Quick answer
Water hammer is a pressure surge in a water pipe that happens when flowing water is stopped or turned suddenly, usually by a fast-closing valve, sending a shock wave through the pipe that you hear as a bang or knock.
The surge adds to normal house pressure and can strain joints, valves and appliances. The standard fix is a water hammer arrestor, a sealed cushion fitted near the fast-closing valve.
Quick facts
The key facts about Water Hammer, with sources:
Key takeaways
- Water hammer is the bang when moving water is stopped suddenly and its energy turns into a pressure spike.
- Fast-closing valves on washers, dishwashers and ice makers are the most common triggers in homes.
- The spike adds to normal house pressure and over time can loosen joints and wear out valves.
- A water hammer arrestor near the fast-closing valve absorbs the shock; many codes require one.
- Not every pipe noise is water hammer; loose pipes and air in lines make similar sounds.
Water hammer meaning: a pressure spike from water that stops too fast
Water hammer is the bang you hear when fast-moving water in a pipe is stopped suddenly.
Water has weight and speed, so when a valve snaps shut, that moving column of water has to stop at once.
The PPFA design guide explains that the energy of the moving water must go somewhere, so it turns into a short spike in pressure 1.
Watts describes the result: sudden stoppage of water flowing at a given pressure and speed causes a surge, and a pressure wave travels back through the piping until it finds a point of relief 5. As the wave bounces, pipes can jump and bang against framing.
The bigger the water speed and the stiffer the pipe, the bigger the spike. PPFA notes that surges can reach instantaneous pressures of 300 to 400 psi and over time damage rigid pipes, fittings, fixtures and water heaters 1.
Not every knock is water hammer
PPFA notes some pipe noise comes from water cavitating as pressure waves move, which is not tied to damaging surges and is more likely at low pressure 1. Pipes that tick as they heat and cool, or rattle in loose straps, are other common causes.
How water hammer happens, step by step
- Water flows: A washer or dishwasher fills, and water moves through the pipe at speed.
- A valve snaps shut: The appliance’s solenoid valve closes in a fraction of a second.
- Pressure spikes: The moving water slams into the closed valve, and its energy becomes a pressure surge 1.
- A wave travels back: The pressure wave runs back up the pipe until it finds relief, such as a larger pipe or a tank 5.
- The pipe moves: Unsecured pipe jumps and hits framing, which is the bang you hear.
- Fast flowWater moving in the pipe
- Quick-closing valveSolenoid or lever snaps shut
- Pressure spikeMoving water’s energy becomes pressure
- Shock waveTravels back to a point of relief
- BangPipe jumps against framing
An arrestor placed near the valve gives the spike somewhere to go. Source: [1] [5]
Water hammer vs other pipe noises
| Type | What you hear | Usual cause |
|---|---|---|
| Classic water hammer | A single loud bang when an appliance or tap shuts off | Quick-acting valve stops fast flow 25 |
| Repeated hammer | Several knocks after shut-off as the wave bounces | Pressure wave reflecting through the piping 5 |
| Cavitation noise | Knocking without high surge pressure | Pressure waves in the water column; more likely at low pressure 1 |
| Thermal ticking | Ticks as hot water runs or cools | Pipe growing and shrinking with heat (PEX moves about 1 inch per 100 ft per 10°F) 1 |
| Loose pipe rattle | Rattle when water runs | Missing or loose pipe supports |
Example: how big the spike gets, with real numbers
This example uses the PPFA design guide’s hydraulic shock table. The shock adds to static (normal) pressure; PPFA says to add the two to get the total at the instant a fast valve closes 1.
| Pipe | Water speed | Added shock (psi, 73°F) | Total with 60 psi static |
|---|---|---|---|
| PEX | 6 ft/s | 87 1 | 147 psi |
| Copper | 6 ft/s | 300 1 | 360 psi |
| Galvanized steel | 6 ft/s | 360 1 | 420 psi |
| Copper | 4 ft/s | 200 1 | 260 psi |
| Copper | 10 ft/s | 505 1 | 565 psi |
Total pressure = Static house pressure + Hydraulic shock
Static house pressure- What a gauge reads with no water running
Hydraulic shock- Extra surge from the sudden stop; grows with water speed and pipe stiffness
PPFA: add the hydraulic shock from its table to the system static pressure measured on site 1.
