Pipe Banging — How to Stop Water Hammer and Knocking Pipes

Common Symptoms

• Loud bang or thud in the walls when the washing machine cycle ends
• Knocking sound when the dishwasher solenoid valve closes
• Bang when an icemaker finishes filling
• Pipes vibrate or knock when any faucet is closed quickly
• Slow rhythmic banging or ticking — not a sharp bang (may be thermal expansion, not hammer)
• Sudden onset of pipe banging after years of quiet operation

Most Likely Causes

1. 1

   Washing Machine or Dishwasher Solenoid Valves (Most Common Cause)

   Modern washing machines and dishwashers use solenoid valves that snap shut in milliseconds when the control board cuts power — much faster than any manual valve. This instantaneous shutoff sends a high-pressure shock wave backward through the supply line. The banging occurs at the end of the fill cycle or when the machine transitions between cycles.

2. 2

   Air Chambers Have Lost Their Air Cushion

   Many homes built before 1970 have vertical dead-end pipe sections (air chambers) installed in the walls near appliances. These air pockets absorb the shock of water hammer. Over time, the air dissolves into the water and the chamber fills with water, losing its cushioning function entirely. The fix: drain and repressurize the system to restore the air pocket — a completely free repair.

3. 3

   High House Water Pressure (Over 80 PSI)

   Water hammer severity scales with pressure squared — double the pressure means four times the shock force. If your house pressure exceeds 80 PSI (normal is 50–70 PSI), every valve closure causes amplified hammer. High pressure also stresses appliance inlet valves, accelerates washer wear, and can void appliance warranties. Test pressure with a $10 gauge at an outdoor hose bib.

4. 4

   Quick-Close Ball Valves on Main Lines

   Ball valves installed on 1-inch or larger main supply lines close in 90° of handle rotation — extremely fast. A plumber who upgraded valves during a repair may have introduced this as a new hammer source. Consider installing a slow-close ball valve or adding a water hammer arrestor on the downstream side.

5. 5

   Loose Pipe Strapping in Walls

   When pipes are not adequately strapped to framing, the shock wave causes the pipes to physically jump and strike the framing or other pipes. This creates a thud or rattle that differs slightly from pure hammer — it tends to move around (you hear it from multiple wall locations rather than a single sharp point).

6. 6

   Thermal Expansion (Slow or Rhythmic Banging)

   If the banging is slow and rhythmic rather than a sharp sudden knock, it may be thermal expansion — pipes expanding as hot water flows through them and contracting as they cool. This is especially common in CPVC and copper supply lines not adequately supported. The fix is additional pipe hangers, not hammer arrestors. On closed plumbing systems (check valves, backflow preventers), thermal expansion from the water heater may require a thermal expansion tank.

Quick DIY Checks

1. 1Identify the source of the bang. Listen carefully: does it occur at a specific moment (end of washing machine fill, dishwasher valve close, icemaker fill)? That pinpoints the solenoid valve causing it. Does it happen when any faucet is turned off quickly? That's a pressure or air chamber issue. Is it slow and rhythmic (tick-tick-tick) rather than a sharp bang? That's thermal expansion, not hammer — see the thermal expansion section below.
2. 2Check house water pressure: buy a water pressure gauge ($10–$15) at any hardware store and thread it onto an outdoor hose bib. Turn the bib fully on and read the gauge while no other water is running inside. Normal range is 50–70 PSI. If you read above 80 PSI, high pressure is amplifying your hammer and you'll need to address the pressure reducing valve (PRV) regardless of what other fixes you do. If pressure is above 100 PSI with no PRV installed, this is a priority repair.
3. 3Try the air chamber drain procedure (free fix — works if air chambers are installed): this procedure drains all the water out of the vertical dead-end air chambers and allows air to re-enter. (1) Turn off the main water supply valve. (2) Open every faucet in the house, starting from the highest (upstairs) and working down to the lowest (basement or outside hose bibs). Let all water drain completely — this takes 2–5 minutes. (3) Flush every toilet once. (4) Turn the main supply valve back on slowly. (5) Let the faucets run until steady water pressure resumes, then close them from lowest to highest. The air chambers should now be recharged. Test the washing machine — if hammer is gone, the air chambers were the problem. This fix may need to be repeated annually.

