselves. Designation e.g. R 3/4" (external) or Rc 3/4" (internal).
Metric threads (ISO)
They are marked with the letter M and a number indicating the actual external diameter in millimeters – e.g. M8, M10, M12. Metric threads commonly appear in sanitary installations not as media connections, but as fastening elements: anchoring screws, mounting brackets, supports for tank mounting. That is why the mounting bracket M8 does not carry the medium – its M8 thread is used exclusively for mechanical fastening of the tank to the wall or bracket.
This basic confusion – using a metric dimension where a pipe thread is required, or vice versa – is one of the most common mistakes made by beginning installers. Both types of threads look similar at first glance, but they are not interchangeable or substitutable.
Standard connections for expansion tanks: what you find in practice
The vast majority of expansion tanks for closed heating systems and hot water systems have a connection with an external 3/4" BSP thread. This applies to tanks from 2 liters up to about 200 liters, regardless of the manufacturer – whether it is Reflex, Zilmet, Aquasystem, Flamco, or other brands. This is the de facto European standard for residential installations.
Smaller tanks (up to about 8 liters) are sometimes supplied with a 1/2" external BSP thread, but their share is decreasing in practice – most manufacturers have switched to a uniform 3/4" size to simplify the range and installation. Larger industrial tanks (over 200–300 liters) may have 1" or 1 1/4" connections, but these are rarely encountered in standard residential practice.
Table: Common expansion tank sizes and their standard connections
| Tank volume | Common connection | Note |
|---|---|---|
| 2 – 8 liters | 1/2" or 3/4" external BSP | Depends on the manufacturer, check the catalog |
| 8 – 80 liters | 3/4" external BSP | European standard, most common |
| 80 – 200 liters | 3/4" external BSP | 3/4" still dominates, some manufacturers offer 1" |
| 200+ liters | 1" or 1 1/4" external BSP | Industrial and commercial applications |
From this follows a practical conclusion: if you are dimensioning a standard family house with a boiler up to 30 kW and an expansion tank up to 80 liters, you will almost certainly be working with a 3/4" connection. And precisely for this dimension, the expansion tank mounting kit 3/4" has been designed, which contains all the necessary elements for reliable mounting and connection in one package.
Connection size vs. pipe size: how they fit together
This is another moment of confusion. In residential heating installations, the pipe is most often copper or plastic (PEX, multi-layered) with an internal diameter of 15 mm or 18 mm, corresponding to a pipe size of 1/2" or 3/4". However, the expansion tank has a 3/4" BSP connection – how does this combine, when the pipe is only 1/2"?
The answer is simple: when connecting to the expansion tank, a branch or T-piece is almost always used on the existing pipe, plus a connecting element (ball valve, union or mounting kit), which transitions from the pipe size to the tank connection size. The tank itself is not connected directly to the main flow circuit – it is connected from a branch, usually with a shut-off valve for the case of membrane or entire tank replacement.
Therefore, the tank connection is dimensioned not according to the pipe diameter, but according to the volume of the tank and the manufacturer's standard. The pipe leading to it can be 1/2" – this does not affect the function, because the expansion tank is not a flow device. Pressure is transferred to it statically.
Conical vs. parallel thread: what is the difference and what to watch out for
Most expansion tanks from European manufacturers have a connection with a cylindrical (parallel) external thread G. The sealing is therefore not achieved by the thread itself, but by a flange – a flat gasket in the flange surface or a wrapped gasket (hemp with paste, teflon, etc.).
Conical threads (R/Rc, NPT) are more common in industrial applications, in the American NPT standard, or in some older installations. If you have doubts, check the technical data sheet of the tank – it always states whether it is a G or R thread.
Practical consequences:
- Always use a sealing on a G (cylindrical) thread: either PTFE tape (teflon), hemp rope with sealing paste, or a rubber/fiber gasket in the flange surface. Without sealing, the connection will leak.
