Noisy Pipes: Water Hammer, Air Locks, Expansion & Flow Rate Issues

Summary

Pipe noise is one of the most common complaints in domestic plumbing, and one of the most misdiagnosed. The noise type — bang, tick, whistle, rattle, or gurgle — directly corresponds to the underlying cause, and correct diagnosis avoids wasted time replacing the wrong components. Most pipe noise problems have inexpensive solutions, but some (particularly severe water hammer in pressurised systems) can cause joint failures and pipe damage if left unresolved.

The UK's shift from low-pressure gravity-fed hot water systems to mains-pressure unvented cylinders and combi boilers has increased the prevalence of water hammer. These systems operate at 1–3 bar; even a small sudden valve closure sends a significant pressure wave through the pipework. Older properties with gravity-fed cold water tanks had much lower operating pressures (typically 0.3–1 bar), which generated correspondingly smaller pressure waves.

Understanding the distinction between noise during flow and noise on flow stoppage is the key diagnostic first step. Noise only during flow (whistling, vibrating) points to a flow restriction or high velocity issue. Noise only on valve closure (banging) is water hammer. Noise during hot water use only (ticking, clicking) is thermal expansion — always present to some degree, only a problem when excessive. Noise in cold water only (gurgling, intermittent) suggests air lock.

Key Facts

• Water hammer pressure — the Joukowsky equation shows the pressure surge from sudden valve closure = ρcv, where ρ is density, c is wave speed (~1000 m/s in copper), and v is flow velocity; at 1 m/s flow, a sudden closure creates ~10 bar pressure surge
• Wave speed in pipes — approximately 1000–1400 m/s in copper, 300–600 m/s in plastic; plastic pipe attenuates water hammer better than copper
• Hydraulic shock arrestors — piston-type devices absorbing the pressure wave; installed at the end of pipe runs near quick-closing valves
• Air locks — air trapped in pipework causes intermittent flow, gurgling, and spitting from taps; more common in gravity-fed systems; rarely occur in mains-pressure combi systems
• Thermal expansion noise — 1°C temperature rise expands 1 metre of copper by 0.017mm; a 50°C rise over 3m of pipe = 2.5mm of movement; this movement at pipe clips causes ticking
• Loose pipe clips — the most common cause of rattle and vibration; pipes should be clipped at 1.2m centres for 15mm copper, 1.8m for 22mm copper (BS EN 806-4 guidance)
• High velocity whistle — water velocity above approximately 1.5 m/s in 15mm pipe can cause noise; reduce velocity by increasing pipe size or reducing flow rate
• Worn tap washer causing vibration — a worn or partially dislodged washer can flutter in the flow, causing a vibrating hum; the noise changes when you adjust the flow
• PRV (Pressure Reducing Valve) — if mains pressure exceeds 3 bar, a PRV reduces supply pressure and significantly reduces water hammer; recommended where mains pressure exceeds 3 bar
• Expansion noise in central heating — normal during heat-up and cool-down; loud cracking in suspended timber floors often indicates pipes touching joists without adequate insulation at clip points
• Air in central heating — air in radiators causes gurgling and cold spots at top; bleed radiators at the start of each heating season
• Boiler pump cavitation — a high-pitched whine or screech from the boiler/pump area may indicate pump cavitation from insufficient system pressure or a blocked pump

Quick Reference Table

Detailed Guidance

Decision Tree

NOISY PIPES — START HERE
        |
        v
WHEN does the noise occur?
        |
    ----+----+-----+------
    |        |     |      |
    v        v     v      v
FLOW       VALVE  HOT    HEATING
only      CLOSURE water   system
    |        |    only    only
    v        |      |      |
Whistle/  BANG    Tick/   Gurgle/
Vibrate          Click   Knock
    |        |      |      |
    v        v      v      v
Go to    WATER   THERMAL  Go to
Flow     HAMMER  EXPAN.   Heating
Section  Section Section  Section

========================
WATER HAMMER
========================
        |
        v
Which appliance causes it?
        |
    ----+--------
    |            |
    v            v
Tap         Washing machine /
closure     dishwasher solenoid
    |            |
    v            v
Slow-closing  Shock arrestor
tap or        on hot/cold
shock         inlet to
arrestor      appliance

