The washing machine finishes filling and a loud bang echoes through the wall. Or you shut off the kitchen faucet quickly and hear a thud somewhere under the floor. Banging pipes are startling, and it’s natural to worry that something is about to burst.
In most homes, the cause is water hammer, a pressure shock that happens when moving water stops suddenly. This guide explains what causes it, how to tell it apart from other pipe noises, and how to fix it by checking your water pressure, draining the system to restore air chambers, installing water hammer arrestors and securing loose pipes.
Quick answer: Water hammer happens when a valve closes fast and the moving water slams to a stop. Start by testing your water pressure and lowering it if it’s above about 60 psi. Then drain your plumbing to refill old air chambers with air, install water hammer arrestors at the washing machine and other fast-closing valves, and strap down any loose pipes.
What water hammer is (and what it isn’t)
Water flowing through a pipe has momentum. When a valve snaps shut, that water has nowhere to go, and the energy bounces back through the pipe as a pressure spike. The bang you hear is that shock wave, often made louder by a pipe slapping against a joist or stud.
Not every pipe noise is water hammer. Use this table to match the sound to the cause.
| What you hear | When it happens | Likely cause | First fix |
|---|---|---|---|
| Loud bang or thud | Right after a faucet or appliance valve closes | Water hammer | Check pressure, add arrestors |
| Repeated banging or shuddering | While the washer or dishwasher fills | Fast solenoid valves, high pressure | Arrestors at the appliance |
| Rattling or knocking | While water is running | Loose or unsupported pipes | Add pipe straps or cushioned clamps |
| Ticking or creaking | As hot water runs or cools | Pipes expanding against framing | Loosen tight straps, add cushioned hangers |
| Whistling or squealing | When a faucet or toilet fills | Worn washer, partly closed valve, failing fill valve | Repair the fixture or open the valve |
| Humming or vibration | Constant, or when the well pump runs | High pressure or pump vibration | Test pressure, check the pump |
Common causes of banging pipes
Fast-closing valves
Washing machines, dishwashers, refrigerator ice makers and some toilet fill valves use electric solenoid valves that shut off almost instantly. Single-handle faucets that you can slap closed do the same. That’s why water hammer often shows up right after you install a new washer or switch to lever faucets.
High water pressure
The higher the pressure, the harder the shock. The Las Vegas Valley Water District notes that most plumbing professionals recommend a pressure regulator setting of 35 to 60 psi, and that sustained pressure above 80 psi can damage plumbing and fixtures.
A failing pressure regulator
If your home has a pressure reducing valve (PRV) on the main line, it can wear out and let pressure creep up. The City of Lincoln, California lists a hammering noise when you turn on water, along with bursts of high pressure and a water heater relief valve that discharges, as signs of a failing regulator.
Waterlogged air chambers
Many older homes have air chambers: short, capped vertical pipes behind fixtures that hold a cushion of air to absorb shock. Over time, that air dissolves into the water and the chamber fills up, so it stops working. Banging that slowly got worse over months or years often points here.
Loose or unsupported pipes
Pipes that aren’t strapped securely move when the shock wave hits, knocking against joists, studs or ductwork. Loose pipes don’t cause water hammer, but they make it much louder and can wear through over time.
Step 1: Check and adjust your water pressure
Screw a water pressure gauge onto an outdoor spigot or the washing machine cold faucet, turn off all other water use, and open the valve. If you have a PRV, it’s usually a bell-shaped brass valve on the main line just after the shutoff.
- If the reading is over 80 psi, you need a working regulator. Call a plumber if you don’t have one.
- If it’s between 60 and 80 psi, lower it. Loosen the locknut on top of the PRV and turn the adjusting screw counterclockwise a quarter turn at a time on most models.
- Open a faucet briefly after each adjustment, then recheck the gauge.
- Aim for roughly 50 to 60 psi, then tighten the locknut.
- If the reading won’t change, or it creeps back up overnight, the regulator has likely failed.
Our guide to low water pressure in the house explains pressure testing in more detail, including what readings mean and how to check for leaks.
Step 2: Drain the system to restore air chambers
If your home has air chambers, draining the plumbing lets air back into them. It costs nothing and takes about 30 minutes.
- Turn off the main water shutoff valve.
- Open the highest faucet in the house, such as an upstairs bathroom sink, in both hot and cold.
- Open the lowest faucet or spigot, such as a basement laundry tub or outdoor hose bib, and let the water drain completely.
- Flush each toilet once to empty the tanks, and leave all faucets open until the water stops.
- Close the lowest faucet, then slowly turn the main shutoff back on.
- Let each open faucet sputter until it runs smoothly, then close them one by one, starting with the lowest.
