Why Is the Heating Pipe Leaking or Cracking – Common Issues

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Why is the heating pipe leaking or cracking – common faults, causes and solutions

A water stain on the ceiling, a damp wall near the radiator, mysterious cracking at night or a sudden pressure drop in the system – these are situations many homeowners and apartment dwellers have experienced. Most people initially underestimate them. A small drop? Maybe just condensation. A crack? Perhaps the house is just "settling". The reality, however, is that faults in the heating pipe always have a specific cause, and the longer they are ignored, the more expensive the repairs and the greater the damage.

In this article, we will look at all the common reasons why a heating pipe is leaking or cracking. We will examine the physical causes, installation errors, material problems, as well as what you can identify yourself and what should be left to a professional. If you are interested in how to prevent problems from the start, also take a look at our other articles from the Knowledge Centre – for example Common mistakes in the installation of plastic heating pipes or Installation of HEPWORTH heating pipe step by step.

Basic overview: where and why leaks occur in heating systems

A heating distribution system is a closed pressure system. In standard apartment buildings, it operates at a working pressure of 1.5 to 2.5 bar, with tightness tested during hydraulic testing at 1.5 times the working pressure (typically 3.0 – 4.0 bar). Every leak means that the system's closed integrity has been compromised somewhere – mechanically, chemically or due to incorrect installation.

Leaks and faults most commonly occur at these locations:

Connections, fittings and elbows – the most risky parts of any distribution system
Places passing through walls or ceilings – temperature changes, vibrations
Closing valves, air vents and drain cocks
Direct pipe routing – due to long-term corrosion or mechanical damage
Connections to the boiler, buffer tank or manifold
Locations where repairs or interventions in the system have been carried out

Most common causes of leaking heating pipes

1. Improperly or incorrectly made connections and fittings

This is by far the most common cause of leaks we encounter in practice. A properly made connection – whether crimped, threaded or push-fit – must be carried out according to the manufacturer's technological procedure on clean, degreased and smooth pipe. Common errors include:

Under-crimped fitting: Crimp connections on plastic pipe must be crimped with a special tool (crimping pliers) for a specific diameter. If the pliers are set to a different diameter or the operator incorrectly positions the jaws, the fitting remains partially unsealed. A minor leak is not immediately visible – it appears only after months or under increased pressure.
Missing edge bevel or calibrated cut on the pipe: Plastic pipe must be cut straight and the inner edge bevelled before inserting into a fitting. If the cut is angled, the O-ring in the connection will not create a proper sealing surface under pressure.
Damaged O-ring: In push-fit fittings, the seal depends on the rubber O-ring. If it is damaged, misaligned or contaminated during installation, a leak is only a matter of time.
Threaded connection without sealing or with incorrect sealing: Metal threaded connections on plastic pipe require proper sealing – most commonly Teflon tape or liquid sealant. An insufficient or overfilled number of Teflon tape turns is equally problematic.

This applies to all dimensions – equally to HEPWORTH heating pipe 15 mm in apartment distribution systems, as well as to HEPWORTH heating pipe 22 mm on main branches or HEPWORTH heating pipe 28 mm in the boiler room.

2. Thermal expansion and insufficient compensation

Plastic heating pipe typically operates in a temperature range from 20 °C (room temperature when heating is off) to 70–90 °C (heating water temperature). Every material expands with temperature change. For polybutylene pipe (HEPWORTH), the linear expansion coefficient is approximately 1.3 × 10⁻⁴ m/(m·K). With a bit of math: a 10-metre pipe with a temperature increase of 60 °C will extend by:

ΔL = 10 m × 1.3 × 10⁻⁴ × 60 = 0.078 m = 7.8 cm

That nearly 8 cm cannot be ignored. If expansion joints (flexible loops, bends or expansion loops) are not installed along the route, this extension creates mechanical stress at points where the pipe is fixed. The result is either cracks in the pipe or leaks at the connection, where the plastic pipe "pops out" of the fitting.

In practice, this problem is mainly manifested in long horizontal runs along walls or ceilings without a sufficient number of sliding supports and without compensating loops. Proper pipe routing includes fixed (anchoring) points and sliding supports between them, which allow for movement. For more information on this issue, see the article Common mistakes in the installation of plastic heating pipes.

3. Water hammer and pressure surges in the system

Water hammer (hydraulic shock) is a phenomenon in which the sudden closure of a valve or the rapid stop of water circulation causes a pressure surge in the pipe. This impulse can momentarily multiply the operating pressure several times. In modern thermostatically controlled systems, this problem occurs with incorrectly dimensioned balancing valves, fast-acting thermostatic heads without an opposing balancing loop, or when the circulation pump is started too quickly.

