If you hear a loud bang, thud, or hammering noise from your pipes when your washing machine finishes filling or when you shut off a faucet quickly, you have water hammer. It is one of the most common residential plumbing problems and, in most cases, one of the easiest to fix. Here is a complete guide to what causes it, why it matters, and the most effective solutions — including how to install a washing machine water hammer arrestor in about 15 minutes.
What Is Water Hammer?
Water hammer — technically called hydraulic shock — occurs when flowing water is suddenly stopped or forced to change direction rapidly. Water in a pipe is under pressure and moving with momentum. When a valve closes suddenly, that momentum has nowhere to go and sends a shockwave through the pipe system. The result is the characteristic bang, thud, or hammering sound that gives the phenomenon its name.
The physics: water is essentially incompressible — it cannot absorb kinetic energy by compressing the way air can. When moving water hits a suddenly closed valve, the pressure spike can be several times the normal operating pressure of the pipe system. In household plumbing, normal operating pressure is typically 40-80 psi; a water hammer event can momentarily spike that to several hundred psi at the point of stoppage.
What Causes Water Hammer?
Fast-Closing Solenoid Valves (Most Common)
The number one cause of water hammer in residential homes is the fast-closing solenoid valve. These are the electrically operated valves inside washing machines, dishwashers, and refrigerator ice makers. Unlike a manually operated tap that you close slowly over several seconds, a solenoid valve opens and closes almost instantaneously — in milliseconds — under electronic control. Every time your washing machine finishes a fill cycle, its solenoid snaps shut and sends a shockwave down the supply lines. Repeat this hundreds of times per year and you have a persistent water hammer problem.
Fast-Closing Ball Valves
Quarter-turn ball valves — the type where you turn a lever 90 degrees to go from fully open to fully closed — can cause water hammer if operated too quickly. Unlike older compression or gate valves that close gradually, a ball valve can be shut in a fraction of a second by a quick wrist movement. Plumbers are trained to close ball valves slowly; homeowners often do not know this.
High Water Pressure
Homes with water pressure above 60-80 psi are significantly more susceptible to water hammer. Higher pressure means more energy in the system — a greater potential for a damaging pressure spike when flow suddenly stops. Most municipalities supply water at 80-100 psi at the meter; a pressure reducing valve (PRV) on the main supply line steps this down to a safer household pressure. If your home lacks a PRV or if the PRV has failed, high operating pressure worsens every water hammer event.
Loose Pipe Straps
Water hammer does not just create pressure spikes — it also causes pipes to physically jolt. If the pipes are not securely strapped to the framing, they can bang against walls, floor joists, or each other when the shockwave hits. Sometimes what sounds like a water hammer noise is actually a secure pipe system producing minimal actual hydraulic shock — but the slight vibration is being amplified because a pipe strap has come loose.
Why Water Hammer Is a Problem
Water hammer is not just annoying — repeated hydraulic shocks cause cumulative damage to a plumbing system:
- Loosened pipe joints and fittings: Repeated pressure spikes gradually loosen threaded connections and solder joints, eventually causing leaks
- Damaged appliance inlet valves: The solenoid valves in washing machines and dishwashers are not designed to absorb the reflected shockwave. Over time, repeated water hammer can crack the valve seat or damage the valve mechanism, causing leaks or failure
- Water heater damage: The connections at the top of a water heater are vulnerable to repeated hammering; tank-type water heaters can develop leaks at the inlet/outlet connections
- Pipe damage: In severe cases with very high pressure, water hammer can crack pipes — particularly older copper or galvanized steel pipes that have become brittle
- Noise and disruption: Persistent loud banging from pipes is disruptive and a sign that the system needs attention
What Is a Water Hammer Arrestor?
A water hammer arrestor (also spelled ‘arrester’) is a device that absorbs the pressure shockwave before it can damage the pipe system. It consists of a sealed chamber containing a spring-loaded piston that separates the water side from an air-cushion side.
How it works:
- The arrestor is installed on the water supply line close to the valve that is causing the hammer
- When the valve closes suddenly and the pressure spike travels back through the pipe, it enters the arrestor
- The pressure forces the piston to compress the sealed air chamber — the air acts as a shock absorber, absorbing the kinetic energy of the water
- The piston returns to its resting position once the pressure normalizes, ready for the next cycle
The piston design is important: older-style ‘air chambers’ (simply a capped section of pipe installed upright, trapping air above the water) work initially but fail over time because the air gradually dissolves into the water and the chamber becomes waterlogged. A proper hammer arrestor has a sealed piston that permanently separates the air from the water — it never needs recharging and provides consistent protection throughout its service life.
Washing Machine Water Hammer Arrestor: The Most Common Fix
Because solenoid-valve appliances are the leading cause of residential water hammer, the most effective and affordable fix is a washing machine water hammer arrestor — a compact device that screws directly into the hot and cold water supply line connections behind the machine.
