Why Does a Gas Water Heater Make a Knocking Noise?

If your water heater started producing unusual sounds, then it’s a tell-tale sign that something in the system is wrong.

Knocking noise in a gas water heater usually indicates that there is sediment build-up in the unit. Once the water gets heated, it will make the deposits float or roll inside the appliance, and you will hear a noise each time the chunks hit the tank.

Why Does a Gas Water Heater Make a Knocking Noise?

The most common reason for a knocking sound in a gas water heater is sediment build-up.

If your area has issues with hard water, then the chances are high that at one point or another you are going to face such a problem. Hard water is full of minerals that are, in general, not harmful to your health, but they can cause a lot of damage to the plumbing system and the appliances that need water to operate.

These mineral deposits gather at the bottom of the tank. Water will get stuck between the actual unit and the layer of sediment, and when it will get heated, the steam bubbles will start making their way through the build-up causing the sediment to ‘knock’ on the tank.

Read: Gas Water Heater Not Maintaining Temperature

Why Is My Hot Water Heater Making Loud Banging Noise?

The gas burner will be heating up the water that is stuck under the sediment build-up until the bubbles manage to escape. Every time this happens, you are going to hear a ‘bang’.

If the boiling water managed to crack open a large layer of sediment, this big chunk will start floating around the tank and creating a loud knocking noise.

Read: Why Is Gas Water Heater Humming?

New Hot Water Heater Making Knocking Noise

Let’s imagine that you have just bought your water heater and you haven’t been using the appliance long enough to give the sediment build-up a chance to form. What can be causing the knocking sound in such a case?

Your pipes or water hammer might be to blame. If the pipes are loose or too tight on mounting fittings, they might start making a knocking noise.

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The system can also make a loud noise when the water coming from the heater suddenly changes direction. This common plumbing problem is known as ‘hydraulic shock’ or ‘water hammer’

Read: Why Gas Water Heater Keeps Cycling ON And OFF?

Gas Heater Making Popping Noise

A popping noise is a bit different than a knocking sound, and when it comes to gas water heaters, the two noises will have drastically different causes.

Dirty burners will delay the ignition process. As a result, gas will have a chance to build up in the system, and when the unit finally ignites, you will hear a loud pop – these are small explosions that happen because of the collected gas.

This is certainly a serious issue that needs to be addressed as soon as possible.

Electric Heater Making Popping Noises

An electric water heater doesn’t have a burner, so the popping sound would never be caused by gas build-up.

If your unit sounds like a popcorn-making machine, then it’s because of sediment build-up. The boiling water will cause the formations to roll around in the tank.

Tankless Water Heater Making Knocking Noise

Tankless water heating systems can also suffer from mineral deposits – even though the water doesn’t get stored in a tank, the whole system still uses the liquid that is full of minerals. Sediment in a tankless system can build up right on the heating element, for example.

Other possible reasons for a knocking sound in a tankless water heater include:

  • Water hammering
  • A dirty flow sensor
  • A malfunctioning vacuum that is siphoning the water away from the unit

Is Hot Water Heater Knocking Dangerous?

Now that you know that a knocking sound indicates the presence of sediment build-up in your water heater, you might be thinking that the whole situation is not that bad after all.

But the truth is that such a layer of minerals can cause extremely serious damage.

  1. Lower effectiveness – mineral deposits reduce the efficiency of your water heater by either taking up space in the tank or by blocking the unit’s components.
  2. The water heater would have to work harder – in order to keep up with the regular hot water demand, your unit will start working harder. This will make your energy bills skyrocket and, at one point, the unit might simply overheat.
  3. Interior damage – the actual sediment can cause damage to the heater’s lining when it’s floating around the tank. Once the lining wears out, the steel beneath it will start to corrode.
  4. Water contamination – once the tank begins corroding, the water can get ‘infused’ with rust. There might be plenty of other harmful elements released from the pipes, so you might not want to have a shower under such water.
  5. Leaks – if you fail to take care of the issue, the water heater might start leaking and then flood your home. In the worst-case scenario, the overheated unit might explode.

Is It Bad If Your Pipes Are Knocking?

There are a few reasons why your pipes might be banging. It is definitely an issue that you would want to fix for your own peace of mind, but, in a lot of cases, nothing bad is going to happen, if you choose to ignore the noise for a bit.

Loose pipes

Over the years, the pipes might become loose from the straps that secure them to the building. Once the water passes through them, the pipes will make a knocking noise.

High water pressure

If you hear the sound while the water is running, but you know that the pipes are properly secured to the wall, then the water pressure might be too high.

Anything that is over 80 psi is too high, so you might want to install a pressure-reducing valve (to lower the pressure of hot water, simply try turning the temperature down a bit).

Water hammer

If you can hear the noise only when you shut the water off, then it might be because the liquid has been forced to bang into the valve.

Read: Why Gas Water Heater Making A Humming Noise

How Do I Stop My Water Heater from Knocking?

Sediment build-up is the main cause of knocking sounds. To get rid of this layer, you would have to flush the water heater.

If you don’t really know where to turn the water and gas supply off and how to drain your tank, then you might want to leave the job to a professional. 

How to Fix a Knocking Water Heater?

You can buy an inexpensive hard water test kit to find out the hardness of the water that’s running through your pipes.

Water between 7 and 10.5 gpg is considered ‘hard’. If that’s your case, then investing in a system that is going to protect your appliances from hard water might be the right decision.

  • Scale inhibitor systems have a filter that is installed on the incoming supply.
  • Whole-house water softeners treat all the water that enters your house.
  • Regularly flushing the water heater will help you get rid of the sediment that had already built up in the system.

What Is Water Hammer Noise?

Water hammer is a type of hydraulic shock that happens when the shut-off valve located in a high-pressure water line gets closed.

You might hear these loud knocking sounds, if the faucet washers are damaged or worn or if there is a build-up of rust or minerals on the shut-off valve.

The good news is that you can try resolving this issue on your own. All you would have to do is turn off the water supply to your house and then open all the faucets to drain the water.

Can Water Hammer Go Away on Its Own?

It is unlikely for water hammer to go away on its own, but you can try to solve the problem on your own by draining the water.

Here are a few other things that can be done:

  • Replacing intake connections with wider ones
  • Installing water hammer arrestors
  • Securing the loose pipes
  • Installing a pressure-limiting valve
  • Installing soft-close taps

Read: Is A Gas Water Heater Cheaper To Run Than Electric?

Why Do I Suddenly Have Water Hammer?

  1. Loose pipes
  2. New appliances
  3. Worn-out stop valves
  4. Clogged air chambers
  5. Fast-acting valves
  6. Water ripples from the tank

Can a Water Hammer Arrestor Get Clogged?

A water hammer arrestor can get clogged with debris or mineral deposits. The device can also get waterlogged or simply worn out if it’s over 3 years old.