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Plumbing guide
Water Hammer Noise in Fountain Hills, AZ: Causes and Fixes
A sharp knock behind the wall when a washing machine or a faucet shuts off is water hammer. Moving water has weight, and when a valve slams shut the water stops abruptly and the shock travels through the pipe.
By the Fountain Hills Rapid Plumbing team ·
What is happening in the pipe
Water moving through a pipe carries momentum. A fast valve, like the solenoid on a washer or dishwasher, can snap shut in a split second and leave the moving water with nowhere to go. The pressure spikes and sends a shock wave back through the line, which shakes the pipe against its supports.
A single knock is a nuisance. Repeated shocks wear fittings, loosen joints and can eventually cause leaks. It is worth fixing and not worth ignoring.
High pressure makes it worse
The higher the supply pressure, the stronger the shock. Homes on the lower side of a hillside, as covered in the guide on high pressure in hillside homes, are more prone to it.
Test the pressure at a hose bib. If it reads above about 80 psi, a regulator or a replacement is the first fix, and it often stops the hammer on its own.
Loose pipes and missing straps
Pipes in walls and attics are held by straps. When a strap loosens or was never installed, the pipe bangs against framing each time the water shocks. In some cases the noise is the pipe knocking, not the shock itself.
Where pipes are reachable, in a garage or under a sink, you can add padded straps. Pipes inside walls need a plumber who can open a small access.
Air chambers and arrestors
Older homes often had short vertical pipes called air chambers near fixtures. The air in them acted as a cushion. With age they waterlog and lose their cushion, and the knocking comes back. You can recharge them by draining the system. Close the main, run the highest and lowest taps until the lines empty, and then turn the water back on.
Newer systems use water hammer arrestors, small sealed devices with a spring or air cushion. A plumber can install one near the washing machine or at the point where the noise starts.
Fixes by cause
Choose the fix that matches the cause.
| Cause | Sign | Fix |
|---|---|---|
| High pressure | Gauge above 80 psi | Regulator |
| Loose pipe | Knock without a pressure shock | Add straps |
| Waterlogged air chamber | Old home, hammer returns | Drain and recharge |
| Fast valve | Bang from one appliance | Install an arrestor |
Which fixtures cause it
Washing machines, dishwashers and ice makers are common sources because of their quick solenoid valves. Single-lever faucets that close fast are another. Note which fixture sets it off, and that is where the arrestor goes.
If the noise starts after a repair, a loose connection or a valve left partly open may be the cause. Tell your plumber what changed.
The risk to the water heater
Repeated pressure shocks can stress the heater and its connections. If the relief valve drips along with the banging, an expansion tank or a regulator problem may be involved. See water heater repair for the follow-up checks.
A leak that starts at a joint after years of hammer is not a coincidence. Address the cause, not only the leak.
When the noise returns after a fix
If the knock comes back weeks later, an arrestor may have lost its charge or a strap may have worked loose. Both are quick to correct.
A return of the noise after a heater or regulator change can also mean thermal pressure is rising. Ask the plumber to recheck the expansion tank.
Why some fixtures bang and others do not
A tap closed slowly by hand gives the water time to slow down. A washing machine valve closes in a fraction of a second. The faster the stop, the stronger the shock, which is why the appliances are usually the loudest.
Long straight runs of pipe make the effect worse because the moving column of water is longer. A house with a long line from the main to the far bathroom may hammer at the end of the run.
Plumbing materials and noise
Rigid copper carries the shock and rattles against framing. Flexible plastic pipe absorbs more of it but can still knock if it is loose. Either way, supports spaced at the right intervals keep the pipe from moving.
If you hear a different sound, such as a hum or a whistle, the cause is likely a worn valve or a restriction and not hammer. Describe the sound to the plumber, because the cure differs.
A simple test to find the source
Turn off fixtures one at a time and see which one triggers the knock. Run the dishwasher, then the washing machine, then a single-handle faucet. Note which tap sets it off and how loud the bang is.
If the knock happens everywhere, the cause is probably system pressure. If it happens at one fixture, a local arrestor will probably solve it.
What a repair visit looks like
A plumber starts with a pressure reading, checks the regulator and expansion tank, and secures pipes where access is simple. Arrestors then go on the lines of the worst offenders. The work usually finishes in a single visit.
Ask where the arrestor will go, and keep a record of the model and location. They have a finite life and may need replacement years later.
When to call
Call if the banging is loud, if it started suddenly, or if you notice a leak near the noise. A plumber can test pressure, check straps and install arrestors in a single visit.
The repiping page describes what happens when old pipes have been damaged beyond repair.
Common questions
Water hammer questions
The shock can loosen joints and cause leaks over time. Fixing it is worth the effort.
Pressure is often higher at night, and a quiet house makes the sound easier to hear.
You can check pressure and secure exposed pipes. Installing arrestors inside walls is a plumber's job.
Not necessarily. It can lead to one, so watch for damp spots.
Related services
Water Line Repair & Replacement
Fix, or renew, the underground water pipe that serves your house, starting at the meter.
Repiping & Whole-Home Pipe Replacement
When patching stops holding, replace worn supply piping throughout the house.
More plumbing guides
Excess pressure in hillside homes
Houses lower on a slope can see far more pressure than those above, and a regulator keeps pipes and fixtures safe.
Weak water pressure and its causes
Weak flow at a single tap and weak flow at every tap have separate explanations, so work out which you have first.
A hose bib that drips
A leaking outdoor faucet wastes water and may hide something worse, so find out where the drip starts.
Pipes banging in the walls?
Name the fixture that sets it off, and the dispatcher sends the right help.