Pipes Knocking When Water Is Turned On: Complete Guide
Stop the Knocking: Expert Solutions for Noisy Pipes
That alarming knocking sound when you turn on taps isn’t just annoying—it signals a plumbing issue that needs attention. As certified plumbing specialists, we’ll explain why pipes knock and provide proven solutions to restore peace to your home.
Why Do Pipes Knock When Water Is Turned On?
Knocking pipes, professionally known as “water hammer,” occurs when water flow suddenly stops or changes direction. Our experts can fix hidden plumbing leaks and diagnose this issue daily across UK homes.
- Water hammer: Sudden valve closure creates pressure waves
- Loose pipe clips: Pipes move and hit walls or joists
- High water pressure: Exceeding 4 bar stresses the system
- Air in pipes: Creates irregular water flow and noise
- Worn valve washers: Cause vibration and knocking
- Thermal expansion: Hot water pipes expand and contract
- Faulty fill valves: In toilets and appliances
Understanding which type of knocking you’re experiencing helps determine the solution. Different sounds indicate different problems.
Types of Pipe Knocking Sounds
Single Loud Bang
When: Turning tap off quickly
Cause: Classic water hammer
Location: Near the closed valve
Solution: Install arrestors
Rhythmic Tapping
When: Hot water running
Cause: Thermal expansion
Location: Along pipe runs
Solution: Secure pipes properly
Rapid Machine-Gun Sound
When: Using specific taps
Cause: Worn washers vibrating
Location: At the tap
Solution: Replace washers
Gurgling with Knocks
When: Multiple taps used
Cause: Air in system
Location: Throughout pipes
Solution: Bleed the system
Water Hammer Explained
Water hammer—the most common cause of knocking pipes—occurs when fast-moving water suddenly stops. Here’s the physics:
- Water flows through pipes at 1-2 metres per second
- Closing a tap instantly stops this flow
- Water’s momentum creates a pressure surge
- This surge travels back through pipes as a shockwave
- Pipes knock as the shockwave hits bends, valves, and fixtures
Why Pipes Knock When Hot Water Is Turned On
Hot water pipe knocking has specific causes beyond standard water hammer. This differs from issues like constantly running toilets which involve different mechanisms:
Thermal Expansion Issues
- Copper pipes expand 1mm per metre when heated
- Restricted pipes buckle and knock against surfaces
- Common in long straight runs without expansion joints
- Worse in properties requiring heating system leak detection
Scale Build-up
- Limescale narrows pipes, increasing water velocity
- Creates turbulence and noise
- Common in hard water areas
- Can indicate need for system cleaning
- May also cause water meter issues
If you’re also experiencing boiler pressure dropping, the issues may be related.
DIY Solutions for Knocking Pipes
Many pipe knocking issues can be resolved without professional help. Try these solutions first:
1. Secure Loose Pipes
Steps:
- Locate knocking areas by having someone turn taps on/off
- Check all visible pipes for movement
- Install clips every 1.2m for horizontal pipes
- Use 2m spacing for vertical pipes
- Add foam padding between pipes and clips
- Leave room for thermal expansion on hot pipes
2. Reduce Water Pressure
High pressure exacerbates knocking. UK homes should maintain 3-4 bar pressure:
- Locate your pressure reducing valve (PRV) near the stopcock
- Turn adjustment screw anticlockwise to reduce pressure
- Check with a pressure gauge (£10-£15 from DIY stores)
- If no PRV exists, consider professional installation
For more on pressure issues, see our guide on water pressure problems.
