Why is the expansion tank leaking and how to fix it

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Why is the compression fitting leaking and how to fix it: a complete technical guide

A leaking compression fitting on a heating system is not just an aesthetic issue – it is a potential cause of floor damage, drywall wall damage, corrosion of surrounding components, and a gradual drop in system pressure. From experience, I know that most customers complain about replacement parts by saying something like, "It's leaking at that screw thing near the boiler" or "The pipe connection to the valve is dripping." Behind this vague description lies a whole range of causes – from a simple lack of tightening of the nut to a fundamental error in installation, a poor choice of sealing material, or a mechanically damaged part.

In this article, I will cover all common causes of leakage in compression fittings in Hepworth heating systems, show what each type of leak looks like, what it indicates, and how to properly fix it – without unnecessarily replacing the entire system and with minimal interference with the installation.

How a compression fitting works and where the sealing occurs

Before we dive into the faults, it is important to understand exactly where the compression fitting seals and why. The whole principle is based on the compression of the sealing surface: when tightening the nut, a mechanical contact is created between the fitting body and the opposite threaded part (or the valve seat), which prevents water or antifreeze from escaping the system.

In the Hepworth system, there are two types of fittings that you encounter when working with copper piping:

Compression fitting – the sealing is ensured by an olive (compression ring), which plastically deforms and wraps around the pipe when the nut is tightened
Threaded compression fitting – the sealing occurs at the threaded joint, where a sealing material (PTFE, hemp fiber, anaerobic sealant) is applied

In most standard installations, when connecting copper pipe to a valve (valve body, manifold, boiler), both principles are encountered at the same time: the compression olive seals the pipe–fitting connection, while the threaded joint with sealing material solves the fitting–valve connection. Precisely this combination of the two places is the source of most faults.

In the schematic, you can see two critical points – the olive (compression ring) and the threaded joint. A leak at each of them has a different cause and a different solution.

Five most common causes of leakage in a compression fitting

1. Under-tightened or over-tightened nut

This is by far the most common reason why I get customer complaints about a new fitting. An under-tightened nut does not compress the olive enough on the pipe, and the joint drips immediately after the system is turned on. On the other hand, an over-tightened nut (usually when using a long pipe wrench without feeling) crushes or deforms the olive asymmetrically – the joint may look sealed, but under pressure of 1.5–3 bar (typical operating pressure of heating systems) it leaks to the side.

Practical rule: after tightening by hand, add another 1.5 to 2 turns with a wrench for soft copper pipes, and 1 to 1.5 turns for hard copper pipes or stainless steel fittings. For most 15 mm Hepworth fittings (which corresponds to 1/2" thread), the correct tightening torque is approximately 20–25 Nm. For 22 mm pipes (3/4" thread), we are around 35–40 Nm.

2. Damaged, incorrectly installed or missing olive

The olive is a small, inconspicuous compression ring, but it is absolutely key. In practice, I often encounter the situation where, during repeated assembly (when the fitting is disassembled and reassembled several times), the old olive is either left on the pipe or a new olive is installed next to the old one. The result is either insufficient compression or eccentric seating of the ring and leakage along the pipe.

Another common problem: the olive is installed upside down. Most olives have a conical shape – one end is smooth, the other slightly rounded. The correct orientation is to have the conical part facing the fitting body. When installed in reverse, the sealing surface does not seat properly and the joint leaks even after the nut is fully tightened.

3. Damaged thread or threaded surface

On the threaded joint (typically when connecting the fitting to a valve or boiler), sealing is achieved using a threaded material. If the thread on the fitting or on the valve body is even slightly damaged (stripped thread, scratches from a screwdriver, corrosion), no amount of PTFE will save it. Thread inspection should be the first step in every installation.

Special case: customers who thread plastic fittings onto brass 1/2" (G 1/2") threads or vice versa – here different thread profiles and tolerances are combined, which can lead to leakage even in a new joint. More on compatibility can be found in the article Compatibility of Hepworth fittings with other pipe systems and materials.

