The quick answer
Yes, as long as the tank stays full. Shutting off the water at the main or a city outage doesn't empty a tank. What ruins an electric heater is power reaching the elements while the tank is empty or partly empty. Rheem says this burns elements out almost instantly and isn't covered by the warranty. If any hot tap has run while the water was off, or you've drained the tank, switch the breaker off first and don't switch it back on until the tank is full again.
The question usually comes up at the wrong moment: a plumber is replacing a valve, a pipe is leaking, or the city has posted a shutoff notice for the afternoon. A burned-out element is a repair bill you can avoid with one breaker flip, so it's worth knowing which situations need it and which don't.
The steps below follow Rheem's own manual for a residential electric tank heater. Other makers' manuals say much the same, but read yours.
Why it's usually fine
A storage tank heater is a sealed, insulated tank that stays full of water. The cold supply line keeps it topped up, but closing that line doesn't drain what's already inside. As long as nobody draws hot water, the tank sits full and the thermostat keeps it at temperature.
So a two-hour shutoff for a faucet repair, or a city outage where you simply stop using water, is no problem for the heater. Leaving it on costs a little standby heat and nothing else.
The one real risk: dry firing
Dry firing means the heating elements get power while they aren't covered by water. An element depends on the surrounding water to carry its heat away. Without it, the element overheats. Rheem's installation article says elements in that state "overheat and burn out almost instantly," and it names the cause: "power is turned on before the tank is fully filled."
Rheem's manual puts the rule in a warning box: the tank must be full of water before the heater is turned on, and the warranty doesn't cover damage from running with an empty or partly empty tank. The same manual lists two "do not" lines: don't turn the heater on unless it's filled with water, and don't turn it on if the cold water shutoff valve is closed. Some Rheem electronic models flash a "Dry Fire (Heating without water)" alarm code.
A tank can end up low while the water is off. This part is my reading of the plumbing, not a line from the manual. With the supply closed, an open hot tap pulls water out of the tank and nothing replaces it. Run a shower or fill a bathtub during an outage and you can drain a good part of the tank without noticing. Draining the tank for service, or a large leak, does the same.
The rule that follows: if you're not sure the tank is full, turn off the breaker and treat it as empty.
What to do by situation
| Situation | Heater power | Why |
|---|---|---|
| Water off for a few hours, nobody draws hot water | Can stay on | The tank stays full |
| Water off and you'll need to use hot taps or flush toilets | Breaker off | Running taps can lower the tank level |
| Plumber will drain the tank or work on its piping | Breaker off | Rheem requires power off before draining |
| Heater is leaking | Breaker off, then water off at the heater | Stops heating a tank that may be losing water |
| Flood, fire or physical damage near the heater | Power and water off | Rheem says don't run it until it's been checked |
| Away for a week or more | Vacation setting or breaker off | Saves standby heat; see below |
A leak at the heater
Water on the floor around the heater is a different problem from the water being off. Switch the breaker off first, then close the cold supply valve above the heater. Rheem's manual says to turn off power and water to a heater that has been through flood, fire or physical damage, and not to operate it again until qualified service personnel have checked it.
Where the water comes from matters. A drip from a pipe fitting or from the relief valve discharge pipe is a plumbing repair. Water seeping from the base of the tank usually means the tank has corroded through. Rheem's repair-or-replace guidance puts leaks from the base on its list of signs a unit should be replaced, and rates most tank heaters at about 8 to 12 years. Rheem also suggests photographing any leaks, rust or unusual condensation to show a plumber. A little condensation on a newly filled tank is normal.
Vacation and long absences
For a weekend away, leave the heater alone. For longer trips, there are two reasonable choices, and the difference is small enough that it comes down to how much you want to fiddle.
Vacation or low setting. Some electronic Rheem models have a VACATION mode on the control that drops the set point to a lower preset. Older models with dial thermostats can be turned down by hand. The tank stays full, so there's nothing to refill and no dry-fire risk when you come back.
Power off at the breaker. Rheem's manual covers an "extended shut-down": if the heater will sit idle for a long period, turn off the power and the water to conserve energy and prevent a build-up of dangerous hydrogen gas. If the pipes could freeze, drain the heater and the piping. The manual doesn't say how many days count as extended, so that's a judgment call.
