Why is my water heater popping or rumbling?

What the noise is

A tank water heater heats from the bottom, either with a gas burner under the tank or with a lower electric element. Hard water carries dissolved calcium and magnesium. When it heats, some of those minerals drop out and settle on the tank floor as sediment.

Water gets trapped under and inside that layer. When the burner or element fires, the trapped water overheats, flashes to steam, and the bubbles burst through the sediment. That is the popping, crackling or rumbling. It is loudest at the start of a heating cycle and in homes with hard water.

Sediment also acts like insulation between the heat and the water, which makes the heater work harder and slows recovery. On electric tanks, heavy buildup can bury the lower element.

How hard is your water?

Hardness is reported as milligrams per liter (mg/L) of calcium carbonate or as grains per gallon (gpg). A few utility-reported examples:

City	Utility figure	Source
San Antonio, TX	15 to 20 gpg	SAWS[3]
Chicago, IL	130 to 150 mg/L (7.6 to 8.8 gpg)	City of Chicago[4]
Cleveland, OH	about 120 mg/L (7 gpg)	Cleveland Water, 2022[5]

Your water utility's annual water quality report lists the figure for your area. The higher it is, the more often a tank needs flushing.

What to do about it

Flush the tank on the maker's schedule. Most owner's manuals describe draining a few gallons, or the whole tank, through the drain valve to carry sediment out. If you do it yourself, follow the manual exactly: shut off the gas (control to "pilot" or "off") or the breaker first, let the water cool, and route the hose somewhere hot water cannot hurt anyone. Never power an electric tank while it is empty.

Expect less from a first flush on an old tank. Hard-packed sediment that has built up for years often will not drain out through a small valve, and old plastic drain valves can crack or fail to reseat. That is a reason to have a plumber handle the first flush on a tank more than a few years old.

Cost. HomeGuide's national range for a flush and clean is $75 to $250^[1].

Is it the heater or the pipes?

Stand next to the heater right after someone runs a hot shower for a few minutes. Popping that comes from the base of the tank while the burner or elements are running is sediment. Knocking in the walls when a tap shuts off is usually the pipes (water hammer), which is a plumbing fix, not a water heater one. Ticking along the pipes as hot water flows is usually expansion and is mostly harmless.

When noise means it is time to replace

Rheem puts typical tank life at 8 to 12 years and suggests replacement becomes more sensible past 10 years^[6]. Loud rumbling on a tank that age, especially with slow heating, rusty hot water, or moisture at the base, suggests the tank is near its end. Flushing a badly scaled old tank rarely brings it back. Check the age with our decoder before paying for service.

Tankless units: the same minerals, a different symptom

Tankless heaters do not have a tank floor for sediment. Instead, minerals form scale inside the heat exchanger. DOE's Building America guide notes that this scale restricts water flow and can shorten the unit's life, which is why makers specify regular flushing^[2]. The unit usually warns you with a code before noise appears: Rinnai shows LC while it keeps running^[7]. See tankless no hot water.

Other noises, briefly

• Ticking or tapping: often pipes expanding and contracting as hot water flows. Usually harmless.
• Sizzling on a gas tank: water dripping onto the burner. It can come from condensation or from a leak. Find the source.
• Screeching or whistling: a partly closed valve or a relief valve starting to open. Check the relief valve pipe.

See whistling, hissing and ticking noises for more.

When to call a pro

• Rumbling plus slow heating on a tank more than 8 to 10 years old.
• A drain valve that will not close after a flush, or no water comes out at all.
• Noise along with a relief valve releasing hot water or steam.
• Popping on an electric tank plus a tripping breaker (possible buried element).