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Why Do My Pipes Bang When I Turn Off the Faucet?

Wall cutaway of a copper supply pipe and shut-off valve with shockwave rings where water hammer jolts the line

That bang is water hammer: a valve stops moving water so abruptly that a pressure surge slams back through the pipe. The lasting fix is a code-listed arrestor near that valve, with house pressure checked against the 80 psi code limit.

Flowing water has momentum. When a valve snaps shut, the moving column of water has nowhere to go, the pressure spikes, and the surge jolts the pipe against the framing. That jolt is what you hear.

The National Plumbing Code of Canada sums up why it matters: the longer the pipe and the faster the water, the bigger the surge. The pressure can reach many times normal static pressure.

Sioux Chief, a manufacturer of water hammer arrestors, points out that the damage isn’t limited to the pipe. Its list of parts that fail early includes water heaters, temperature-and-pressure relief valves, pressure-reducing valves, backflow preventers, faucets, appliance solenoid valves, fittings, and pipe hangers. So the bang is worth fixing, not just tolerating.

Match the sound to the cause

What you hear Likely cause First check Fix
One sharp bang the instant a washer, dishwasher, ice maker, or faucet shuts off Water hammer at that fixture Test each fixture alone ASSE 1010 arrestor at that valve, hot and cold
Bangs at many fixtures around the house Pressure too high, or a failing pressure-reducing valve Pressure gauge reading Bring pressure to 45–60 psi; a PRV is required above 80 psi
Banging stopped after you drained the pipes, then came back within weeks Old air chambers filling with water again Look for capped pipe stubs near fixtures Replace them with manufactured arrestors
Dull knock or rattle while water runs, or as the pipe settles Loose pipe hitting framing Look at strap spacing on exposed pipe Add supports to code spacing
Water heater relief valve drips, often after a PRV was installed Thermal expansion in a closed system Has a PRV or check valve been added? Pressure-control device such as an expansion tank
Popping or rumbling from the water heater itself Sediment in the tank Where the sound comes from Flush the tank (see water heater popping)

Fix 1: Put an arrestor where the fast valve is

The two International Code Council model codes say the same thing. The International Residential Code (section P2903.5) and the International Plumbing Code (604.9) require a water-hammer arrestor wherever quick-closing valves are used. It must conform to the ASSE 1010 standard and be installed according to the manufacturer’s instructions.

Common quick-closing valves are the electric ones that snap shut on their own, in washing machines, dishwashers, and ice makers. A faucet you flick closed fast can do it too.

What size and where. Sioux Chief’s guideline for homes is a point-of-use AA-size ASSE 1010 arrestor on both the hot and cold lines at each ice maker, dishwasher, washing machine, and tub or shower valve. AA is the small residential size in that standard. The arrestor works by giving the surging water somewhere to go. Inside is a sealed cushion of air or gas, kept permanently separate from the water by a piston, diaphragm, or bellows, which compresses and absorbs the surge.

Installing one:

  • Washing machine: arrestors with hose threads screw on between each supply valve and its hose. Shut both valves, disconnect the hoses, add the arrestors, and reconnect. Laundry boxes are also sold with arrestors built into the valves, which is the cleanest option in new or remodeled walls.
  • Dishwasher or ice maker: tee-style arrestors fit at the supply stop under the sink or behind the fridge. Ice-maker boxes with built-in arrestors exist too.
  • In-wall valves (tub/shower): these need the line opened. Many homeowners hand that off to a plumber; our guide on when a repair is worth calling a pro can help you decide.

Look for “ASSE 1010” on the package. Codes accept nothing less.

Supply line with an AA water hammer arrestor on a tee beside a quick-closing valve; aim for 45 to 60 psi, PRV required above 80 psi

Fix 2: Check the pressure, because it multiplies everything

Higher pressure means faster water and a harder stop. The EPA’s WaterSense program recommends 45 to 60 psi at the house. The IRC (P2903.3.2) and IPC (604.8) cap static pressure at 80 psi and require a pressure-reducing valve (PRV) above that. EPA notes that supply mains can run 100 psi or more, and Department of Energy building researchers at PNNL note that some exceed 200 psi.

