Tankless Water Heater Banging? Shut It Down

The Short Answer: A bang at a tankless is not a 40-gallon sediment cannon. A whoosh at light-off is delayed ignition — shut it down. A kettle knock while it fires is scale. A slam when a fixture or recirc valve closes is water hammer. Name when it bangs. Do not keep resetting 11/12 into a dark cabinet. Hire a licensed tech.

Search Intent & The Real Problem

The closet slammed when you opened hot. People paste tank “flush the rocks” onto a Rinnai, Navien, Rheem RTG, Noritz, or Takagi. There is no gravel pile. What you have is a heat exchanger, a combustion fan, a gas valve that opens on a timer, and sometimes a recirc pump. A bang at ignition is unburned gas lighting late. A knock while it is already burning is mineral boiling in a restricted passage. A hit when flow stops is a check valve or solenoid slamming.

Cousins, different sounds: groaning, clicking, Navien knocking, tank delayed ignition: loud bang. This page is generic tankless banging.

What other guides get wrong

They call every bang “sediment” and sell vinegar. Tankless kettling is scale in copper, not a lime brick on a bottom head. They treat a light-off boom as the same noise as a recirc slam, so you descale a delayed-ignition problem. They tell you to keep clicking through 11 / 12 / E003. Repeated late lights dump fuel, then spark. That is a fire event, not a flush.

Step-by-Step Troubleshooting

Step 1: Name when it bangs

• What to look for: Stopwatch vs the in-use lamp. Helper at a faucet. Photo of 11 / 12 / 10 / 14 / 16 or Navien E003 / E012 before you clear it.
• Required tools: Ears, phone. Cover stays on for the first listen.

A light fan whoosh every fire is normal. A door-rattling bang is not.

Step 2: Delayed ignition is shutdown, not a descaler

• What to look for: Whoosh-then-bang, scorch at the cabinet, flame-failure after a late light. New pipe, a meter relight, 1/2” flex on a 180–199 kBTU burner, LP regulator frosting. Exhaust that smells like fuel. Tank delayed-ignition physics is the same emergency — loud bang — different box.
• Required tools: Phone photo of the code. Hand on the switch to leave it off. No wrenches.

Do not purge the gas line yourself. Inlet and manifold pressure are licensed (NG often ~3.5–7 in. w.c. / LP ~10–11 in. w.c. — confirm the rating plate). One bang is enough. A second trial is how cabinets fill.

Step 3: Scale kettling versus a dirty inlet screen

• What to look for: Hard water, skipped annual flush, overheat / thermal-fuse codes, lukewarm showers with the knock. Used to be quiet, now it bangs as fire rate climbs. Inlet screen packed with grit more often cuts flow than it bangs — still rinse it with power and gas off.
• Required tools: Isolation-valve flush kit if the install has service ports; manufacturer descaler. Flashlight on the cold-side filter.

Flush through factory isolation valves. Vinegar in a garden sprayer with the gas on is how people eat an exchanger. No service valves is a licensed flush. Tank popcorn is a different machine — sounds like rocks.

Step 4: Water hammer, recirc, and isolation valves

• What to look for: Slam in the walls when a solenoid or a shower valve snaps shut. ComfortFlow / RUR-style recirc, or an external pump, hammering a check. Isolation valves left at 45° cavitating. House static over ~80 psi with a waterlogged or missing expansion tank — the cabinet becomes the drum. Recirc off for one shower: if the bang dies, you found the pump path.
• Required tools: Hose-bib gauge, recirc menu, eyes on both service valves fully open.

A hammer arrestor on the hot is a licensed add if the piping was never designed for a fast-closing tankless valve. Do not “fix” hammer by cranking the setpoint.

Step 5: Fan, flue, and when the exchanger is the problem

• What to look for: Silent blower on a heat call — do not fire it (blower). Termination full of snow, a nest, or a contractor cap still on the intake — starved air lights late and bangs. Groan plus a wet heat exchanger, green copper, or repeating overheat after a proper flush is licensed exchanger or gas-valve work. Rinnai 12 during a bang is flame loss, not “just noise” — Rinnai code 12.
• Required tools: Flashlight on the cap. Rating plate (BTU/hr, NG vs LP).

Need it dead while you wait? Apartment shutoff. Closet odor now is leave.

When DIY Ends: Calling a Licensed Pro

Stop at delayed ignition, gas smell, an overheat code that returns after a correct flush, a wet PCB, a grinding fan, and any bang with rollout. You may photograph codes, fully open isolation valves, rinse the inlet filter, turn recirc off for a test, and descale only through factory service valves with the manufacturer’s procedure. Heat-exchanger work without ports, gas-valve work, and fan replacement are licensed. “It has always ticked a little on heat-up” can be pipe expansion. “It started banging and the showers went lukewarm” is scale or ignition — get a tech before the 6 a.m. lockout.

Frequently Asked Questions

Q: Why is my tankless water heater making a banging noise? A: A boom at light-off is delayed ignition. A kettle knock while it fires is scale in the exchanger. A slam when flow stops is water hammer or a recirc check. Name when it bangs before you buy a part.

Q: Is a banging tankless dangerous? A: Delayed ignition is a fire and carbon monoxide hazard — shut gas and power. Kettling is a maintenance problem that can overheat the exchanger. Pipe-expansion ticks with the unit off are usually not.

Q: Will flushing stop tankless banging? A: If the knock tracks the burner and you have skipped descaling in hard water, a correct isolation-valve flush often quiets it. It will not fix a whoosh at ignition, a dead fan, or a slamming check valve.