· Michael Blair
A water heater that bangs, pops or rumbles is not about to explode. It is almost always sediment, and in Middle Tennessee it arrives faster than the manufacturer assumed. What matters is telling ordinary scale noise apart from the two sounds that mean the tank is finished.
What the rumble actually is
Middle Tennessee water is moderately hard. Calcium and magnesium drop out of solution as the water heats and settle on the bottom of the tank, directly over the burner on a gas heater. Water gets trapped underneath, flashes to steam, and forces its way up through the crust. That is the bang.
The noise is a symptom of lost efficiency rather than damage. The sediment layer insulates the water from the burner, so the burner runs longer to hit the same thermostat setting. You are paying for that gap every cycle.
Flushing it, and when flushing stops working
An annual flush is the cheapest maintenance in the house and the most commonly skipped. Kill the gas or breaker, connect a hose to the drain valve, and run it until the water leaves clear. On a heater that has never been flushed, expect grit and expect it to take a while.
Past roughly five years without a flush, the deposit can harden into a sheet that will not break up and will not pass the drain valve. At that point flushing buys very little, and the honest answer is to run the heater until it fails rather than spend money on a service that cannot reach the problem.
The two noises that are not sediment
A sharp bang in the pipework rather than the tank, right after a tap or an appliance valve shuts, is water hammer. That is pressure, not the heater, and it is fixed with an arrestor or a pressure-reducing valve.
A hiss or a sizzle at the base of the heater is water hitting something hot, which means it is escaping the tank. A tank leaking from the body cannot be repaired. Turn off the water and the gas and plan on replacement.
What it tells you about the heater’s remaining life
A noisy heater under about eight years old is worth flushing and keeping. Between eight and twelve, the noise is a reasonable prompt to start pricing a replacement while you still have hot water and can choose rather than react.
Past twelve, a heater that has begun rumbling has usually been running with an insulating layer for years. The anode rod is likely spent and the tank is corroding from the inside. Replacing on your schedule costs the same as replacing on its schedule, minus the flooded floor.