Why is my water heater making a knocking noise? Common causes and simple solutions

Your water heater is making a weird knocking noise, and now you’re wondering what’s going on. It might sound a little alarming, but don’t worry, that knocking sound is pretty common and it’s something you can usually fix without too much hassle. We’ll go over the common causes and give you some simple solutions to get your water heater back to quietly doing its job.

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Important: The information provided in this article may not apply to your specific appliance model. While we strive to offer helpful advice for DIY appliance repairs, variations in models and configurations can lead to differences in troubleshooting steps, error codes and part replacements. For the most accurate guidance, we strongly recommend consulting the technical sheet for your specific appliance model.

Common causes for knocking noise

A water heater making a knocking noise is typically heard when the system is heating water or during cooling periods. The sound may vary from a rhythmic knocking to a loud bang and might even cause your pipes to vibrate. Several factors could explain why your water heater is making a knocking noise:

Sediment buildup in the tank

One of the leading causes of a water heater knocking sound is sediment buildup at the bottom of the tank. Over time, minerals like calcium and magnesium, especially in hard water areas, settle at the base. When the water heats up, steam bubbles form underneath the sediment, causing a knocking or popping sound as they escape. This buildup not only leads to noise but can also reduce the efficiency of your water heater.

Water hammer effect

The water hammer effect occurs when water flow suddenly changes direction, causing pipes to knock against the walls or other surfaces. This usually happens when an appliance, like a washing machine or dishwasher, quickly shuts off the water supply. The pressure imbalance can cause the knocking sound to reverberate through your pipes, making it seem like it’s coming from the water heater.

Expansion and contraction of pipes

As your water heater operates, the metal pipes connected to it expand and contract with the temperature changes. This movement can cause the pipes to knock against walls or the brackets securing them, producing the knocking sound you’re hearing. While it might sound like the noise is coming from the water heater, it’s often the connected pipes that are the source.

Loose pipe straps or supports

If the pipes around your water heater aren’t properly secured, they may vibrate and create a knocking noise as water flows through them. This is especially common in older homes where pipe supports may have loosened over time.

Overpressure in the system

Excessive pressure in your water heater system can also lead to knocking or banging noises. When the water pressure is too high, the movement of water through the pipes becomes turbulent and noisy. An expansion tank or pressure-reducing valve can help manage this issue.

Solutions for a water heater knocking sound

Fortunately, most causes of a water heater knocking noise can be resolved with simple fixes. Here’s how you can tackle each problem:

1. Flush the water heater

1. Turn off the water heater and let it cool down. For gas heaters, set the gas control valve to the “pilot” position.
2. Locate the drain valve on the tank. It is typically located near the bottom and looks like a hose bib with a threaded end.
3. Attach a garden hose to the valve and run it to a drain, utility sink, or outside.
4. Open a hot water faucet in your home to allow air into the system and help with drainage.
5. Open the valve and allow the tank to drain completely. Be cautious as the water can be hot.
6. To further flush the tank, turn the cold water supply back on for a few minutes while the drain valve is still open. This helps to flush out any remaining sediment that may be settled at the bottom of the tank.
7. Once the water runs clear and you have flushed the tank adequately, close the drain valve by turning it clockwise.
8. Disconnect the hose from the drain valve.
9. Turn on the water supply to refill the tank. Keep the hot water faucet open to release any air trapped in the system.
10. For electric heaters, switch the power back on at the breaker box. For gas heaters, set the gas control valve back to the desired temperature.

2. Install water hammer arrestors

These devices absorb the shock caused by sudden water flow changes, preventing the pipes from knocking.

1. Determine the appropriate location for installing the water hammer arrestors. This is usually near the source of the noise, such as where water pipes connect to appliances like your water heater.
2. Turn off the water supply to prevent any leaks or water damage during the process.
3. Open faucets in your home to release any built-up pressure in the plumbing system.
4. Follow the manufacturer’s instructions for installing the water hammer arrestors. This typically involves cutting into the water line and securing the arrestor in place using appropriate fittings.
5. Once the arrestors are installed, turn the water supply back on and check for any leaks or issues.

3. Secure loose pipes and straps

Check the pipes around your water heater for loose or insecure straps. Tightening or adding additional supports can eliminate the knocking noise caused by pipe movement. Installing foam or rubber insulation around the pipes can also reduce noise from expansion and contraction.

4. Install an expansion tank

1. Choose an expansion tank that matches the water heater’s capacity and pressure specifications. Refer to the manufacturer’s guidelines for compatibility.
2. Turn off the power supply to an electric water heater or set the gas control valve to the “pilot” position for a gas water heater. Shut off the main water supply to the house.
3. Connect a garden hose to the drain valve at the base of the water heater and drain a few gallons of water to relieve pressure.
4. Install the expansion tank on the cold water supply line near the water heater.
5. Position the tank vertically with the air valve facing upwards.
6. Cut the cold water supply line and solder a tee fitting into the line.
7. Connect the expansion tank to the tee fitting. Ensure the tank’s inlet is connected to the cold water line and the outlet to the water heater.
8. Use pipe straps to secure the expansion tank to the wall or a nearby support structure.
9. Use a tire pressure gauge to check the air pressure in the expansion tank. It should match the water pressure in the plumbing system, typically around 50-60 psi (pounds per square inch).
10. Turn on the main water supply and check for any leaks around the expansion tank and fittings.
11. Once the expansion tank is installed and there are no leaks, restore power to the water heater and turn on the gas supply if applicable.

FAQs

Why is my water heater making a knocking noise?

A water heater making a knocking noise is often caused by sediment buildup inside the tank. Over time, minerals settle at the bottom, and when the water heats up, steam bubbles trapped under the sediment create a knocking sound. Other causes can include water hammer effects, loose pipes, or high water pressure.

Is a knocking noise in my water heater dangerous?

In most cases, a water heater knocking noise is not dangerous, but it can indicate issues like sediment buildup or overpressure, which can reduce efficiency and potentially lead to more severe damage if not addressed. Regular maintenance, such as flushing the tank, can help prevent problems.

How do I stop my water heater from making a knocking noise?

To stop a water heater knocking sound, you can flush the tank to remove sediment, install water hammer arrestors to fix pressure surges, and secure any loose pipes that may be vibrating. Checking the system’s water pressure and adjusting it if necessary can also reduce the noise.

What is sediment buildup in a water heater, and how does it cause knocking sounds?

Sediment buildup occurs when minerals from hard water, like calcium and magnesium, settle at the bottom of the water heater tank. This sediment traps steam bubbles, which escape and create knocking or popping noises as the water heats. Flushing the tank regularly can prevent this.

Can high water pressure cause my water heater to make a knocking noise?

Yes, high water pressure can cause a water heater to make a knocking or banging noise. Excessive pressure can create turbulent water flow and cause vibrations in the system. Installing a pressure-reducing valve or expansion tank can help manage high pressure and stop the noise.

Will flushing my water heater stop the knocking noise?

Yes, flushing your water heater can often stop the knocking noise caused by sediment buildup. By draining the tank and removing the mineral deposits, you allow the heater to run more smoothly, reducing the likelihood of knocking or popping sounds.

Additional information

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