Quick answer
Banging pipes are usually water hammer: a fast-closing valve stops moving water suddenly and the shock slams through the pipes. Loose pipes, high water pressure and failed air chambers make it worse. Safe checks include noting when it happens and securing visible pipes. Persistent banging, leaks or high pressure readings call for a plumber.
What water hammer is
Water hammer happens when a closing valve suddenly stops moving water. Sioux Chief, a plumbing products manufacturer, explains that the water column behind the valve slams into the valve and the pipe. Watts, another manufacturer, describes it as a bounceback caused by sudden stoppage, and compares it to a car hitting a wall: the faster the flow, the greater the impact.
The usual sign is a bang or thump, but Sioux Chief notes that water hammer is sometimes silent. It can occur on both hot and cold lines. Silence alone does not rule it out.
The main causes
Most banging comes from one or more of the following causes.
- Fast-closing valves. Sioux Chief lists washing machines, dishwashers, tub and shower valves, ice maker lines, faucets, toilet fill valves and sprinkler systems as common sources.
- Loose pipes. Oatey, a plumbing products manufacturer, notes that pipes without enough support can move and rattle against walls or framing.
- High water pressure. Oatey says pressure above 80 psi can make pipes vibrate and bang, and fast flow increases the force of the shock.
- Failed air chambers. Sioux Chief states that air in a capped standpipe or air chamber is absorbed over a few weeks, leaving the chamber waterlogged and useless.
- Sediment in the water heater. Oatey notes that buildup can cause knocking as steam bubbles form and release.
Why it matters
Sioux Chief reports that on a 50 psi line, uncontrolled water hammer commonly produces spikes of 250 to 400 psi. The spike starts at the valve and travels back through the plumbing until it is absorbed, which can damage pipes, fittings, water heaters and other components.
This is why repeated banging is worth acting on.
Air chambers and arrestors
Many older homes have short capped pipes near fixtures that were meant to cushion the shock with trapped air. Sioux Chief says these are not an effective long-term fix because nothing separates the air from the water, so the air is absorbed and the chamber stops working.
A water hammer arrester works differently. Watts describes an arrester with a pre-charged air chamber separated from the water by a sealed diaphragm that flexes to absorb the shock. Sioux Chief says arresters should be installed within six feet of the valve, because effectiveness drops with distance. Watts publishes a chart for the number of units needed based on pipe size, length and pressure, and states its residential arrester is rated for a maximum working pressure of 150 psi.
Oatey describes fitting an arrester as a simple job for a handy homeowner: shut off the water, hand-tighten the arrester onto the shut-off valve, snug it with a wrench without overtightening, then restore water and listen for a change. Sioux Chief cautions that a single large central arrester or an expansion tank does little to control hammer throughout the system, since the spike has already done its damage by the time it reaches them. If you are unsure where the arresters belong, a plumber can place them at the valves that cause the problem.
Safe checks you can do yourself
These checks do not involve cutting pipe or touching gas or electrical connections.
- Notice when the bang happens. Does it follow a specific appliance, a toilet, or a faucet? Note hot or cold side.
- Make sure faucets and valves are fully open. Oatey notes that partly closed valves restrict flow and create turbulence and noise.
- Look at exposed pipes in basements, crawl spaces and garages. Can you see pipes that move or rest against framing? Oatey advises supporting pipes at changes of direction and cushioning them where they pass through holes. Do not bend or force pipes.
- Measure your pressure with a gauge on an outdoor hose bib with nothing running. Readings above 80 psi are a concern, and PNNL advises static pressure of no more than 60 psi.
- Check the water heater for signs of sediment noise, such as rumbling. Draining the tank involves hot water, so follow the manufacturer's instructions and take care.
What not to do
Do not cut into walls to find an air chamber, and do not tighten or loosen fittings under pressure in hopes of stopping the noise. Do not turn a pressure reducing valve adjustment screw without knowing the setting you are aiming for, because that can lead to pressure that is too high or too low.
If you have a water heater that makes banging or rumbling and also leaks, or whose relief valve is discharging, treat that as a separate issue. The temperature and pressure relief discharge pipe should never be capped.
When banging signals a bigger problem
Occasional hammer from a single appliance can be a simple fix. These signs point to something larger.
- Pressure that tests above 80 psi, which suggests a failed or missing pressure reducing valve.
- Banging along with leaks at joints, damp drywall, or a water bill that has climbed.
- Banging that gets worse or spreads across the house.
- A meter that moves when everything is off, per the EPA's two-hour meter test.
- Older pipe that is corroded, undersized or poorly supported throughout the house. If pipes need support or replacement in many places, repiping may be the practical fix.
When to call Pyper Plumbing
If the banging is constant, linked to leaks, or paired with a high pressure reading, it is worth having someone look at the whole system rather than silencing one fixture. Pyper Plumbing provides emergency repair, leak detection and repair, water heater service and repiping in Snohomish, Skagit, Whatcom and Island counties.
If a pipe has shifted, a joint is spraying or you see water where it should not be, turn off the main valve first, then call. Pyper does not do gas line, septic, well pump, HVAC or electrical work, so if the noise comes from one of those systems, contact the right specialist.
Common questions
Is water hammer dangerous?
The bang is not dangerous in itself, but Sioux Chief reports spikes of 250 to 400 psi on a 50 psi line, which can damage pipes, fittings and water heaters over time.
Why do my pipes bang when the washing machine stops?
The machine's valve closes quickly and stops moving water suddenly. Sioux Chief and Watts both list washing machines among the common sources of water hammer.
Can high water pressure cause banging pipes?
Yes. Oatey says pressure above 80 psi can make pipes vibrate and bang. Test with a gauge on an outdoor hose bib and have a plumber check the pressure reducing valve if the reading is high.
Do plastic pipes still get water hammer?
Yes. Sioux Chief states that water hammer must be controlled regardless of pipe material, including plastic pipe.
Sources
Facts in this guide come from the following references. Local rules and manufacturer instructions can differ, so follow your utility and your equipment manual.
- Water Hammer Arrester FAQ (Sioux Chief)
- Series 150A Water Hammer Arrester (Watts)
- What Causes Banging Pipes and How to Stop It (Oatey)
- Service Water Pressure (PNNL Building America Solution Center)
- Fix a Leak Week (U.S. EPA WaterSense)
This guide is general information, not a substitute for an on-site inspection. If you smell natural gas, leave the building and call 911 or your gas utility. Pyper does not perform gas line work.
