Why a elbow is leaking and how to fix it without replacing the entire joint

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Why an elbow is leaking and how to fix it without replacing the whole fitting

A leaking elbow is one of the most common issues I encounter during service visits to heating pipe systems. A customer calls to say that there is a leak at a joint somewhere behind the boiler, near a radiator, or in a floor heating manifold. The first impulse is to replace the whole elbow – to dismantle it, buy a new one, and go to the hardware store. In practice, however, most leaks have a rather mundane cause and can be eliminated without you having to reach for a wrench for an hour. In this article, I will explain why this actually happens, how to correctly diagnose the problem, and when a small intervention is sufficient instead of replacing the whole piece.

How sealing works in an elbow – the physical basis

Before we look at the faults, it is good to understand what actually keeps the joint sealed. Depending on the type of elbow and the pipe material, there are several different mechanisms involved.

Compression joints (press-fit) – in elbows for plastic pipe HEPWORTH (e.g., K – piece 15 x 15), the sealing is done by an O-ring made of EPDM rubber, which wraps around the outer diameter of the pipe when the pipe is inserted into the fitting and creates a pressure seal. The system is designed so that the higher the internal pressure of the medium, the more the O-ring is pressed against the wall.

Threaded joints – in elbows with a thread (e.g., Elbow with a nut 1/2" x 15), the sealing is ensured by a combination of metal threading and sealing compound or Teflon tape applied to the thread. The mechanism is different – pressure is created by tightening the thread, where the thread fibers press against each other and the sealing material fills the microscopic gaps between them.

Compression nuts (nutted joints) – here it is about the compression of the sealing between two seating surfaces when the nut is tightened. The sealing can be made of a softer metal, fibrous material, rubber, or a combination of materials.

Five most common reasons for a leaking elbow

Over the years of practice, I have encountered the same problem in various forms. Here are the causes, listed from the most common to the less common:

1. Improper insertion of the pipe (under-insertion)

This is by far the most common reason for failure of a push-fit joint. The installer (or a DIY enthusiast) inserts the pipe into the fitting, but not fully. The joint appears to be in place, the O-ring is slightly compressed, the system is pressurized – and after a few hours or days, it starts to leak. The problem is insidious precisely because during installation it "looks fine". A pipe inserted by 5–8 mm instead of the required 15–18 mm (depending on the diameter) is held mechanically, but the sealing is insufficient.

For a 15 mm diameter, the pipe must be inserted at least 17 mm, for 22 mm at least 22 mm – the manufacturer HEPWORTH specifies these values directly in the technical documentation and you usually find them also on the fitting packaging. In practice, I recommend always marking a line on the pipe with a marker at a distance corresponding to the insertion depth before inserting. After installation, the line is visible right at the edge of the fitting and you have a visual check.

2. Damaged or missing O-ring

O-rings made of EPDM rubber are very durable, but not indestructible. They can be damaged during improper installation (e.g., inserting the pipe with a skew without removing burrs – sharp edges can cut the rubber), or during long-term operation in an environment with aggressive water. Symptoms: it leaks directly at the fitting location, not at the thread or the nut.

O-rings are commonly available as spare parts and their replacement does not require replacing the whole fitting – it is enough to remove the pipe (if the construction allows), replace the O-ring, and put the pipe back in. However, if the fitting shows other damage (cracks, deformations), replacing the whole piece is appropriate.

3. Insufficient or incorrect sealing on the thread

Threaded joints require sealing material. The most common mistake is using too little Teflon tape, incorrect winding (in the wrong direction), or an unsuitable material for the given type of medium. In heating pipe systems, in addition to Teflon tape, flax string with paste (Hemp + Fermit) or liquid sealants (e.g., Loctite 577) are commonly used. Each of these materials has different applications and different temperature and pressure limitations.

With elbows with a nut, such as Elbow with a nut 1/2" x 15, it is important to remember that the threaded part of the joint (usually G 1/2" or G 3/4") is sealed with standard materials for threaded joints, while the push-fit side relies on the O-ring. A mistake occurs when the installer applies sealant to the push-fit side or, conversely, forgets to apply it to the threaded part.

