Your tankless water heater stopped making hot water. Before you call anyone, there are a handful of things that cause this, and most of them follow a pattern. A licensed plumber can diagnose and fix all of them, but understanding what you’re looking at helps you describe the problem clearly and avoid a costly misdiagnosis.
The Most Common Reasons Tankless Units Go Cold
Flow sensor failure is the most frequent culprit. Tankless heaters only fire when they detect water moving through the unit above a minimum flow rate (commonly somewhere around 0.5 to 0.75 GPM, depending on the model). If the flow sensor is dirty or failing, the unit thinks no water is moving and never triggers ignition. You’ll get cold water with no error code, or an intermittent hot/cold/hot cycle.
Ignition failure is second. The unit tries to fire, can’t establish flame, and shuts down. Navien units will typically display an error code here. Write down exactly what the display shows (including any letter prefix) before calling anyone. The Navien manual, available free on Navien’s website, has the full code list and will tell you what each code means.
Gas pressure problems. Tankless heaters demand a high volume of gas in a short burst. If your gas pressure is low, or the supply line is undersized for the unit, the burner won’t sustain. This is common in homes where multiple gas appliances run simultaneously. Signs: works fine when nothing else is running, fails under load.
Venting obstruction. Blocked or improperly installed exhaust venting causes the unit to shut down on a safety switch. Check that the vent termination outside isn’t blocked by a bird nest, debris, or snow. If your unit is direct-vent (concentric pipe through the wall), also check the intake screen.
Scale buildup inside the heat exchanger. Water hardness in the Bay Area varies significantly by location; some areas have quite hard water. Over time, calcium deposits build up inside the heat exchanger, reducing flow and heat transfer. This usually shows up as gradually declining hot water performance before it becomes a full failure.
Condensate drain issues (on condensing models, which includes most current Navien units). If the condensate drain is blocked or frozen, the unit can trip a fault. Check that the drain line isn’t clogged and drains freely.
How a Tech Actually Diagnoses This
A plumber working on a Navien (or Rinnai, Noritz, Rheem, etc.) will typically do this in order:
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Pull the error code history from the unit’s service menu. Most modern units log multiple faults, not just the current one. This tells the tech whether it’s an intermittent problem or a hard failure.
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Check gas pressure at the unit under firing conditions, not just static pressure. A unit can have adequate static pressure but still starve under demand.
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Test the flow sensor with a meter and verify the minimum flow threshold is being reached. A partially closed isolation valve upstream can cause this.
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Inspect the igniter and flame sensor (flame rod). These wear over time and are often the actual failure even when the gas pressure looks fine.
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Look for scale. On units with buildup, a tech will descale the heat exchanger using the appropriate service procedure for that model.
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Check the venting for proper clearances, correct pipe diameter, and any obstructions.
What You Can Safely Check Yourself
These are safe, no-tools checks before you call:
- Check the error code. Write down exactly what the display shows. The Navien manual is available free on Navien’s website and has a full error code list. If you can describe the code to your plumber, the call will go faster.
- Check the cold water inlet filter. Navien units have a small screen at the cold water inlet that traps debris. It can be removed and rinsed without special tools, but turn off the water supply first. Your manual shows the exact location and steps.
- Check the gas shutoff at the unit. Make sure it’s fully open. Sounds obvious, but it gets overlooked after any work near the unit.
- Check the circuit breaker. Tankless units need electricity to operate the controls, igniter, and fan. A tripped breaker will shut the unit down completely.
- Check other gas appliances. If your stove and furnace also have issues, the problem is upstream at the meter or regulator, not the heater itself.
What You Should Not Touch
Do not attempt to adjust the gas valve, clean the burner assembly, or replace the igniter yourself unless you are a licensed plumber or gas technician. Gas work requires a permit in California and must be done by someone with the right license. Same goes for the venting: improper repairs to exhaust venting create carbon monoxide hazards.
Do not attempt to bypass a safety lockout. If the unit is refusing to fire, there’s usually a reason. Forcing ignition while a fault exists can damage the heat exchanger or create a hazard.
When to Call a Licensed Plumber
If your checks above don’t find an obvious culprit, call a licensed plumber. Specifically:
- Any error code you can’t resolve by cycling power and clearing the fault once
- Suspected gas pressure issues
- No ignition after verifying flow and gas supply
- A burning smell, any smell of gas, or visible rust or corrosion on the unit
- The unit is more than 10 to 12 years old and showing multiple symptoms
When you hire anyone for this work, verify their California contractor license at cslb.ca.gov. Plumbing work (including gas line work connected to plumbing fixtures) requires a C-36 license. Water heater replacement also typically requires a permit from your local building department.
Tankless heaters are reliable when they’re maintained. Most no-hot-water calls turn out to be a flow sensor, a scale flush, or an ignition component. The diagnosis matters though, and a licensed plumber with the right tools will get there faster and safer than working through it yourself. Verify any plumber’s C-36 license at cslb.ca.gov before work starts.