Water Hammer: Knocking sounds coming from pipes.

If you hear a strange knocking sound when you turn off a faucet or use your washing machine, then you are probably experiencing “water hammer.”

This is a common issue that affects many people’s houses.

What is causing this knocking sound?

Water hammer is what happens when a fast-moving water supply comes to a halt.

It is the sound of water banging up against the valves in your pipes.

When you or one of your appliances shuts a valve, a surge of pressure will build up in the pipes.

Water Hammer Graph

This graph highlights how shutting a valve can increase the water pressure. As you can see, there is a big surge at 0.1 seconds.

Remember that water is relatively heavy (dense) and that it can gain a lot of momentum while it is travelling through the pipes inside your home.

When you turn off your faucet or your washing machine closes its water inlet valve, that momentum has nowhere to go. As a result, it slams up against the valve.

The next thing you know, you’re hearing strange knocking sounds reverberating inside your walls.

Can these banging noises damage your pipes?

Yes, water hammer can damage pipes, joints and other supports that hold your pipes together. Over time, the increased water pressure and shock waves can loosen the joints and fittings that keep your pipes secure.

It can also cause small cracks to appear in your pipes.

As you can imagine, small leaks can quickly turn into bigger leaks. Consequently, you may end up with structural damage and dampness, which can create the perfect breeding ground for other serious issues such as mold.

It is worth noting that the pipe structure is not the only thing that is at a risk of failing.

As the pipes vibrate from the resulting shock wave, they will pull at the fittings that keep them secured to the wall.

If this continues, the constant movement could pull the fittings out of the wall.

In conclusion, yes, water hammer can damage your pipes. This is an issue that you should investigate before it becomes more than just a knocking sound.

What causes water hammer?

Most modern plumbing systems have air chambers.

These air chambers act like cushions. When a sudden surge of water hits the air chamber, the air in the chamber helps to soften the blow.

Take the following air chamber as an example:

Air chamber

A diagram of an air chamber behind a sink.

In the diagram above, there is an air chamber behind the wall. This air chamber helps to soften the surge of water that comes rushing towards the sink.

Unfortunately, in some cases, these air chambers can become waterlogged.

This happens when a surge of water manages to get into the chamber and absorb the air. As a result, it will no longer function as a cushion.

How to fix it.

If these loud sounds weren’t there before, then it is possible that one or more of your air chambers have become flooded.

To fix this issue, you will need to completely drain the water from your pipes.

Thankfully, you do not need to be a plumber in order to do this.

You don’t even have to be an expert at DIY. It is actually a pretty easy fix to attempt all by yourself.

To re-fill your chambers with air, you will need to take the following steps:

  1. Completely shut off the water supply to your house. The valve to shut off the water supply can be typically located in the basement, under your kitchen sink or on the outside of the property.
  2. Once you have shut off your water supply, you should turn on the faucet in your kitchen, as well as the faucet in your upstairs bathroom (if you have one). This will allow the remaining water in your pipes to drain out. If you have a faucet in your basement or in your yard, then you should turn this one on. The goal is to turn on two faucets: The highest one and the lowest one.
  3. After a while, the water should come to a stop. If it doesn’t, then it means that you didn’t restrict the water supply properly.
  4. Once the water has stopped flowing, turn off the bottom faucets and turn the water supply back on. Do not turn off the upstairs faucet yet.
  5. When the top faucet starts flowing again, it means that the water has risen through the system. You can now turn it off.

If the fix was successful, you should no longer hear any knocking sounds.

“That didn’t fix my issue.”

If the solution above did not resolve your issue, then you might need to call a plumber. In some cases, air chambers can become clogged with sediment and limescale.

It is also possible that the air chambers are unable to handle the water pressure. In other words, the water pressure coming into your property might be too high.

That, or the previous plumber didn’t install the air chamber in a practical location.

A plumber can investigate the issue and install a Water Shock Arrestor, which is a mechanical device that contains springs and an air bladder to help soften the flow.

These are popular in commercial buildings and apartment blocks, where air chambers can be ineffective.

If the pressure coming into your property is too high, a plumber can also install a regulator that keeps the PSI (pressure) at a recommended level. Plumbers typically install these devices close to the water input so that they can control the supply of water that is coming into your home.

If pressure is the issue here, then you should definitely consult a professional, as a PSI that is above normal levels can cause damage to pipes, fittings and kitchen appliances.