- On a R/Rc (conical) thread, it is sufficient to use either the thread itself (with a suitable material pair), or a minimal layer of teflon tape. A flange gasket is not needed.
- Never mix conical and cylindrical threads in the same pair – the different geometry will cause the thread to either not tighten at all, or to leak even after tightening.
Mounting components: where the M8 thread comes into play
Once the media connection is resolved, the second task comes up: the tank itself must be mounted to the wall or to a load-bearing structure. Expansion tanks are metal vessels filled with water and pressurized air – even an 18-liter tank when full can weigh 20–25 kg. This is not a weight that should be supported only by the connection thread.
For mounting, either of the following is used:
– bracket hangers (for smaller tanks up to 25 liters, where the manufacturer integrates the mounting bracket directly into the tank shell),
– or separate mounting kits with a bracket, rod, and anchoring component for the wall.
Wall or beam anchoring screws are almost always metric threads – most commonly exactly M8. Therefore, the M8 mounting adapter is a standard part of mounting kits: it provides the correct threaded hole for anchoring into various types of substrates.
A more detailed comparison of metric threads and their use in practice can be found in the article Metric thread M8 vs. other sizes: which mounting adapter do I need in our Knowledge Center. Here we emphasize only the key point: M8 (metric) and G 3/4" (pipe) are two completely different functions – one anchors, the other tightly connects media. Both are typically present simultaneously in an expansion tank installation.
Practical scenarios from everyday customer practice
Scenario 1: Replacing an old expansion tank at the boiler
The customer has a 12-year-old 18-liter tank at a condensing boiler in an apartment block. The old tank has a G 3/4" connection and is hung on a bracket with M8 screws. A new tank of the same volume from another manufacturer – identical connection dimensions (G 3/4" external) and the same M8 anchoring holes. The replacement goes smoothly without any reduction; it is enough to replace the gasket in the flange connection and properly pressurize. This is about 80% of cases in practice.
Scenario 2: Adding an expansion tank to a floor heating system
New construction, a floor heating manifold without a primary expansion tank (the designer forgot to include it in the project – unfortunately not an unusual situation). The installer adds a 12-liter tank. The manifold has a 1/2" internal connection. The tank has 3/4" external. Solution: a reducing adapter 1/2" internal → 3/4" external, a closing ball valve 3/4", and then directly onto the tank thread. Mounting to the wall via a bracket with M8 anchoring screws. Here it is important not to forget the correct tank position – the connection points downward to prevent air from forming in the pipe leading to it.
Scenario 3: Tank for a domestic hot water (DHW) system
Expansion tanks for DHW have the same connection dimensions (G 3/4"), but a different type of membrane – a membrane made from a material suitable for potable water (EPDM food grade or butyl). Technically, the connections are identical, but never mix a heating tank with a DHW tank – not because of the thread, but because of the hygienic compatibility of the membrane. The 3/4" connection is the same, but for DHW it is even more important to have a proper seal, as potable water is more aggressive to some sealing materials.
Scenario 4: Solar circuit with an expansion tank
Solar circuits operate with heat transfer media based on glycol mixtures at higher temperatures and pressures. The tank connection is again usually G 3/4", but here the choice of sealing materials is critical: classic hemp with standard paste is not suitable – use PTFE tape or specialized sealing for glycol media. The thread and diameter do not change, only the sealing material changes.
How to verify the connection size before purchase or installation
Verifying the connection size of your existing (or planned) system is a step that will save unnecessary trips to the store. Here are verified methods:
- Technical specification sheet of the tank or boiler – always the first choice. Manufacturers list the connection size in the "technical parameters" or "connection" section. Look for the term "connection", "Anschluss" (German), or directly "G 3/4" BSP".
- Physical measurement – if you have access to the old thread, measure the external diameter of the threaded part: 20.96 mm = G 1/2", 26.44 mm = G 3/4", 33.25 mm = G 1". Measuring with a caliper (sliding gauge) is sufficiently accurate.