Plastic pipes are generally less sensitive to hydraulic shock than copper ones, as their walls have a certain elasticity that dampens the pressure surge. Nevertheless, even plastic pipes have their limits – if the surges act repeatedly in the same place (typically at a fitting or valve), they can cause material fatigue and eventually lead to a rupture.

4. Corrosion and chemical degradation of material

Plastic pipes are resistant to corrosion of the plastic itself. However, the heating system is not just plastic – it includes metal radiators, boiler, circulation pump, and fittings. The water in the heating system contains oxygen, salts, and other substances that can attack metal parts. The result is the formation of sludge, iron oxides, and other deposits. This sludge settles in the pipes and can:

Increase hydraulic resistance and thus change pressure conditions in the system
Attack metal fittings or valves and cause corrosion
At higher concentrations, cause mechanical damage to the circulation pump
Directly affect rubber O-rings in fittings and cause their degradation

Along with corrosion, the problem can also be caused by unsuitable water chemistry. Water that is too hard causes limescale deposits, while soft aggressive water attacks metal parts. Heating systems should have treated water with a pH in the range of 7.5 – 9 and a total hardness of up to 2.0 mmol/l. These values are adjusted according to the EN 14868 standard, and most boiler manufacturers list them directly in the warranty conditions.

5. Frost and freezing of water in the pipes

Freezing of water in the pipes belongs to the most destructive causes of malfunctions. Water expands by approximately 9 % when it freezes, which in a closed system creates pressure capable of bursting any commonly used pipe. This typically happens:

In unheated recreational buildings where the system was not drained before winter
In peripheral parts of the distribution that pass through unheated areas (garage, basement, attic)
During heating failure during strong frosts
On radiator connections that are closed to a minimum by a thermostat

After thawing, the fault becomes immediately apparent – a crack in the pipe or a pushed-out fitting will leak water without warning. Therefore, in recreational buildings, it is essential to decide: either drain the system and fill it with an antifreeze mixture, or ensure year-round minimum heating.

6. Mechanical damage – drilling, nails, clamps

One of the most common causes of leaks in the interior of apartments and houses, which is often overlooked: accidental damage to the pipe during minor construction work. When someone comes to hang a picture and does not know where the pipe is running behind the wall, a screw or drill can penetrate directly into the pipe. This type of fault may not become apparent immediately – if the hole is small and the pressure does not force the water out right away, the first symptoms may appear only after a few days or weeks, when the moisture becomes noticeable.

Equally problematic are metal clamps, which are sometimes used to secure cables and are mounted directly on plastic pipes. The edge of the metal clamp in combination with thermal expansion can gradually cut into the pipe wall.

We recommend always – before any drilling work in a room with embedded piping – to verify the location of the pipes using a pipe detector. Especially in older renovations, where the actual layout of the piping may not match the original plans.

Cracking of the pipe – it is not always a leak

Cracking is most often heard at night or in the morning, when the boiler turns on or off. People associate this with a fault, but most of these sounds are quite normal and have nothing to do with a water leak. Nevertheless, sound manifestations should be correctly interpreted.

Normal sounds vs. signs of a fault

Normal cracking and knocking occurs due to thermal expansion of the pipe in the supports. The pipe heats up and elongates – if it passes through a tight hole in the wall or is fixed in a rigid metal clamp, it produces a cracking or knocking sound. This is annoying, but not dangerous – provided it is properly compensated. Solution: replace rigid metal supports with soft plastic ones with a sliding washer, or enlarge the diameter of the wall hole and fill it with a flexible material.

Signs of a real fault are different:

Loud hammering or knocking in the pipe (hydraulic shock)
Hissing sound near a fitting or valve (leak under pressure)
Cracking at a location where the pipe passes through a structural element, accompanied by wet stains
Bubbling or gurgling in the radiator – a sign of air release or incorrect pipe slope

Air in the system – problems with air entrapment

Air in the heating system causes characteristic bubbling or gurgling sounds in radiators, cold spots despite warmth in the rest of the system, and unstable circulation. Air enters the system during the first filling, during repairs or water top-ups, or diffuses through unsuitable plastic pipes (without an oxygen barrier). The solution is to bleed the system using air vents on radiators and automatic air vents at the highest points of the piping layout.

Leaks in specific types of installation

Pipes embedded in the floor (underfloor heating and under plaster)

Hidden piping is the most critical in terms of faults – when a leak appears, water always finds a way and can affect the building structure for a long time before anyone notices. Typical causes of hidden leaks in the floor are: improperly made joints before concrete pouring, insufficient pressure testing before pouring (recommended minimum test of 6 bar for 60 minutes in the cold state), or mechanical damage during concrete pouring (e.g. from heavy machinery).

