Popping and rumbling almost always means sediment: mineral scale on the bottom of the tank traps water that boils through it as the burner heats. Elk River's city water is hard (16 grains per gallon, according to Elk River Municipal Utilities), so it builds up quickly here. It's not usually dangerous right away, but it makes the heater work harder and can shorten its life.
What the noise actually is
Hard water carries dissolved calcium and magnesium. When it heats, some of those minerals come out of solution and settle on the bottom of the tank, or crust onto the lower element of an electric heater. Water gets trapped under and inside that layer. When the burner fires, that trapped water flashes into steam bubbles that pop through the sediment. That's the crackling and rumbling you hear.
Elk River Municipal Utilities lists the city's water hardness at 16 grains per gallon, and suggests setting a softener at or below 16 (ERMU, Water Hardness). At that hardness, untreated water leaves scale fast.
Is it dangerous?
The popping itself isn't a sign the tank is about to burst. But it does mean:
- The burner is heating through an insulating layer, so it runs longer and uses more energy.
- The bottom of a gas tank can overheat, which speeds up corrosion.
- There's less room for hot water, so it runs out sooner.
Different sounds mean different things. A high-pitched whine or screech can be a partly closed valve. A hiss or sizzle on a gas heater can be water dripping onto the burner, which you should check for a leak. Banging in the pipes when a tap shuts off is water hammer, not the heater.
Older tank that's never been flushed? Ask a plumber before opening the drain valve. Old valves can clog or fail to close.
Call nowWill flushing fix it?
On a newer tank, flushing a few gallons through the drain valve every year or so helps keep sediment from building up. On an older tank that has never been flushed, it's more complicated:
- The sediment may have hardened and won't flush out.
- Loose sediment can clog the drain valve so it won't close.
- Stirring it up can send debris into faucet aerators.
If your tank is over about 8 years old and has never been flushed, it's worth having a plumber look first.
Preventing it
- A water softener, set to your water's hardness.
- Regular flushing from early in the heater's life.
- Anode rod checks. Softened water tends to wear the anode rod faster, and the rod is what protects the tank from rusting.
- A sensible temperature. Hotter water drops more scale.
Tankless heaters and hard water
Tankless units don't collect sediment the same way, but scale builds up inside the heat exchanger. Most manufacturers call for periodic descaling, which matters even more with water this hard.
Nearby towns
St. Michael, Monticello, Rogers, Big Lake and Zimmerman each have their own water supplies, and homes on private wells vary a lot. Your city's annual water quality report usually lists hardness, or you can have your water tested.