Navien Water Heater Leaking — What’s Happening
When a Navien tankless water heater is leaking, it’s usually not a fault code by itself. Navien’s own troubleshooting identifies flow sensor issues and recommends checking whether the flow sensor is spinning properly by opening and closing the pressure relief valve. What most people call a “leak” falls into three categories: external water leakage from fittings, valves, or condensate lines; internal leakage from the heat exchanger or pump; or overflow and discharge from the pressure relief valve or condensate system that looks like a leak.
A leak complaint often overlaps with flow, pressure, or valve problems rather than a cracked component. The water could be coming from the cabinet floor, service valve unions, the relief valve discharge pipe, the condensate drain on condensing models, or from internal component seepage. Identifying the exact source is the first step in any repair.
Most Likely Causes
- Loose or failing water connections Inlet and outlet unions, service valves, or threaded fittings can loosen over time or fail from thermal cycling and vibration.
- Pressure relief valve discharge The relief valve can drip or discharge water due to overpressure, thermal expansion, or a stuck valve, and this is often mistaken for a unit leak.
- Clogged inlet filter or restricted flow Navien’s published guidance for flow issues includes checking for a clogged inlet filter, which can create abnormal operation and water discharge problems.
- Scale or sediment damage to internal components Hard water scale is a recurring manufacturer-recognized problem in tankless service and can damage internal water-path components, especially in hard-water areas.
- Faulty flow sensor or internal water control component Navien identifies the flow sensor as a common internal issue when the unit is not detecting normal flow, which can overlap with apparent leak symptoms.
- Condensate drain issues on condensing models Condensate can drip or back up from a clogged trap or drain line and be mistaken for water leakage from the unit itself.
- Heat exchanger leakage If water is coming from inside the cabinet or combustion side, the heat exchanger may be leaking, which is a serious internal failure and usually a replacement-level repair.
How to Diagnose and Fix
- Identify exactly where the water is coming from: cabinet floor, fittings and unions, relief valve discharge pipe, condensate drain, vent or exhaust area, or internal component seepage.
- Shut off power and gas if needed, then close the cold-water inlet and hot-water outlet service valves to isolate the unit and depressurize safely.
- Inspect the easiest leak points first: inlet and outlet unions, service valves, drain plugs, relief valve piping, and condensate trap and drain tubing on condensing models.
- Remove and inspect the inlet filter for clogs and verify water supply pressure, as Navien’s published guidance for low-flow issues includes checking for a clogged inlet filter.
- Open and close the pressure relief valve while checking for flow sensor response, as Navien’s FAQ references this step when diagnosing flow-related symptoms.
- Check for scale, corrosion, or freeze damage on internal water-path components, especially if the unit has evidence of mineral buildup or is in a cold climate.
- Pressure-test the water path by isolating sections and testing the heat exchanger and internal assemblies if the leak is internal or intermittent.
- Repair or replace the failed component: re-seat or replace fittings, gaskets, O-rings, valves, drain parts, or sensors; replace the heat exchanger if it is leaking; replace the relief valve and correct overpressure or thermal expansion; or clean and replace the condensate trap and tubing.
Parts You Might Need
| Part | Notes |
|---|---|
| Navien inlet filter screen | Amazon | Check model-specific part; clean or replace if clogged or damaged. |
| Pressure relief valve (T&P valve) | Amazon | Replace if discharging or leaking; verify correct pressure rating for your model. |
| Condensate trap and drain kit | Amazon | For condensing models; replace if cracked or clogged. |
When to Call a Pro
If the leak is coming from inside the unit cabinet, from the heat exchanger, or from an internal component you cannot access safely, call a licensed technician. Heat exchanger leaks and internal water-train failures are replacement-level repairs that require disassembly, pressure testing, and warranty verification. If you suspect freeze damage, scale buildup affecting internal parts, or a faulty flow sensor, a qualified tech can diagnose and repair correctly. Gas and venting work on tankless units should always be handled by a professional. For gas line, burner, or igniter work, or if you ever smell gas, stop and call a licensed technician.