Plumbers sometimes call it kettling, and the comparison is exact: the same thing happens in a tea kettle with scale on its base. Minerals — mostly calcium from hard water — settle out every time the tank heats, and over the years they bake into a crust on the floor of the tank. Heat has to push through that crust, water caught beneath it flashes to steam, and each little eruption is a pop, a rumble, or a knock. The noise is the symptom. The sediment is also costing you: the burner or elements run longer to heat through it, and the tank floor overheats in the process, which shortens the tank's life.
Cut the heat and let the water cool. For a gas heater, turn the control dial to Pilot. For an electric heater, switch off its breaker. Then run a hot tap somewhere in the house until the water comes out warm rather than hot — draining a tank of near-scalding water through a garden hose is a bad plan. This cooling step can take a couple of hours after heavy use.
Close the cold-water inlet and open a hot tap. The inlet valve is on the cold pipe entering the top of the tank. Shut it. Open the hot side of the nearest faucet and leave it open — that lets air in so the tank can drain instead of vacuum-locking.
Attach the hose and open the drain valve. The drain valve sits near the bottom of the tank. Run the hose to a floor drain or downhill outside, then open the valve. If nothing flows, sediment may be plugging the outlet; brief blasts of water back through the hose sometimes clear it. Our full guide to draining a water heater covers stubborn valves and the yearly routine in more detail.
Flush until the water runs clear. With the drain open, crack the cold inlet back on for a few seconds at a time to stir the tank floor and push out loosened sediment. Repeat until the hose water runs clear and grit-free. Gritty, sandy output is exactly what you were hoping to see — that's the layer that was popping.
Refill completely before restoring heat. Close the drain valve, leave the inlet open, and wait until water flows smoothly from that open hot tap — that means the tank is full and the air is out. Only then turn the gas dial back or flip the breaker on.
No tank means no sediment floor — but the heat exchanger inside a tankless unit grows mineral scale on its hot surfaces, and the result is similar popping and kettling sounds, often paired with fluctuating water temperature. The fix is a descaling flush: a small pump circulates a vinegar or descaling solution through the unit via its service valves. Plenty of owners do this themselves with a kit; plenty also pay for it during an annual service visit, since the same scale that's making noise is quietly cutting efficiency. Either way, don't ignore it — a scaled-up exchanger works harder and ages faster, same as the tank.
A flush works when the tank itself is still sound. It's the wrong move when the popping comes with company: rusty or discolored hot water, moisture around the base, a tank that's already 10–12 years old, or noise so loud it sounds like gravel in a cement mixer — that last one can mean the sediment has hardened into chunks too big to flush out. Check the anode rod while you're at it, too; a depleted rod is why tanks rust from the inside, and what an anode rod does explains the inspection. If the tank is old and has never been flushed, read our rundown of how long a water heater lasts before spending a Saturday on it — flushing a dying tank buys months, not years.
Turning the thermostat up to "burn it off." Higher heat settings don't dissolve sediment. They make the popping louder, raise scald risk at the taps, and stress the tank floor further.
Draining without opening a hot tap. A closed system vacuum-locks. You'll get a trickle, conclude the valve is broken, and miss the sediment entirely.
Flushing once and forgetting it for a decade. In hard-water homes, sediment starts rebuilding immediately. Yearly is the standard advice; every couple of years is the realistic minimum if your water is only moderately hard.
Popping and rumbling means your heater is boiling water through a layer of baked-on sediment. On a reasonably healthy tank, a careful flush — heat off, water cooled, tank refilled before the power comes back — removes the layer and the noise with it. On an old tank with rust, leaks, or a decade of neglect, the same noise is the tank telling you its remaining time is short, and the honest fix is planning a replacement instead of a flush.
Usually not immediately — the noise itself is just steam escaping through sediment. The danger is indirect and slow: the overheated tank floor weakens over time, efficiency drops, and a neglected tank fails sooner. The exception is any noise accompanied by leaking, a tripped pressure-relief valve, or a strong gas smell — stop and call a plumber (or your gas utility, for the smell) rather than flushing anything.
Once a year is the standard recommendation, and it matters most with hard water or a water softener that's out of salt. If your water is soft and the drained water runs clear, stretching to every two years rarely causes harm.
You can try, with two cautions: the drain valve may not reseal afterward (have a cap ready), and stirring up a decade of sediment sometimes reveals how thin the tank floor has become. If the heater is otherwise performing and not leaking, many plumbers advise leaving a very old, never-maintained tank alone and budgeting for replacement instead.
Commonly $150–$300 as part of a service visit, depending on the area. It's a reasonable price if your drain valve is corroded, the heater is hard to access, or you'd rather someone else discover the stuck valve.
The noise happens when the burner or elements fire and heat pushes through the sediment — typically right after heavy hot-water use, when the tank is reheating a full load of cold water. At idle, or when reheating a small top-up, the same tank may be completely quiet.