Faucet & Sink Repair
Repair drips, weak flow, stuck shutoff valves and leaking outdoor spigots.
Plumbing guide
Water Hammer: Why Pipes Bang and How to Stop It
When moving water stops suddenly, it has nowhere to go, and the shock travels through the pipes as a bang. It is annoying and it also wears out joints and fixtures.
By the Urban Comfort Plumbing team ·
What water hammer is
Water in a pipe has weight and momentum. When a valve closes quickly, such as the solenoid in a washing machine or a dishwasher, or a single-lever faucet that is snapped shut, the moving water slams into the closed valve. The pressure spike travels back through the pipe as a shock wave, and the pipe knocks against its supports.
The sound is a thump or a series of bangs, and sometimes a rattle that runs along the wall. A single bang is not an emergency. A pattern that repeats every time the same appliance stops is a sign something needs attention.
Why it happens
Four things cause most cases. First, quick-closing valves in appliances. Second, high water pressure, which gives the water more force. Third, pipes that are not strapped and are free to move. Fourth, air chambers that have lost their air.
An air chamber is a short vertical pipe capped at the top, installed near a fixture. Air in it acts as a cushion, absorbing the shock. Over time, the chamber fills with water and the cushion is gone. That is the most common reason a house that was quiet becomes noisy.
Recharging air chambers
This fix costs nothing. Turn off the water at the main valve. Open the highest faucet in the house, and then open the lowest one, such as an outdoor spigot, to drain the system. As the water drains, air enters the pipes and refills the air chambers.
Close the faucets, turn the water back on slowly and let air bleed out of the highest tap as water returns. The air chambers are now recharged. The effect can last for months or years. If the noise returns within weeks, there is another cause.
Check the pressure
A pressure gauge on a hose spigot shows the house pressure. If it reads well above the usual range of roughly 40 to 80 psi, a failing regulator is a likely cause. High pressure makes water hammer worse and shortens the life of hoses, valves and fixtures.
A plumber can test the regulator, replace it if needed and set it to a safe pressure. An expansion tank can also be added if the water heater is on a closed system, which absorbs thermal pressure swings.
Secure the pipes and add arrestors
Pipes that run through open framing, in an attic or a garage, should be held by straps every few feet. A pipe that bangs against a stud when it stops can be quieted by adding a strap or a foam sleeve. A plumber can find where the knocking occurs by listening along the run. An article in Plumbing & Mechanical magazine, citing the Plumbing & Drainage Institute's PDI-WH201 standard for arrestors, says to place an arrestor at the end of a branch between the last two fixtures, to add another on branches longer than 20 feet, and to put it as close as possible to the quick-closing valve.
A water hammer arrestor is a small device with a sealed chamber that absorbs the shock. It screws onto the supply line to a washing machine or dishwasher or goes in the wall. It does not need recharging. This is the dependable fix for a particular appliance that always bangs.
Finding which fixture causes it
Listen while someone opens and closes each fixture in turn. Washing machines, dishwashers, toilets and a single-lever kitchen faucet are the usual culprits. The one that makes the loudest bang when it shuts is where the cure starts. A bang that comes from the other end of the house means the shock travels a long way, and the cause may be at the fixture or at the regulator by the main.
If the bang comes after a toilet fill valve closes, replace the fill valve with a quieter model. If it comes from the washing machine, the supply hoses and the machine's valve are the place to look.
Washing machines and dishwashers
These appliances use solenoid valves that snap shut in a fraction of a second, which is the most common cause of a sharp bang. An arrestor on the supply, close to the appliance, absorbs the shock. They are inexpensive and simple to install, with a threaded fitting between the supply valve and the hose.
Check the hoses for age. Rubber hoses that are old can bulge and burst, and a braided stainless hose is a better choice. Always turn the supply valves off when you are away for a long time.
What not to do
Do not ignore a bang that gets louder over time, since loose fittings can leak. Do not tighten fittings in the wall without cutting the water first. Do not set the regulator by feel. And do not assume the noise is harmless in a house with old galvanized pipe, which can crack at threaded joints when it is shocked repeatedly.
Hot water and the water heater
Sometimes the bang comes from the hot side because heat expands the water, and a closed system has nowhere for it to go. A thermal expansion tank on the water heater absorbs that growth. If the tank is waterlogged or missing, pressure swings can cause knocks and make the relief valve drip. A plumber can test the tank's air charge in a minute and replace it if it has failed.
Faucets and outdoor spigots
A single faucet that bangs when closed may have a worn cartridge or washer that allows a slam. An outdoor spigot with a quick-closing handle can do the same. Replacing the part, or closing the valve more gently, quiets it. See faucet and sink repair for how these parts are replaced.
A banging noise combined with a leak, a drop in pressure or a pipe that visibly moves is worth a faster call, since a joint may be working loose.
When it is more than noise
Repeated hammering can loosen fittings and crack old pipe over time. A house with old galvanized or brittle pipe is more at risk. If the noise started after plumbing work, ask the plumber who did it to check. If the pipe is corroded and the hammering is hard, the pipe may need replacing, and repiping covers the options.
Common questions
Water hammer questions
Over time, yes. Repeated shocks can loosen joints and shorten the life of fixtures and appliances.
Recharge the air chambers, check the pressure, strap loose pipes and add arrestors on appliances that cause it.
It is common but not harmless. An arrestor on the supply line usually fixes it.
Use a gauge on a hose spigot. A reading far above the typical range suggests a failing regulator.
Related services
Repiping & Whole-Home Pipe Replacement
Replace worn galvanized, copper or older plastic pipe through the whole house.
Water Line Repair & Replacement
Repair or replace the pipe that carries water from the meter into the house.
Pipes banging all the time?
Call and describe when it happens. The dispatcher will connect you with a local contractor.