Air in the lines will spit and splash for a minute, so keep a towel nearby. If the banging stops, the chambers were waterlogged. If it returns within weeks or never improved, arrestors are the more permanent fix.
Pro tip: Turn off a gas water heater’s burner or set it to “Pilot,” and switch off an electric water heater’s breaker, before you drain the whole system. That keeps it from firing while the lines are empty.
Step 3: Install water hammer arrestors
A water hammer arrestor is a small sealed device with a cushion of air or gas separated from the water by a piston or diaphragm. Unlike a plain air chamber, it doesn’t fill with water over time. It works best installed as close as possible to the fast-closing valve.
At the washing machine
This is the most common and easiest spot. Screw-on arrestors fit between the hot and cold valves and the washer fill hoses.
- Turn off the hot and cold washer valves and unplug the washer.
- Unscrew the fill hoses from the valves, catching drips in a bucket.
- Screw an arrestor onto each valve, hand-tight plus a slight turn with pliers. Make sure the rubber washer is in place.
- Reattach the hoses to the arrestors, turn the valves on, and check for leaks.
- Run a fill cycle and listen. The bang should be gone or much softer.
At other fixtures
Dishwashers, ice makers and fixtures without hose connections need in-line arrestors installed on the supply line, often with push-fit or compression fittings under the sink. If the banging comes from inside a wall with no access, a plumber may need to add an arrestor through an access panel.
Step 4: Secure loose pipes
Go to the basement, crawlspace or any spot with exposed pipes, and have someone run the fixture that bangs. Feel for pipes that jump or vibrate.
- Add pipe straps or hangers where runs are unsupported, using straps made for your pipe type. Use copper or plastic-coated straps on copper pipe to avoid corrosion from mixed metals.
- Use cushioned clamps or rubber-lined hangers where pipes touch framing.
- Don’t clamp hot water pipes, especially plastic ones, so tightly that they can’t move as they expand and contract.
- Add foam pipe insulation where pipes rub against wood. It also helps protect pipes in cold areas, which our guide on how to prevent frozen pipes covers.
Tools and materials
- Water pressure gauge with a hose thread fitting
- Adjustable wrenches and groove-joint pliers
- Screw-on washing machine water hammer arrestors (a pair)
- In-line arrestors for other fixtures, if needed
- Pipe straps, hangers or cushioned clamps
- Foam pipe insulation
- Bucket, towels and a flashlight
Safety note: Repeated pressure shocks stress joints, valves and appliance hoses. A washing machine hose that bursts can release a lot of water fast, so check yours for bulges or cracks, and turn off the washer valves when you’re away for more than a day or two.
When to call a pro
Call a licensed plumber if you notice any of these red flags:
- Your pressure reads above 80 psi, or the regulator won’t adjust.
- The water heater’s relief valve is dripping or discharging.
- Banging comes from inside walls where you can’t reach the pipes.
- You see a new leak, damp drywall or a stain near where the banging happens.
- Draining the system and adding arrestors at the washer didn’t help.
According to HomeAdvisor, a plumbing service call typically costs $100 to $350 and usually includes the first hour, which covers many arrestor installations. HomeAdvisor’s regulator cost guide puts pressure regulator replacement at typically $200 to $700. Jobs that require opening walls cost more.
Frequently asked questions
Is water hammer dangerous?
It’s not an emergency, but it shouldn’t be ignored. Each shock puts stress on joints, valves, appliance hoses and fittings, and over time that can lead to leaks. High pressure combined with water hammer is especially hard on plumbing.
Why do my pipes bang when the washing machine fills?
Washers use solenoid valves that close almost instantly, which is a classic trigger for water hammer. Screw-on arrestors between the valves and the fill hoses usually solve it. Check your water pressure too, since high pressure makes the shock worse.
Why did my pipes suddenly start banging?
Common triggers include a new appliance with fast valves, air chambers that finally became waterlogged, or a pressure regulator that’s starting to fail and letting pressure rise. Test your pressure first, since that’s the easiest way to tell.
Does draining the pipes fix water hammer?
It can, but only if your home has air chambers and they’ve filled with water. Draining lets air back in. If the noise returns or never improved, install water hammer arrestors instead.
Why do my heating pipes or radiators bang?
That’s usually a heating-system issue rather than water hammer in your supply lines. Steam systems bang when condensate collects in pipes that have lost their slope, while hot water systems can knock or gurgle when air is trapped. Our guide on how to bleed radiators covers the air side.
The bottom line
Banging pipes are usually water hammer, a pressure shock from fast-closing valves that high pressure and loose pipes make worse. Test your pressure and keep it around 50 to 60 psi, drain the system to refresh old air chambers, add arrestors at the washing machine and other quick-closing valves, and strap down loose pipes. If pressure is above 80 psi or the noise comes from inside walls, call a plumber.
