"No hot water" is the #1 water heater service call. The diagnostic sequence depends on what type of heater you have. Work through this top-down; you'll usually find the cause inside 15 minutes without calling anyone.
Electric tank — no hot water at all
- Check the breaker. Most common cause. Water heater is on a dedicated 240V double-pole breaker. Tripped breakers can look "on" — flip fully off, then back on. If it trips again immediately, you have a short (likely element)
- Press the ECO reset button. Behind the upper access panel. If it had popped, the upper thermostat tripped. Press the red button. If it pops again within hours, the upper thermostat or element needs replacement
- Test the upper element. See element page. If burned out, replace. $25-60 part, DIY
- Test the upper thermostat. See thermostat page
- If all of above test fine: wiring fault or tripped GFCI somewhere upstream
Electric tank — lukewarm only (not fully cold)
Indicates the upper element is working but the lower isn't:
- Test the lower element
- Test the lower thermostat
- Check for sediment — flush the tank. Sediment buildup covers the lower element and prevents proper heating
Gas tank — no hot water at all
- Check the pilot light. Look through the sight glass. If out: relight per the instructions on the unit. If it lights and stays — done. If it lights then goes out — thermocouple problem
- Bradford White units specifically: clean the intake air filter at the bottom of the unit BEFORE replacing thermocouple. This is the #1 cause of "pilot won't stay lit" on Bradford White Defender / ICON units. See BW intake filter cleaning
- Replace the thermocouple if pilot light fails to stay lit after relighting. $15-25 part, 45 min DIY
- Gas supply — is gas to the home working? Test another gas appliance (furnace, range)
- Gas valve failure — if pilot lights but main burner won't fire when calling for heat: gas control valve. Dealer-installed replacement
Tankless — no hot water
Tankless units almost always display an error code when something's wrong. Note the code first:
- Rinnai error codes — most common: Code 11 (no ignition), Code 12 (flame failure)
- Navien error codes — most common: 003, 012, 016
- Rheem error codes
If no error code is displayed:
- Verify the unit has power. Display should be lit. If not, check breaker or outlet
- Verify minimum water flow. Tankless units need 0.4-0.5 GPM to activate. Low-flow aerators on faucets can drop flow below activation threshold
- Verify gas supply. Other gas appliances working?
- Inspect the inlet water filter for sediment buildup — restricts flow below activation
Heat pump / hybrid — no hot water
- Check the operating mode. Most hybrids have Hybrid / Heat Pump Only / Electric Only / Vacation modes. If accidentally on "Vacation" or "Heat Pump Only" in cold ambient, output may be inadequate
- Check the breaker — 30A double-pole on most units
- Check the air filter on top of the unit — clogged filter shuts down heat pump mode
- Check the condensate drain — full drain pan triggers a safety shutoff on many models
- Check the upper resistance element — heat pumps include backup resistance elements. Test like a standard electric unit
Common mistakes during diagnosis
- Replacing parts before testing — buy a $15 multimeter and test elements/thermostats before replacing
- Forgetting to refill the tank after partial drain — energizing a dry electric tank burns elements in seconds
- Replacing the gas valve before checking the thermocouple on gas tanks — thermocouple is the more common failure
- Ignoring the error code on tankless units — every code has a specific cause; "no hot water" without checking the code wastes time
When to call a plumber
- Gas valve failure (gas tank or tankless)
- Heat exchanger issues (tankless)
- Any work involving gas line connections
- Tankless error codes that persist after Level 1 reset
- Heat pump compressor failures
- Inside warranty period — to keep warranty intact
Related guides
Bottom line
Electric: breaker → ECO reset → upper element → upper thermostat. Gas: pilot → thermocouple (or intake filter on Bradford White) → gas supply → gas valve. Tankless: error code first; if no code, check power, flow, gas. Heat pump: mode setting → breaker → air filter.