4. Incorrect sealing of the thread (PTFE, hemp fiber, anaerobic sealant)

This is a topic I cover in more detail in the article How to seal a compression fitting on a heating system: PTFE vs. hemp fiber, but in short: the most common mistake is either insufficient amount of sealing material or, conversely, overstuffing with hemp fiber, which during installation pushes the fibers into the pipe and narrows the flow. With PTFE (PTFE tape), a typical problem is incorrect winding direction – the tape must always be wound in the direction of tightening the thread, otherwise it unwinds during installation instead of being compressed.

At water temperatures above 85–90 °C (which is common for boiler connections), hemp fiber behaves differently than in low-temperature heating – it dries out, shrinks and after a few seasons loses elasticity. In such cases, I recommend using anaerobic sealant (Loctite 577, Tangit Uni-Lock) or professional hemp fiber with a special linseed oil-based sealant.

5. Mechanical damage to the fitting or pipe

Less common, but quite insidious: micro-cracks in the brass fitting body (typically a result of excessive force during installation or frost expansion), a cut groove on the pipe (imprecise cutting with a pipe cutter), an oval pipe (due to incorrect bending or mechanical impact). With an oval pipe, the olive cannot seat evenly around the entire circumference and the joint will always be leaky on one side.

How to identify exactly where the leak is coming from

Before reaching for the wrench, it is essential to precisely locate the leak. This may sound trivial, but with compression fittings on a manifold or at the boiler inlet, where multiple joints are close together, water can run down from the actual source of the leak 10–20 cm further away. Statistically, I have seen several cases where a customer tightened a completely different joint than the problematic one.

Leak localization procedure:

Use a piece of dry paper towel to touch each joint individually – a wet spot will clearly indicate the location
If in doubt, drain the system (close the expansion valve, reduce pressure to 0), touch the joints, then gradually increase pressure and observe where moisture appears first
Distinguish whether the water is dripping from the surface of the nut (problem with the olive or pipe), from the threaded joint (problem with the thread seal) or from the body of the fitting (crack)

Step by step: repairing a leaking compression fitting with olive

The procedure depends on the cause. Here is a systematic approach for the most common scenario – a dripping compression fitting on a copper pipe:

Case A: The joint is only slightly dripping, the olive is in good condition

If you see only minimal moisture and the joint is less than 2–3 seasons old, it may be sufficient to slightly tighten the nut. Important: do not over-tighten when cold and under full pressure. Close the circuit, drain the system, tighten by a maximum of 1/4 turn, restore pressure and observe for 30 minutes.

If this does not help, or the joint starts to leak again after a few days, a complete disassembly and replacement of the olive is necessary.

Case B: Replacing the olive and reassembling

Close the branch or the entire system, drain water from the problematic section
Loosen the nut of the fitting, pull the pipe out of the fitting
Remove the old olive (usually firmly seated on the pipe) carefully using a special tool or gently loosen it with a sharp knife around the edge – avoid scratches on the pipe
Check the end of the pipe: it must be cut straight (use a pipe cutter, not a saw), without burrs, with a perfect circular cross-section. Chamfer the end of the pipe both internally and externally
Put on the nut and then the new olive – the conical side should face the body of the fitting
Insert the pipe fully into the fitting, hand-tighten the nut, then tighten with a wrench by 1.5 turns (15 mm pipe) or 1.75 turns (22 mm pipe)
Refill the system, slowly increase pressure and observe the joint for 15 minutes

If you are reconstructing an existing installation and working with a compression fitting 1/2" x 15 on thin copper pipes near the boiler, check whether you have replacement olives of the appropriate size available – these are usually included in the packaging when purchasing a new fitting.

Repairing a leaking threaded joint

This depends on the condition of the thread and whether it was originally sealed with PTFE tape or hemp fiber:

If the thread is leaking with PTFE tape

PTFE tape does not ensure a lasting seal if applied incorrectly or in insufficient quantity. Repair procedure:

Unscrew the compression fitting from the valve or boiler thread
Mechanically remove the old PTFE (soft cloth, scraper, never sandpaper on the thread!)
Wind new PTFE tape with at least 3–4 layers in a clockwise direction (i.e., in the tightening direction of the thread)
Screw the compression fitting back – for G 1/2" thread approximately 3–4 turns after contact, for G 3/4" thread approximately 3 turns

If you are connecting the fitting to a valve with an internal thread, also look at products such as straight transition 1/2" x 15 with internal thread or straight transition 3/4" x 22 with internal thread – these adapters will allow you to easily connect the pipe to a valve with an external thread without the need to improvise with adapters.

