Tankless Water Heater Making Noise

Noises that are normal

Worth ruling these out first, because they get reported as faults constantly:

• A fan spinning up before ignition, and running on afterwards. The unit purges the combustion chamber before and after firing. It is supposed to.
• A click, then a whoosh. Igniter, then the burner lighting.
• A valve or motor sound when you open or close a tap, or when the unit changes output.
• A pump running when nobody is using water, on units with recirculation.

None of those need action. If the noise arrived at the same time as a change in how the water behaves, that is different.

Rumbling, kettling, or popping

This is the one that matters. A deep rumble or a sound like a kettle coming to the boil, usually loudest a few seconds after the burner lights, is classic scale.

What is happening is that mineral deposit has built up on the inside of the heat exchanger passages. Water gets trapped under the scale layer, flashes to steam against metal that is far hotter than the water flowing past it, and collapses — repeatedly. That is the noise.

The reason to act on it is not the sound. It is what the sound tells you about heat transfer: the heat is going into the metal instead of into your water. Rinnai’s own troubleshooting for high exhaust temperature (code 54) asks the technician to check the heat exchanger surface for hot spots and says plainly that hot spots indicate blockage from scale build-up, with flushing as the remedy and replacement if the exchanger is already damaged.

The fix is a flush, and it is roughly $30-80 of supplies if you do it yourself. How to descale a tankless water heater

If you have hard water and have never flushed the unit, that is the answer before anything else is investigated.

Your unit may already have told you

Every brand on this site has a way of saying “scale” before it says “failure”, and they are easy to dismiss because the heater keeps working:

• Rinnai shows LC, or LC0-LC9 on the generations that detect scale directly, and SS as a time-based service reminder. Rinnai notes LC is the only code family that lets the unit keep running — which is exactly why it gets ignored. The digit counts how many times the flush has been postponed.
• Navien shows E760, a flushing and service alarm.
• Rinnai’s older units have code 58, where the manufacturer states outright that the code indicates scale build-up inside the secondary heat exchanger.

Banging or hammering in the pipes

If the bang is in the plumbing rather than in the unit — a thud when a tap or an appliance shuts off — that is water hammer, not the heater. It is a plumbing fix (arrestors, or securing a loose pipe run) and the heater is an innocent bystander.

Grinding, screeching, or metal on metal

A fan or a pump with a failing bearing. Not scale, and not something to run until it stops on its own — a seized combustion fan takes the unit out mid-winter. Note when the noise happens (at ignition, continuously, only when recirculation runs) and book a service call.

When to stop using it

• Any noise together with a smell of gas or exhaust — see gas or exhaust smell and leave first.
• Banging accompanied by an overheat code. Turn it off and call someone; the unit is telling you it is running hotter than it should.