How to measure it (based on the EPA’s method):

  1. Screw a water pressure gauge onto an outdoor hose bibb, the cold-water washing machine hookup, or a first-floor cold faucet that takes one.
  2. Make sure nothing else in the house is using water.
  3. Turn that faucet fully open and read the gauge.
Gauge reading What it means What to do
Under 45 psi Below EPA’s recommended range Banging here is unlikely to be a pressure problem; focus on arrestors and supports
45–60 psi EPA’s recommended range Pressure is fine; fix at the fixture
61–80 psi Within the code limit, but above EPA’s range If you have a PRV, set it to 45–60 psi, as EPA advises; if not, it’s worth considering one
Over 80 psi, no PRV Above the code maximum A PRV is required (ASSE 1003 or CSA B356)
Over 80 with a PRV already installed The PRV is failing or set wrong Have it adjusted or replaced

On a private well, the EPA’s guidance is to set the pressure tank at 60 psi.

The PRV catch: thermal expansion. A PRV turns your plumbing into a closed system. When the water heater warms a tank of water, that water expands and has nowhere to go, so pressure climbs.

The IRC (P2903.4.1) requires a pressure-control device when thermal expansion pushes pressure downstream of the PRV above its setting. The same goes for homes with a check valve or backflow preventer (P2903.4.2). In practice that often means a thermal expansion tank at the water heater.

A relief valve on the water heater that drips after a PRV goes in is a warning sign. Never cap or plug a dripping relief valve; fix the expansion problem instead.

Fix 3: Don’t rely on the old drain-and-refill trick

Older homes were often built with air chambers, short capped stubs of pipe near fixtures that trap a pocket of air. Over time the chamber can fill with water, which is one reason a house can go quiet for years and then start banging.

The common advice is to shut the main, open every faucet from the top floor down to drain the system, then refill. That can refill the chambers with air and quiet things for a while. Treat it as a test, not a fix.

The National Plumbing Code of Canada says plainly that air chambers made from a piece of vertical pipe do not provide acceptable protection, and requires manufactured arrestors instead. If draining helps and the bang returns, you’ve confirmed the cause: replace the chamber’s job with an ASSE 1010 arrestor at that fixture.

Fix 4: Strap the pipes to code spacing

Even with water hammer under control, a pipe that’s free to move will knock. The IRC sets maximum spacing between supports (Table P2605.1):

Pipe Max. spacing, horizontal runs Max. spacing, vertical runs
Copper tubing, 1¼ inch and smaller 6 feet 10 feet
PEX, 1 inch and smaller 32 inches 10 feet, plus a guide midway
CPVC, 1 inch and smaller 3 feet 10 feet, plus a guide midway
PVC 4 feet 10 feet, plus a guide midway for 2 inch and smaller

Walk the exposed runs in the basement or crawlspace with a tape measure. Check PEX first: on horizontal runs it needs a support every 32 inches, far closer than copper’s 6 feet. Cushioned clamps help stop the knocking where a pipe rubs a joist.

When to call a plumber

  • The pressure is over 80 psi, or you have a PRV and it’s still high. The PRV and the expansion control that goes with it are main-line work.
  • The water heater’s relief valve drips.
  • You see dampness, stains, or drips near a pipe that bangs. A joint may already be stressed.
  • The bang comes from inside a wall at a shower or tub valve.
  • You’ve added arrestors at the fast-closing valves and checked pressure and supports, and it still bangs.

FAQ

Is water hammer dangerous or just loud?

It’s more than noise. Each surge hits the whole system, and the arrestor maker Sioux Chief lists water heaters, relief valves, PRVs, faucets, and fittings among the parts that fail early from repeated hammer. Codes also require arrestors at quick-closing valves.

Why did it start suddenly in a house that was always quiet?

Common triggers are a new washing machine or dishwasher (a new fast-closing valve), air chambers that finally filled with water, or a change in supply pressure, including a PRV that’s wearing out. Testing each fixture and taking a pressure reading usually points to which one.

Do I need arrestors on both the hot and cold lines?

Yes, if both have a quick-closing valve. The residential guideline from Sioux Chief is an AA-size arrestor on both lines at each washer, dishwasher, ice maker, and tub/shower valve.

Sources

Written and reviewed by peter.bae for Hearth & Habit · Last reviewed October 11, 2026

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