You're halfway through a shower and the water goes cold. Or the first two people get hot showers and the third gets a cold one. Both feel like "the water heater ran out," but they usually point to very different problems.
Knowing which one you have can save you from paying for the wrong fix.
Running out during one shower
If hot water runs out during a single shower, even when nobody else has used any, something is usually wrong with the water heater itself. A working tank should easily get one person through a normal shower.
Common causes:
- A failed heating element (electric). Electric tanks have an upper and a lower element. If the lower one fails, only the top of the tank gets hot, so you get a few minutes of hot water and then it's gone.
- A broken dip tube. This tube sends cold water to the bottom of the tank. If it breaks, cold water mixes in near the top, and your hot water turns lukewarm fast.
- Sediment buildup. Minerals settle at the bottom of the tank over time. They take up space and make it harder for the heater to warm the water.
- The thermostat is set too low. Sometimes it really is that simple.
On an older tank, these problems are often a sign it's close to the end of its life.
Running out after several showers
If the first shower or two are fine but the third or fourth runs cold, your water heater may be working exactly as designed. It's just not keeping up with how much hot water your household uses at once.
This comes down to two numbers: recovery rate and first-hour rating.
What is recovery rate?
Recovery rate is how fast a water heater can heat a fresh batch of water after you use some. It's measured in gallons per hour, usually at a 90°F temperature rise.
The bigger the gas burner or the higher the wattage of the electric elements, the faster the recovery. That's why fuel type makes a big difference:
| Type (50-gallon tank) | Typical recovery rate |
|---|---|
| Standard electric | About 21 gallons per hour |
| Standard gas | About 40 gallons per hour |
Those figures come from manufacturer spec sheets for standard 50-gallon electric and gas models. In plain terms, a standard gas tank refills its hot water about twice as fast as an electric one the same size.
What is first-hour rating?
The first-hour rating (FHR) is how many gallons of hot water the heater can deliver in one hour, starting with a full tank of hot water. It depends on the tank size, whether it's gas or electric, and how big the burner or elements are.
This is the number that matters most when you're choosing a replacement. It combines the hot water already stored in the tank with how much the heater can make during that hour.
You'll find it on the yellow EnergyGuide label on the side of the tank, listed in the top left corner as "Capacity (first hour rating)."
How to size your replacement
The U.S. Department of Energy recommends this simple method:
- Figure out your busiest hour. For most homes, that's the morning rush.
- Add up the hot water used in that hour. The DOE's worksheet counts about 20 gallons per shower. Its example of three showers, a shave, and hand-washing dishes adds up to 66 gallons.
- Pick a water heater with a first-hour rating that matches or beats that number. In the DOE example, that household would need a first-hour rating of 66 gallons or more.
If your current heater keeps running out after several showers, your busiest hour is probably bigger than its first-hour rating.
Your options if you need more hot water
- Go bigger. A larger tank stores more hot water to start with.
- Switch from electric to gas, if you have a gas line. Faster recovery means more hot water through the busy hour.
- Choose a high-recovery model. Some gas water heaters have larger burners that recover much faster than a standard tank.
- Consider tankless. A tankless water heater doesn't run out the same way, but it has to be sized for how many fixtures you run at the same time.
- Spread out your usage. Waiting a bit between showers, or running the dishwasher at night, gives the tank time to recover.
Don't forget about winter
When the weather gets cold, the water coming into your home is colder too. Your water heater has to work harder to heat each gallon, so you may notice it running out sooner in the winter than in the summer.
Not sure which problem you have?
The Water Heater Heroes specialize in water heater repair and replacement across Metro Atlanta. We'll figure out whether your water heater needs a repair or is simply too small for your home, and give you an estimate before any work starts. We offer same-day service.
Call (770) 641-9474 or request service online.
Source: U.S. Department of Energy: Sizing a New Water Heater