Lukewarm showers at 6 a.m. are usually the first clue something is off. Knowing how to tell if water heater is working properly — before the tank fails completely — saves you from cold rinses and the panic call to a plumber at holiday rates. Most water heaters give two to three weeks of warning before total failure if you know what to watch for. This guide walks through the seven checks I run on my own units every spring and fall, plus the symptoms that mean you need a pro on the phone today.
Check 1: Hot Water Temperature at the Tap
Run the kitchen sink on full hot for 90 seconds, then aim a meat thermometer or infrared gun at the stream. A healthy unit delivers 118 to 125 degrees Fahrenheit at the tap. Anything below 110 means the thermostat is wrong, the heating element is partially failed, or sediment is blocking heat transfer.
Above 130 degrees is a scald risk and usually means the upper thermostat is stuck closed. Adjust the dial down one notch and recheck after two hours. The target setting on the tank itself is 120 degrees, which the Department of Energy and most pediatricians both recommend.
Check 2: Recovery Time
Drain the hot water by running the shower or tub until it goes cold. Note the time. A standard 40-gallon electric tank should fully reheat in 60 to 80 minutes. A 50-gallon gas tank reheats in 30 to 45 minutes. Tankless units do not have recovery time but should produce hot water within 8 to 15 seconds of opening a tap.
If recovery takes more than double the expected time, suspect either a failing lower heating element (electric) or a partially clogged burner orifice (gas). Both are repair jobs, not replacement jobs, if caught early.
Check 3: Listen to the Tank
Stand next to the tank during a heating cycle. A healthy unit makes a low, steady hum (electric) or a soft whoosh of burner ignition followed by quiet flame noise (gas). Warning sounds include:
- Popping or rumbling — sediment is boiling under the burner; flush the tank
- High-pitched whining — scale buildup on an electric element; replace the element
- Tapping or knocking — thermal expansion in CPVC plumbing, usually harmless
- Sizzling or hissing — water is dripping onto a hot surface, indicating a leak
The sizzle is the one to act on immediately. A small drip onto a gas burner can corrode the assembly within weeks.
Check 4: Pilot Light or Indicator LED
For older gas heaters, lift the inspection cover and look for a steady blue flame. Yellow or orange flickering indicates incomplete combustion and possible carbon monoxide production — shut the gas off and call a plumber. Newer gas units use electronic ignition with a status LED. Two flashes is normal on most Rheem and AO Smith models; specific codes are printed on the inside cover.
Electric units do not have a pilot but most have a small green LED on the upper access panel. If it is off or red, the high-temp limit switch may have tripped. Press the small red reset button once. If it trips again within 24 hours, the thermostat or element is failing.
Check 5: Inspect the Tank for Moisture
Run your hand around the base of the tank, the cold and hot supply lines, and the temperature and pressure relief valve. Any moisture is a problem. A wet TPR valve discharge tube indicates excess pressure — replace the valve for $15 and verify the expansion tank is functioning. Water at the tank base usually means the inner liner has rusted through, and that means full replacement.
Take a photo of the tank’s data plate while you are down there. The serial number tells you the manufacture date — most tanks are designed for 8 to 12 years. If yours is past 10, start budgeting for a replacement even if it seems fine.
Check 6: Test the TPR Valve Annually
Place a bucket under the discharge tube. Lift the lever on the temperature and pressure relief valve briefly — water should rush out, then stop completely when you release. If water continues to dribble after release, the valve is fouled with mineral deposits and needs replacement. If no water comes out at all, the valve is seized closed, which is more dangerous than a stuck-open valve.
Check 7: Anode Rod Condition
The anode rod is a sacrificial magnesium or aluminum rod that corrodes in place of the tank’s steel liner. Pull it once every two to three years using a 1-1/16 inch socket. A healthy rod is mostly intact with some pitting. A rod that looks like a thin wire surrounded by white residue needs immediate replacement, around $35 for a flexible aftermarket rod that fits low-clearance basements.
Replacing the anode every three to four years can extend a tank’s life from 10 years to 18 or more. It is the single highest-return maintenance task you can do.
When to Stop Diagnosing and Replace
Some symptoms mean the tank is done. Rusty hot water at every tap (cold runs clear) means the inner liner has perforated. A puddle that returns within hours of mopping means a slow leak from the tank itself, not a fitting. And total failure to heat after replacing both elements and both thermostats on an electric unit usually points to a cracked dip tube or a failed lower heating chamber.
Replacement runs $1,400 to $2,800 installed for a standard 40 or 50-gallon tank in 2026, depending on whether codes require a new expansion tank, drain pan, or seismic strapping in your area. Tankless conversion is $3,500 to $5,500 and only pays back over 12 to 15 years for households of three or more. Knowing how to tell if water heater is working buys you time to plan that decision instead of making it under pressure on a Saturday morning.