Source: [1]
Two lessons stand out. Slower water means a smaller spike, so oversized pressure and undersized pipe make hammer worse. And flexible pipe absorbs more of the shock than rigid metal. The 60 psi static figure is an example; read your own with a gauge.
The water hammer arrestor: the standard fix
A water hammer arrestor is a small sealed device on the supply line near a quick-acting valve. Inside is a pocket of air or gas kept apart from the water by a piston or bellows.
When the surge arrives, it pushes against that cushion instead of the pipe. The Watts Series 15, for example, uses a pre-charged, permanently sealed air chamber with a piston and a 60 psi air pre-load 5.
Codes want a listed mechanical device, not a homemade air chamber. The 2025 California Plumbing Code text quoted by the City of Dublin requires arrestors that comply with ASSE 1010 or PDI-WH 201, installed as close as possible to quick-acting valves 2.
Dublin’s own policy adds that they should go at the dishwasher, clothes washer and ice maker, and it marks §609.11 as not adopted by HCD, the state housing agency, so Dublin applies it to homes through local policy 2. ASSE’s current edition is ASSE/IAPMO/ANSI/CAN 1010-2025 4.
Sizing follows the PDI fixture-unit method. Watts’ table maps PDI sizes A through F to fixture-unit ranges, from A at 1–11 units to F at 155–330 5.
On a branch line up to 20 feet, one arrestor goes at the end of the branch between the last two fixtures; longer branches get a second unit 5.
What fixing water hammer protects, and what it cannot
What an arrestor does
- Absorbs the surge near the valve that causes it 25.
- Cuts noise and protects joints near quick-acting valves; Bainbridge Island notes fittings close to such valves can be harmed by surge 3.
- Sealed models need no recharging and may be installed in concealed spots, per Watts 5.
- Fits on a standard tee, at any angle, for that model 5.
- Works with any pipe material.
What it cannot do
- Does not fix loose pipe straps or thermal ticking.
- Has a rated working pressure (150 psi for the Watts Series 15) 5.
- Must be sized and placed by fixture units, not guessed 5.
- Will not fix very high house pressure; that needs a pressure reducing valve.
- A plain capped pipe stub is not an approved mechanical device, which the CPC text requires 2.
Why high pressure, fast water and rigid pipe make hammer worse
An arrestor treats the symptom at one valve. If the whole house is at high pressure, every fast valve will produce a bigger spike. Our page on residential water pressure covers how to read and lower it, and a pressure reducing valve is the usual fix.
Pipe material changes the size of the problem. PPFA’s lab tests found half-inch PEX had lower peak surge than copper at every flow rate tested, and its shock table shows far lower added surge in PEX 1.
PPFA says arrestors are not necessary in normal PEX installations but may be needed where PEX is mixed with mostly metal piping 1.
Codes do not always follow that view. Bainbridge Island, Washington requires arrestors at quick-acting valves regardless of pipe material, unless the PEX maker’s instructions say one is not needed 3.
What drives the cost of fixing water hammer
How many valves need arrestors, how easy they are to reach and whether a pressure fix is needed set the cost. We do not list prices, because fair numbers need dated local quotes.
The drivers are how many quick-acting valves need arrestors, whether the valves are easy to reach (a washer box with built-in arrestors vs a dishwasher line behind a cabinet), whether walls must be opened, whether house pressure must be lowered with a new or adjusted reducing valve, and whether loose pipes need new straps.
Diagnosing the real cause first often saves money.
Our plumbing cost guide explains how quotes are built, and the fixture installation service covers appliance hookups that often include arrestors.
How a plumber diagnoses and fixes water hammer
- Find the trigger: Note which appliance or tap causes the bang, and when.
- Check pressure: Read static pressure with a gauge on a hose bibb or test port.
- Inspect supports: Look for loose straps where the pipe moves.
- Fit arrestors: Install listed arrestors (ASSE 1010 or PDI-WH 201) as close as possible to the quick-acting valve, sized by fixture units 25.
- Correct pressure if high: Adjust or install a pressure reducing valve.
- Test: Run the appliance cycle and listen again.
High pressure is a hidden risk
A pressure spike stresses every joint, valve and water heater connection; PPFA notes surges of 300–400 psi can damage rigid pipes, fittings, fixtures and water heaters over time 1.
If banging comes with leaks, a dripping relief valve on the water heater, or a gauge reading far above normal, call a plumber. Never cap or plug a water heater relief valve to stop it dripping.
