A tankless water heater that will not fire almost always fails one of three checks: not enough water flow to trigger the unit’s flow sensor, a venting or ignition fault, or a genuine gas or power supply issue. Tankless units work fundamentally differently from a tank system, so the usual tank-heater troubleshooting steps do not apply here.
Beyond the checks below, diagnosing gas valve, burner, or venting faults on a tankless unit should be left to a qualified technician. Tankless units also have specific, sometimes complex venting requirements — improper venting is a carbon monoxide risk.
How ignition works on a tankless unit
Unlike a tank heater that is always ready with stored hot water, a tankless unit only fires when it detects sufficient water flow through it — typically triggered by opening a hot tap. If flow is below the unit’s minimum activation threshold, it simply will not turn on, which can look identical to a genuine fault.
Common causes
| Cause | How likely | What to look for |
|---|---|---|
| Water flow below the unit’s minimum activation rate | Very common | A single low-flow fixture (some faucet aerators, for example) may not trigger it |
| Clogged inlet water filter | Common | Reduced flow throughout, gradually worsening |
| Gas supply issue (gas units) | Common | Other gas appliances also affected |
| Power supply issue (electronic ignition needs power even on gas units) | Common | No display or response at all from the unit |
| Blocked or improperly installed vent | Occasional | Error code specific to venting, if the unit has a display |
| Failed ignition components | Less common | Flow and supply confirmed fine, unit still will not fire |
How to diagnose it
- Check the flow rate at the tap
Open the hot tap fully. If flow is weak, the unit may genuinely not be detecting enough water movement to activate — check for a clogged aerator or partially closed valve first.
- Check for an error code
Most tankless units have a digital display or a series of blinking lights that indicate a specific fault. Note the exact code or pattern before calling for service — it saves diagnostic time.
- Confirm gas and power supply
Check other gas appliances are working normally, and confirm the unit itself has power (many gas tankless units still need electricity for their ignition control and display).
- Check the inlet water filter
Many units have a small filter screen at the water inlet that can be removed and cleaned — check your manual for its location.
When to call a professional
If flow, gas supply and power all check out but the unit still will not fire, or if it displays an ignition or venting-specific error code, have a technician qualified in tankless systems diagnose the ignition components and venting.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