3. Install Water Hammer Arrestors
These devices absorb pressure surges:
- Cost £15-£40 each from plumbing suppliers
- Install near washing machines, dishwashers, and quick-closing valves
- Screw directly onto appliance connections
- No special tools required for most installations
4. Bleed Air from System
- Turn off water supply at mains
- Open all taps starting from ground floor
- Work up to highest taps
- Turn mains back on slowly
- Close taps in reverse order once water runs smoothly
- Repeat if gurgling persists
Professional Solutions for Persistent Knocking
When DIY methods fail, professional plumbers provide advanced solutions:
Pressure Reducing Valve Installation
- Regulates incoming mains pressure
- Protects entire plumbing system
- Reduces water hammer throughout property
- Includes pressure gauge for monitoring
Expansion Vessel Fitting
- Absorbs thermal expansion in hot water systems
- Prevents knocking from temperature changes
- Essential for unvented cylinders
- Requires annual maintenance checks
Pipe Reconfiguration
- Adding expansion loops to long runs
- Installing flexible connectors at key points
- Rerouting pipes to eliminate problem areas
- Upgrading old pipework to modern standards
- Essential for leaking underfloor pipes and complex systems
Professional Pipe Knocking Solutions
Don’t let noisy pipes disrupt your home. Our certified plumbers diagnose and fix knocking pipes quickly, preventing damage and restoring peace.
Call 07897 020 680 or +44 1440 840 772
Email: hello@completeleakdetection.co.uk
Cost of Fixing Knocking Pipes UK
Understanding repair costs helps you budget effectively:
| Service | Typical Cost | Time Required |
|---|---|---|
| Diagnostic inspection | £80-£150 | 30-60 minutes |
| Installing pipe clips | £150-£250 | 1-2 hours |
| Water hammer arrestor fitting | £200-£350 | 1-2 hours |
| PRV installation | £350-£500 | 2-3 hours |
| Expansion vessel installation | £400-£600 | 2-4 hours |
| Complete pipe reconfiguration | £800-£1,500 | 1-2 days |
| Thermal imaging leak detection | £350-£550 | 1-2 hours |
| Emergency callout | +£50-£100 | Same timescales |
These prices include labour and materials but may vary based on location and system complexity.
Preventing Pipe Knocking
Prevention is always better than cure. Follow these maintenance tips:
Regular System Checks
- Monitor water pressure monthly with a gauge
- Check pipe clips remain secure annually
- Listen for new noises developing
- Service PRVs and expansion vessels yearly
- Replace tap washers at first sign of wear
- Maintain correct pressure (3-4 bar)
- Be aware of condensation issues that may mask leaks
Upgrade Considerations
- Install soft-close taps in new bathrooms
- Choose appliances with built-in arrestors
- Upgrade old gate valves to modern ball valves
- Consider services to fix hidden bathroom leaks during renovations
Specific Scenarios: Knocking in Different Situations
Knocking When Shower Is On
Shower-specific knocking often indicates:
- Loose shower valve behind the wall
- Thermal expansion in hot water pipes
- Pressure imbalance between hot and cold supplies
- Worn shower cartridge creating vibration
Note: This differs from toilet water running issues which involve fill valve problems.
Banging When Cold Water Is Turned On
Cold water knocking suggests:
- Higher mains pressure (cold comes direct from mains)
- Quick-closing valves need arrestors
- Possible underground pipe issues
- May require professional buried pipe leak detection
Knocking When Tap Is Turned On
Tap-specific issues include:
- Worn tap washers or ceramic discs
- Loose tap body needs securing
- Air trapped in tap mechanism
- Scale build-up in tap internals
- May require professional leak detection using audio methods to locate exact source
When Knocking Indicates Serious Problems
Some knocking patterns signal urgent issues requiring immediate attention:
- Knocking accompanied by visible pipe movement
- Water leaks appear after knocking starts
- Walls or ceilings show damp patches
- Knocking gets progressively louder
- Multiple knocking locations develop suddenly
- You notice issues with your heating system simultaneously
These symptoms may indicate imminent pipe failure. Our infrared inspection for leaks can identify hidden damage before catastrophic failure. This is especially important in concrete slabs—see our slab leak detection guide for specific risks.