How to prevent water hammer
- Make sure washers, dishwashers and ice makers have arrestors at their supply valves 2.
- Keep house pressure in a normal range; have a plumber test it if hammer starts suddenly.
- Secure loose pipes in basements and crawlspaces with proper straps.
- Close lever valves and shutoffs gently rather than snapping them.
- Leave sealed arrestors alone; Watts’ Series 15, for one, is factory charged and not field rechargeable 5.
Washer boxes with built-in arrestors
Some laundry outlet boxes come with arrestors built into the valves. When you replace a washer, check whether yours has them before adding more.
Water hammer warning signs, and when to call
- A loud bang when the washer or dishwasher stops filling.
- Banging that started suddenly, which can mean a failed pressure reducing valve.
- Leaks at joints near appliances that bang.
- A water heater relief valve that drips or discharges.
- Banging that continues after arrestors are installed.
For a leak, shut off the water and see our plumbing emergencies guide. Our leak detection and repair service finds hidden leaks that pressure spikes can cause.
Arrestor codes and ASSE 1010 / PDI-WH 201 standards
California Plumbing Code, 2025 edition, §609.11 (quoted by the City of Dublin, CA, policy BDP 12, revised March 23, 2026): supply systems with quick-acting valves need water hammer arrestors that comply with ASSE 1010 or PDI-WH 201, installed as close as possible to those valves; §609.11.1 says to follow the maker’s placement instructions.
The policy notes the section is not adopted by HCD, California’s housing agency, so its reach in homes depends on local rules like Dublin’s 2.
Uniform Plumbing Code §609.10 (quoted by Bainbridge Island, WA, policy effective October 31, 2013): the same arrestor rule for quick-acting valves; the city requires it regardless of pipe material unless the PEX maker documents otherwise 3.
Product standards: ASSE/IAPMO/ANSI/CAN 1010-2025, Water Hammer Arresters 4; PDI-WH 201 for sizing and certification, which Watts lists alongside ASSE 1010 and ANSI A112.26.1M for its Series 15 5.
Local adoption: The IPC and IRC have their own water hammer sections; we could not read them at source, so we do not quote them. Check the code edition your jurisdiction adopted.
Water hammer fixes compared: arrestor, PRV, straps and air chambers
| Fix | What it does | When it helps |
|---|---|---|
| Water hammer arrestor | Sealed cushion absorbs the surge 5 | Bang at a specific quick-acting valve 2 |
| Pressure reducing valve (PRV) | Lowers static house pressure; ASSE 1003 covers PRVs 4 | Hammer everywhere, or high gauge readings |
| Pipe straps | Stops pipe from moving | Rattles where pipes hit framing |
| Air chamber (capped pipe stub) | Older air pocket method | The CPC text calls for approved mechanical devices instead 2 |
| Expansion tank | Absorbs thermal expansion from the water heater | Pressure creep in closed systems, not valve slam |
Water hammer myths
- Myth Banging pipes are just noise.
- Reality PPFA notes surges of 300–400 psi can damage pipes, fittings, fixtures and water heaters over time 1.
- Myth Any capped pipe stub works as an arrestor.
- Reality Codes call for approved mechanical devices to ASSE 1010 or PDI-WH 201 2.
- Myth PEX never needs arrestors.
- Reality PPFA says normal PEX systems do not, but some cities require them anyway unless the maker says otherwise 13.
- Myth One arrestor at the main fixes the whole house.
- Reality Arrestors go as close as possible to each quick-acting valve 2.
- Myth All pipe noise is water hammer.
- Reality Cavitation, thermal movement and loose straps make similar sounds 1.
How state and city code change arrestor rules
Arrestor rules depend on which code your city adopted and how it reads it.
California’s 2025 Plumbing Code §609.11 names ASSE 1010 or PDI-WH 201 devices, but it is not adopted by HCD for housing; Dublin applies it to homes through its own policy and names the dishwasher, clothes washer and ice maker as locations 2.
Bainbridge Island, Washington, under the UPC, requires arrestors at quick-acting valves even with PEX unless the maker’s instructions say otherwise 3. Ask your building department which edition and interpretation apply before a remodel.
When water hammer matters, and what to do first
Water hammer matters when you hear a bang every time an appliance stops filling, when banging starts suddenly, or when you add a dishwasher, washer or ice maker. Banging that starts all at once can point to a pressure change, such as a failed pressure-reducing valve, so check pressure before blaming the appliance.