| Detail | Information |
| Thread size | 3/4 inch hose bib (standard washing machine connection) |
| Installation location | Between the supply valve and the washing machine hose |
| Hot and cold | Two arrestors needed — one per line |
| Price | $15-30 each; $25-50 for a pair |
| Tools required | Adjustable wrench; plumber’s tape (Teflon tape) |
| Time to install | Approximately 15 minutes |
| Standard | ASSE 1010 certified (required for code compliance in most US jurisdictions) |
| Top brands | Watts, Sioux Chief, Zurn, SharkBite |
How to Install a Washing Machine Hammer Arrestor
This is a straightforward DIY job that requires no special plumbing skills:
- Step 1 — Turn off the supply valves: Locate the hot and cold water supply valves behind the washing machine (typically red and blue handles). Turn both clockwise until fully closed
- Step 2 — Disconnect the supply hoses: Unscrew the supply hoses from the wall valves (or hose bibs). Have a towel ready for a small amount of residual water
- Step 3 — Wrap threads: Wrap 2-3 layers of Teflon (plumber’s) tape around the threads of the wall valve (the male threads the hose screws onto). This ensures a watertight seal
- Step 4 — Thread the arrestor onto the valve: The arrestor has a female thread on one end and a male thread on the other. Thread the female end onto the wall valve and hand-tighten, then snug with an adjustable wrench (do not overtighten)
- Step 5 — Reconnect the supply hose: Thread the washing machine supply hose onto the male end of the arrestor. Hand-tighten, then 1/4 turn with pliers
- Step 6 — Turn water back on: Open both supply valves slowly. Check for leaks at all connections
- Step 7 — Run a wash cycle: The next time the machine fills and stops, you should notice the banging has reduced significantly or eliminated entirely
The same process applies for dishwasher hammer arrestors (usually 3/8-inch compression or 1/2-inch connections, so check the thread size before purchasing) and for ice maker supply lines.
Water Hammer in Other Locations
Banging When Faucets Are Turned Off
If the banging occurs when you shut off a faucet or valve rather than when an appliance fills, the fix is the same in principle — a hammer arrestor installed on the supply line serving that fixture. For sink faucets and toilets, smaller in-line arrestors are available in 3/8-inch and 1/2-inch sizes. In some cases, replacing an old fast-closing faucet with a newer model that has a built-in soft-close mechanism eliminates the problem without additional hardware.
Banging From Pipes in the Walls
If the noise appears to come from pipes inside the walls rather than at specific appliances, the cause may be a combination of water hammer shockwaves and loose pipe straps. Check accessible pipe runs in the basement, crawlspace, or utility room for pipes that are vibrating against framing or against each other. Adding pipe straps (sold in packs at any hardware store) every 4-6 feet on horizontal runs and at each floor penetration on vertical runs absorbs the physical movement. This alone sometimes resolves the noise even if the underlying hydraulic shock remains.
Whole-House Water Pressure Too High
If water hammer is widespread throughout the house — happening at multiple fixtures and appliances — check the home’s water pressure with an inexpensive pressure gauge (screws onto any hose bib). Residential plumbing is designed for 40-60 psi. If your reading is above 80 psi, having a plumber install or adjust a pressure reducing valve (PRV) on the main supply line will reduce the severity of hammer events throughout the entire system. This is more expensive than a washing machine arrestor ($200-500 installed) but addresses the root cause in high-pressure situations.
Air Chamber vs Hammer Arrestor: What Is the Difference?
| Air Chamber | Hammer Arrestor | |
| How it works | Capped pipe section trapping air above water level | Sealed piston separates air cushion from water |
| Durability | Air dissolves into water over 1-3 years; becomes waterlogged | Piston permanently seals air — maintenance-free |
| Maintenance | Must be drained and refilled periodically | None required |
| Installation | Requires cutting and soldering pipe section | Screws onto existing connections |
| Cost | Low (materials only) | $15-30 per unit |
| Recommended? | No — temporary solution | Yes — current standard |
Frequently Asked Questions
What is a hammer arrestor?
A water hammer arrestor is a plumbing device with a sealed piston and air chamber that absorbs pressure shockwaves when water flow stops suddenly. It prevents the banging noise and pipe damage associated with water hammer. Modern arrestors use a piston to permanently separate the air from the water, making them maintenance-free and long-lasting.
What causes the water hammer sound?
The banging or hammering sound comes from the kinetic energy of moving water hitting a suddenly closed valve, creating a pressure shockwave that physically jolts the pipes. The most common cause in residential homes is the fast-closing solenoid valve in washing machines and dishwashers. High water pressure worsens the effect.
Do I need a hammer arrestor for my washing machine?
If your washing machine causes a banging sound in the pipes when it finishes filling, a washing machine hammer arrestor is the most effective and affordable fix. Two arrestors (one hot, one cold) costing $25-50 total and taking about 15 minutes to install will typically eliminate the problem.
Where is the hammer arrestor installed on a washing machine?
Between the wall supply valve and the washing machine supply hose — screwed directly onto the hose bib (the threaded connection coming out of the wall). The supply hose then connects to the bottom of the arrestor. One is installed on the hot supply line and one on the cold supply line.
What is the difference between a water hammer arrestor and arrester?
‘Arrestor’ and ‘arrester’ are both used — ‘arrestor’ is the more common spelling in US plumbing literature. They refer to the same device. The ASSE 1010 standard, which governs these devices in the US, uses ‘arrestor.’
Final Thoughts
Water hammer is one of the most fixable home plumbing problems. In the vast majority of residential cases, the cause is a washing machine or dishwasher solenoid valve, and a pair of ASSE 1010-certified hammer arrestors costing under $50 and installed in 15 minutes eliminates the problem permanently. If arrestors do not fully resolve the issue, check home water pressure — anything above 80 psi warrants a pressure reducing valve. Loose pipe straps are worth checking too if the banging seems to be physical pipe movement rather than pure hydraulic shock. For most homeowners, this is firmly in DIY territory: no soldering, no pipe cutting, and no special plumbing knowledge required.