- Counting the threads – BSP 1/2" has 14 threads per inch (25.4 mm), BSP 3/4" also has 14 threads per inch. If you want, place a piece of paper, make an imprint of the thread, and count.
- Testing with a nut or coupling – if you have a precise G 3/4" nut at hand, a simple slide over the thread will verify compatibility. It should not be too loose or stuck.
- Reading the label on the tank – most manufacturers indicate the connection size directly on a label or stamped plate on the tank. Look for the format "G 3/4" or "3/4" M" (M = mâle, French term for external).
Sealing: materials and correct application
Even the best-chosen thread will not seal properly if the sealing is applied incorrectly. This is one of the topics we also cover in the article Common mistakes when installing connecting components in the bathroom and how to avoid them. Here we list only the basic rules:
- PTFE tape (Teflon): wind in the direction of the thread (it winds up when tightening, not unwinding), at least 3-4 layers for G thread. Start from the first thread, not from the edge. For heating, 4-5 layers are more reliable; for drinking water and TÚV, use a tape with approval for drinking water (certification for Food/Drink).
- Hemp + paste: a classic combination, still very reliable. Wind the hemp fiber in the direction of the thread and impregnate with paste (e.g., LOCTITE 577 or similar). Advantage: self-healing effect under micro-vibrations. Disadvantage: harder to disassemble after years.
- Liquid sealing pastes: (e.g., TANGIT Uni-Lock, Hapflex) suitable for cylindrical and conical threads, approved for most media. Apply a thin layer to the thread and screw in without waiting for it to dry – the seal hardens only in the thread.
- Flange gasket: if the fitting has a flange surface (typically a flat end of the thread), use the appropriate gasket (EPDM, NBR, fibreglass). This gasket is not replaced by Teflon – it requires a flat surface to be applied to.
Dimensioning: Do I need to increase the pipe size for a larger tank?
This is a question that comes up in larger installations. The answer for residential systems is simple: no, you don't need to. An expansion tank is not a flow device – you don't pump media through it, you only transfer pressure. A G 3/4" connection is hydraulically sufficient for any tank in a family home, because "flow" through the connection only occurs during rapid inflation or deflation, which is an extremely slow process in a closed system.
For larger commercial or industrial systems (apartment building boilers, administrative buildings), a different rule applies – there, the designer sizes the connection according to the calculated volume and pressure change rate, and sometimes a 1" or larger connection is needed. In standard residential practice (family home, apartment), however, G 3/4" covers everything from 8 to 200 liters without limitation.
How to correctly choose a mounting kit for your installation
A proper mounting kit must address three things at once: the connection (correct thread size), the mounting (correct type of bracket and anchoring elements), and the tank position (vertical or horizontal mounting). For standard residential installations with a 3/4" connection, it is most convenient to choose a complete expansion tank mounting kit 3/4", which addresses these three aspects at once.
When choosing a mounting kit, check:
- Maximum load capacity of the bracket (kg) – compare with the weight of the tank full of water
- Type of wall anchor thread (M8, M10) – according to the substrate (drywall, concrete, brick wall)
- Compatibility with tank diameter – the bracket must fit the tank of corresponding diameter
- Material – galvanized steel is standard, for wet areas prefer stainless steel
More about types and compatibility of mounting systems can be found in the article Comparison of types of mounting systems for plumbing and a practical step-by-step procedure is processed in Mounting of mounting elements for expansion tanks and plumbing step by step.
Most common mistakes and how to avoid them
Over the years of contract work, the same mistakes keep repeating. A brief overview:
- Mixing up G (cylindrical) and R (conical): look similar, but do not seal. Always check the technical data sheet.
- Too little sealing: "for testing" is enough, but during pressure testing or first heating, the joint will leak. Better to add an extra layer of Teflon.
- Over-tightening: cast iron and bronze parts are more brittle than they look. Tightening "with force" will crack the valve or distributor body.
