How to Reset a Tankless Water Heater (and When Not To)

What a reset actually does

It clears the code from the display. That is all. It does not repair anything, and on the faults that matter it does not even clear the condition — the unit simply re-checks, finds the same problem, and stops again.

That is worth saying plainly because “how do I reset it” is usually the wrong question. The right one is “should this be reset”, and for a fair number of codes the answer is no.

Write the code down before you clear it. It is the only diagnostic information you have, and the first thing anyone you call will ask for.

The two-attempt rule

If the unit will not run after two resets, stop. Every manufacturer’s troubleshooting for a gas appliance that will not light ends in inlet gas pressure measurements and combustion checks that need instruments and a licence. Repeatedly forcing an ignition attempt on a unit that has decided not to light is not troubleshooting, and it is the behaviour that turns a $200 visit into a bigger one.

Resets you can do from the controller

These are on the temperature controller, on the wall or the front of the unit. Nothing here means opening the cabinet.

Rinnai — clearing a diagnostic code. Press the ON/OFF button on the controller twice. Rinnai documents this for code 03, the power-interruption-during-bath-fill code, and adds a useful limit: if the code comes back after three resets, stop resetting and treat the control board as the next suspect.

Rinnai — the SS service reminder. SS is a time-based reminder that a flush is due, not a fault. On the SENSEI generation, press the controller’s on/off button five times within five seconds. On the SENSEI RX, hold the “A” button until SS disappears. Rinnai lists SS for the SENSEI and SENSEI RX generations only — on older non-SENSEI units the equivalent is LC, which is a real scale detection rather than a timer.

Rinnai — buying 70 hours on an LC lockout. This one is documented and genuinely useful, and it is also the one most worth understanding before you use it. Pressing the on/off button five times on a unit showing LC gives you about 70 more hours of hot water. It is not a fix and it is not free: the digit after LC counts how many times you have done it. LC2 means the unit has been reset for three 70-hour periods with no flush. What is being protected is the heat exchanger, the most expensive part on the machine. Use it to get through a weekend while you arrange a descale, not as a way of living with the code.

Navien — the 30-minute wait. On the first-generation NPE (NPE-A / NPE-S), Navien opens the remedy for E016 and E030 with turning the system off for at least 30 minutes before restarting it. It is a real, documented step and it is worth doing properly — the wait is the point, since what it does is let an overheated heat exchanger cool. The NPE-2 manual drops this step.

Resets that are not yours

Anything on the control board. Several Rinnai procedures — clearing an LC count after a flush, the gas-valve reset sequence on the SENSEI — use two buttons marked A and B on the PC board, inside the cabinet. Rinnai notes that the LC hard reset is not published in the owner’s or installation manual at all. That is the manufacturer telling you who it is for. Opening a sealed gas appliance to press them is not a homeowner job, and on most units it also means disturbing the seal that keeps combustion products where they belong.

The “hard reset” after a safety switch trips. On Rinnai’s non-SENSEI generation, code 57 and code 58 need what Rinnai calls a hard reset — power off and back on. But read the instruction in full: correct the problem first, then hard reset. The order is the whole content of the procedure. Cycling power without fixing the cause simply re-arms a device that has already told you something overheated.

Codes where resetting is the wrong move

• Overheat codes. Rinnai’s code 14 is a bi-metal switch or a thermal fuse. The switch resets itself once the heat exchanger cools, so the unit coming back to life on its own is not a repair — it is a warning with a delay on it. If a thermo-fuse has blown instead, it is a one-shot device and the unit will not restart at all.
• Scale lockouts. See the LC note above. The lockout exists to stop a scaled heat exchanger destroying itself.
• Anything involving gas, ignition, venting or exhaust. These stop the unit because continuing is worse than stopping. If you smell gas, leave the building and call your gas utility from outside — do not reset, and do not restart the unit.
• A leak code. Water and mains electricity in the same cabinet. Switch it off at the breaker rather than clearing the code.

The plain power cycle

Switching the unit off at the wall or the breaker for a minute and back on is the most common thing people mean by “reset”, and it is legitimate in a narrow set of cases — Rinnai documents it as the first step for code 05 on its older units, for example, where the code either clears or proves the valve has failed.

Used that way it is a diagnostic step with a defined outcome, and that is the distinction worth keeping: a power cycle that tells you something is useful, and a power cycle repeated because the code keeps coming back is just delay.

After the reset

If the code cleared and has not returned after a day of normal use, it was transient and you are done. If it comes back, note how long it took — a code that returns in minutes and one that returns in a fortnight point at different things, and it is the most useful sentence you can give whoever comes out.

And if the reset was on an LC or SS code, the reset was never the job. Flush the heat exchanger — that is what the unit was asking for.