Hot water runs out fast: 7 reasons and what to do

1. A failed lower element or thermostat (electric)

On an electric tank, A. O. Smith's troubleshooting guide points to the lower heating element, then the lower thermostat, when there's some hot water but not enough. It's the most common cause on electric heaters and a routine repair (see how to test an element).

2. Sediment taking up space

Rheem says heavy sediment can reduce the amount of usable hot water in a tank, since deposits take up space and make the heater work harder. Rumbling or popping is the clue. A flush may help if it isn't too far gone (see how to flush a water heater).

3. A broken dip tube

The dip tube carries incoming cold water to the bottom of the tank. Rheem lists a damaged dip tube among the causes of short hot water. If it breaks, cold water mixes in at the top where hot water leaves.

4. Thermostat set too low

Rheem lists faulty thermostats as a cause, and a setting that's been bumped down does the same thing. A. O. Smith's guide suggests setting both thermostats on an electric heater to 120°F (see what temperature to set).

5. Colder water in winter

A. O. Smith notes that seasonal changes in incoming water temperature can cause "not enough hot water" complaints. Colder water takes longer to heat, so the same tank goes further in July than in January. If the problem only shows up in winter, the heater may be fine but small.

6. A faulty shower valve

A. O. Smith's guide also lists faulty shower controls and defective mixing valves. If the sink stays hot while the shower fades, look there first (see hot water in the sink but not the shower).

7. The heater is too small

A household that's grown, a new soaking tub or more back-to-back showers can outrun a heater that used to be enough. Tank heaters are rated by first-hour rating, the gallons of hot water they can deliver in an hour starting full. University of Florida Extension suggests choosing a tank whose first-hour rating is within 1 to 2 gallons of your household's peak-hour demand. The first-hour rating is listed in the heater's specifications.