Usually the tank is too small for your household's demand, or part of it has stopped heating. A dead lower element on an electric unit, sediment on the tank bottom, a broken dip tube, or a thermostat set too low each shrink the hot water you get before the shower turns cold.
Is the tank simply too small?
A tank that ran fine for two people can fall short for four. Before you blame a broken part, rule out capacity.
### What is a first hour rating?
The U.S. Department of Energy says to size a storage water heater by its first hour rating, not its gallon size. The first hour rating is how much hot water the heater can deliver in one hour, starting with a full tank of hot water. It depends on the tank capacity and on the size of the burner or element. DOE's advice is to estimate your peak one-hour hot water demand, then pick a model with a first hour rating within about 2 gallons of it. Our page on what size water heater you need walks through that sizing.
### Did your household or your season change?
Demand changes over the years. A new soaking tub, teenagers, a second dishwasher cycle or a guest room that's suddenly full can push your peak hour past what the heater was sized for. If the shower has always run out after the same amount of use, and nothing is broken, that points to size.
The season matters too. A. O. Smith's troubleshooting guide says seasonal changes in cold-water temperature, along with hot-water use, can produce "not enough hot water" complaints. Rheem's gas water heater manual says the same thing: incoming water can be colder than when the unit was first installed, which takes a longer heat-up cycle. So a heater that felt fine in August may feel short in January.
Rheem's advice for a temporary shortage is simple. If usage has exceeded the heater's capacity, wait for it to recover. Our page on water heater recovery time explains how long that takes.
What broken parts make the hot water run out early?
If size isn't the answer, one of four parts is usually to blame.
### A dead lower element (electric heaters)
This is one of the most common causes on an electric heater with two elements. A. O. Smith's guide lists "check lower heating element" as a step for not enough hot water, and notes that the lower thermostat does most of the work in a dual-element tank. When the lower element fails, the top of the tank can still heat, but the rest of the tank stays cool. You get a short burst of hot water, then cold. You can test the element with a multimeter, and we cover the steps in how to test a water heater element.
### Sediment in the tank
Sediment takes up space and blocks heat. Sediment is minerals that settle to the bottom of the tank, and it builds up fastest where water is hard. On a gas unit that layer sits between the burner and the water, so the heater recovers slowly. The Department of Energy's Building America guideline notes that higher temperature settings can accelerate sediment buildup, and it cites a standard calling for draining twice a year to remove scale and sediment. On an electric unit, Rheem's manual lists scale or mineral deposits on the heating elements as the cause of a rumbling noise, and says to remove and clean the elements.
Phoenix water is hard, so this one is common here. Our page on how hard Phoenix water is has the numbers. A. O. Smith's annual maintenance checklist includes draining and flushing the tank and marks it very important for gas water heaters. Our page on how often to flush a water heater in Phoenix gives a schedule, and popping or rumbling from the tank is covered on why a water heater pops or rumbles.
### A broken dip tube
The dip tube is a plastic pipe inside the tank that sends incoming cold water to the bottom, where the heater can warm it. InspectApedia says a leaking dip tube can make hot water run tepid, because incoming cold water mixes with the hot water even though the heater is working. It also describes the other sign: small pieces of plastic from the failed tube clogging faucet strainers and aerators. Replacing a dip tube is a plumbing job, not a quick DIY swap, because the tank has to be drained and the fittings opened.
### A thermostat set too low, or a bad shower valve
Check the setting. A. O. Smith's guide says to set both thermostats to 120 degrees F on an electric unit, and it points out that a higher setting increases hot water delivery but also raises the scald risk. If you set the tank above 120, the guide says to install a thermostatic mixing valve. Rheem warns that water above 125 degrees F can cause severe burns or death from scalding, and it recommends a mixing valve that meets ASSE 1017 when you need to lower the temperature at the point of use. The Department of Energy says 120 degrees F is high enough for washing and bathing while keeping scald risk down. Our page on what temperature to set a water heater covers the trade-offs, including Legionella.
A. O. Smith also says faulty shower controls or defective thermostatic mixing valves can produce the same complaint. If only one shower runs out of hot water and the rest of the house is fine, look at that shower valve first. We cover that on why one shower has no hot water.
How do you tell which cause you have?
Match what you see to the likely cause. This table pulls together the manufacturer and plumber guidance above.
| What you notice | Likely cause | First check |
|---|---|---|
| It has always run out after about the same amount of use, or the household grew | Heater too small | Compare the first hour rating on the label to your peak hour |
| Electric heater, hot water lasts far shorter than it used to | Failed lower element or thermostat | Test the elements with a multimeter, power off |
| Gas heater, slow to recover, popping or rumbling from the tank | Sediment | Flush the tank |
| Plastic bits in the showerhead or aerators | Broken dip tube | Have the tank inspected |
| Only one shower runs cold | Shower valve or mixing valve | Compare with other showers |
| It got worse when the weather cooled | Colder incoming water | Consider the thermostat and heater size |
What can you check yourself, and when should you call?
- 1Note when it runs out. Does it always run out after the same amount of use? That points to size or a dead element. Did it change suddenly? That points to a failed part.
- 2Check the thermostat. Look at the setting on the gas control dial or, for an electric unit, at the thermostats behind the access panels, with the power off first.
- 3Check other showers. If only one is affected, it's the valve.
- 4Look at the tank's age. A heater that's past its expected life and has never been flushed is a strong suspect for sediment. Our page on whether to repair or replace a water heater helps you decide.
Rheem's electric heater manual says not to attempt repair of electrical wiring, heating elements or other safety devices, and to refer those repairs to qualified service personnel. So testing is fine, but replacing an element, a thermostat or a dip tube is a plumber's job. If those checks don't explain it, a plumber can test the elements, flush the tank, check the dip tube and confirm the heater is the right size.
HQ Plumbing and Air is licensed and insured, gives free estimates, and quotes an upfront price before any work starts. You can see our water heater service page or call anytime, since we take emergency calls 24/7.