Navien Knocking, Banging or Vibrating: What Causes It
On this page (9 sections)
The Short Answer: The timing of a Navien’s noise tells you the cause. A thud when a tap shuts off is water hammer. Knocking while the burner runs suggests scale. A clunk or buzz when recirculation runs points to a check valve or the pump. A rough hum with the burner is often the fan or loose mounting. A bang at light-off is delayed ignition: turn it off and call for service.
Match the knock to its timing
| When you hear it | Likely cause | How urgent |
|---|---|---|
| A thud or bang in the pipes the moment a tap, washer or toilet valve closes | Water hammer from fast-closing valves, often worse with high house pressure | Not urgent for the heater; fix the plumbing |
| Popping, ticking or knocking that continues while hot water runs | Scale in the heat exchanger causing localized boiling | Schedule descaling soon |
| A clunk when the recirculation pump starts or stops, with no tap open | Check valve slamming or a pump issue (A2 models and external recirculation) | Service when convenient |
| A loud bang or “whump” just as the burner lights | Delayed ignition: gas built up before lighting | Stop using it and call |
| Ticking as pipes heat up and cool down | Copper pipes expanding against straps or framing | Usually harmless |
| Rough hum, buzz or rattle that rises and falls with fan speed while heating | Dirty or worn combustion fan, loose cover or mounting | Service soon; clean the intake filter |
| Whine or rattle with no tap open, stops when recirculation is turned off | Recirculation pump, air in the pump circuit | Service when convenient |
| Noise only in strong wind | Vent terminal or air pressure disturbance | Note it for the technician; see Navien exhaust blockage |
A soft fan whoosh that rises and falls as the burner modulates is normal, and so is a faint pump hum on an NPE-A2 while recirculation runs.
Water hammer
Water moving through pipes has momentum. When a valve closes quickly, especially the solenoid valves in washing machines, dishwashers and some faucets, the water stops suddenly and the pipes jump. The tankless heater is often blamed because the noise travels through its connections, but it isn’t the cause.
High static water pressure makes hammer worse. Pressure much above 80 psi is generally considered high for a house. A pressure-reducing valve, a correctly charged expansion tank where the system is closed, and water hammer arrestors near quick-closing valves are the usual fixes. A plumber can measure your pressure. If you have a hose-bib pressure gauge, reading it at an outside tap is an owner-level check.
Scale in the heat exchanger
In hard water, minerals deposit inside the heat exchanger. Water trapped behind scale can overheat and flash into steam bubbles that collapse with a knock or crackle. Other signs are hot water that isn’t as hot as it used to be (see Navien not getting hot) and codes such as E016 (overheating of heat exchanger) or E030 (exhaust overheating). Navien’s NPE-2 manual lists flushing the heat exchanger and cleaning the inlet filter for both.
Descaling uses isolation valves, a pump and a descaling solution, following the manual’s procedure. If the heater has no isolation valves, a plumber should add them. Scale returns in hard water, so ask about a water softener or scale-reduction system if it builds up quickly.
Recirculation check valves
NPE-A2 models have a built-in recirculation pump, and many homes have external loops. Check valves stop water from flowing backward. A spring check can clunk each time the pump starts or stops. Turning recirculation off for a day is a simple test. If the clunk stops, the recirculation side is the source.
Hum, buzz or vibration
A Navien has two moving parts that can vibrate: the combustion fan, which runs whenever the burner fires, and on NPE-A2 models the built-in recirculation pump, which can run with no tap open.
Combustion fan. Dust and lint drawn in with combustion air can build up on the fan and unbalance it, and the fan bearing can wear. Navien’s NPE-2 manual lists cleaning the intake air filter for code E109 (abnormal operation: fan motor) and recommends cleaning that filter every 3 to 4 months. A clean filter protects the fan.
Recirculation pump (NPE-A2). The A2’s built-in pump moves water through a small buffer tank and the recirculation loop. Air trapped after the unit was drained or after plumbing work can make a pump rattle or whine, and a worn pump can buzz. A failed pump may show E438 (abnormal operation: circulation pump). External pumps have the same issues.
Mounting and piping. A heater mounted on thin drywall without solid backing, rigid pipes touching framing, or a loose front cover can turn normal operating vibration into a noticeable buzz. If gently pressing on the side of the cabinet or a pipe while it runs changes the noise, the source is a loose cover, mounting or pipe strap.
Delayed ignition
If gas builds up for a moment before it lights, the result is a muffled bang at the start of heating. Causes include a weak or dirty igniter, low gas pressure, a partly blocked air intake, or a burner problem. Ignition codes such as E003 may also appear; see Navien E003. Don’t keep running a heater that bangs on start-up. Turn it off at the front panel and arrange service. If you smell gas, leave and call from outside.
What you can check safely
- Note exactly when the noise happens, and record it if you can.
- Turn off recirculation for a day to see whether the noise stops.
- Check that the front cover screws are snug, nothing is leaning on the cabinet, and pipe straps aren’t loose. Don’t remove the cover.
- Clean the air intake filter if you’re comfortable following the owner’s manual steps, with the heater off and unplugged.
- Check house water pressure with a hose-bib gauge, if you have one.
- Photograph any code on the display.
What the technician will check
A technician will check the ignition sequence and gas pressure for start-up bangs, flush and inspect the heat exchanger for scale knocks, check check valves, bleed air from the pump circuit and confirm pump settings for recirculation noise, and inspect and clean the combustion fan, comparing its readings with Navien’s specifications. They also check the wall mounting and isolate pipes from framing where needed. For water hammer, a plumber measures static pressure, checks the pressure-reducing valve and expansion tank, and adds arrestors where needed.
Frequently Asked Questions
Q: Will flushing stop every knock? A: No. Flushing helps only when scale is the cause. It won’t fix water hammer, a slamming check valve, or delayed ignition.
Q: Is a knocking or vibrating Navien dangerous? A: Water hammer and pipe ticking are nuisances that can loosen fittings over time. Scale knocking means the heat exchanger is overheating and should be descaled soon. Fan vibration is usually not a hazard by itself, but a failing fan can lead to airflow faults and shutdowns. A bang on ignition, or vibration with a burning or exhaust smell, is a safety issue: turn the heater off and call for service.
Q: Can I lubricate the fan or pump? A: No. They aren’t designed for owner lubrication and are behind the front cover. Have a technician inspect them.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- Navien NPE-2 Series user's information manual (opens in a new tab)Navien NPE-A2 and NPE-S2: built-in pump (A2), air intake filter, codes E003, E016, E030, E109 and E438, and heat exchanger flushing.
- Navien: manuals and product documents (opens in a new tab)Model-specific owner information; service manuals are intended for qualified professionals.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
- A. O. Smith technical bulletin 45: thermal expansion (opens in a new tab)Background on pressure rise in a closed water system. Pressure testing and repairs are professional work.
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