No Hot Water From a Tankless Water Heater
You open a hot tap and get nothing but cold — on a tankless unit that almost always means the burner never fired, and the reason is usually gas, power, or flow, not a mechanical breakdown.
Most likely causes, ranked
- Ignition failure. The unit called for heat but the burner never lit. This is the single most-reported tankless fault across every brand: Rinnai 11, Navien E003, Rheem 11, and AO Smith 111 all describe the same underlying failure.
- Gas supply interrupted. Gas shut off at the unit, meter, or propane tank; an empty or near-empty propane tank; or, less commonly, inadequate gas pressure or the wrong gas type after a swap.
- No power to the unit. A tripped breaker or GFCI outlet — tankless units need continuous power to run their control board and ignition even though the burner itself is gas.
- Water flow below the unit's minimum activation threshold. Tankless heaters only fire once flow crosses a minimum rate, commonly around 0.4 GPM. A low-flow fixture, a partially closed valve, or a clogged inlet filter can keep that threshold from ever being reached — see Rheem P1 and Navien E439.
- Freeze protection shutdown. Some installations shut the unit down in extreme cold as a protective measure until temperatures recover.
- Clogged inlet water filter. Sediment and scale can restrict flow enough that the unit never senses a valid draw.
Check this first
- Confirm the gas valve is fully open at the unit and at the meter or propane tank; check the propane gauge if applicable.
- Confirm the unit's display is lit — if it's completely dark, check the breaker panel and any dedicated GFCI outlet.
- Open the hot tap fully rather than a trickle — some fixtures alone can't reach the unit's minimum flow rate to trigger it.
- Look at the unit's display for an error code or blink pattern and write it down before you reset anything.
- Power-cycle the unit once (off, wait, back on) and retry a full-flow tap.
- Clean the inlet water filter per the owner's manual if the unit has one and it's accessible.
Which fault code are you seeing?
If a code or blink pattern is showing, look it up directly: code lookup, or jump to a brand: Rinnai, Navien, Rheem, AO Smith.
When to call a professional
Call a licensed plumber or HVAC technician if ignition still fails after you've confirmed gas supply, power, and a full-flow tap; if the same error code returns after a power cycle; or if you smell gas at any point. Gas pressure testing, igniter and flame-rod diagnosis, and gas valve work all require a licensed professional — this isn't a parts-swap repair a homeowner can safely do.
Gas supply lines, gas control valves, and venting should only be worked on by a licensed plumber or HVAC technician. Mains/line-voltage electrical work should be done with the breaker off and, beyond a simple reset, by a qualified electrician or plumber.