On savings: the U.S. Department of Energy's home checklist recommends the 120°F warm setting for scald safety and energy, and I found no official dollar figure for shutting a tank heater off during a trip. Expect a modest saving, not a big one.
Hydrogen deserves a plain warning. Rheem says a hot water system that hasn't been used for about two weeks or more can build up hydrogen gas, which is extremely flammable. Before using any electrical appliance connected to the hot water system, open the hot faucet at the kitchen sink for several minutes. A hissing sound like air escaping means hydrogen may be present. No smoking or open flame near the faucet while it runs.
Turning off an electric heater
- Find the heater's breaker in the main panel. Rheem tells owners to learn where it is and how to shut it off. It's often a double-width breaker labeled "water heater."
- Flip it off. If there's a separate disconnect switch near the heater, switch that off too.
- Close the cold water shutoff valve on the supply line if the heater is leaking or being worked on.
- Leave the access panels closed. Anyone who opens them for service should do so only with the breaker off. Rheem tells owners not to attempt repairs on wiring, thermostats or elements, and to send those to a qualified technician.
If your house has both gas and electric appliances, see our page on how to tell if your home is gas or electric. A gas heater has a gas valve and a pilot or electronic ignition, so the breaker doesn't switch it off. I didn't read a gas model's manual, so follow the one printed on yours.
Turning it back on
Do this in order, especially if the tank could have been drained.
- With the breaker still off, open the cold supply valve.
- Open a hot faucet nearby. Rheem says to open each hot faucet slowly so air can escape from the heater and piping.
- Wait for a steady stream of water from the hot faucets. Rheem calls that the sign of a full tank.
- Close the faucets, then switch the breaker on.
Give a heater that has been idle for a long time a look before you rely on it. Rheem recommends that a qualified technician check the controls after a long shutdown, and that the tank be completely filled before it goes back into operation.
Check the temperature setting while you're there. Rheem's label shows how fast hot water scalds: at 120°F it takes more than 5 minutes to cause a serious burn, at 125°F about 1.5 to 2 minutes, and at 130°F about 30 seconds. Homes with small children or older adults should stay at 120°F or lower.
If the heater won't heat after a restart, the cause is often the red reset button or a tripped breaker. Our Rheem electric water heater troubleshooting page goes through those steps.
Frequently asked questions
Can I leave the water heater on if the water is off for a few hours?
Yes, if nobody draws hot water. The tank stays full when the supply is closed, so the elements stay covered. If you do run hot taps during the outage, switch the breaker off first.
What happens if the heater runs with no water in the tank?
The elements overheat and can burn out almost instantly, according to Rheem. Its manual says the warranty doesn't cover damage from running the heater with an empty or partly empty tank. Replacing elements is a job for a qualified technician.
Should I turn the water heater off when I go on vacation?
For a weekend, no. For a longer trip, either use the vacation or low setting or switch the breaker off. Rheem's manual advises turning off power and water during an extended shutdown, and draining the heater and pipes if they could freeze. It doesn't define how long counts as extended.
Is there a hydrogen risk when the heater sits unused?
Rheem warns about hydrogen gas in a hot water system that hasn't been used for two weeks or more. Before using electrical appliances connected to it, run the kitchen hot faucet for several minutes and keep flames and cigarettes away from the faucet.
How do I know the tank is full again?
Open a hot faucet with the breaker off. Rheem says a steady flow of water from the hot faucet means the heater is full. Close the tap, then switch the power on.
Sources
- Rheem Use and Care Manual for residential electric water heaters (AP21980) (full tank before power, warranty, extended shut-down, hydrogen gas, vacation mode, scald chart)
- Rheem: The Top Installation Mistakes Plumbers See With Water Heaters (dry firing and burned-out elements)
- Rheem: What to Do Before Calling a Plumber or Replacing Your Water Heater (breaker off before work, leaks, photographing damage)
- Rheem: Water Heater Lifespan, When to Repair vs. Replace (8 to 12 year lifespan, base leaks)
- U.S. Department of Energy: Home Energy Checklist (120°F warm setting)