4. Damage to the surface of the pipe at the sealing area

Every scratch, groove, or cut on the outer surface of the pipe in the area where the O-ring seats can cause a leak. This happens especially when the pipe is disassembled and reassembled multiple times (e.g., during repairs), or when during installation of the pipe in the wall, mechanical damage occurs. The surface of the pipe in the sealing area must be smooth and clean – ideally cut at a right angle (with pipe cutters, not a saw) and deburred.

5. Thermal movements and vibrations

Heating pipe systems operate in a temperature range typically of 20–80 °C (for condensing boilers) or up to 90 °C (for older radiator systems). Each cycle of heating and cooling means linear expansion of the pipe. Plastic pipes expand significantly more than copper or steel ones. If the piping is not properly anchored and expansion loops or compensators are not ensured, the movement is transferred to the fittings and the joints gradually loosen. The leak appears after a season or after the first thorough heating of the system.

Diagnosis: Where exactly is the leak?

The first step before any repair is to precisely determine where the source of the leak is. This may sound simple, but in practice it can be tricky – water can run along the pipe and drip at a completely different location than where the problem is. Here is a method that works:

Visual dry check: Wipe the entire joint dry with a cloth or paper towels. Then let the system remain under pressure (without turning off the pump) and observe. The first sign of dampness will reveal the source. If it drips from the edge of the fitting – it is likely a push-fit joint. If it drips from the thread – the problem is there.

Paper test: Wrap a white sheet of paper around the suspicious area and wait a few minutes. The moisture on the paper will clearly indicate where the leak is.

Insertion check: If you have access to the joint, place a ruler against the edge of the fitting and check whether the marking line (drawn during installation) is still visible. If the line is not visible, or if the tube moves (has play when pulled), the joint is not fully inserted.

Repair of a push-fit joint without replacing the fitting

The good news is that a HEPWORTH push-fit joint is designed to be disassembled and reassembled. Not indefinitely, but with careful handling, several times.

Disassembly procedure:

Drain the system and release the water from the relevant section (don't forget to close the shut-off valves).
Locate the release ring of the fitting (usually a white or gray plastic ring just at the edge of the fitting). Press it inward toward the fitting while pulling the tube outward.
If the tube is stuck, use special HEPWORTH release pliers – these tools are available at heating supply stores and will significantly ease the work without the risk of damaging the fitting.
After removing the tube, inspect the O-ring. If it has a cut, deformation or hardening (the rubber is not elastic when pressed), replace it.

Tube preparation for reassembly:

Cut the tube 2–3 cm before the damaged area. Use tube cutters, not a saw – the cut must be perpendicular.
Deburr the outer and inner edge of the tube with a deburring tool (1–2 seconds, just to remove the edge).
Measure the required insertion depth and draw a line with a marker.
Moisten the O-ring in the fitting with clean water – dry insertion can damage the rubber.
Insert the tube fully (until it stops at the stop) and check that the line is at the edge of the fitting.

Repair of a threaded joint without disassembling the fitting

This is an area where you can save a lot of work. If a threaded joint on an elbow is only slightly leaking, it is not necessary to replace it immediately. There are two proven methods:

Method 1 – Tighten the thread: If the sealing was applied correctly during installation, but the joint was only slightly under-tightened, a simple tightening may help. Hold the fitting firmly (possibly using a wrench) and tighten the threaded side by an additional sixth to quarter of a turn. For elbows of 1/2" or 3/4", this is usually sufficient. Caution: over-tightening can damage the thread or crack the bronze/brass body.

Method 2 – Liquid thread sealant under pressure: There are special anaerobic sealants on the market (e.g. Loctite 55, UNIPAK), which can be applied to the thread even if the system is still slightly wet and under mild pressure (up to about 2 bar). Apply the sealant through a small gap, tighten and wait the specified curing time. This solution is temporary or emergency – a full repair requires disassembly and a new sealing.

