Water hammer: what is it & 5 possible causes

Water hammer occurs when the water in a pipe is suddenly forced to stop or change direction. It’s often the result of a valve closing abruptly, such as a tap or a ball valve. It typically produces banging, vibrating, and knocking sounds.

Water hammer isn’t just an annoyance. It can generate leaks or even cracks or friction damage in walls if left untreated.

Water hammer explained

When the water in a pipe suddenly stops or changes direction, a shockwave is created. This phenomenon is known as hydraulic shock. It’s the result of the sudden pressure increase inside a pipe when a valve is closed quickly. The pressure wave resonates along the affected pipes and fittings until it dissipates. This causes unwanted noises.

Water hammer is less likely to occur on things that use low-pressure water, like things supplied by a cold water storage cistern. However, it can also occur on pumped systems, such as on pipework for a pumped shower, and on central heating systems.

What causes water hammer?

Water hammer typically occurs on pipework and valves which carry water at high pressure, such as on sealed central heating systems, pipework belonging to a pumped shower, and pipework carrying mains pressure water. Here are several things that can generate water hammer:

  • Ball valves
  • Quick operation valves
  • Airlocks
  • Worn valves/stopcocks
  • Improperly secured pipework

Ball valves

A common suspect is the ball valve in the cold water storage cistern. As the cistern fills, the float can bob up and down on the surface of the water, repeatedly opening and closing the valve.

A tried and tested way to prevent this from happening is to tie a small empty yogurt pot to the arm of the valve next to the float with steel wire. The yogurt pot should sit upright, just below the surface of the water. This will dampen the effect of the float bouncing on water ripples.

If there isn’t one already there, another good idea is to fit a small plastic backing plate on the ball valve. This strengthens the cistern wall, protecting it from the stress of the upward force of the float, and makes the float less likely to bounce. The backing plate is secured to the outside of the cistern via a nut on the stem of the ball valve.

For the cold water storage cistern, make sure that the nozzle inside the valve is the high-pressure white nozzle. Using a red low-pressure nozzle on a high-pressure pipe will result in an extremely high flow of water and will make the system much more vulnerable to water hammer. Replacing a Part 1 ball valve with a Part 2 valve may also solve the problem.

Quickly operated valves

Fast filling toilet cistern valves on mains-fed toilets with a quick shut-off feature can also cause the problem.

Valves operated by an electronic solenoid can easily cause water hammer due to their instantaneous action.

Turning taps off quickly

Even turning taps off too quickly can create unwanted vibrations and noises in pipes.

Air locks

Air trapped inside a sealed central heating system, such as a system with a combi boiler, can cause or exacerbate water hammer.

Improperly secured pipes

Lengths of pipe which have not been secured in place with pipe clips are much more vulnerable to water hammer.