No Hot Water From a Tank Water Heater
The tank is full of cold or lukewarm water and nothing's changing it — on a storage tank unit this is almost always a dead pilot, a tripped breaker, a failed heating element, or a gas control valve fault, not a problem with the tank itself.
Most likely causes, ranked
- Gas pilot light is out. The single most common cause on an older standing-pilot gas tank heater — no pilot flame means no burner, means no heat. See pilot won't light or won't stay lit for the full relighting and thermocouple troubleshooting.
- Thermopile or thermocouple failure (electronic-ignition gas models). On AO Smith FVIR tank models, this shows up as a specific flash pattern on the gas control valve's status light — AO Smith 2 Flashes (low thermopile voltage) is the direct match.
- Gas control valve failure. The valve itself can fail internally and simply stop passing gas to the burner even with a good pilot — AO Smith 7 Flashes documents this fault.
- Energy cut-off (ECO) or high-limit switch tripped. This safety device shuts the gas off entirely if the tank overheated once — see AO Smith 4 Flashes. It should not simply be reset over and over.
- Flammable vapor sensor lockout (FVIR models). If gasoline, paint, or solvent vapors were ever near the unit, the safety sensor can lock the gas valve out entirely — AO Smith 6 Flashes.
- Tripped breaker or GFCI (electric tank models). Simple, common, and free to check.
- Failed upper or lower heating element, or a tripped high-limit reset button (electric tank models). Elements do wear out, especially in hard-water areas.
- Thermostat set low or in vacation mode. Easy to overlook after someone else adjusted it, or after a power outage reset it.
Check this first
- Look at the thermostat dial or display — confirm it wasn't turned down or left in "vacation"/"pilot only" mode.
- Gas models: check the pilot viewing window for a flame, and check the gas control valve for a blinking status light — count the flashes and match it against the brand's code list.
- Gas models: confirm the gas valve is fully open at the water heater and at the meter.
- Electric models: check the breaker panel for a tripped breaker, and look for a red high-limit reset button on the thermostat (behind the access panel) that may have tripped.
- Note whether the loss of hot water was sudden or gradual — sudden usually points to a tripped safety device or breaker; gradual points to a failing element or sediment buildup.
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 if the pilot won't stay lit after a correct relight attempt, a flash-code lockout repeats after you've cleared the obvious cause, an ECO or high-limit switch has tripped more than once, or you smell gas at any point. Electric-side element and thermostat replacement is also a job for a licensed pro once breaker and reset-button checks are ruled out — it involves working inside an energized (or drained) tank.
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.