When replacement is necessary: If the thread is visibly damaged (cut threads, corrosion, crack in the body of the fitting), liquid sealant will not help. In this case, replacement of the elbow is necessary. For HEPWORTH elbows for plastic piping, such as K – piece 22 x 22 or Elbow 45° – 15 x 15, replacement of the fitting is quick (no soldering or welding is required) – it is enough to drain the section, remove the old fitting and insert a new one.

Special case: Reduced elbow and leakage at the transition of diameters

Reduced elbows connect two pipes of different diameters in one angle. This brings a specific risk: each side of the fitting has a different O-ring and different prescribed insertion depths. A fitter used to standard elbows sometimes automatically applies the same procedure to both sides – and the smaller side may be insufficiently inserted because it visually looks "fuller".

Another issue is mechanical: at a reduced elbow, the larger side is heavier (larger diameter = higher hydrostatic pressure on the joint), while the smaller side has a higher share of thermal expansion (thinner pipe expands relatively more). This means that both sides are stressed differently and, in case of insufficient pipe anchoring, the stresses concentrate precisely at this fitting.

For more detailed information on dimensioning at the transition between diameters, I recommend also reading the article Dimensioning elbows at the transition between different pipe diameters and Reduced elbow or reduction separately – what is more cost-effective in this Knowledge Centre.

Tightness after repair: How to perform a pressure test

After every repair (even a seemingly trivial one), I recommend performing a basic pressure test before covering the joint with drywall or insulation. It is not a complicated procedure:

Fill the repaired section with water and vent the air (open the vent valve if present).
Close the section valves and use a hand pump (or the system pump) to increase the pressure to 1.5 times the operating pressure – for a standard heating system (6 bar operating pressure), this would be around 9 bar, for a household system with 2.5 bar operating pressure around 4 bar.
Let the pressure act for 30 minutes. A pressure drop greater than 0.2 bar indicates a leak.
Visually inspect all repaired areas.

If you do not have a pressure pump, a simpler test will suffice: close the valves, let the system remain under normal operating pressure, and after 24 hours check the pressure gauge. A drop without another explanation = a leak somewhere in the system.

When it is worth replacing the entire elbow instead of repairing it

Not every leak is worth repairing. Here are the situations when replacing the entire fitting is clearly a better choice:

Crack in the body of the fitting: A visible or palpable crack in the plastic or brass body. No thread repair or O-ring replacement will solve this.
More than three previous disassemblies: Push-fit fittings are designed for repeated assembly, but each insertion and removal slightly wears the O-ring and the release mechanism. After three cycles, I recommend replacing the fitting preventively.
Corrosion of the brass thread: Green or white coating on the thread (zinc corrosion due to galvanic corrosion) indicates that the thread is weakened. Tightening in such a case can simply tear the thread.
Wrong angle or size of the fitting: If it turns out that the original elbow was installed with the wrong angle (90° where it should have been 45°) or the wrong diameter, repairing the leak is pointless – the fitting must be replaced anyway. For these cases, I recommend reading the article 45° or 90° elbow – when to use which type, which discusses angles in detail.

Replacing an HEPWORTH elbow, e.g., 45° Elbow – 22 x 22, is much faster compared to repairing a classic soldered copper system and does not require any special tools except for pipe cutters and a deburring tool. With the correct procedure, an experienced DIY enthusiast can manage the replacement.

Preventive maintenance: How to avoid leaks

The best repair is the one you don't have to do. A few practical recommendations from practice:

Use pipe cutters, not a saw: A sawed cut leaves burrs and the cut is not precise. Pipe cutters (e.g., for 15 and 22 mm diameter) make a cut in seconds and without burrs.
Always deburr: Even with a cut from cutters, a small "lip" may form on the inner edge. A deburring tool is a cheap tool and saves the O-ring.
Mark the insertion depth: Always draw a line – even if you are experienced, even if the joint is behind a wall and you cannot see it. The line is your guarantee.
Do not insert the pipe cold at low temperatures: Plastic pipe at temperatures below 5 °C is stiffer, harder to insert, and the O-ring is less flexible. If you are working in winter in an unheated boiler room, let the pipe and fittings warm up a bit (e.g., using a space heater).
Pipe anchoring: Always follow the prescribed spacing for supports (for HEPWORTH plastic pipe usually every 50–60 cm for smaller diameters). Insufficient anchoring is the silent killer of tightness.
Pressure test before covering: Never cover joints with drywall or insulation without a prior pressure test. Repairing a joint behind drywall costs five times more.

