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Water Hammer: Why Pipes Bang and How to Fix It
When a valve slams shut, moving water has nowhere to go and hits the closed end with a shock. The bang is the pipe jumping against the framing.
By the Westminster Plumbing Co. team ·
What water hammer is
Water moving through a pipe has momentum. A dishwasher or washing machine valve closes quickly, the flow stops in an instant and a pressure wave travels back through the pipe. The result is a knock, a bang or a shudder.
It can wear fittings over time, so it is worth fixing, not tolerating.
Common causes
High water pressure is the most common cause. Loose pipes that can move against studs make the sound louder. Quick-closing valves in appliances start the shock. Old air chambers that have filled with water no longer cushion it.
Check pressure with a gauge on a hose bib. Pressure above about 80 psi is a concern.
Fixes you can try
Secure loose pipes with clamps or straps where they run through framing. Add foam insulation where they touch wood. Drain the system to restore the air in the old air chambers: close the main, open the highest and lowest taps, let the pipes empty and then restore water slowly.
If you have a pressure reducing valve, a plumber can adjust it or replace it when worn.
Causes of banging pipes and fixes
Match what you hear to the likely cause.
| What you notice | Likely cause | Fix |
|---|---|---|
| A knock when the washer stops | A quick-closing valve | Add a hammer arrestor |
| Rattling along a wall | Loose pipes | Secure with straps |
| Loud bang and a dripping relief valve | High pressure or no expansion tank | Check pressure, add an expansion tank |
| Hammer after a drain-down | Air chambers filled with water | Drain the system to restore air |
Water hammer arrestors
An arrestor is a small device with a piston or bladder that absorbs the shock. It installs near the appliance, such as a washing machine or dishwasher. Modern code requires them in many new installations.
They are inexpensive, and they fit on the supply line with a few fittings.
Expansion tanks and pressure
A closed plumbing system with a pressure reducing valve needs an expansion tank on the water heater. Without one, pressure spikes when the heater runs. Dripping relief valves and banging pipes can come together.
A plumber can test pressure and add an expansion tank where needed.
When to call a plumber
Call if the noise is loud, if it comes with leaks or if you cannot find the source. A plumber can test pressure, secure the pipes and fit arrestors.
If you recently had work done and the banging began afterward, mention it. A shutoff left partly closed can cause noise.
Where to listen for the source
Run each fixture and listen. A bang at the washing machine points to its supply valves. A bang when a toilet finishes filling points to the fill valve. A knock when a faucet closes quickly suggests a single-handle fixture with no air chamber nearby.
Check the straps too. A pipe that moves when pressure changes will tap against framing, and that noise is often mistaken for hammer. Looking along exposed basement pipes for loose or missing hangers often solves part of the problem.
Why it gets worse over time
Many older homes have vertical air chambers that act as small cushions. Over the years those chambers fill with water and stop working, so a house that was quiet becomes noisy. Draining the system briefly can refill them with air.
Rising pressure can also worsen the problem. If a pressure reducing valve wears out, pressure climbs, and every closing valve produces a harder hit. A gauge on a hose bib confirms whether pressure is above a safe range.
Costs and what to expect
A simple arrestor on a washing machine supply is a modest part and a short job. Whole-house solutions, such as a new pressure reducing valve or an expansion tank, cost more but protect the entire system.
Ask the contractor to explain where each device will go and what it is meant to correct. Fixing hammer protects pipes, fittings and appliance valves, so the work usually pays for itself over time.
Check these before you call
Confirm the washing machine hoses are firmly attached and the shutoff valves open all the way. A valve left partly closed forces water through a small opening and can cause more noise than a fully open one.
Look at the water heater too. A missing or waterlogged expansion tank can make pressure spikes worse, and the heater's relief valve may drip when pressure rises. Mention these details to the plumber.
Record when the bangs occur: after a flush, when a washer stops, or on first use in the morning. A timeline points to the cause far faster than a general description.
Why ignoring it costs more later
Each hammer event stresses joints, valves and hoses. Over months, a washing machine hose can burst or a joint can begin to weep behind a wall, creating water damage that costs far more than the original fix.
If you hear loud knocks, address them soon. Cheap devices such as arrestors, straps and a pressure check solve most cases, and they protect the plumbing and your appliances.
Noise that is not hammer
Pipes also tick and creak as hot water expands them. That sound is normal, and it usually fades within a minute. A sharp knock that repeats right as a valve closes is the sign of true hammer, and it is the one worth fixing.
Common questions
Answers about water hammer
Eventually they can. Each shock loosens fittings and wears out valves.
The machine's valve closes fast. An arrestor on the supply absorbs the shock.
Often. A gauge shows it. A pressure reducing valve brings it down.
Some fixes, such as securing pipes, are easy. Pressure work and arrestors are better left to a plumber if you are unsure.
Related services
Water Heater Repair
Track down the cause of cold showers, tank leaks, popping noises, pilot trouble or burned-out elements.
Leak Detection
Trace a hidden leak behind a wall, under a slab or along the buried line before any wall has to be opened.
Keep reading
Low water pressure: causes and fixes
Weak flow at one tap is a different problem from weak flow everywhere. Work through the usual causes before you call.
PEX vs copper pipe: which is better?
PEX and copper both work. The choice depends on cost, freezing risk, water chemistry and how the pipe will be run.
Water heater leaking from the bottom
Water at the base of the heater is either a loose fitting or a failed tank. A few checks tell you which, and what to shut off first.
Pipes banging loudly?
Phone in and say when the banging occurs. The dispatcher then matches you with a nearby contractor.