Navien Knocking Noise: What It Means
The Short Answer: A knock on an NPE-A2/S2 or NPN is not one part. A metallic ping that tracks the burner is scale kettling in the exchanger. A bang only at ignition is delayed ignition — shut it down. A slam when the recirc pump or a fixture valve closes is water hammer. A light fan whoosh is normal. Do not “tighten the case” or replace the heater until you name the sound.
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Search Intent & The Real Problem
The closet sounds like a pot on a hard boil, or the pipes take a hammer hit when the shower stops. Tankless units do not have a 40-gallon pile of rocks, so “flush the sediment” from a tank article is the wrong first move. Navien A-series units add ComfortFlow — a small buffer and an onboard pump — so some knocks are the pump and check valve, not the burner. Same family of noises as a humming Rinnai, different hardware.
What other guides get wrong
They treat every knock as mineral buildup, then recommend swapping the Navien for a tank or a heat pump. Scale can kettle. It does not slam the copper when a solenoid closes. They also tell you to adjust the gas line and a “pressure-balancing valve” as if those were customer settings. Gas-pressure work is licensed. A shower balancing valve is a fixture part, not a Navien menu.
Step-by-Step Troubleshooting
Step 1: Name when it knocks
| When | Likely source |
|---|---|
| Only while the burner is modulating, kettle ping | Scale kettling in the heat exchanger |
| One bang at light-off, then quiet | Delayed ignition (air, weak gas, dirty burner) |
| Slam when a fixture or recirc stops | Water hammer / check-valve slam |
| On a recirc schedule, no one using water | ComfortFlow pump or external recirc |
| Tick with condensate, not a knock | Trap / drain — not the exchanger |
| Constant, even with power off | Pipe, nearby appliance, or expansion against a stud |
- Required tools: Ears, one helper at a faucet, the controller clock/recirc menu.
A hum that never knocks is fan or pump electrical — different job.
Step 2: Delayed ignition is a shutdown, not a flush
- What to look for: A whoosh-then-bang, scorch at the cabinet, error E003 (ignition failure), E004 (false flame), or E012 (flame loss). New pipe or a meter relight in the last day. That is air or manifold pressure, not scale.
- Required tools: Phone photo of the code. No wrenches.
Do not purge the gas line yourself. One bang is enough. A second trial is how you warp a burner door.
Step 3: Scale kettling versus a dirty filter
- What to look for: Hard water, skipped annual flush, overheat E016 / high-limit E046, lukewarm water at the same time as the ping. Inlet screen packed with grit will more often cut flow than knock.
- Required tools: Isolation-valve flush kit if the install has service ports; flashlight on the cold-side filter.
Flush through factory isolation valves with the manufacturer’s descaler procedure. Vinegar in a garden sprayer with the gas on is how people eat a heat exchanger.
Step 4: ComfortFlow, recirc, and water hammer
- What to look for: NPE-A / A2 pump starting on a schedule. A failed or missing check valve that slams. City static over 80 psi (hose-bib gauge) with no expansion tank on a closed PRV system — the Navien becomes the drum.
- Required tools: Gauge, recirc menu set to a short window or off for a test.
If the house goes quiet with recirc off, you found it. Arrestors belong at quick-closing valves, not as a substitute for a stuck check.
Step 5: Mounting, valves, and pipe tick
- What to look for: Isolation valves at 45° cavitating, a unit hung on drywall with no lag into studs, copper expanding against a strap. Those ticks follow temperature, not burner RPM.
- Required tools: Eyes, fully open the service valves.
When DIY Ends: Calling a Licensed Pro
Stop at delayed ignition, gas smell, E016 after a flush, a wet PCB, and any knock with rollout. You may photograph codes, open isolation valves, rinse the inlet filter, and turn recirc off for a test. Heat-exchanger descaling on a unit with no service valves, gas-valve work, and pump 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 Navien tankless making a knocking noise? A: Scale kettling, water hammer from recirc or fixture valves, or delayed ignition. A fan whoosh while it fires is normal. A bang at light-off is not.
Q: Is a knocking Navien 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.
Q: Will flushing stop Navien knocking? A: Only if the sound is kettling from scale. Flushing will not fix a slamming check valve, high city pressure, or air in a new gas line.