Water hammer is the bang, thud or rattle you hear in the walls when water is stopped suddenly: a washing machine finishing a fill, a faucet snapped shut, a toilet tank topping off. Moving water has weight, and when a valve slams it closed, that water slaps into the closed end and sends a shock back through the pipe. Most cases are fixable with a cheap part. The rest point to high pressure or worn plumbing that deserves a proper look. If you are in Burbank or the Valley and want a plumber to see it, Lankershim Plumbing can arrange a visit.
What causes the banging sound when I turn off a faucet?
The noise comes from moving water halted quicker than the pipe can take it. A valve shuts, the flowing column has nowhere to go, and the pressure spike throws the pipe against a stud or strap. Quick-acting valves in washers and dishwashers set it off most often, followed by single-lever faucets and toilet fill valves.
Think of it as a car braking hard with a truck behind it. Three things make the spike worse in a house:
- High supply pressure. The higher the pressure, the harder the slam.
- Long straight runs of pipe. A longer column of water carries more momentum.
- Nothing to cushion the stop. Homes built with vertical air chambers behind fixtures got a built-in shock absorber. Over time those chambers fill with water and stop working, so a house that was quiet for decades can start knocking.
Loose pipes make it louder. Even a mild pressure spike sounds like a hammer when the pipe is free to swing against framing. Pipes that were strapped tightly in 1955 may have loosened after decades of seasonal heat and cold.
How do I find out which fixture is causing it?
Begin with a walk-through. Run each appliance and fixture singly, shut it off and listen. The usual suspects are the washer, dishwasher, ice maker line, toilet, kitchen faucet and outdoor hose bib. Note which one triggers it, and whether the knock sits in the wall beside it or somewhere distant, because pipes carry sound far.
A few cheap checks narrow it down:
- Close the shutoff valve to the suspect appliance halfway and run it again. A quieter result means the appliance valve slams too hard or the supply is too strong.
- Put a hand on the pipe as the water stops. Often you can feel the jump at its source.
- Look at exposed pipe in the garage, crawl space or under the sink for loose straps and pipes touching wood or metal.
- Noise with nothing running, or only when the water heater fires, is a different problem: ticking from expansion is common and harmless, but a knock tied to the heater deserves a plumber.
Smell gas near a water heater or any gas appliance? Skip every check. Get outside first, then ring SoCalGas on 1-800-427-2200, or dial 911 if someone is at risk.
How do I stop water hammer in my house?
Fixes tend to be small and cheap. Typical ones: water hammer arrestors, sealed devices with a cushion that soaks up the spike, on the washer or dishwasher supply; recharging old air chambers; replacing a worn fill valve; taming high supply pressure with a regulator; and re-strapping loose pipes so they stop swinging.
Arrestors thread on between the supply valve and the appliance hose, so the washing machine is a reasonable DIY job. Draining the whole system to recharge old air chambers helps for a time but often relapses, which is why many plumbers fit arrestors on the troublesome line.
Pressure matters more than most people expect. Houses at the foot of a hill, or on streets where the utility runs high, can see far more than the building was designed for. A plumber reads it in a minute using a hose-bib pressure gauge. A high reading calls for a regulator near the main shutoff, which protects every fixture and appliance. An aged regulator can also fail and pass full pressure, another reason to test it.
Quieter parts help as well. Current toilet fill valves close gently, and some makers sell quiet-fill and slow-closing versions. See our guide to replacing a toilet fill valve if the knock starts as the tank refills.
Is water hammer dangerous for my pipes?
An occasional knock is mostly a nuisance, yet repeated spikes can fatigue joints, loosen fittings and wear out valves and appliance hoses. The hammer seldom bursts a healthy pipe. In old, thin or corroded pipe, however, it can split a weak spot that was one shock from leaking.
So treat a new or worsening hammer as a warning. Postwar tract homes across Burbank, North Hollywood and Sherman Oaks often still have galvanized steel supply lines in the walls, and the inside of that pipe narrows as it corrodes. Narrow pipe raises velocity, and higher velocity makes the shock worse. A house with pinhole leaks, rusty water and hammer is saying one thing three ways. See signs it is time to repipe if that sounds familiar.
Keep an eye on the bill and meter too. Hammer that begins as the water bill climbs may signal a leak or failing valve, and the meter test in our high water bill guide takes two hours. Per EPA, the typical home wastes over 9,300 gallons a year through leaks.
When should I call a plumber about water hammer?
Phone one when the banging is loud, worsening, or paired with other symptoms: a dripping valve, rust-coloured water, weak pressure at some fixtures, or damp drywall near a pipe. Also ring if washer-line arrestors did not cure it, or if you suspect excess pressure. The visit can test pressure, pick spots for arrestors and inspect the regulator.
Anything that alters the main water line, moves pipe or touches gas piping is not for DIY. Burbank permits for such work go through the city's building division. Homes in the Los Angeles neighborhoods of the Valley, such as Van Nuys or Encino, go through LADBS. Whoever does the work deals with those rules.
A typical visit: you describe the sound, the plumber measures pressure and listens at each fixture, and the quote comes in writing first. Ask for a quote and someone local comes out. Our emergency page explains what to do if a pipe has already burst.
We are serving homes from Burbank to Sherman Oaks, and the washer and dishwasher hookups are where we would look first.
Questions people ask
Why do my pipes bang only when the washing machine stops?
A washer's solenoid valve cuts off within a split second, about the fastest any household valve can stop flow, and that sends the shock down the supply pipe. Arrestors on the washer hookup usually cure it. If the noise persists, check the pressure and look for a loose pipe behind the wall.
Can water hammer damage a water heater or appliance?
Repeated shocks can wear fittings, supply hoses and valve seats and shorten the life of an appliance connection. The tank is rarely the first part to fail. Some homes have a thermal expansion tank on the water heater, which also cushions pressure swings. Have it checked to see whether yours works.
Is water hammer covered by the plumbing code?
Codes do address it, and quick-closing fixtures are expected to have shock protection. Exact rules depend on the code edition and the job. Find out from the contractor what applies to your house, and whether the permit office must see the change.
How much time does water hammer repair take?
One arrestor on a washing machine line often takes under an hour. Pressure testing, a regulator or chasing a loose pipe through a wall takes longer, and more again if walls must open. An estimate comes after listening and measuring, and the written quote lists what is included.
Does someone have to be home for the visit?
Yes, an adult ought to be on hand to give access to the main shutoff, the washer and the water heater, and to hear the findings. If that is impossible, send a trusted adult to open the door, and agree in writing what work is approved before it begins.
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