Running Out of Hot Water Fast? Why a Water Heater Loses Capacity

"My showers used to last twenty minutes, now they go cold in ten." "Brand new water heater and we're still running out." "The tank is big enough, so why is it empty already?" These are some of the most-discussed water heater problems among homeowners and plumbers online. When there's some hot water but much less than before, the heater is usually working, just not at full capacity. (If you have no hot water at all, start with water heater not working instead.)

First, rule out the simple things

The temperature setting. If someone turned the thermostat down, the tank holds the same amount of water but less usable heat, so it runs out sooner. Don't fix it by cranking the setting high; very hot water is a scald risk, especially for kids and older adults.
A change in demand. More people in the house, a new high-flow showerhead, a soaking tub, or back-to-back showers and laundry can outrun a tank that was fine before.
The season. Incoming water is colder in winter, so the same tank delivers less hot water at the tap. A modest drop in winter is normal; a sudden, large drop isn't.

Sediment taking up space

Minerals settle to the bottom of a tank over the years, and in a hard-water area like ours that happens faster — see hard water and your plumbing. A thick layer of sediment takes up room, insulates the water from a gas burner, and on electric units can bury the lower element. The classic companion symptom is popping or rumbling as the heater runs, covered in water heater making noise.

A broken dip tube

Cold water is supposed to enter the tank through a dip tube that carries it to the bottom. If that tube cracks or breaks off, cold water dumps in at the top, right next to the hot water outlet, and mixes with it. The result is lukewarm water much sooner than it should be. Small bits of white plastic in faucet aerators or showerheads can be a clue.

A failed element (electric heaters)

Most electric tanks have two elements. When the lower one fails, the heater only warms the top portion of the tank, so you get a short burst of hot water and then it goes cold. Our guide to thermostat vs. element problems explains how plumbers tell the two apart. Testing elements involves live electrical parts, so it's a job for a professional.

Burner or gas supply issues (gas heaters)

On a gas unit, a dirty burner, a weak flame or a problem with the gas control can mean the tank recovers slowly and never fully reheats between uses. If the pilot keeps going out, see pilot light keeps going out. Any work on the gas valve, burner or venting belongs with a licensed plumber, and if you ever smell gas, leave the house and follow our gas leak warning signs guide.

Cold water crossing into the hot lines

Sometimes the heater is fine and the problem is downstream. A faulty single-handle faucet or shower valve, or a mixing valve that has failed, can let cold water push into the hot side of the plumbing. A sign of this is hot water that's lukewarm at several fixtures even right after the heater has had hours to recover.

An aging tank

Tanks lose efficiency as they age and sediment builds up. If yours is getting old and the drop has been gradual, it may be closer to replacement than repair — see how long water heaters last. If you're replacing it anyway and running out is a regular problem, it's worth talking through tank size or a tankless option.

When to call a plumber

Call if the drop in hot water was sudden, if you hear rumbling or see plastic bits at the taps, if the heater is electric and you suspect an element, if anything involves the gas side, or if the tank is leaking. In Torrance, call Rush Plumber at (209) 860-5004 or see our plumbing services.