Tankless water heaters are quieter than tank units when they’re working correctly. When they’re not, the noise they make is usually trying to tell you something. Here are the ten most common sources of tankless water heater noise — what they mean, and what to do about them.
1. Mineral Scale Buildup
Hard water leaves mineral deposits inside the heat exchanger. As scale grows, it traps water that boils against superheated metal — the result is popping, crackling, or kettling sounds. Annual descaling fixes this and extends the life of the unit.
2. Loose Mounting
Tankless units vibrate slightly when the burner fires. If mounting bolts loosen, the whole unit can rattle against the wall. A simple hardware tightening usually solves it.
3. Air in the Gas Line
After gas line work, residual air in the line can cause a low-frequency rumble or stutter as the unit ignites. It usually clears itself over a few cycles; if it persists, call a licensed plumber to bleed the line properly.
4. Insufficient Gas Supply
Undersized gas lines or a partially closed shut-off can starve the burner. The result is incomplete combustion — popping, whooshing, or surging sounds. This is a safety issue and needs licensed attention.
5. Cold Water Sandwich
If you turn the hot water on and off in quick succession, you can get a slug of cold water mid-flow. Some units make a clicking or whoosh sound as they re-fire to compensate. This is normal but can be reduced with a buffer tank if it bothers you.
6. Combustion Chamber Issues
A whistling or roaring sound can indicate a partially blocked combustion chamber, dirty burner, or restricted exhaust. This is a safety issue — turn the unit off and call a licensed plumber.
7. Inadequate Venting
Tankless venting must be sized and routed to manufacturer specs. A blocked vent or improper slope can cause backdrafting, which sounds like puffing or whooshing — and it’s dangerous (carbon monoxide risk). Stop using the unit and call us.
8. Water Hammer
If the noise is a hard banging when fixtures shut off, you have water hammer — pressure waves slamming through the supply lines. The fix is usually a hammer arrestor, not a tankless repair.
9. Failing Fan or Igniter
The internal fan and igniter both move or click during operation. A grinding fan or persistently clicking igniter is a sign of imminent failure. Replace before the unit quits entirely.
10. Heat Exchanger Damage
Severe scaling, prolonged neglect, or a manufacturing defect can crack a heat exchanger. The sound is often a hissing or sizzling. This usually means replacement, not repair.
If your tankless water heater is making any of these sounds, call Stollwerck Plumbing & Sewer at 425-374-3909. We’ll diagnose the issue, walk you through the options, and get you back to quiet hot water.