Short answer: tankless heaters leak at isolation valves, water connections, the condensate drain on condensing models, and the heat exchanger. Scale from hard water is the most common slow killer of the heat exchanger. A descaling flush, done on the manufacturer’s schedule, helps prevent it. For water heater leak repair services in Diamond Bar on a tankless unit, tell the provider the brand and model.
Where tankless heaters leak
| Where | Likely cause | Is it urgent? |
|---|---|---|
| Isolation (service) valves | Worn washers or a valve left half-open after service | Close the supply and call. Often a quick fix |
| Inlet and outlet connections | Loose or corroded fittings, scale buildup | Yes. Water can reach the electronics |
| Condensate line or neutralizer (condensing models) | Clogged drain, cracked tubing or a full neutralizer | Not an emergency, but the water is acidic and damages floors |
| Inside the cabinet or under the unit | Heat exchanger crack or pinhole, often related to scale or freeze/thaw | Yes. Turn off power and water and call |
| Relief valve or discharge line | High pressure or thermal expansion | See relief valve leaks |
Why hard water is hard on tankless heaters
As water is heated, dissolved calcium and magnesium come out of solution and bake onto the heat exchanger. Even a thin layer reduces heat transfer, so the unit runs hotter and cycles more, until it overheats, throws error codes or cracks. Because a tankless unit’s passages are narrow, scale has less room to build before it affects performance.
Southern California water is commonly hard. Walnut Valley Water District’s 2025 report lists average hardness of 234 mg/L for part of its supply, and Pomona’s groundwater averaged 260 mg/L in its 2024 report. Both are above the 180 mg/L the USGS treats as very hard. Details for each area are on our Walnut and Pomona pages. Your own supply depends on which provider serves your street, so check the latest water quality report.
What a descaling flush involves
- Shut off power, gas and water to the unit. The plumber also checks that the isolation valves are present. Units installed without them need valves added first.
- Connect a pump and hoses to the service ports. The hoses run from the pump and a bucket to the hot- and cold-side service valves.
- Circulate descaling solution. A manufacturer-approved solution, or white vinegar where the manufacturer allows it, is circulated through the heat exchanger for the period the manual specifies, commonly 45 minutes to an hour.
- Flush with clean water. The system is rinsed thoroughly so no solution is left in the lines.
- Clean the inlet filter and check for leaks. Debris screens are cleaned, valves reopened, the unit restarted and the connections checked dry.
How often to flush depends on water hardness, the model and the warranty. Many manufacturers suggest roughly once a year in hard-water areas, but follow the manual for your unit, and keep records, because some warranties require proof of maintenance.
When a flush is not enough
A flush clears scale. It does not repair a cracked heat exchanger or a leaking fitting. If you see water inside the cabinet or an error code that returns right after a flush, the unit may need parts or replacement. Compare options in water heater replacement. If you are unsure whether the water is coming from the unit or from a pipe above it, start with the emergency leak checklist, or ask one of the plumbers serving Diamond Bar and nearby cities to look at it.
Need a plumber for this? Describe the leak and your address. If a provider covers your area, you can ask about fees, timing and scope before agreeing to anything.
Call +1 (909) 760-1466Frequently asked questions
Do tankless water heaters leak?
Yes, although they have no storage tank to fail. Common sources are isolation valves, water connections, a clogged condensate line on condensing models, and a cracked heat exchanger.
How often should a tankless water heater be descaled?
It depends on your water and the model. Many manufacturers suggest about once a year in hard-water areas. Check the manual, since the warranty may require regular maintenance.
Can I descale a tankless heater myself?
Some owners do it with a pump, hoses and descaling solution. It involves gas, power and hot water, so many people hire a plumber, and a unit without isolation valves needs them installed first.
Is the condensate from a condensing tankless heater harmful?
It is mildly acidic. A neutralizer cartridge is usually installed to treat it. A clogged or leaking condensate line can stain or damage flooring, so it should be checked.
Sources and further reading
Related guides
- Replacing a leaking water heaterWhen a leak means replacement, what a California-compliant installation includes, and how permits and bracing fit in.
- Leaking from the bottom: tank corrosionHow tanks wear out, what the anode rod does, and how to tell a failing tank from a fitting leak or condensation.
- Emergency water heater leak repairWhat to do in the first ten minutes, how to find where the water is really coming from, and when to call right away.