Water Heater Not Heating
Check power or gas, then the reset and thermostat.
By Jonathan West · reviewed 2026-10-21 ·how we research these
When a water heater is not heating, check the power or gas supply first, then the high-limit reset and thermostat, then the element on an electric heater or the pilot and thermocouple on a gas heater. Stop and call a licensed professional for anything inside the 240-volt wiring or any gas smell.
The symptom narrows the list before you open anything. No hot water at all points to a different part than water that is lukewarm or runs out fast. Both branches are covered below, along with tankless units, sediment, and the thermostat setting, which can produce a not-hot-enough complaint with no failure at all.
The costs here are typical US figures for orientation only. They are not quotes.
Starting With Electric or Gas
Find out which kind of heater you have before any other check, because the first steps differ. An electric tank heater runs on a dedicated 240-volt circuit. A gas tank heater runs on a gas supply line, with a gas control valve that has a thermostat, and a pilot flame or electronic igniter.
The nameplate on the heater is the reference for element wattage, and the electric section covers breaker sizing. Tankless units do not have a tank, so they follow the rules in the tankless section instead.
The first three checks, in order, are the power or gas supply, the high-limit reset on an electric tank heater, and the thermostat setting. For the supply, check the breaker on an electric heater and the gas supply on a gas heater. The reset and the thermostat setting come next, and the thermostat setting is covered in its own section below.
Checks for an Electric Water Heater Not Heating
Electric tank heaters usually have two heating elements, an upper and a lower, and each element has its own thermostat. The upper thermostat also carries the high-limit cutoff, a safety switch with a red reset button. Where the hot water stops tells you which part to suspect. No hot water at all typically points to the upper element, the upper thermostat, or a tripped high-limit. Hot water that is lukewarm or runs out fast typically points to the lower element or heavy sediment.
Check the breaker first. The heater runs on a dedicated 240-volt circuit, and its sizing follows NFPA 70, the National Electrical Code, published by the National Fire Protection Association (NFPA). A 4,500-watt element draws about 18.75 amps (4,500 divided by 240). NFPA 70 sets a continuous-load branch circuit at no less than 125 percent of the nameplate load, which is 23.4 amps (18.75 times 1.25). A 30-amp breaker is the common match. Always follow the nameplate on the heater and your local code, because adoption and amendment of NFPA 70 vary by jurisdiction.
If the breaker keeps tripping, the circuit breaker keeps tripping guide covers that case separately. Element testing uses a multimeter set to resistance, with power off and the element wires disconnected. Working inside the 240-volt compartment is electrician or plumber work if you are not experienced with it.
The water heater reset button is the red button on the high-limit cutoff. If it trips again, stop resetting it and call an electrician or plumber instead. To reset it, work through these steps in order:
- Switch the breaker off.
- Confirm the power is off.
- Remove the access panel and the insulation.
- Press the red button.
- Restore the power.
The 240-volt compartment carries serious hazards. Turn the breaker off and confirm the power is off before you remove any panel or touch the wiring. If you are not experienced with 240-volt work, call an electrician or plumber.
Checks for a Gas Water Heater Not Heating
A gas tank heater relies on a pilot flame or an electronic igniter, a thermocouple (or flame sensor), and a gas control valve with a thermostat. If the pilot will not stay lit with the button held, the thermocouple is a common cause. A replacement thermocouple typically costs about $10 to $30 for the part. That is a typical US figure for orientation only.
On a flammable-vapor-ignition-resistant (FVIR) heater, a dirty or lint-clogged flame arrestor can cause the unit to lock out. A lockout stops the heater from firing.
Gas control valve, venting, and combustion-air problems belong to a licensed plumber, a heating, ventilation and air conditioning (HVAC) technician, or a gas fitter. These are not steps for the homeowner to attempt.
If you smell gas, do not touch switches or relight anything. Leave the house, then call the gas utility or 911 from outside.
Tankless Heaters
A tankless heater needs a minimum flow rate to fire. If the water flow through it is below that minimum, the unit will not heat. Check the manual for your model.
In hard water, tankless heaters scale up. Manufacturers publish descaling intervals for each model, and the interval is in the manual.
