Start with how the heater gets its heat. On a gas heater, check whether the pilot is lit. On an electric heater, check the breaker. If the water is lukewarm rather than cold, an electric element or thermostat has usually failed. If hot water runs out much faster than it used to, think sediment or a broken dip tube.
Smell gas? Stop. Leave the house, don't use switches or phones inside, and call your gas utility or 911 from outside.
First: gas or electric?
Look at the bottom of the heater. A gas heater has a burner compartment with a small access cover, a control knob or valve on the front, and a metal flue pipe out of the top. An electric heater has a thick power cable going into a junction box on top, and one or two panels on the side covering the heating elements. Tankless units are wall-mounted boxes, gas or electric.
Gas water heater: no hot water
Is the pilot lit?
Most standard gas heaters have a small window near the burner. If you can't see a small blue flame, the pilot is out. Relight it by following the instructions on the label on the tank, exactly as written. Don't improvise.
The pilot lights but won't stay lit
This usually means the thermocouple or flame sensor, the part that tells the gas valve the pilot is burning, is worn or out of position. It can also be a dirty pilot tube or not enough combustion air around the heater. Relighting it over and over isn't a fix. Call.
Newer heaters with electronic ignition
Many newer gas heaters have no standing pilot, but an igniter and a status light that blinks a code. The label or manual explains the codes. Note the blink pattern before you call, because it saves time.
Power-vent heaters
If your heater has a fan on top with a power cord, it needs electricity to run. Check that it's plugged in and the outlet works.
Electric water heater: no hot water
Check the breaker
Electric heaters are usually on a double breaker. If it's tripped, reset it once. If it trips again, stop. Something is shorting, and repeated resets aren't safe.
The high-limit reset
Many electric heaters have a red reset button behind the upper panel, which trips if the water overheats. Resetting it may bring the hot water back, but it tripped for a reason, often a failed thermostat or a shorted element. Working behind the panels means exposed live wiring, so if you're not comfortable with that, leave it to a plumber.
Lukewarm water
Electric tanks have two elements. When the upper one fails you get no hot water. When the lower one fails you get a small amount of warm water that runs out quickly.
Pilot won't stay lit, or the breaker keeps tripping? That's a part failure, and a plumber can usually tell which one over the phone.
Call nowHot water, but it runs out fast
- Sediment. Minerals settle on the bottom and take up space, so there's less hot water and it heats slower. Elk River's city water is hard: Elk River Municipal Utilities lists it at 16 grains per gallon (source). That speeds this up.
- A broken dip tube. The tube carries cold water to the bottom of the tank. If it cracks, cold water mixes in at the top and the hot water seems to vanish.
- A failed lower element on electric heaters.
- The thermostat set low, or someone turned it down. Many households use around 120°F.
- More demand. A new shower head, more people in the house, or winter's colder incoming water all mean the tank runs out sooner.
Hot water in some taps but not others
If only one faucet or shower has gone cold, the problem is probably that fixture's mixing valve, not the heater.
What to tell the plumber
- Gas or electric (or tankless), and roughly how old.
- What you've already tried.
- Any blinking light codes.
- Whether there's any water around the base.