Water Heater Runs Out of Hot Water Fast

First: did something change?

The cause usually lines up with what changed.

• It used to be fine and got worse over weeks: a failing element, sediment or a dip tube.
• It went from fine to short overnight: an element or thermostat died.
• It's been short since you moved in, or since the family grew: the heater is probably too small.
• Only in winter: colder incoming water means the tank delivers less mixed water per shower. DOE notes that cooler inlet temperatures can reduce the water temperature at the most distant faucet.

Cause 1: A dead lower element (electric)

An electric tank has two elements. The upper one heats the top part of the tank. The lower one heats the rest. When the lower element burns out, the upper element still heats the top part, so you get a short burst of hot water and then it goes lukewarm. That pattern is the giveaway.

Rheem lists failing heating elements and bad thermostats as causes of reduced hot water. Test the elements with a multimeter, breaker off. Our guide to testing a water heater element has the steps, and no hot water from an electric water heater covers the rest of the electric side.

Cause 2: A broken dip tube

Cold water enters through the top of the tank and should travel down a plastic dip tube to the bottom. If the tube cracks or breaks off, cold water mixes right at the top where hot water leaves. The tank is full of hot water, but you can't get at it. Rheem includes damaged dip tubes on its list.

Look for small white plastic flakes in faucet aerators and shower heads. That's the classic sign. A plumber can swap the tube without replacing the heater, and HomeGuide puts it at $100 to $300 installed.

Cause 3: Sediment

Sediment takes up room on the bottom of the tank, and it insulates the water from a gas burner or the lower element. Rheem names sediment as a cause of reduced hot water. If the tank also pops or rumbles, you've found it. A.O. Smith suggests draining a few gallons every year to keep it in check. Our noises page explains the flush.

Cause 4: The tank is too small

Tanks are sized by peak demand, not by average use. The Department of Energy says to pick a heater with a first hour rating at least equal to your household's busiest hour. The first hour rating is the number of gallons a tank can deliver in one hour starting full, and it's on the yellow EnergyGuide label.

DOE's average hot water use per activity:

DOE's example is three showers, one shave and some hand dishwashing in the same hour, which adds up to 66 gallons. Add up your own busiest hour and compare it to the label. If you're well over, no repair will fix it. Spreading out showers and laundry helps, and so does a new heater sized right. Our water heater sizing guide goes further.

Cause 5: Recovery time

A tank that ran empty needs time to heat again. A.O. Smith gives 30 to 40 minutes for a gas tank and 60 to 90 minutes for electric. If showers are back to back with no gap, the last person gets cold water even from a healthy heater.

One fix that adds hot water

A thermostatic mixing valve lets you store water hotter and blend it down to a safe temperature at the taps. A.O. Smith says these valves can increase usable hot water and reduce scald risk. It's a reasonable option for a tank that's close to big enough. Have a plumber install it, and don't raise the tank temperature without one.

Tankless: does it run out of hot water?

A tankless heater doesn't run out the way a tank does, because it heats water as it flows. What it has is a flow limit. DOE says a typical gas tankless unit gives about a 70°F temperature rise at 5 gallons per minute, and an electric one at 2 gallons per minute. Open too many taps at once and the water comes out cooler.

A.O. Smith lists an undersized unit and scale buildup among tankless problems, and a "cold water sandwich" (a short burst of cold between uses). Run fewer fixtures at once, and descale the unit once a year.

When to call a plumber, and what it costs

Call if an element tests bad and you don't want to do the electrical work, if you find dip tube plastic in the aerators, or if you're thinking about a bigger heater. HomeGuide's repair figures:

Rheem suggests planning replacement around year ten on a standard tank. If yours is that old and short on hot water, put the repair money toward a new heater.