If the thread is leaking with hemp fiber

Hemp fiber, when applied correctly (in combination with linseed oil or special thread sealant), provides excellent long-term sealing. If it leaks, the hemp is usually too old and dry (after 10–15 years), or it was applied incorrectly. The procedure is the same as with Teflon tape – complete disassembly, cleaning the thread, and new application.

Important warning: never add Teflon on top of old hemp fiber. The combination of these two materials does not create a better seal – on the contrary, it can cause uneven seating of the thread.

Concrete scenarios from practice

Scenario 1: A new fitting leaks immediately after the first filling

This is a typical situation in DIY installation without experience. The customer bought a 3/4" x 15 coupling with a union to connect a new radiator valve, installed it, filled the system, and immediately noticed a dripping connection. The cause is usually one of two things: either the pipe was not cut straight and the olive did not seat evenly around the circumference, or the nut was tightened by hand without a wrench. Solution: drain the system, check the end of the pipe, install a new olive, and tighten properly with a wrench.

Scenario 2: A connection leaks after 3–5 years without an obvious cause

In most cases, the culprit is natural aging and thermal cycling. Each season brings hundreds of thermal cycles – the pipe expands and contracts, and the nut gradually loosens. I recommend checking all threaded connections during each annual boiler and system inspection. More on this topic can be found in the article Checking and Tightening Unions During Annual Heating Service. In this case, it is usually sufficient to slightly tighten the nut (max. 1/4 turn) without disassembling the joint.

Scenario 3: The coupling leaks at temperatures above 70 °C, but is dry with cold water

This is a tricky case of thermal expansion. Some brass fittings (especially cheaper variants) have smaller manufacturing tolerances and, at higher temperatures, the material expands, the olive loses compression, and the joint starts to leak. After cooling, the water evaporates or is absorbed by the insulation, and the customer comes with the issue that "it leaks sometimes, not always." Solution: replace it with a quality fitting with correct tolerances, and check whether the pipe is bent at an angle near the fitting (which creates lateral load on the olive due to thermal expansion).

Scenario 4: Leaking after repeated installation of the same joint

Customers who regularly disconnect radiators (for example, during painting) often encounter the issue that after the 3rd or 4th tightening of the same nut, the joint stops sealing. The reason is simple: the olive is a compression element for one-time use. With each disassembly, it loses part of its elasticity. By the third installation, the olive is usually so "fatigued" that even a new application of force is not enough. Solution: always keep a stock of spare olives of the appropriate size. For the 15 mm size, olives are standardized and available in packs of 10 pieces.

When is it necessary to replace the entire fitting

Not every fault can or is economically viable to fix with a repair. The entire fitting must be replaced in the following cases:

Visible crack or fracture in the body of the fitting – pressure in the system will widen the crack, sealing with thread sealant is only temporary and unreliable
Stripped thread – if the thread on the fitting or the nut cannot be properly screwed, the repair is not possible
Corrosion exceeding the surface – with green corrosion layers on copper, or white deposits from zinc corrosion on brass, the material is weakened internally
Fitting was deformed by pliers or an incorrect wrench – beveled edges of the nut are a sign that the body of the fitting may have been stressed internally
Age over 20–25 years – in such old installations, replacement is preventive and more economically advantageous than repeated patch repairs

If you are replacing in 3/4" distribution lines (for example, main supply lines to a floor heating manifold), check out a coupling with a union 3/4" x 22 for connecting to a 22 mm copper pipe. This is a standard size for main horizontal distribution lines in single-family homes.

