Why is my water heater not working? A symptom-by-symptom guide

Start with safety

If you smell gas (a rotten egg or sulfur smell in the air), stop reading. Leave the building on foot, do not use a phone, switch or anything that could spark, and call the gas utility and 911 once you are safely away^[5]. Our gas smell page has the full checklist.

Also stop and call a licensed plumber if the relief valve is blowing hot water or steam, if you see scorch marks around the burner door, or if water is pouring from the tank.

Match your symptom

What you notice	Most likely causes	Safe first check	Full guide
No hot water, gas tank	Gas supply off, pilot or igniter fault, safety lockout	Is the gas valve on and do other gas appliances work?	No hot water (gas)
No hot water, electric tank	Tripped breaker, failed element or thermostat, tripped high-limit	Check the breaker panel	No hot water (electric)
No hot water, tankless	Error code, low gas, clogged inlet filter, blocked vent, condensate backup	Read the code on the display	No hot water (tankless)
Water on the floor	Relief valve discharge, drain valve, condensation, failed tank	Find where the water comes from	Leaking from the bottom
Relief valve dripping	Thermal expansion in a closed system, high pressure, overheating	Note when it drips (after heating?)	Relief valve dripping
Popping or rumbling	Sediment on the tank bottom	Is your water hard? When was it last flushed?	Popping and rumbling
Hot water smells like rotten eggs	Bacteria reacting with the magnesium anode	Is the smell only in hot water?	Rotten egg smell
Pilot will not stay lit	Thermocouple, dirty pilot, flammable vapor lockout, draft	Follow the lighting label once	Pilot won't stay lit
Gas smell in the air	Possible gas leak	None: leave and call	Gas smell

Why the type of heater changes the diagnosis

Gas tanks fail in the fuel path most of the time: gas supply, the pilot or electronic igniter, the flame sensor or thermocouple, and the gas control valve. Many newer gas tanks also have a flammable vapor sensor that locks the burner out after it detects fumes. All of the parts past the gas valve are pro work.

Electric tanks have two heating elements and two thermostats, plus a high-limit cutoff. When the upper element or the high-limit fails, you get no hot water at all. When the lower element fails, you get a small amount of hot water that runs out fast. Testing elements means opening panels with 240 volts behind them, so it belongs to a licensed plumber or electrician.

Tankless units tell you what is wrong. Rinnai, Navien and Rheem all display numbered codes, and the makers' service documents list which ones a homeowner can address (a closed valve, a dirty inlet filter, a blocked vent terminal) and which need a technician^[2][3][4]. Start at our error code hub.

What a homeowner can safely check

These need no tools and no opening of sealed panels:

• Fuel and power. Gas valve handle in line with the pipe (open), gas service on, breaker not tripped, tankless unit plugged in.
• Water valves. The cold supply valve above the heater should be fully open.
• Settings. The thermostat or setpoint was not turned down by accident.
• Codes and lights. Write down any code or status light pattern before you reset anything.
• Vents and drains (tankless). Clear snow, leaves or nests from the outside vent terminal; make sure the condensate line drains and is not frozen. Rinnai code 10 and Rheem codes 10, 90 and 99 all point to venting^[2][4].
• Filters (tankless). Clean the cold water inlet filter, and on Navien units the intake air filter, using the owner's manual steps^[3].

One reset is a fair test. If the fault returns, stop. Rinnai says to treat its overheat code (14) seriously and not keep resetting it^[2].

What belongs to a licensed pro

• Gas valves, gas lines, pilot assemblies, burners and venting changes.
• Heating elements, thermostats, wiring and circuit boards.
• Relief valve replacement and expansion tank installs. Where a check valve or pressure-reducing valve closes off the water system, the residential code calls for a thermal expansion control device^[6].
• Anything inside a tankless cabinet: sensors, fans, gas valves, heat exchangers.
• Chlorinating a tank or changing the anode for odor. Bradford White warns that removing the anode entirely voids the warranty and shortens tank life^[8].

Two root causes behind many symptoms

Age. Rheem puts typical tank life at 8 to 12 years and says repairs make the most sense on units under 6 to 8 years old^[1]. Before paying for a repair on an older tank, check its build date with our age decoder.

Hard water. Minerals settle as sediment in tanks, which causes popping noises and slower heating. In tankless units the same minerals form scale in the heat exchanger, which restricts flow and can shorten the unit's life^[7]. That is why Rinnai (LC and SS), Rheem (1L) and Navien (E760) all have scale or service reminder codes^[2][3][4].

What repairs usually cost

HomeGuide's national figures (Aug 2025) put a typical tank repair at $150 to $700. Common line items: thermocouple $100 to $325, gas control valve $150 to $550, heating element $100 to $350, and relief valve $50 to $300. Tankless repairs run $300 to $1,300^[9]. These are ranges, not quotes, and service call fees can add to them. If the quote is more than half the cost of a new unit, read repair or replace first.

When to call a pro

• Any gas smell, scorch marks, or steam or hot water blowing from the relief valve.
• Water coming from the tank body, or a leak you cannot trace.
• A fault code that returns after one reset.
• Any work on gas, wiring, venting or the relief valve.