A tankless water heater about three years old stops making hot water, and inside the cabinet the heat exchanger is corroded and leaking. That's what we found on a Navien NPE-240S in a Westford home. A repair like this raises four separate questions: what failed, what contributed, what the manufacturer covers, and what has to pass before the heater goes back into service.
If your tankless is leaking
Stop using it and arrange a diagnosis. Don't open the cabinet, remove venting, touch wet electrical equipment, handle gas parts or keep resetting it. Follow only the user-level shutdown steps in your exact model's manual when the controls are dry and safely accessible and there is no suspected gas leak, CO alarm, possible CO exposure, flame, smoke or fire. Any of those hazards means leave immediately and call 911 from outside; for suspected gas, call the gas utility or propane provider from outside as well. Don't operate switches, phones, appliances or shutoffs inside during a suspected gas leak. Stay out until officials clear the house.
What the case record shows
The unit had stopped providing hot water. Its heat exchanger was visibly corroded and actively leaking. One hardness result recorded for the property was 4.6 grains per gallon. We documented the failure with photographs, coordinated the claim with Navien, and Navien covered the exchanger part, while labor remained the homeowner's responsibility. The work included teardown, exchanger replacement, internal cleaning and restored operation.
What isn't covered here: a claim letter, the final invoice, commissioning readings, any treatment the homeowner chose afterward and long-term follow-up. The record supports a documented warranty case. It doesn't show that every deposit came from hardness, that the exchanger failed from one cause, or that another serial number or installation would get the same coverage.
What 4.6 grains per gallon can and can't tell you
Hardness measures dissolved calcium and magnesium, expressed as calcium carbonate. A result of 4.6 grains per gallon is about 79 milligrams per liter, which falls in the moderately hard band of the USGS hardness classification. Those minerals can form scale in a heat exchanger's water passages, restrict flow and affect maintenance needs.
Hardness isn't corrosivity, though, and a 4.6 result can't by itself explain visible corrosion. A fuller analysis would look at pH, alkalinity, other chemistry, pressure, flow, temperature control, condensate, venting, combustion, installation, maintenance history, freeze exposure and where the leak started. In this case, pH, alkalinity and total dissolved solids were described only in general terms, without numbers or a test method, so they can't confirm or clear a corrosion mechanism. The careful conclusion is narrower: moderate hardness created scale potential worth addressing in the prevention plan, and the active leak justified the repair.
How the warranty claim was handled
The value in this case wasn't a promise that every claim gets approved. It was one coordinated path from field evidence to the manufacturer's review to the approved repair. A claim starts with the exact model, serial number, installation information, any displayed or stored codes, the leak location, visible condition, service history and relevant water, venting and operating evidence. The homeowner should know which component is being submitted, what diagnostic and connected work falls outside the part claim, and what may need correcting first. We price repair work at a flat rate, with the total and any fees shown for approval before work begins.
Navien reviews the evidence and decides eligibility. Heat exchanger terms depend on the model, application and recirculation setup, registration, installation, maintenance, water conditions, the cause of failure and Navien's current written NPE-S warranty terms. A covered part doesn't automatically include diagnosis, labor, access, shipping, seals, other damaged parts, venting, water treatment or permits.
Jeneral Plumbing is owner-operated by a Licensed Master Plumber (MA #9628831-PL-M), factory trained by Navien and Navien Service Specialist (NSS) certified. That supports model-specific diagnosis, claim documentation, repair and commissioning. It doesn't make us the warranty decision-maker.
The venting note and return to service
The venting setup in this home was described as needing improvement. That calls for checking the installed model's venting requirements alongside its operating and combustion checks. A combustion reading alone can't confirm the vent's materials, clearances or physical condition. The observation isn't proof of a code violation, and it doesn't establish whether venting contributed to this exchanger failure.
Restored operation also isn't a full test sheet. A qualified technician must complete the installed model's required reassembly, leak, operating and safety checks before returning it to service. Keep that record with the repair paperwork; the case summary doesn't supply those readings.
What we recommended next
After the repair, we discussed maintenance, softening, inline filtration and conditioning. Which option the homeowner chose, if any, isn't described here. The logic behind each option is useful for anyone in the same spot.
Descaling removes mineral scale when the model's procedure calls for it, but it doesn't reverse corrosion or repair a leak. Annual service is a practical review point, while the manual, measured water, use, recirculation and the scale found during service set the actual interval. A properly sized softener can reduce hardness when testing and observed scale support it. A standard sediment filter doesn't remove dissolved hardness, and conditioning equipment should be judged against its verified performance claim. Whatever's chosen, retest the treated water to confirm the target was met. Our test-first water treatment service and water testing service explain that sequence.
Keep the records too: the model and serial number, invoices, maintenance dates, water results, treatment settings, photos, warranty correspondence and the commissioning record. They make the next diagnosis and any future claim more specific.
Repair or replace a young leaking unit?
A young unit, a covered exchanger and restored operation supported repair in this case. That's not a rule. The decision weighs the confirmed damage, parts availability, exact coverage, labor and connected corrections, whether the installation suits the model, repeated faults, current demand and whether commissioning can pass. Our Tyngsborough heat exchanger replacement walks through a documented repair, and our Lowell procedure guide covers isolation, reassembly and commissioning.
Does hard water automatically void a Navien warranty?
No town name or hardness number decides a claim automatically. Navien reviews the cause and the evidence under its current written terms, including any exclusions. Measured water conditions, installation and maintenance records, observed scale, the failure findings, the model and the application all matter. The 4.6 result in this case neither guaranteed coverage nor showed that an exclusion applied.
Navien service in Westford
We're based in Westford, MA and serve homes across Middlesex & Worcester County. A regular-hours visit carries an $80 dispatch fee — the cost of the visit, waived when you hire us for the work. Call (978) 392-7789 with your model, serial number and what the heater is doing, and we'll explain the safe next step and the evidence a claim will need. Our tankless water heater service page covers repair and replacement in more detail.
Written by Jeneral Plumbing.
This article was drafted with AI writing tools and edited before publication. AI tools did not inspect any job or validate technical conclusions.
Last updated .