What each noise usually means
| Sound | Usual cause | Action |
|---|---|---|
| Popping or rumbling while heating | Sediment on the tank bottom; water trapped under it flashes to steam | Flush the tank |
| Crackling or sizzling (electric) | Scale on the heating element | Flush, possibly a new element |
| Sizzling (gas) | Water dripping onto the burner | Find the drip, see leaking heater |
| Ticking or tapping | Pipes expanding, or heat trap nipples | Usually harmless |
| Screeching or whistling | Partly closed valve or a sticking T&P valve | Have the valves checked |
| Knocking or banging in pipes | Water hammer from fast-closing valves | Arrestors on the lines |
| Humming (electric) | Loose element vibrating | Tighten during service |
Of all of these, popping and rumbling are the ones to act on. They mean the bottom of the tank is running hotter than it should, and that heat stresses the glass lining and the steel.
Why sediment builds up
When water is heated, dissolved calcium and magnesium carbonate fall out of solution as solid particles. In a tank heater they sink to the bottom. Over a few years they form a layer of grit and then a hard crust. Hard-water regions build that layer much faster. In parts of the Southwest, the Plains and much of Florida, a neglected tank can have several inches of sediment within five years.
On a gas heater, the burner heats the tank from below, so sediment sits right between the flame and the water. The bottom of the tank overheats, efficiency drops, and pockets of water trapped under the crust boil with a pop. On an electric heater, the lower element can end up buried, overheat and fail.
Our guide on hard water and your water heater explains how to find your local hardness and how fast sediment builds at different levels.
What a professional flush includes
- Gas set to PILOT or power off at the breaker.
- Cold supply closed and a hot faucet opened to break the vacuum.
- Tank drained through a hose to a drain or outside.
- Cold supply opened in bursts to stir up and push out sediment until the water runs clear.
- On badly scaled tanks, the drain valve is removed for a larger opening, or a descaling solution is used.
- Drain valve checked or replaced; plastic valves often crack after a flush and are worth upgrading to brass.
- Tank refilled with air bled, heat restored, and the anode rod checked if the plumber has the clearance to pull it.
A flush typically costs $100 to $300. It is often combined with an anode rod inspection, which is the best predictor of how much life the tank has left.
Noise on a brand-new heater
A heater that pops within its first year is unusual but not unheard of in very hard water. It usually means the old tank's loose sediment was carried over through the pipes, or the water is hard enough to build a layer fast. Flush it once now and plan on an annual flush. If you are on a well, check for sand: a sediment filter on the main line protects the new heater and every faucet in the house.
A ticking sound that starts after a new install is normally the heat trap nipples or new pipe straps settling as the metal expands and contracts. It tends to fade within a few weeks.
When a flush is too late
Flushing a tank that has been neglected for ten years can backfire. Hardened sediment sometimes protects weak spots in the tank bottom, and stirring it up can clog the drain valve or reveal a leak that was sealed under the crust. If a heater is old, very noisy and has never been flushed, a plumber may honestly recommend leaving it alone and planning a replacement.
The best time to start flushing is in the first year, then once a year after that. In soft-water areas, every two or three years is usually enough.
The anode rod: the part that decides tank life
Every glass-lined steel tank has an anode rod, a magnesium or aluminum rod screwed into the top. It corrodes on purpose so the tank does not. Once it is down to its steel core wire, the tank starts rusting from the inside. Replacing the anode before that point, usually around year four to six, can add years to a tank.
Magnesium anodes protect best but can react with sulfur-reducing bacteria to cause a rotten-egg smell in hot water. Aluminum-zinc anodes or powered anodes are common fixes for smelly water. An anode replacement usually runs $150 to $450, depending on whether a flexible segmented rod is needed for low ceilings.
Doing a quick flush yourself
If your tank is less than five years old and has been flushed before, a mini flush between professional visits is a reasonable homeowner job. Put a bucket under the drain valve at the bottom of the tank, open the valve for a few seconds, and close it again. Repeat until the water runs clear. You are not draining the tank, just pulling the loose grit off the bottom while the water pressure does the work.
Stop if the drain valve weeps after closing, or if it will not open by hand. Forcing an old plastic valve is the most common way homeowners turn a maintenance job into a leak. Also stop if the water comes out brown and stays brown, which suggests rust from inside the tank rather than mineral grit.
Keep in mind that the water at the drain valve is scalding. Turn the heater to PILOT or switch off the breaker an hour before you start, and keep children and pets away from the bucket.
Tankless units get flushed differently
Tankless heaters do not collect sediment in the same way, but scale builds inside their heat exchangers. They are cleaned by circulating a descaling solution through the unit rather than draining it. See tankless repair and descaling for intervals and costs.
Whichever type you have, ask the plumber to write the service date on a tag on the heater. It makes the next flush easy to schedule and helps whoever services it next.
Questions people ask
Why is my water heater making a popping noise?
Sediment on the bottom of the tank. Water trapped under the layer turns to steam and pops as it escapes. A flush usually quiets it.
Is a noisy water heater dangerous?
Popping itself is not an immediate danger, but it means the tank bottom is overheating, which shortens tank life. Screeching from a valve or sizzling on a gas burner should be checked soon.
How much does it cost to flush a water heater?
Usually $100 to $300. Badly scaled tanks that need a descaling solution or a new drain valve sit at the top of the range.
How often should a water heater be flushed?
Once a year in hard-water areas, every two or three years where the water is soft.
Can flushing an old water heater cause a leak?
It can on a very old, never-flushed tank, because hardened sediment may be covering weak spots. A plumber can judge whether it is worth the risk.