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Water moving through a pipe has momentum. When a valve closes suddenly, the water stops, and the shock travels back through the pipes as a bang. Modern appliances make it worse: a washing machine or dishwasher shuts its valve in a fraction of a second. Left alone, water hammer loosens joints, damages valves and wears out appliance hoses.
Is it water hammer?
- A single loud thud when a tap, a washing machine or an ice maker shuts off: water hammer.
- A rattling or ticking while water is running: a loose pipe vibrating against a joist or a strap. Find it and secure it with a pipe clamp or a bit of foam.
- A humming or moaning: usually a worn washer in a tap or a toilet fill valve, or a pressure regulator that is failing.
- Banging from steam radiators is a different matter, covered in our heating advice.
Fix 1: recharge the air chambers
Older houses have short vertical stubs of pipe, capped at the top, near each fixture. They hold a cushion of air that absorbs the shock. Over the years the air dissolves into the water and the chambers fill. Recharging them costs nothing:
- Close the main water shut-off valve.
- Open the highest tap in the house, and the lowest, and every other tap and outdoor faucet. Flush the toilets.
- Wait until water stops running from the lowest tap. The pipes are now drained and the chambers full of air.
- Close every tap. Open the main valve slowly.
- Open each tap briefly to let the air out of the pipes. The chambers keep theirs.
If the banging stops, that was it. It may return in a year or two, and you simply repeat the process.
Fix 2: fit a water hammer arrestor
A water hammer arrestor is a small sealed cylinder with a piston and an air cushion that does not dissolve. For a washing machine, the type that screws between the valve and the hose fits without tools in a few minutes, one on the hot and one on the cold. For a dishwasher, an ice maker or a fixture, the arrestor is fitted on the supply line under the sink or behind the appliance.
Fit it as close to the valve that is causing the bang as you can.
Fix 3: check the water pressure
High pressure makes water hammer worse and wears everything out faster. Screw a pressure gauge onto an outdoor faucet or the washing machine cold valve. Between 40 and 60 psi is normal. Above 80 psi is too high, and the house needs a pressure-reducing valve on the main supply, which is a plumber's job. Many parts of Rockland with town water run well above 80, especially lower down in a valley.
Fix 4: secure loose pipes
Even with the hammer cured, a pipe that can move will knock. In the basement, look for pipes that run long distances between supports, and add straps every few feet. Where a pipe passes through a joist, a strip of foam insulation stops it rattling.
Slow-closing valves
Some newer faucets and appliance valves close gradually to avoid the problem. If you are replacing a washing machine, look for one whose inlet valves are described as soft-closing.
When to call a plumber
- The pressure is above 80 psi, or fluctuates a lot
- The banging continues after the chambers are recharged and arrestors fitted
- You find a joint weeping after a bang, which means the hammer has already done damage
- The banging comes from the main supply pipe when the water meter or a neighbor's fixture operates
Find a licensed plumber near your town.