========================
FLOW NOISE
========================
        |
        v
Whistle or vibrate?
        |
    ----+-------
    |           |
    v           v
Whistle:    Vibrate/Hum:
Check all   Identify
valves on   which tap
circuit     by opening
-- fully    each slightly
open?       |
    |       v
    v   Worn washer
Upsize  or loose
or PRV  component
        -- repair

========================
AIR LOCK
========================
        |
        v
Cold water system (gravity)?
        |
    YES | NO
        |  \
        v   v
Reverse   Mains pressure
pressure   system -- air
purge:     locks rare --
connect    check for
hosepipe   system leak
cold to
hot, then
open both

Water Hammer — Causes and Cures

Water hammer is the pressure wave created when flowing water is suddenly stopped. The physics are straightforward: a column of water moving at velocity v suddenly brought to rest by a closing valve creates a pressure surge. This surge travels back along the pipe at the speed of sound in that material (~1000 m/s in copper), bouncing between the valve and the first open vessel (cistern, expansion vessel) until it dissipates.

The practical consequences range from an annoying bang to, in severe cases, joint failure, valve seat damage, and pipe fatigue. Modern quick-closing solenoid valves (washing machines, dishwashers, auto fill valves) are the most common cause — they close in milliseconds, generating maximum hammer.

Cures in order of preference:

1. Slow-closing valves — quarter-turn ball valves are actually faster-closing than compression taps; ceramic disc taps with smooth action close faster than old washered taps. For appliance inlets, use solenoid valves with built-in anti-hammer feature.

2. Hydraulic shock arrestor — a piston-type device (e.g., Honeywell DA) installed at the end of the pipe run closest to the problem valve. The piston absorbs the pressure surge. Install on both hot and cold supplies to the appliance. Size by pipe diameter (15mm or 22mm) and system pressure.

3. Mini expansion vessel — a small diaphragm vessel (0.3–0.5 litre) installed at the end of the pipe run provides similar function to a shock arrestor. Useful where space is limited.

4. Pressure reducing valve (PRV) — if mains pressure exceeds 3 bar, a PRV reduces operating pressure and significantly reduces hammer severity. Install after the stopcock. Most PRVs are pre-set to 3 bar; adjustable models allow fine tuning.

5. Adding cushion/air chamber — a capped pipe stub (dead-leg) filled with air provides a compressible cushion. This works temporarily but the air is eventually absorbed into the water. Modern shock arrestors are preferable.

Thermal Expansion Noise in Hot Water Systems

Thermal expansion is unavoidable — all materials expand when heated. In copper pipework, the coefficient of linear thermal expansion is 0.017mm/m/°C. A 50°C temperature rise (cold supply at 10°C, hot at 60°C) in a 3m pipe run creates 2.5mm of movement.

This movement at pipe clips causes the familiar ticking or creaking. The noise is more pronounced in:

• Plastic pipe (higher expansion coefficient than copper)
• Long pipe runs with rigid clip attachment
• Pipes passing through joist holes that fit tightly
• Pipes touching metal brackets or structural elements

Fixes:

• Slacken pipe clips to allow sliding movement (they should hold the pipe, not clamp it)
• Fit rubber-lined pipe clips (Ofix or similar) to isolate vibration
• Where pipes pass through timber, enlarge the hole and insulate with foam pipe lagging
• On long horizontal runs, use an expansion loop or offset fitting to accommodate movement

In underfloor pipe runs — particularly in screed — thermal expansion can cause popping or cracking sounds as the pipe screed interface moves. This is normal for the first few heating cycles in new installations. If it persists, the pipe may not have been adequately insulated from the screed.

Air Locks in Gravity-Fed Systems

Air locks form when air is trapped in a section of pipework, particularly on pipes that have an upward run to a high point. Water cannot push past the trapped air bubble; flow becomes intermittent or stops entirely.

In gravity-fed cold water systems (with a cistern in the loft), air locks typically occur:

• After maintenance where the system was drained
• In pipework with inadequate flow to carry air through
• At high points in the pipework where air naturally accumulates

Diagnosing an air lock:

• Tap delivers good flow initially, then slows or stops
• Gurgling sound from tap when flow is reduced
• Spitting and intermittent spurts from the tap

Purging an air lock:

1. Direct purge — connect a hosepipe from the mains (kitchen tap) to the affected tap using a hose connector. Turn on mains pressure; it pushes water backwards through the pipe, carrying the air lock back to the cistern.
2. Gravity drain — for an air lock in a rising pipe, temporarily disconnect the pipe at the high point, drain the trapped air, reconnect.
3. System drain and refill — for severe or persistent air locks, drain the entire affected section and refill slowly.