1. 4Install water hammer arrestors on the washing machine supply lines: if the air chamber drain doesn't solve the problem, or if your house has no air chambers (most homes built after the 1980s don't), install water hammer arrestors on the hot and cold supply lines to the washing machine. A water hammer arrestor (Sioux Chief 660-G or equivalent) contains a small air chamber sealed by a piston — it absorbs the shock wave without the air dissolving into the water over time. Cost: $15–$25 each, and installation takes about 15 minutes. The arrestor threads onto the same connection as the washing machine hose — shut off the supply valves, unscrew the hose, screw in the arrestor, then reconnect the hose to the arrestor's outlet. Do the same for the dishwasher and icemaker supply lines if those are also banging.
2. 5Install water hammer arrestors on the dishwasher supply line: the dishwasher cold water supply is usually a 3/8-inch or 1/2-inch compression or braided line under the sink. Install a hammer arrestor on the hot supply valve under the sink (dishwashers draw hot water). Most arrestors have a 3/8-inch or 1/2-inch female thread — match to your supply line. This is a 10-minute job with two pairs of pliers.
3. 6Check and adjust or replace the pressure reducing valve (PRV): if house pressure is over 80 PSI, locate the PRV on the main supply line entering the house (usually in the basement or utility room — it's a bronze bell-shaped device with an adjustment screw on top). Turn the screw clockwise to increase pressure, counterclockwise to decrease. Make half-turn adjustments and recheck with your gauge after each adjustment, waiting a minute for pressure to stabilize. Target 60–65 PSI. If the PRV is old, stuck, or you can't reach the target pressure, replacement is needed. PRV replacement is a $50–$100 part job — it requires cutting the water main briefly (shut off at curb stop) and soldering or using push-fit fittings depending on your pipe type. Professional installation runs $150–$350.
4. 7Address loose pipe strapping: if pipes are banging or moving in the walls (you can feel vibration in the framing or hear thumping from multiple locations rather than one spot), the pipes need additional strapping to the framing. For accessible pipes in basements or crawl spaces, add plastic pipe hangers every 4–6 feet for copper and every 3–4 feet for CPVC, secured to framing with 1-1/4-inch wood screws. The pipe should fit snugly but not so tight that it can't expand slightly — leave 1/16-inch clearance.
5. 8Thermal expansion tank for closed systems: if your plumbing system has a check valve or backflow preventer on the main line (standard in most newer construction), you have a 'closed system' — water heated by the water heater has nowhere to expand except back through the plumbing, which causes pressure spikes. Signs: pressure relief valve dripping periodically, rhythmic banging or ticking from pipes, pressure gauge readings that rise significantly after the water heater runs. The fix is a thermal expansion tank ($40–$80) installed on the cold water inlet to the water heater. This is a plumber job if you're not comfortable soldering or using push-fit connectors.

Repair vs Replace

Recommended Tools & Parts

• Water Hammer Arrestor (Sioux Chief 660-G or equivalent)

  Piston-type water hammer arrestor for washing machine, dishwasher, or icemaker supply lines. Size A (660-A) fits 3/8-inch lines; Size G (660-G) fits standard 3/4-inch washing machine hose connections. Install one on hot and one on cold for washing machines.

  $15–$25 each

  Buy on Amazon →

• Water Pressure Test Gauge

  Attaches to any standard hose bib. Reads static house pressure in psi. Essential for diagnosing pressure-related hammer before spending on parts.

  $10–$15

  Buy on Amazon →

• Adjustable Pressure Reducing Valve (PRV)

  Replacement adjustable PRV for homes with high water pressure or a failed existing PRV. Available in 3/4-inch and 1-inch sizes. Reduces municipal supply pressure (80–150 PSI) to safe household levels (55–70 PSI).

  $50–$100

  Buy on Amazon →

• Thermal Expansion Tank

  Pre-charged bladder tank installed on the cold water inlet to the water heater on closed plumbing systems. Absorbs expansion pressure from heated water, prevents pressure relief valve dripping and thermal expansion banging.

  $40–$80

  Buy on Amazon →

• Plastic Pipe Hangers and Straps

  Cushioned plastic pipe hangers for securing copper and CPVC supply lines to framing in basements and crawl spaces. Proper strapping prevents pipe movement that amplifies hammer noise.

  $8–$15 per pack

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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