Tankless Water Heater Leaking

Do this first

1. Switch the unit off at the breaker. Not at the controller — at the breaker. There is mains electronics directly above whatever is leaking.
2. Close the cold water isolation valve to the unit if you can reach it safely. If the unit was installed with a service valve kit, this is a quarter turn.
3. Do not run it to see whether it stops.

Then work out where the water is coming from, because that is the question that decides everything else.

Not every drip is a leak

Two normal behaviours get reported as leaks constantly:

• Condensate. A condensing heater produces water as a by-product of burning gas — that is what makes it condensing. A steady drip or trickle from the condensate drain line during and after operation is the unit working correctly. Navien’s own manual lists frequent discharge from the condensate drain pipe as normal.
• Pressure relief discharge. If water comes from the relief valve, that is not a leak either, it is a warning: the system is over pressure or over temperature. Do not cap it or plug it. Call someone.

Where the water is genuinely coming from inside the cabinet, keep reading.

Where it comes from, cheapest first

Fittings and unions. Connections at the inlet, outlet, and the service valves. Common after any work on the unit, and usually the least expensive outcome.

The condensate trap or its line. A cracked trap or a split line puts acidic water where it should not be. Related but different: a blocked condensate line backs water up and triggers a shutdown rather than a leak — Rinnai gives that code 25.

A frozen and burst pipe or exchanger. If this follows a hard freeze, especially a freeze during a power cut, assume this until proven otherwise.

The heat exchanger. The expensive one. On a unit old enough to be out of warranty, a leaking heat exchanger is often the point where replacing the heater beats repairing it.

Your unit may have caught it before you did

Rinnai’s older non-SENSEI generation has a leak detector in the bottom pan and gives it code 79. The order Rinnai diagnoses it in tells you how the manufacturer ranks the odds: wiring first, then the heat exchanger and all internal piping, then pooled water in the pan — and only after all of that, the idea that the sensor itself failed.

There is a second signal worth knowing. Rinnai’s checklist for code 71 asks the technician to check for moisture in the tube running from the combustion box to the gas valve, and if it is there, to find out whether the heat exchanger is leaking or condensate is running back down the vent. A gas valve code that turns out to be a leak is not a rare story.

Why none of this is DIY

Water plus mains electricity plus gas, in a sealed cabinet. Even the cheap outcome — a weeping fitting — needs the unit isolated and drained to reach, and the inspection that rules out the expensive outcome has to happen anyway.

Get a written diagnosis that says which of these it is before agreeing to any repair. The gap between a fitting and a heat exchanger is the gap between a call-out and a new water heater.