- Wrong tank orientation: the connection must point downward (or sideways according to the manufacturer). A tank with the connection pointing upward does not work properly – an air bubble blocks the membrane.
- Missing shut-off valve: always install a ball valve between the tank connection and the circuit. It allows tank replacement without draining the entire system.
- Wrong membrane type for TÚV: (not dependent on the thread, but on the material) – never use a heating expansion tank as a TÚV expansion tank.
Frequently asked questions (FAQ)
What is the difference between G 3/4" and 3/4" BSP – is it the same?
Yes, for all practical purposes, G 3/4" and 3/4" BSP are identical designations for the same cylindrical pipe thread according to ISO 228 standard. "BSP" is the British designation, "G" is the European ISO designation. You will find both terms in catalogs – they always refer to the same size with an outer diameter of 26.44 mm and a pitch of 14 threads per inch.
Can I connect a tank with a 3/4" thread to a 1/2" pipe?
Yes, this is a common situation. Use a reducing adapter from 1/2" internal thread to 3/4" external, or a T-piece with the appropriate dimensions for the branch. Hydraulically, it is problem-free, because an expansion tank is not a flow device and the pipe size has no effect on its function in standard installations.
Should I use Teflon or hemp rope for sealing the expansion tank connection?
Both options are correct for heating. For TÚV (drinking water), prefer PTFE tape with certification for drinking water or liquid sealing pastes approved for potable media. Hemp rope with standard paste is technically sufficient for heating, but for drinking water, check the product's approval for potable use. For solar circuits with glycol, use PTFE or special pastes for glycol exclusively.
What is the M8 thread in the expansion tank mounting kit for, if the connection is 3/4"?
The M8 thread and the G 3/4" connection are two different functions of the same installation. The G 3/4" connection connects the tank to the pipe circuit and transfers the media pressure. The M8 thread on the mounting bracket is used solely for mechanically fastening the tank to the wall or supporting structure. Without mechanical fastening, a full tank would hang on the connection, which would lead to a leak at the thread or damage to the pipe. Both elements are therefore necessary in a standard installation.
How do I tell if my old tank has a G 3/4" or another size?
The fastest way: measure the outer diameter of the threaded part of the connection with a caliper. If it measures around 26.4 – 26.5 mm, it is G 3/4". A value around 20.9 – 21.0 mm means G 1/2". Second option: look at the label on the tank itself, where the manufacturer states "Connection: G 3/4" or similar. Third option: look up the manufacturer and model in the technical documentation online.
Is it necessary to change the connection size when replacing an old tank with a new one of a larger volume?
In the vast majority of cases, no. If you are replacing an 18-liter tank with a 25-liter or even 50-liter one within a residential installation, the connection remains G 3/4" external – this is a pan-European standard for these volumes. You only need to change the bracket and mounting kit if the new tank has a different shell diameter or weight.
Conclusion: rules that always apply
If you take only three things from this article, let them be these:
First – the connection of an expansion tank for residential heating and TÚV is almost always G 3/4" external cylindrical BSP thread. Check the technical data sheet, but you will rarely be wrong with this value.
Second – M8 thread and G 3/4" thread are two different functions. One fastens, the other seals the media. Both are necessary in every properly executed expansion tank installation.
Third – sealing is as important as the correct thread size. The correct sealing material, applied properly, is the difference between an installation that lasts 15 years and one that leaks after a year.
If you are unsure about the choice for your specific system, also check related articles in our Knowledge Center – for example, How to mount an expansion tank: choosing a kit and the correct connection size or Maintenance and inspection of mounting elements in plumbing installations. And if you are looking for complete equipment for installation, the expansion tank mounting kit 3/4" in combination with the mounting bracket M8 covers most standard residential installations without the need to search for additional parts.
Do you have a question about this topic?
Having trouble making a decision or dealing with a specific situation in your household? Write to us - we'll be happy to help.