Why Are My Pipes Knocking When the Toilet is Flushed?

Are your pipes making loud knocking noises every time you flush your toilet? Are you worried that you might have a major plumbing issue on your hand? Wait before you frantically call your plumber. While the loud banging noises can be scary, they are quite easy to fix if you understand the reason behind them. 

The most common reason your pipes knock when the toilet is flushed is the water crashing onto itself due to the sudden closing of the automatic flush valve. As you pull the flush, the water from the pipes enters the tank with force. If the flush valve closes suddenly, the water has nowhere to go, dissipating energy as the loud knocking sound. 

The loud knocking sound is called a ‘water hammer’ in the plumbing industry and is caused by a spike in the water pressure. But why does it happen in the first place? How can you fix it? Let’s find out!

What Makes the Pipe Knock When Flushing the Toilet?

Let’s see how your toilet flush works to understand the reason behind the occasional loud knocking noise when flushing your toilet. The water feed in your house is maintained under pressure to allow it to reach every room in your home. When the toilet fill valve of your flush tank is closed, the water on the other side of the pipes is still under pressure. 

replacing faulty fill valve with a new one
Image from Your Santa Clarita Handyman

As soon as you open the valve, the water rushes into the empty tank with force till the tank is filled and the input valve gradually closes. 

However, if the input valve of the flush tank abruptly closes, the water is suddenly cut off while still in motion. All the kinetic energy of the moving water makes it crash onto itself, generating the characteristic knocking noise.   

How Serious is the Water Hammer Problem?

Water hammer is not a catastrophic plumbing problem, although it might sound like one! However, if you don’t address it soon, the water pressure might loosen or even break the joints and fittings, which may cause potential leaks and water damage. 

The best way forward is to fix the issue as soon as possible before it gets out of hand. Fortunately, fixing the water hammer issue is not at all complicated. You don’t need to use any tools or need to have any plumbing knowledge. 

Modern plumbing has a Built-in Cure for Preventing Water Knocking.

One of the best ways to prevent the water from crashing onto itself is to provide air gaps in the plumbing. Sending the water into an air pocket strategically placed in your plumbing system can act as a shock absorber and prevent knocking. 

water arrestor installed in plumbing
Image from Jesse Jones

Plumbing installed before the 1960s dealt with the potential knocking issue by including T-section pipes near automatic and rapid closing valves containing air. As the water pressure increases, the water is pushed vertically upward into the air gap, creating cushioning and preventing air hammering.  

If your plumbing was installed after the 1960s, your plumbing most certainly has devices called “hammer arrestors” installed strategically near rapidly closing valves. The hammer arrestors contain a pressurized air chamber and a piston. As the water pressure spikes after the sudden closure of the intake valve, the water is driven up against the piston relieving the pressure. 

Modern hammer arrestors are designed to last for a long time, and they rarely fail. So, knocking when you flush is not an issue you face daily. 

How to Fix Knocking Pipes When Toilet is Flushed?

If you hear a loud knocking noise from the pipes after flushing, it usually means that the air chamber is compromised or the hammer arrestors are not working properly. If you have done some DIY work on your plumbing, you might have skipped installing a hammer arrestor leading to the knocking sound. 

Whatever the reason for the knocking, there is a simple fix. Here is how you fix knocking pipes in six simple steps-

Step 1

Find the water main in your house and close off the valve that supplies water. You can find your main water valve near the foundation of your wall or under a metal plate if the valve is underground. 

Step 2

Open the highest faucet in your home and drain all the water in your house’s plumbing system. Flush the toilets and run your dishwasher and washing machine. The goal here is to remove all the water that is present in all the pipes in your house. 

Step 3

Once the water is drained from the pipes, the air re-enters the air chambers. If you have water arrestors, then the water that accidentally entered the arrestor assembly is also drained out. Make sure to drain all the water in your plumbing system. 

Step 4

Close the lower-level faucets and turn on the water supply from the water main. You will observe that the water from the top faucets sputters for a while before establishing a steady stream. 

Step 5

Allow the water to fill the flush tank by opening the lower-level faucets. That’s it! 

Step 6

Check your toilet flush. It should work properly without any knocking noise. 

As you can see, fixing the knocking pipes issue is not a big deal. The most difficult part of the process is to locate the main valve supplying water to your home. If you can’t find the valve, you can consult with any plumbing company that can help you identify the valve over the phone. 

a plumber checking the plumbing system

What if the Solution Doesn’t Work?

Following the six steps we saw above, most situations should solve the knocking issue. However, if the knocking persists or the problem is recurrent, it might indicate a plumbing issue. It is best to seek professional help and call in the experts at this point. 

Reasons to Call a Plumber

Here are a few reasons why you should consider calling the plumber-

  • If the knocking has rattled the pipes loosening them from their straps, they need to be strapped back to position. A professional plumbing service can help you identify and secure such lines with padding. 
  • If you have had pipe knocking issues for a long time, you should get your pipes checked out by a professional plumber. The prolonged knocking might cause minor leaks, later becoming a major risk factor. 
  • If the water pressure in your main plumbing line is above 50 PSI, there is a high chance of plumbing issues, including knocking pipes. Although you can adjust the water pressure yourself, I recommend calling in a professional if you are unsure of your DIY skills! 

Frequently Asked Questions

Should I install water hammer arrestors in my home? 

different sizes of water hammer arrestor
Image from Jngtech – Plumbing/Fire Protection Supplier of Valves, Pipes & Fittings

It pays to install the hammer arrestors if you have older plumbing in your home. Not only will they prevent the knocking noise, but they can also save the pipes from undue stress. 

Is knocking pipes after flushing a major plumbing issue?

In most cases, it is not. The problem is quite easy to fix. Draining your plumbing system usually fixes the problem. However, you should get your plumbing checked if the problem persists for a long time. 

To Conclude

Ignoring the knocking sound from your pipes every time you flush your toilet is not good for your plumbing system. Thankfully, there is an easy fix to this problem; however, if you are in doubt, you should call the professionals to get a permanent solution. 

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