Tankless Water Heater Keeps Shutting Off? 6 Causes, Starting With the Cheapest

1. Flow below the unit's minimum

Every unit needs a minimum flow to fire and stay lit. A figure around 1.5 gallons per minute is sometimes quoted, and yours is in the manual. Below it, the unit can trip off because there is not enough water moving to run on. Opening a tap only a crack, a low-flow shower head or weak pressure at the unit can all do it. Open the tap a little further and see whether it holds.

The opposite problem exists too. Two or three fixtures running together can ask for more than the unit can heat. That usually shows as lukewarm water rather than a shutdown, but it is worth ruling out.

2. Clogged aerators and shower heads

Scale chokes the small openings first. Unscrew the faucet aerator or the shower head, and if it is crusted with white mineral, soak it in white vinegar and scrub it. If the shower is still weak with a clean head, the cartridge inside the shower valve may be restricting the flow, and that is a plumber's job.

3. A dirty inlet filter

The screen at the cold inlet catches grit before it reaches the heat exchanger, and it clogs. Power the unit down, shut the two water valves beneath it, lift out the screen, rinse it under a tap and refit it. Expect a little water to spill. Your manual shows where the filter sits on your model.

4. A flow sensor on its way out

The flow sensor tells the board that water is moving. When it fails, it can report flow on and off while a tap runs steadily, and the burner follows it. One owner whose unit kept turning on and off while water ran found a bad flow sensor behind it. Replacing the sensor is technician work and falls in the estimated $150 to $800 range for part repairs.

5. Scale and overheating

A heat exchanger coated in scale cannot shed heat well, so the unit trips a temperature limit and stops to protect itself. Overheat codes are the sign: Navien's E016, and code 14 on several brands. Cycling between warm and hot before it quits points the same way. A flush is the fix, and the flush guide covers it step by step.

6. Blocked venting or a clogged condensate drain

Owners who chased a unit that turned itself off after about ten seconds found causes like a clogged exhaust or intake screen, a clogged condensate drain, a dirty flame rod and a plugged heat exchanger. Condensing units make acidic condensate that has to drain away, and a blocked trap or a failed pump can shut the unit down with a code before any water shows. Clear snow or debris from the outside termination by hand. The rest is for a technician, and repeated resets are not a fix.

When to call, and what it costs

If cleaning the aerators and the filter does not stop it, or a code comes back within a day, call. A diagnosis visit is estimated at $100 to $250, part repairs at $150 to $800 and a descale at $150 to $350. All of these are estimates, and a written quote comes before any work.