Start by noting which fixture causes the bang. Then have a plumber check pressure and fit arrestors where they are missing. The guide below matches four common patterns to a first step.
- If Bang only at the washer or dishwasher
- Then Add or replace an arrestor at that valve
- If Bangs everywhere, or it started suddenly
- Then Check the house water pressure
- If Rattle while water runs
- Then Secure the loose pipe
- If Leaks near the noise
- Then Call a plumber now
Questions about Water Hammer
What causes water hammer?
Water hammer happens when flowing water is stopped suddenly, usually by a quick-closing valve such as the solenoid in a washer, dishwasher or ice maker. The moving water’s energy turns into a pressure spike, and a shock wave travels back through the pipe until it finds relief. That wave can make pipes jump and bang.
Is water hammer dangerous?
It can be harmful over time. The PPFA design guide says surges can reach 300 to 400 psi and can damage rigid pipes, fittings, fixtures and water heaters. The sound itself is harmless, but repeated spikes can loosen joints and cause leaks. A sudden start of banging can also signal a failed pressure reducing valve.
How do I stop water hammer?
Find which fixture triggers it. Then install a listed water hammer arrestor as close as possible to that quick-acting valve, check that house pressure is normal, and secure any loose pipes.
Model code text, such as 2025 California Plumbing Code §609.11, calls for arrestors that meet ASSE 1010 or PDI-WH 201; whether it applies to your home depends on local adoption.
What is a water hammer arrestor?
A water hammer arrestor is a small sealed device on the supply line with a cushion of air or gas behind a piston or bellows.
When a valve slams shut, the surge pushes into the cushion instead of the pipe. Sealed models such as the Watts Series 15 are factory charged and do not need recharging.
Where should a water hammer arrestor be installed?
As close as possible to the quick-acting valve it protects, which is what the UPC-family code text says. Dublin, CA’s policy lists the dishwasher, clothes washer and ice maker.
For a branch serving several fixtures up to 20 feet long, Watts’ placement rule puts one arrestor at the end of the branch between the last two fixtures.
Do PEX pipes need water hammer arrestors?
It depends on your code. The PPFA design guide says arrestors are not necessary in normal PEX systems because the pipe absorbs much of the surge, but may be needed where PEX is mixed with metal pipe. Some cities, such as Bainbridge Island, WA, require them anyway unless the PEX maker documents otherwise.
Why did my pipes suddenly start banging?
A sudden change often means something in the system changed. Common causes are a new appliance with a fast valve, a failed pressure reducing valve letting street pressure in, or an old arrestor that has stopped working. Have a plumber check static pressure with a gauge before replacing parts.
Do water hammer arrestors wear out?
They can. Codes now call for approved mechanical devices rather than plain air chambers, and sealed piston models such as the Watts Series 15 are factory charged and not field rechargeable. If banging returns after years of quiet, the arrestor may need replacing.
Is pipe knocking always water hammer?
No. PPFA notes some knocking comes from water cavitating as pressure waves move, which is not linked to damaging surges. Pipes also tick as they heat and cool, and rattle when they are loosely strapped. Noting when the sound happens, such as right after an appliance stops, helps tell them apart.
Sources
- PPI and PPFA (prepared by Home Innovation Research Labs), Design Guide: Residential PEX Water Supply Plumbing Systems, 2nd ed. (November 2013), retrieved .
- City of Dublin, CA, Building Policy BDP 12: Water Hammer in Residential Buildings (revised March 23, 2026; quotes 2025 CPC §609.11), retrieved .
- City of Bainbridge Island, WA, Building Department Policy: Water Hammer Arresters (effective October 31, 2013; quotes UPC §609.10), retrieved .
- ASSE International, Current Product Standards list, retrieved .
- Watts, Series 15 Water Hammer Arrestor specification sheet (ES-15), retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: How surges form, their size, the hydraulic shock table and PEX findings are from the 2013 PPI/PPFA PEX design guide (an industry publication).
Code text is quoted as reproduced in two city policies: Dublin, CA (2025 CPC §609.11) and Bainbridge Island, WA (UPC §609.10). Standard editions are from ASSE’s current list.
Arrestor construction, sizing bands and placement rules are from one manufacturer’s spec sheet (Watts Series 15), labeled as such. IPC/IRC water hammer sections were not readable at source and are not quoted. No prices are given.
Suggest a correction. We fix errors and say what changed.