Therefore, performing a hydraulic pressure test BEFORE covering the pipes is an absolute requirement for any installation of piping in the floor or under plaster. This applies without exception and is also a legal obligation according to the design documentation and STN standards for heating.

Wall-mounted piping (visible layouts)

Visible piping is easier to access for repairs, but has other issues: improper fastening (too few clamps or, conversely, too tight), exposure to mechanical impacts, or improper compensation for thermal expansion in long runs. The recommended distance between supports for plastic piping at temperatures up to 70 °C is:

DN 15 (outer ⌀ 15 mm): max. 500 mm between supports
DN 22 (outer ⌀ 22 mm): max. 700 mm between supports
DN 28 (outer ⌀ 28 mm): max. 900 mm between supports

At temperatures above 70 °C or for horizontal runs, it is advisable to reduce these distances by 20–30 %. More technical details on sizing piping layouts can be found in the article What pipe diameter do I need for heating: 15, 22 or 28 mm.

Pipes in reels versus sticks – are they related to faults?

Many people ask whether the delivery form of the pipe affects the rate of faults. The answer is partial: HEPWORTH heating pipes in sticks 15 mm or HEPWORTH heating pipes in sticks 22 mm are delivered in straight sticks of usually 3 or 5 meters in length. They have a lower risk of so-called "memory of shaping" – i.e. the tendency to curl into an arc, which occurs with pipes from reels. If an installer incorrectly straightens a pipe from a reel and fixes it in a stressed state, this shape memory can cause unexpected movements and leaks at joints during thermal expansion.

In practice, this means that for shorter runs (e.g. radiator connections, risers), sticks are more suitable and reduce the risk of installation errors. For long runs, reels are more suitable as they reduce the number of joints along the route. We dedicate an entire separate article to this topic – HEPWORTH pipes in reels vs. in sticks – what to choose.

Material aging and long-term operation

Polybutylene (PB)-based plastic piping, as used in the HEPWORTH system, has a designed service life of 50 years or more under proper operating conditions. Nevertheless, there are factors that shorten this lifespan:

Continuous temperature exceedance: If the system operates continuously above 80 °C (e.g. with old boilers without regulation), the PB material ages faster. Every additional 10 °C above the designed temperature exponentially increases the aging rate of the polymer – this follows the Arrhenius law for polymer materials.
UV radiation: If plastic piping is exposed to prolonged direct sunlight (e.g. in external runs or in glazed winter gardens), UV degrades the polymer and makes the pipe brittle. PB heating pipes are not UV stabilized and must be protected.
Incorrect water chemistry: Aggressive disinfectants, high chlorine content or unsuitable corrosion inhibitors can interact with the polymer or with elastomeric O-rings.
Repeated freezing cycles: Systems that are started every spring and drained every autumn (e.g. in recreational buildings) are stressed much more than systems in continuous operation. Each cycle represents mechanical stress for the fittings.

More about proper maintenance and expected lifespan can be read in the article HEPWORTH heating pipes – maintenance and lifespan.

How to correctly diagnose the location and cause of a fault

When you notice a pressure drop in the system, a wet stain or other symptoms, the first step is always a calm and systematic diagnosis – not panic or immediately breaking walls open.

1. Monitor the pressure gauge. Most modern boilers have a pressure gauge on the front panel. The operating pressure in the heating system should be around 1.0 – 1.5 bar in the cold state (boiler off). If it regularly drops and you have to refill the system, it is clear that there is a leak somewhere. If the pressure does not drop, the problem is not in the pipe tightness – it could be in the expansion tank or the safety valve.

2. Check the safety valve. The safety valve occasionally "opens and does not close" – it drips continuously and it seems that the system is not hermetic. This is not a pipe fault, but a valve fault or an improperly set operating pressure.

3. Visual inspection of all visible piping and fittings. With a full system and operating temperatures, walk the entire piping route and all fittings. Look for wet spots, white corrosion on metal fittings, damp deposits, or a crystalline white coating (limescale after water evaporation) around the joints.

4. Pressure test. If the source of the leak is not visible, it is time for a pressure test. The system is filled with cold water, bled of air, and using a hand pump the pressure is increased to 1.5 times the operating pressure. The pressure should remain stable for at least 30 minutes. If it drops, monitor the pressure gauge and after some time inspect all accessible connections.

5. Thermal imaging camera and acoustic detectors. For concealed piping (behind plaster, in the floor) a thermal imaging camera is used in professional practice – leaking hot water heats the surrounding structure and the thermal camera displays this as a thermal gradient. An acoustic detector will pick up the sound of flowing water through the structure. These methods allow locating the fault without demolition.

