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Why is My Water Heater Making a Popping/Knocking Noise?

Hearing a popping, knocking, rumbling, or vibrating sound coming from your water heater can be very alarming. However, the reason behind a water heater making a popping or knocking noise is actually quite innocuous and easy to fix. 

Water heaters making a popping or knocking sound are likely suffering from a sediment build-up in their tank.

This is a very common problem in areas that have hard water, like our home state of Minnesota. If your water heater is making unusual noises like these, it’s an indication that you need to flush the system; fortunately, this is a simple process that we’ll explain later in this article.

Why is My Water Heater Making a Popping Noise?

If your water heater is making a popping, knocking, or rumbling noise, the most likely reason is a buildup of sediment or limescale in its tank. This noise may seem ominous, but in reality, there isn’t too much to worry about. As we go on to explain, you can easily fix a popping or knocking water heater yourself by flushing the system. First, for some context, we’ll discuss what sediment actually is and how it ends up in your water heater’s system.

What is Sediment and Where Does it Come From?

Sediment, or limescale, is an accumulation of loose minerals that are naturally present in water. It isn’t the case that sediment infiltrates your tank, but rather it comes from the water itself. The sediment layer accumulates over time as water passes through the system and leaves behind deposits of its loose minerals. 

You’re much more likely to experience a problematic buildup of sediment if you live in an area with hard water. For us living in Minnesota, this is a particularly common issue as our water is typically considered to be hard across the state. In simpler words, ‘hard’ water is the term for water that has a high content of minerals like calcium and magnesium. As the water passes through your heater’s tank, it leaves small deposits of these minerals behind; over time, these mineral deposits settle on the bottom of the tank, gradually building up to create a thick layer of sediment. 

How Does Sediment Cause the Popping Noise in My Water Heater?

In short, the popping noise in your water heater comes from air bubbles escaping the thick, hard sediment layer. You can imagine this as boiling water in a covered pot pushing up the pot’s lid.

In electric water heaters, the heating element is located at the bottom of the tank; it lies directly beneath the part of the tank where sediment settles and accumulates as a layer. The sediment layer isn’t entirely solid, instead trapping pockets of water among the solid material. So, when the heating element warms up, it boils the water trapped within the sediment. The hot water rises through the sediment layer and ‘pops’ out of the solid material, causing the popping noise that you can hear. In gas heaters, the same effect occurs from hot water travelling from the bottom of the tank to the top. 

Can Sediment Damage My Water Heater?

When the sediment layer builds up to significant depth, it can cause some issues for your water heater; these include the tank overheating or leaking, or, in electric water heaters, the total failure of the heating element. At minimum, having an accumulation of sediment will reduce the efficiency of your heating system. 

The first issue that sediment can cause is reducing your water tank’s overall efficiency. When sediment forms a thick layer on the bottom of the tank, it acts as a barrier that traps hot water. Consequently, the system has to work harder to push past the sediment and heat up the water in the tank. This forces the water heater to use more electricity or gas, in turn raising your bills each month. Even a small amount of sediment can have such an impact on your heating system’s efficiency. 

Beyond this, when the sediment builds up to a significant thickness, it can start causing physical damage to your system. A thick sediment layer can cause the tank to overheat which may damage the tank’s steel casing; this can result in leaking, among other issues that come from such damage to the heater’s parts. In electric heaters, a thick sediment layer may cause the total failure of the heating element altogether. 

With these consequences in mind, it’s best to take action to remove the sediment from your water heater. A small amount of sediment isn’t too much of a concern; however, allowing the sediment to build up will likely lead to higher bills and a malfunctioning water heater.

How to Remove Sediment by Flushing Your Water Heater

Luckily, removing sediment from your water heater is a relatively simple process known as ‘flushing’ the system. To avoid any safety issues, it’s best to contact professionals like us at Metro Heating and Cooling to do this job for you; otherwise, you can attempt to carry it out yourself by following the steps below.

How to Flush a Water Heater

  1. Put on a pair of thick gloves to avoid burns to your hands from the hot water.
  2. Turn off your water heater. For electric heaters, disconnect the power to the system at your circuit breaker; for gas heaters, turn the control knob to the ‘Pilot’ setting.
  3. Shut off the cold water supply. Find the cold water valve or lever, then turn it to a 90-degree angle to prevent cold water from entering the tank. 
  4. Wait for 30 minutes to allow the system to cool down.
  5. Locate the drain valve at the bottom of the tank, then attach a garden hose to this valve. 
  6. Place the other end of the hose somewhere that the water and sediment can safely drain, such as a basement drain or somewhere outside. 
  7. Open the heater’s pressure relief valve and one of the hot water faucets in a sink or tub in your home. This is crucial to prevent a vacuum from forming in the lines. 
  8. Open the drain valve to allow hot water and sediment to drain from the heater. Take care for this step as the water may still be hot enough to cause burns. 
  9. Once the hot water has drained completely, you can begin the flushing process. Close the pressure release valve before turning on the cold water supply. Toggle the cold water supply lever between ‘open’ and ‘shut’ a few times to flush any remaining sediment.
  10. When the water is running totally clear, close the drain valve and remove the hose. 
  11. Allow the cold water to fill the tank back up; this should take around 10 minutes.
  12. Once the tank has been refilled, you can power your water heater back up as normal. 

How to Prevent Sediment Buildup in the Future

While there’s no way to prevent sediment from settling in your water heater, you can prevent problematic buildups through regular maintenance of your system. We recommend that your water heater should be flushed at least every 6 months for optimal performance. 

At Metro Heating and Cooling, we can take care of the essential maintenance services your heating system needs to stay safe and functional. If your water heater is making unusual noises, or you have any other concerns with your system, get in contact with our team of professionals today. 



Schedule a HVAC repair anywhere in the Minneapolis or St. Paul area today by calling (651) 294-7798 or by requesting an estimate online.


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