Error codes are model-specific. What a code means is listed in the manual for that model, so look up the code there before you act on it.
Sediment and Age
Heavy sediment is one of two causes that typically produce lukewarm water or hot water that runs out fast. The other is lower element failure.
Manufacturers commonly recommend flushing a tank heater about once a year. The exact interval for your model is in the manual.
Tank heaters often last in the range given on the water heater lifespan page, which is the place to look for how age fits into the decision.
If the tank is leaking rather than just cold, the water heater leaking guide covers that symptom.
Temperature Setting vs Real Failure
The U.S. Department of Energy (DOE) says 120 degrees Fahrenheit is a comfortable setting for most households. The Consumer Product Safety Commission (CPSC) recommends setting the heater no higher than 120 degrees Fahrenheit to reduce scald risk.
A thermostat set too low causes a not-hot-enough complaint, with no failure behind it. If the water heater is not getting hot enough, check the setting before you test any part.
If the setting is already 120 degrees, the setting is ruled out and the fault is in a part.
What a Pro Charges and When to Call
These are typical US figures for orientation only, and they are not quotes. A professional repair visit often runs $150 to $450 all-in. A heating element part typically costs about $15 to $40, a thermostat about $15 to $40, and a thermocouple about $10 to $30. Full replacement is priced on the water heater replacement cost page.
If the heater still makes no hot water, get a written quote from a licensed plumber or electrician. For trade rates, see the plumber cost page and the electrician cost page. Next step: switch the breaker off, follow the reset steps for an electric heater, and call a licensed pro if the cutoff trips again or you smell gas.
| Symptom | Likely part | DIY or pro |
|---|---|---|
| No hot water at all, electric | Upper element, upper thermostat, or tripped high-limit | Breaker and reset: only with power off, and call a pro if unsure. Element testing and 240-volt work: electrician or plumber unless experienced |
| Lukewarm or runs out fast, electric | Lower element or heavy sediment | Sediment: check the manual for the flush interval. Element work: electrician or plumber unless experienced |
| Pilot will not stay lit, gas | Thermocouple (or flame sensor) | Replacement is a job for a licensed plumber or gas fitter |
| Lockout on an FVIR heater | Dirty or lint-clogged flame arrestor | Check the manual, or call a licensed plumber or gas fitter |
| Gas smell | Unknown. Do not investigate | Leave the house, then call the gas utility or 911 from outside |
| Gas valve, venting, or combustion air | Gas control valve, venting, or combustion air | Licensed plumber, HVAC technician, or gas fitter |
| Tankless error code | Model-specific, per the manual | Look up the code in the manual |
| Cutoff trips again after reset | High-limit cutoff | Stop resetting it. Call an electrician or plumber |
Common questions
Why isn't the water heater heating water?
Causes of this symptom include no power or gas supply, a tripped high-limit cutoff, a thermostat set too low, or a failed part. Work through the sections above in that order.
What size breaker does a water heater need?
A 4,500-watt electric element draws about 18.75 amps (4,500 divided by 240). NFPA 70 sets a continuous-load branch circuit at no less than 125 percent of that, or 23.4 amps, so a 30-amp breaker is the common match. Follow the nameplate on your heater and your local code, since adoption and amendment of NFPA 70 vary by jurisdiction.
How often should I flush my water heater?
Manufacturers commonly recommend flushing a tank heater about once a year. The exact interval for your model is in the manual.
How do you reset a hot water heater?
On an electric tank heater, switch the breaker off, confirm the power is off, remove the access panel and insulation, press the red high-limit button, and restore the power. If it trips again, stop and call an electrician or plumber. Gas heaters are covered in the gas section above.
What trips the reset button on a hot water heater tank?
The high-limit cutoff trips because something overheated or failed. On an electric tank heater, the cutoff sits on the upper thermostat, and its red button is the reset. If it trips again, stop resetting it and call an electrician or plumber.
What are the first signs of a water heater going bad?
The symptoms tied to specific parts are no hot water at all, water that is lukewarm or runs out fast, and a pilot that will not stay lit on a gas heater. The water heater lifespan page gives the range for tank heaters.