Pipes Banging When the Water Is Turned Off? Causes and Fixes

Pipes that bang when you turn off a faucet or a machine shuts off are usually suffering from water hammer: moving water stops suddenly and slams into the closed valve. Loose pipes, high water pressure and waterlogged air chambers make it worse. Strap loose pipes, check the pressure, and add arrestors if the banging continues.

The noise can sound alarming, like something heavy hitting your walls. Most of the time it is a plumbing habit that is easy to fix, though it should not be ignored for years.

Why do my pipes bang when the water is turned off?

Water has weight. When it moves fast through a pipe and a valve closes quickly, the water has nowhere to go and slams into the valve. The pressure surge travels back through the pipes with a loud bang. Causes include:

• Fast-closing valves. Washing machines and dishwashers use solenoid valves that snap shut. Single-lever faucets can do the same.
• High water pressure. The higher the pressure, the harder the surge. Pressure above about 60 psi makes banging more likely.
• Waterlogged air chambers. Older homes may have short vertical pipes behind walls that hold air to absorb shocks. Over time they fill with water and stop working.
• Loose pipes. Pipes that are not strapped can swing and hit studs and joists when the surge arrives.
• Long straight runs of pipe. The longer the run, the more momentum the water carries.
• A failing pressure regulator. If the regulator at the main is bad, the pressure may be too high.

Is it water hammer or something else?

A quick test: turn a faucet off fast and listen. A single thud points to water hammer. A prolonged rattle points to loose pipes. If it happens only with hot water, thermal expansion is more likely.

6 fixes, easiest first

1. Close taps more gently. Quick turns and flicks of a lever create the surge. This may be the whole cause at one sink.
2. Strap loose pipes. Where pipes are visible, such as in a basement or crawl space, add pipe clamps or hangers so they cannot swing. Foam pipe insulation where they touch wood can also help.
3. Check your water pressure. Screw an inexpensive pressure gauge onto an outdoor hose spigot, with no water running elsewhere. Aim for roughly 40 to 60 psi. Anything above about 80 psi should be brought down.
4. Recharge the air chambers. In an older house with air chambers, you can drain the system to refill them with air. Turn off the main water, open the highest faucet and the lowest faucet or drain, and let everything drain. Close the faucets, turn the main back on slowly, and open each tap to bleed air.
5. Install water hammer arrestors. These screw onto the washing machine hoses, or a plumber can fit them near the problem fixture. They are one of the most reliable cures for banging that continues.
6. Replace or adjust the pressure regulator. If the pressure is too high, a plumber can adjust or replace the regulator where the line enters your home.

What if the banging happens with the water off?

If you hear banging when no one is using water, check a few things:

• A running toilet. A fill valve that opens and closes can cause a knock. See a toilet that refills by itself.
• A small leak. A leak can cause pressure changes. Look for damp spots, or see finding a concealed leak.
• A pressure swing from the supply. City pressure changes can create a thump, especially at night when demand is low.
• Thermal expansion. If your system has a closed backflow preventer, pressure can build up as water heats. An expansion tank on the water heater helps with this.

Is high pressure a bad thing?

High pressure helps showers feel strong, but it is hard on pipes, fixtures and appliances. Most plumbing codes keep household pressure below 80 psi. If your pressure is high, a regulator protects everything. If pressure is low instead, the cure is different, as in low water pressure at home.

Is it worth fixing?

A few pipe straps and a pressure gauge cost very little. Arrestors are inexpensive. A new pressure regulator costs more, but it protects your whole plumbing system. Repeated banging can loosen joints and shorten the life of valves and appliances. Fixing it early is cheaper than repairing a leak behind a wall.

When to call a professional

Call a licensed plumber if:

• The banging continues after straps and arrestors.
• Your pressure is above 80 psi, or you are not sure how to read it.
• The pipes are in walls, and you cannot reach them.
• You see leaks, stains or moisture.
• You want a regulator installed or replaced.
• You have an older house and suspect failed air chambers.

What not to do

• Do not ignore banging for years.
• Do not turn off the main valve and leave a leak running.
• Do not use too much force on old fittings.
• Do not drain the system without first turning off the water heater power or gas.

Bottom line

Banging pipes are usually water hammer. Start with strapping the pipes and checking the pressure, then add arrestors. If you hear banging with the water off, look for a toilet, a leak or a pressure problem. If your pipes freeze in a cold snap, see winter pipe protection.