Tankless Water Heater Groaning Noise
On this page (6 sections)
The Short Answer: A low groan or moan from a gas tankless heater usually comes from one of four things: the combustion fan (a bearing wearing out or debris on the wheel), scale in the heat exchanger, a gurgling condensate drain on a condensing model, or a recirculation pump. Notice whether the sound follows the fan speed, the hot water flow, or a schedule. A fan that grinds needs service before it stops.
Listen for what the sound follows
| The groan… | Likely source | Urgency |
|---|---|---|
| Rises and falls with the fan speed, even right after a tap closes | Combustion fan bearing or debris | Service soon |
| Starts after the burner lights, with weaker hot water | Scale in the heat exchanger | Descale soon |
| Sounds like gurgling from the plastic drain line | Condensate trap or drain partly blocked | Clear the drain path |
| Runs on a timer or with no taps open | Recirculation pump | Check the pump |
| Comes from the pipes, not the heater | Partly closed valve, high water pressure | Plumber |
| Comes with a burning smell, smoke, or a fan error | Failing fan motor | Turn the heater off; call |
Fan noise
The combustion fan runs before ignition, during firing, and for a short time after the burner shuts off. A soft whoosh that changes with demand is normal. A new groan, growl, or rising whine usually means a bearing is wearing, or leaves or debris have reached the fan through the air intake on an outdoor unit. A fan that fails will stop the heater, typically with a fan error code.
What you can check: look at the outside air intake and vent for debris, nests, or lint. Do not open the cabinet or oil the fan. A technician will inspect the fan, check its speed signal, and replace it if needed. Fans are a common repair part.
Scale in the heat exchanger
In hard water, minerals deposit inside the heat exchanger. Water passing over hot, coated surfaces can boil in small spots, producing a rumbling or groaning tone and reducing output. Overheat-related codes may follow. Descaling through the isolation valves, following the heater manufacturer’s procedure and interval, removes the deposit. A plumber can add isolation valves if the heater has none.
Condensate gurgling
Condensing models send acidic water from the exhaust through a trap and drain line. If the drain line sags, is kinked, or is partly blocked, or a condensate neutralizer is clogged, it can gurgle loudly when the heater runs. Check that the line slopes down to the drain without dips and that any condensate pump runs. If water backs up into the heater, many models stop with a drain or blockage code. See Navien condensate drain leaking for an example.
Pumps and pipes
Built-in or add-on recirculation pumps can hum or groan, especially if air is trapped in them after plumbing work. Turn recirculation off for a day to test. Partly closed isolation valves and high household water pressure can also make the pipes near the heater moan; a plumber can check the pressure and valves.
For sharper sounds, see tankless water heater banging noise. Brand examples: Navien knocking, Rinnai humming, and Takagi vibration.
Frequently Asked Questions
Q: Is it safe to keep using a groaning tankless? A: Usually for a short time, if hot water is normal and there is no smell or error code. A grinding fan, a burning smell, or a fan error means turning it off until it is serviced, since the fan clears combustion gases.
Q: How often should a tankless be descaled? A: It depends on water hardness and use. Your heater’s manual gives the recommended interval; hard-water homes often need it more often.
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.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
Editorial standards and corrections · Prepare a useful service request