Frequently asked questions (FAQ)

Can I reuse a push-fit fitting after disassembly?

Yes, HEPWORTH push-fit fittings are designed for repeated assembly. Before reusing, always check the condition of the O-ring (flexibility, absence of cuts), the condition of the release ring, and the inside of the fitting. If the fitting is older than 10–15 years or has been disassembled multiple times, I recommend using a new one instead.

My joint is only dripping at the first start of heating for the season – is that normal?

It is not normal, but it is not a catastrophe either. It may be a micro-leak that becomes larger with expansion of the pipe and fittings (due to thermal expansion) when heated. Check all joints, tighten threaded connections, and verify the correct insertion of push-fit joints. If the leak disappears after heating and reappears every year, the problem is thermal expansion and the pipe routing anchoring needs to be improved.

Can I use silicone instead of Teflon tape on a thread?

No. Silicone is not suitable for sealing threaded connections at temperatures above 80 °C and is not resistant to hydrostatic pressure in a closed system. Use Teflon tape (PTFE), hemp rope with paste, or approved anaerobic sealing paste for heating distribution threaded connections. Silicone is used for sealing penetrations through building structures, not for metal threads.

How do I know that an O-ring is worn and needs to be replaced?

A worn O-ring is hard (when pressed between fingers, it does not return to its original shape), may be cracked, deformed, or have visible cuts. A new EPDM rubber O-ring is soft, elastic, and uniform around the entire circumference. If you have doubts, O-rings are cheap – preventive replacement is always cheaper than fixing a leak later.

Why is my connection leaking on a 22 mm pipe, but it was always fine on a 15 mm one?

A larger diameter means a larger contact area of the O-ring and higher hydrostatic pressure on the connection. Installation errors (e.g., not pushing the pipe in by 3 mm), which a 15 mm pipe O-ring can "compensate" for, will cause a leak on a 22 mm pipe. At the same time, it is physically more difficult to push a thicker pipe in fully – more force is needed, and sometimes slightly moistening the O-ring helps.

Where can I find replacement O-rings for HEPWORTH fittings?

Replacement O-rings are available at heating wholesale stores and from authorized HEPWORTH distributors. It is important to specify the exact diameter of the fitting (15, 22, 28 mm) and the type of connection. When ordering online, make sure it is the original EPDM material for potable water or heating media – cheap substitutes made from unsuitable rubber may fail after a few months.

Conclusion: Repair or replace – how to decide quickly

Most leaks in fittings you will encounter in practice have simple causes and solutions that do not require replacing the entire fitting. A pipe that is not fully pushed in, a worn O-ring, or insufficient sealing of the thread can be fixed in a few minutes with minimal costs. The key is correct diagnosis – identifying the exact location of the leak before you start disassembling anything.

Reserve the complete replacement of the fitting for cases where the body is damaged, corrosion is advanced, or the problem recurs despite repeated repairs. With the HEPWORTH system, replacing a fitting is quick and cost-effective – the availability of fittings in various sizes and angles (15 mm, 22 mm, 45° and 90° elbows, reduced variants) from the category of reduced elbows means you can always find the right replacement without long waiting times.

For a deeper understanding of installation procedures and choosing the right elbow, I also recommend other articles in this Knowledge Center: Step-by-step installation of an elbow with soldering on copper and plastic pipes, Sealing of the connection – most common problems when installing reduced elbows, or External thread vs. internal thread on an elbow – what difference does it make. Each of them addresses a different part of the issue, together giving a comprehensive picture of what every heating technician (even a DIY one) should know about HEPWORTH fittings.

Do you have a question about this topic?

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