Tankless Water Heater Groaning Noise
The Short Answer: A groan while a tankless fires is usually scale kettling in the exchanger, a dry condensate trap, or a fan bearing — not a 40-gallon pile of rocks. A bang at light-off is delayed ignition; shut it down. Name when it groans. Do not oil the combustion fan. Hire a licensed tech if it is kettling or the fan is grinding.
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Search Intent & The Real Problem
The closet moans when someone opens a hot tap. Tank-type articles tell you to flush sediment. There is no gravel pile in a Rinnai, Navien, Rheem RTG, Noritz, or Takagi. What you have is a heat exchanger, a combustion fan, sometimes a recirc pump, and a condensate trap. A groan that tracks the burner is scale boiling in a restricted passage. A groan that tracks the fan is a bearing. A gurgle in the PVC is the trap, not the copper.
Brand-specific cousins: Navien knocking, Rinnai humming, Takagi vibration. Tank gurgle: gurgling. Hybrid compressor drone: hybrid noise. This page is the generic tankless groan.
What other guides get wrong
They paste a tank vinegar flush onto a condensing exchanger and call the moan “normal modulation.” Modulation changes fan pitch; it does not sound like a teakettle. They tell you to tighten cabinet screws for a failing bearing. They skip the condensate trap, so you descale a dry trap that is sucking air. And they treat a whoosh-bang as the same noise as a low groan — it is not.
Step-by-Step Troubleshooting
Step 1: Name when it groans
| When | Likely source | Next |
|---|---|---|
| Low moan/kettling only while the burner is on | Scale in the exchanger | Isolation-valve flush if you have ports |
| Pitch rises and falls with the fan, no boil | Combustion fan | Normal if light; grinding is licensed |
| Gurgle in the PVC under the unit | Condensate trap / drain | Pitch, trap, pump |
| One bang at light-off, then quiet | Delayed ignition | Shut down — loud bang |
| Groan on a recirc schedule, no one using water | Pump or check valve | Recirc off for a test |
| Tick in the copper with power off | Pipe expansion / strap | Not the heater |
| Groan plus lukewarm showers or an overheat code | Scale until proven otherwise | Photograph the code |
- What to look for: Stopwatch vs the in-use lamp. Helper at a faucet. Photo of any 10/11/12/14/16 or Navien E-code before you clear it.
- Required tools: Ears, phone. Cover stays on for the first listen.
A light whoosh every time it fires is the fan doing its job. A new scream is not.
Step 2: Delayed ignition is shutdown, not a descaler
- What to look for: Whoosh-then-bang, scorch at the cabinet, flame-failure 12 / E012 / 11. New pipe or a meter relight. That is air or manifold pressure, not mineral. Exhaust that smells like fuel: tankless exhaust gas.
- Required tools: Phone photo of the code. No wrenches.
Do not purge the gas line yourself. One bang is enough. Inlet and manifold pressure are licensed (NG often ~3.5–7 in. w.c. / LP ~10–11 in. w.c. — confirm the rating plate).
Step 3: Scale kettling versus a dirty inlet screen
- What to look for: Hard water, skipped annual flush, overheat / thermal-fuse codes, lukewarm water with the moan. Used to be quiet, now it growls as fire rate climbs. Inlet screen packed with grit more often cuts flow than it groans — still rinse it with power/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, not a weekend hose.
Tank popcorn is a different machine — sounds like rocks.
Step 4: Trap, fan, and recirc
- What to look for: Condensing unit: trap dry or the drain looped uphill so it gurgles and the flame can drown. Fan that used to whisper and now groans on every call — bearing; do not oil it, and do not run a unit whose blower is dying (blower). Recirc pump through rigid pipe, missing check, house static over ~80 psi with no expansion tank — the cabinet becomes the drum.
- Required tools: Level on the condensate PVC, hose-bib gauge, recirc menu off for one shower.
If the house goes quiet with recirc off, you found the pump path. A fan that never stops is a different complaint.
Step 5: Mounts, valves, and when the exchanger is the problem
- What to look for: Isolation valves at 45° cavitating. Unit hung on drywall with no lag into studs. Copper tight against a strap. Those ticks follow temperature, not burner RPM. Groan plus a wet heat exchanger, green copper, or repeating overheat after a proper flush is licensed exchanger or gas-valve work — stop cycling it.
- Required tools: Eyes. Fully open the service valves.
Rinnai 12 during a groan is flame loss, not “just noise” — Rinnai code 12.
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 knock with rollout. You may photograph codes, 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 moaned a little on heat-up” can be the fan. “It started groaning 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 groaning noise? A: Scale kettling in the heat exchanger, a dry or clogged condensate trap, a recirc pump, or a combustion-fan bearing. A light fan whoosh while it fires is normal. A bang at light-off is not.
Q: Is a groaning tankless dangerous? A: Kettling is a maintenance problem that can overheat the exchanger. Delayed ignition is a fire hazard — shut gas and power. Pipe-expansion ticks are usually not. Grinding fan plus a silent impeller on the next call is carbon monoxide if you keep firing it.
Q: Will flushing stop tankless groaning? A: If the groan tracks the burner and you have skipped descaling in hard water, a correct isolation-valve flush often quiets it. It will not fix a bang at ignition, a dry trap, or a dying fan.