Water Heater Not Working? A Diagnostic Flowchart

No hot water is one of the most disruptive, urgent home failures — and one of the most misdiagnosed. Many homeowners call an emergency plumber when the fix is a $20 thermocouple or a flicked breaker, and many others "fix" a simple symptom while a costly leak saturates a wall.

Before you pick up the phone, walk this diagnostic flowchart.

Step 1: Is it really the water heater?

Before anything else, verify the tank itself is the problem:

• Other gas/electric appliances working? Yes → utility service is fine.
• Cold water at every hot tap (not just one)? Yes → it's the heater, not a single faucet cartridge. -Pooling water around the base? Yes → possible leak, jump to Step 5.

Step 2: Identify your water heater type

Step 3: Match symptom to cause

A. No hot water at all

Gas tank:

1. Is the pilot out? If yes, light it (or relight per the unit's instructions). If it won't stay lit, thermocouple is the most common replacement — a $20 part.
2. Is the gas valve ON? Check both the knob and the inline valve.
3. If you smell gas — shut gas off at the meter, leave the house, call utility. Do not relight.

Electric tank:

1. Check the breaker for the water heater — typically a 30-amp double-pole. If tripped, reset once, once. If it trips again, you have a short — usually the lower element.
2. Press the red reset button on the upper thermostat. If it clicks and powers on, the high-limit tripped. If it trips again immediately, an element is grounded.

Tankless (gas or electric):

1. Check error code on the display. Tankless manufacturers (Rinnai, Navien, Rheem) use standard codes — Rinnai code 11 means ignition failure, code 10 means air supply issue.
2. Scale build-up in the heat exchanger is the #1 tankless failure. Annual descaling with vinegar (or a descal kit) prevents most error codes.

B. Hot water runs out quickly

• Cause 1: Lower heating element failed (electric). Top element makes the top hot, lower element keeps it hot. If only the top works, you get 5 minutes of hot then cold.
• Cause 2: Dip tube broken. This tube sends cold to the bottom. If broken, cold mixes at the top and runs out fast. Tip: look for plastic chips in aerators.
• Cause 3: Undersized tank for the household. If your family grew, you grew out of the tank.

C. Water is too hot / not hot enough

• Look for the thermostat dial on the gas valve or behind the electric lower access panel.
• Recommended setting: 120°F. Higher risks scalding; lower invites Legionella.
• If adjustments don't take, the thermostat itself may need replacement ($30-100 part + labor).

D. Rusty or smelly hot water

• Rusty hot water only (cold clean) → anode rod consumed or tank corroding internally.
• Replace the anode rod ($25-50 part) — many tanks are tossed when only the anode died.
• Sulfur / rotten egg smell → bacteria in the anode reacting with sulfate. A magnesium → aluminum anode swap and a hydrogen peroxide flush usually resolves it.

E. Leaking from the tank base

This is the end-of-life failure. Replace, do not repair. A slow leak at the base means the inner tank is corroded. Patching the outer jacket doesn't fix the inner tank, and a slow leak becomes a flood.

Step 4: Age check — should I fix or replace?

A tank water heater's lifespan is 8-12 years. Past 10, you are risk pooling a leak that costs 5x the replacement to clean up.

Step 5: When to call a plumber

• Gas valve or gas line involved
• 240V electrical work past the breaker
• Tankless internal components (controller, flow sensor, fan motor)
• Anything inside the tank (dip tube replacement)
• Pressure relief valve (TPR) discharge — implies overpressure, serious safety issue

A surprising number of "no hot water" calls are $20 fixes the homeowner can do safely. The rest — especially the gas and 240V ones — are worth paying a licensed plumber $150-300 to do correctly. The judgment call is the actual skill.