Repair versus replacement – when a patch is not enough

Once the location of the fault is identified, the question arises: repair or replace? For plastic piping the following rules apply:

Local failure of a fitting: The fitting is replaced with a new one. If the fitting was crimped, it is necessary to shorten the pipe, re-flare the edge and crimp on a new one. Patches or adhesives are not approved solutions for pressure heating systems.
Crack in the pipe: The affected section of the pipe is cut out and replaced with a new piece of pipe with two fittings (couplings). A patch with an aluminum tape is only a temporary emergency solution, never a permanent one.
Extensive corrosion or degradation: If the piping system is old (20+ years), the pipes are brittle, the fittings are corroded or the system is full of sludge – consider a complete replacement of the piping. Partial repairs on a degraded system are often more expensive and temporary.
Repeated failures at the same location: If a failure occurs repeatedly at the same location, it means either a systemic cause (unsuitable material, design flaw) or an incorrect first repair. It is necessary to identify the cause, not just apply a patch.

Preventive measures – how to prevent failures

Prevention is always cheaper than repair. Here are specific steps that significantly reduce the risk of failures:

Always require and perform a pressure test after installation and before concealing the piping
Use only certified fittings and valves suitable for the pipe material and for the given temperature and pressure
Regularly (once a year before the heating season) check the pressure in the system and visible piping
Maintain the correct pH and water hardness in the heating system
Install a sludge separator (magnetic filter) to protect the pump and system
Never drill into walls without verifying the location of the pipes
For holiday properties, address winter shutdown of the system (draining or using antifreeze)
Do not tamper with the system yourself if you lack the necessary knowledge and tools

Most frequently asked questions (FAQ)

What should I do if the pressure in the boiler drops rapidly and I cannot find the leak location?

First, check the safety valve and expansion tank – these are the most common causes of apparent "leaks" without a visible location. If they are in order, bleed the system, top up to the correct pressure and monitor the pressure gauge over several days. If the pressure repeatedly drops without a visible cause, call a professional with a thermal imaging camera. Never be satisfied with repeated topping up without diagnosing the cause – water with dissolved substances entering the system with each topping up accelerates corrosion and sludge formation.

Plastic piping cracks at night – is it dangerous?

Most nighttime cracking sounds from plastic piping are due to thermal expansion in brackets and transitions – it is not dangerous, but indicates improper support or missing sliding pads. It would be dangerous if the cracking were accompanied by an odor, moisture or a sudden worsening of the sound (indicating a crack). In that case, shut down the system immediately and call a professional.

Can I repair plastic heating piping with epoxy sealant or self-vulcanizing tape?

No, definitely not as a permanent solution for a pressurized heating system. These materials are not certified for permanent repair of piping under pressure and at temperatures above 60 °C. Epoxy or tape may temporarily stop visible dripping, but the location remains weakened and may fail at any time – potentially causing greater damage than before the "repair". A proper repair means replacing the damaged section of the pipe or fitting.

How long will HEPWORTH plastic piping last in heating without replacement?

Under adherence to the designed operating conditions (maximum temperature 80 °C, maximum pressure 6 bar, proper water chemistry, without UV exposure) the declared lifespan of HEPWORTH polybutylene pipe is 50 years. In practice, systems from the 80s and 90s are still functioning without problems if properly installed and maintained. Shortened lifespan is mainly caused by prolonged elevated temperatures, aggressive water and mechanical damage.

When is it necessary to replace the entire piping and when is a local repair sufficient?

A local repair is sufficient if the failure is clearly localized to a specific fitting or short section of pipe, the rest of the system is in good condition and the system is not older than 25–30 years without a history of chemical problems. Consider a complete replacement if the system shows multiple failures at once or within a short period, the pipes are brittle or discolored, visible sludge or rust coloring is present in the water, or the system is undergoing a complete renovation of the property.

Can piping crack due to excessively high water pH?

Direct cracking of polybutylene pipe due to excessive pH is not typical, but strongly alkaline water (pH above 10) can degrade rubber O-rings and seals in fittings and valves. The result are leaks at the connections. In addition, if corrosion inhibitors not suitable for the material are added to the system, they may chemically interact with the polymers. Always use corrosion inhibitors certified for plastic piping and keep pH within the recommended range of 7.5 – 9.

Conclusion

Failures in heating piping are always the result of a specific cause – not a coincidence. Whether it is thermal expansion without compensation, an improperly made connection, mechanical damage or a frozen system, each cause has a solution. The key is quick diagnosis, professional repair and, above all, prevention through proper installation and regular system inspection.

If you are planning a new installation or replacement of piping, pay attention to the selection of high-quality piping from proven manufacturers. The range of HEPWORTH heating piping in various dimensions and delivery forms can be found directly in our category. Before making a decision, we also recommend reading the article How to choose the right HEPWORTH piping for heating and Difference between HEPWORTH and HEP2O piping for heating – both will help you choose the right material for your specific system.

Do you have a question about this topic?

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