Preventive measures: how to prevent leaks

In the long-term perspective, prevention is much cheaper than repeated repairs. Here are specific recommendations that stem from practice:

Never tighten fittings without sealing the thread – even during test installations. Applying Teflon later on a thread that was already tightened dry is less effective than the first installation with sealing.
Use a calibrated torque wrench – at least for the first installation in critical areas (boiler room, manifold). Simple torque wrenches for piping (e.g., adjustable from 10 to 50 Nm) are not expensive and significantly reduce the risk of over-tightening.
Check the pipe before installation – inspect each end of the pipe for ovality, cutting, and hole. Copper pipe is easily deformed into an oval cross-section when bent improperly.
Use thermally stable sealing when installing near the boiler – Teflon PTFE or professional anaerobic sealant resistant to 150 °C, not regular hemp fiber.
Inspect all visible joints annually – with a paper handkerchief on a cold and hot system. More details can be found in the article Checking and Tightening Unions During Annual Heating Service.
Distinguish between types of transitions – if you are unsure whether you need a coupling with a union or a straight transition, refer to the article Difference Between a Coupling with a Union and a Straight Transition, which compares these types in detail.

Frequently asked questions (FAQ)

Can I tighten a leaking coupling with a union without draining the system?

With a slight leak, it is technically possible – carefully, without over-tightening, by a maximum of 1/4 turn. Never do this under full pressure for more than one turn and never with hot water – the risk of scalding from steam or hot water exists. If the dripping continues after a slight tightening, drain the system and resolve the issue properly.

How many layers of Teflon tape are correct for a G 1/2" thread?

For a G 1/2" thread (a common size for a coupling with a 1/2" x 15 ferrule), it is standard to use 3 to 5 layers of PTFE tape with a width of 12–19 mm. For a G 3/4" thread, increase to 4–6 layers. The tape must be wound tightly, without air bubbles, always in a clockwise direction when viewed from the front of the thread (the same direction as tightening).

What if the thread is leaking, but the fitting is welded or soldered and cannot be unscrewed?

In this case, you can temporarily apply an anaerobic sealant based on acrylates (Loctite 290 for capillary joints) – this product, in a low-viscosity formulation, will penetrate the leak from the outside. This is, however, only a temporary solution. At the next opportunity (for example, during the summer system shutdown), the threaded joint must be properly disassembled and resealed.

Why can't I remove the ferrule from the pipe when I want to disassemble the fitting?

The ferrule plastically deforms after the first tightening and firmly binds to the pipe. This is an intentional design principle – the ferrule can only be removed by mechanical displacement (e.g., using a special ferrule remover, or carefully cutting along one side with a scalpel, taking care not to damage the pipe). Never try to rotate the ferrule or pull it back along the pipe with force – you will scratch the pipe and future installation will be leaky.

Is there a difference in the repair procedure for a 1/2" and 3/4" coupling?

The procedure is the same, only the dimensions (ferrules, nuts) and tightening torques differ. For a 3/4" size (e.g., a coupling with a 3/4" x 22 ferrule for 22 mm pipes), a slightly greater force is required – tighten appropriately using a longer wrench arm, not brute force. More about choosing the correct size can be found in the article Dimensions of ferrule couplings: how to correctly measure pipe diameter.

Can I use silicone instead of Teflon for repairing a threaded joint?

Silicone is not suitable for threaded seals in heating systems. It does not penetrate the thread sufficiently, it tears rather than compresses during tightening, and it loses elasticity faster than PTFE or anaerobic sealants at temperatures above 70–80 °C. Silicone is suitable only for external sealing, i.e., for sealing longitudinal gaps and transitions, not for pressurized threaded joints.

Conclusion: a systematic approach saves time and costs

A leaking ferrule coupling is a problem that can be repaired in most cases within 30–60 minutes with minimal material costs, provided the correct procedure is followed. The key is accurate diagnosis before any intervention – knowing whether the leak is at the compression point (ferrule), the threaded seal, or due to mechanical damage to the fitting. Each of these cases has a different procedure and different spare parts.

At the same time, the quality of installation during the first installation determines how often you will encounter these problems. A properly cut pipe, a centrally mounted ferrule, a precisely tightened nut, and a well-sealed thread – these are the four pillars of a trouble-free joint that can last for decades without repair. If you want to explore the topic in more depth, I also recommend other articles in the Knowledge Center: Step-by-step installation of a ferrule coupling on copper pipe, How to choose the right ferrule coupling for heating, and Internal vs. external thread: what type of transition do I need.

Do you have a question about this topic?

Struggling to decide or dealing with a specific situation in your home? Write to us – we are happy to help.