Why your water heater pops and rumbles (and how flushing helps)

That popping or rumbling is almost always sediment. Minerals from your water settle to the bottom of the tank, and small pockets of water get trapped underneath. When the burner or lower element heats those pockets, they boil and push up through the sediment in little bursts. Flushing the tank clears out the loose stuff and often quiets it down.

Why does sediment build up?

All tap water carries dissolved minerals, mostly calcium and magnesium. Heating the water pulls some of them out, and they settle as grit and scale at the bottom of the tank. The harder your water, the faster it piles up.

In Huntsville, that depends on which water reaches your house. Huntsville Utilities puts its Tennessee River water at about 65 to 75 mg/L of hardness, roughly 4 grains per gallon, which is moderately hard. Its well water runs about 160 to 170 mg/L, roughly 9 to 10 grains per gallon, which is hard. A tank on the harder water will collect sediment faster. Our post on hard water in Huntsville goes into what that means for the rest of your plumbing.

Is a popping water heater dangerous?

Usually not. Sediment noise by itself isn’t a safety problem.

It is costing you, though. The sediment sits between the heat and the water, so the heater has to run longer to do the same job. On a gas tank, the steel at the bottom runs hotter than it should and wears out faster. On an electric tank, sediment can bury the lower element and burn it out early. It adds up to higher bills and a tank that doesn’t last as long as it could.

If the noise comes along with a dripping relief valve, water that’s scalding hot, or a leak, that’s a different situation. Turn the heater off and call.

How to flush a tank water heater

Once a year is a good habit for most tank heaters. Check your owner’s manual too, since some makers set their own schedule.

1. Turn off the heat. For electric, switch off the breaker. For gas, turn the control to “pilot” or “off,” whichever the label on your unit says to use.
2. Let the water cool for a few hours if you can. Tank water is hot enough to scald, and you’re about to run it through a garden hose.
3. Shut off the cold water valve on the line going into the top of the heater.
4. Hook a garden hose to the drain valve near the bottom of the tank. Run the other end to a floor drain or outside, somewhere that can handle hot, gritty water.
5. Open a hot water faucet somewhere in the house. That lets air in so the tank can drain.
6. Open the drain valve and let the tank empty. Cloudy water and bits of grit coming out are normal.
7. With the drain still open, turn the cold supply back on for a minute or two. The fresh water stirs up the bottom and pushes more sediment out. Repeat until the water runs mostly clear.
8. Close the drain valve and take off the hose.
9. Leave the cold supply on and let the tank refill. Keep that hot faucet open until water runs from it steadily with no sputtering. That’s how you know the tank is full.
10. Now turn the heat back on. Flip the breaker, or set the gas control back to its normal setting and relight the pilot per the label if it went out.

Never power up an empty electric tank. The elements can burn out almost as soon as they heat up if they aren’t under water. Refill completely, with hot water running steadily from a faucet, before you turn the breaker back on.

Be careful with old tanks

If your tank is ten years old and has never been flushed, think twice before you start. Plenty of drain valves are plastic, and after years of sitting closed with grit behind them, they don’t always close tight again once you open them. Some just break. Then you’ve got a tank that won’t stop draining, probably on a Sunday night.

Heavy sediment can also clog the drain so almost nothing comes out. And flushing only removes the loose stuff. Scale that has hardened onto the bottom of the tank isn’t going anywhere.

On an older tank, it can make more sense to have a plumber do the first flush, or to leave it alone and start planning for a replacement. How long water heaters last can help you decide which way to go.

What other water heater noises mean

• Ticking or tapping is usually pipes expanding and contracting as hot water moves through them. Mostly harmless.
• Hissing or sizzling on a gas heater is often condensation dripping onto the burner, which is common when the tank is full of cold water. It can also be a leak dripping onto something hot, so look for water. On an electric tank, it can be sediment built up around an element.
• Screaming or whistling usually means water squeezing through a valve that’s partly closed, or a relief valve letting water or steam out. Check the relief valve pipe. If it’s hot or wet, call.
• Humming on an electric tank can come from an element that’s loose and vibrating. Tightening it means draining the tank with the power off, so only take it on if you’re comfortable with that.

If it’s still rumbling after a good flush, the sediment has probably hardened in place, and it’s time to ask how much life that tank has left.