Mains-pressure combi systems are much less susceptible to air locks because the supply pressure carries air through. However, air can enter after maintenance or if the system pressure drops below zero (creating a vacuum).

Noise in Central Heating Systems

Heating system noise is usually distinct from domestic water pipe noise. Common sources:

Kettling (boiling/rumbling sound from boiler): Caused by scale build-up on the heat exchanger causing localised overheating and steam micro-bubbles. More common in hard water areas. A powerflush and descaler, followed by inhibitor dosing, cures most cases. Severe scale may require heat exchanger replacement.

Pump noise (whine/hum): A correctly operating system pump makes a slight hum. A whine or screech indicates pump cavitation — the pump is not receiving sufficient water flow. Causes: system pressure too low, pump speed too high for system, blocked strainer. Check system pressure (1–1.5 bar cold), clean pump strainer.

Radiator gurgling: Air in the system. Bleed all radiators starting from the ground floor, working upward. Top up system pressure after bleeding. Persistent air ingress suggests a leak somewhere on the system — air enters to replace water as it escapes.

Ticking floor/ceiling when heating runs: Copper pipes in suspended floors expand into tight joist holes. Enlarge holes to 5mm more than pipe diameter and insulate with foam lagging at the crossing point.

Frequently Asked Questions

Is water hammer dangerous?

Mild water hammer is annoying but typically harmless. Severe water hammer — particularly in older systems with soldered or olive compression joints — can eventually loosen joints and cause leaks. If a hydraulic shock arrestor alone doesn't resolve it, check system pressure and consider a PRV if mains pressure is high.

My pipes only bang in winter — what causes seasonal water hammer?

Cold water pipes contract in winter, making them tighter in their clips and against structures. This reduces the damping of the pressure wave. Also, mains water temperature drops in winter, making the water denser, which increases the magnitude of the hammer effect. Insulating exposed cold pipes reduces the temperature variation and often reduces seasonal hammer.

Can plastic pipe cause less water hammer than copper?

Yes. The wave speed in plastic (MDPE, PEX, polybutylene) is 300–600 m/s compared to ~1000 m/s in copper. This means plastic pipe naturally attenuates the pressure wave more effectively. This is one reason that plastic pushfit pipework systems (Speedfit, Hep2O) are often quieter than equivalent copper installations. However, plastic pipe has greater thermal expansion, which increases ticking noise.

My boiler makes a kettling noise but there's no scale — what else could it be?

If scale has been ruled out (system is soft-water area, recently power-flushed), check for:

• Pump speed set too high, causing turbulent flow through the heat exchanger
• Partially closed lockshield valves restricting flow, causing localised high velocity
• Air pocket in the heat exchanger — power-flush and inhibitor treatment may clear this
• Restricted heat exchanger due to magnetite (black sludge) — MagnaCleanse or powerflush

I can hear water running when nothing is on — is there a leak?

A constant sound of running water with all taps and appliances off is a strong indicator of a concealed leak. Check the water meter — remove the cover and watch the dial. If it is moving with all fittings off, you have a leak. Isolate the internal stopcock and check again; if the meter still moves, the leak is on the supply pipe before the stopcock (contact your water company). If the meter stops, the leak is internal.

Regulations & Standards

• Water Supply (Water Fittings) Regulations 1999 — fittings must not cause waste; pipework must be adequately supported

• BS EN 806:2004 (Parts 1–5) — specifications for installations inside buildings conveying water for human consumption; Part 4 covers pipework installation including clip spacings

• BS 6700:2006 — specification for design, installation, testing and maintenance of services for water supply

• BS 8558:2015 — guide to the design, installation, testing, and maintenance of services supplying water for domestic use within buildings

• CIPHE Plumbing Engineering Services Design Guide — pipe sizing, clip spacing, and hydraulic design

• BSRIA: Water Hammer Technical Note — pressure surge theory and mitigation

• Water Regulations Advisory Scheme — approved fittings and installation requirements

• Caleffi: Hydraulic Shock Arrestor Technical Guide — sizing and selection of anti-hammer devices

• dripping tap — tap washer and cartridge diagnosis

• low pressure — boiler pressure loss and system pressure management

• powerflush — system cleaning for kettling and sludge

• radiator balancing — flow balancing to prevent pump noise and uneven heating