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Why Do My Water Heaters Make a Loud Knocking Noise?

Noise coming from any appliance can be a cause for panic and worry for homeowners. It’s common with some appliances while others sit quietly forever. However, sometimes the presence of any noise squeaking out of an appliance shouldn’t necessarily cause great concern. Your water heater making a knocking or popping sound might be just that.

The knocking or rumbling noise in the water heater tank is most likely because of a settlement of a thick layer of sediment at the bottom of the tank, also called “scale.” It’s a common occurrence in some areas. The solution isn’t difficult, either. The tank may need flushing.

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What is Scale?

The sediment or mineral deposits at the bottom of the tank are called scale. The minerals (like calcium and lime) may be present in your water and are the source.

Water inherently doesn’t contain any minerals. When it rains or snows, water is mostly in pure form.

However, minerals like calcium, magnesium, and potassium are present in rocks. They get picked by the water when it gets in contact with soil or rocks. Don’t worry; in most cases, the minerals are harmless. They might even benefit your health.

Depending on the local geography, the water might be hard or soft (mineral-less). Some minerals are essential to the human body but not so necessary for your water heater. The deposit of minerals builds over time and ends up damaging your water heater.

How is the Knocking Noise Caused?

Once the sediment buildup is significant, it starts causing problems. The heating element in the heater is at the bottom of the tank, where the sediment is deposited. The heating element hardens the sediment during regular usage.

The hardened sediment traps the water under it and, under high temperatures, creates pressure. The water trapped under the sediment tries to escape, rattling around and making knocking or popping sounds inside the tank. It’s similar to how the untightened lid of a boiling kettle makes noise when it comes under high-temperature pressure.

A quick solution is flushing the tank. Flushing removes this sediment, and water can no longer get trapped.

What If the Heater is Left Without Flushing?

It’s strongly recommended that you flush the water heater whenever necessary. You could either do it yourself with the steps explained below or get a technician for help. The sediments not only cause the knocking noise but make the water heating process difficult.

In simple words, your heater is struggling. It now runs longer than the usual time required to heat water, leading to overheating issues. Overheating is what you don’t want – it damages the tank’s protective inner lining and deteriorates the effectiveness of the heating element in the long run.

How to Flush the Water Heater Tank?

Here are the steps required to be followed to flush the tank properly. Luckily, the steps aren’t difficult and should be simple enough, even for beginners. Put on some heat-resistant gloves, preferably, before you get started.

  1. Turn off the power supply of the water heater. If you have a gas water heater, set the gas supply to pilot or vacation mode.
  2. To prevent further cold water from entering the water heater, close the cold water shutoff valve.
  3. Turn on the nearby hot water faucet (kitchen or washroom). This will prevent any vacuum and ease the draining. Leave the water heater for about 20-30 minutes to cool down.
  4. Connect a garden hose to the water heater’s drainage spigot at the bottom of the tank.
  5. Make sure to put the other opening of the garden hose in a safe location for drainage, like a large bucket or basement drain
  6. The water heater tank is ready to drain. Open the drainage valve and let the water flow out. The initial water might be dirty; that’s the sediment coming out – just some of it.
  7. Time to flush the tank. Carefully flush out the sediment. Turn the cold water on and off alternatively to make sure no residue remains.
  8. Once the cleansing is done, close the drain valve, remove the hose and let the cold water flow in the heater.
  9. Turn the power supply back on. You’re all set now.

How Often Should You Flush the Water Heater?

Ideally, the water heater should be flushed once a year. If you notice any hint of knocking or popping sound before that, it would be wise to flush. It’s a straightforward process, as explained, and won’t take much of your time.

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Should You Treat the Hard Water?

If your tank is sedimented too often, the hard water might be too hard. It might damage other appliances, clog the plumbing, damage your clothes, irritate skin and hair.

It would help if you considered treating the hard water by installing a water filtration system, which could be expensive, but a one-time fix. A cheaper alternative is to use a water softener for daily purposes.

If your water heater is making strange sounds, it may also mean you’re overdue for service. We here at Absolute Airflow can come out and inspect your water heater and let you know if it’s time to replace it. Please contact us today to learn more or schedule a service check with our team of professional technicians.

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Customer Reviews

Called Absolute for my 81 year old landlord in the height of Summer heat. Scott came out to inspect the old beast of an AC and found many issues. He took the time to educate us, note everything, photograph it and provide several recommendations...and my stubborn landlord has done nothing LOL. A couple months later we asked that Scott come back out and he was there that same day.

Emily H.

Santa Ana, CA

I contacted Absolute Airflow to check my mother's AC, as it was not getting cold. The tech (Scott) arrived earlier than his scheduled time, and was very knowledgeable, professional, and most of very personable with my mother in letting her know that the issue was. Superb customer service Scott and thank you Absolute Airflow! I highly recommend Absolute Airflow.

Billy A.

Billy A.

Bellflower, CA

I have called in Absokite Airflow for my tankless water heater and A/C unit for maintenance. They were professional and took the safety precautions necessary including wearing a mask and cornering their shoes when entering my home. I recommend them highly. Shout out to Zack who took care of the tankless water heater.  They were professional and friendly.

Beatriz S.


Santa Fe Springs, CA

Carlos came to our house to access the A/C project and to give us an estimate. He was very knowledgeable and took his time to explain all the opinions in detail as well as the installation process and warranties. Tanner and Chris were the installers, they were very professional taking their time and attention to details. To keep our house clean while installing the new A/C system.

Robert M.

Robert M.

Whittier, CA