The Science Behind Different Pipe Materials
Pipe material affects knocking characteristics:
Copper Pipes
- Excellent sound conductor—knocks travel far
- Significant thermal expansion
- Require proper support spacing
- Common in UK homes built 1960-2000
Plastic Pipes (MDPE/Polybutylene)
- Better sound dampening than copper
- More flexible, less prone to water hammer
- Still require proper clips and support
- Standard in modern installations
Lead Pipes (Older Properties)
- Heavy, less prone to movement knocking
- May indicate need for pipe replacement
- Health concerns override noise issues
- Require professionals to fix water main leaks for safety
Smart Home Solutions
Modern technology offers advanced monitoring:
- Smart water valves: Gradual closing prevents hammer
- Pressure monitors: Alert to dangerous spikes
- Leak detection systems: Identify problems early with water leak tracer technology
- Automatic shut-offs: Prevent damage from failures
These systems particularly benefit properties requiring leak inspection for commercial sites where downtime costs are high.
Insurance and Knocking Pipes
Understanding insurance implications:
- Gradual damage from knocking rarely covered
- Sudden pipe failure may be covered
- Document knocking issues when they start
- Preventive maintenance shows due diligence
- Keep receipts for all plumbing work
For detailed coverage information, see our insurance guide.
Regional Considerations
Hard Water Areas
London, Southeast, and East Anglia face additional challenges:
- Limescale accelerates wear on valves
- Narrowed pipes increase water velocity
- More frequent washer replacement needed
- Consider water softeners for prevention
High Pressure Areas
Properties near pumping stations or on hills may experience:
- Mains pressure exceeding 5 bar
- Increased water hammer risk
- Essential need for PRV installation
- More robust arrestors required
If you’ve received communication about water issues, see our guides on Anglian Water notifications or Thames Water notifications.
Conclusion
Knocking pipes when water is turned on ranges from minor annoyance to serious plumbing issue. Understanding the causes—from simple loose clips to complex water hammer—empowers you to choose appropriate solutions. While many fixes are DIY-friendly, persistent or severe knocking requires professional assessment to prevent costly damage.
Act promptly when knocking starts. Early intervention prevents pipe joint damage, extends system life, and maintains your home’s peace. Whether securing a loose pipe or installing pressure-reducing valves, addressing the root cause ensures long-term resolution.
- Water hammer is the primary cause of pipe knocking
- Secure pipes and reduce pressure for quick fixes
- Hot water knocking often indicates thermal expansion
- Professional solutions prevent recurring problems
- Early action prevents expensive damage
For expert diagnosis and repair of knocking pipes, contact our complete leak detection service. Our certified plumbers provide fast, effective solutions across the UK, ensuring your plumbing system operates quietly and efficiently.
Call 07897 020 680 or +44 1440 840 772 for immediate assistance.
Frequently Asked Questions
Why do my pipes knock when I turn the water on?
Pipes knock due to water hammer—a pressure surge when fast-flowing water suddenly stops. Common causes include loose pipes, high water pressure, faulty valves, or air in the system. The knocking sound occurs when pipes move or water pressure waves hit closed valves.
How do I stop my pipes from knocking?
Install water hammer arrestors, secure loose pipes with clips, reduce water pressure to 3-4 bar, replace worn tap washers, and bleed air from the system. For persistent knocking, professional plumbers can install pressure-reducing valves or expansion vessels.
Is pipe knocking dangerous?
While not immediately dangerous, persistent knocking can damage pipe joints, cause leaks, and stress your plumbing system. Address it promptly to prevent costly repairs. Severe water hammer can burst pipes or damage appliances like washing machines.
How much does it cost to fix knocking pipes in the UK?
DIY fixes cost £10-£50 for clips and arrestors. Professional repairs range from £150-£500 for basic fixes to £500-£1000 for pressure-reducing valve installation. Emergency callouts add £50-£100. Most plumbers charge £40-£100 per hour.
Why do pipes only knock with hot water?
Hot water pipe knocking often indicates thermal expansion issues. As pipes heat up, they expand and move against clips or joists. This is common in older properties with copper pipes. Installing expansion loops or flexible connectors usually resolves this.