For most trips, don't switch a tank water heater fully off. Turn it to its vacation or lowest setting instead. Manufacturers say that keeps energy losses low and keeps the tank from freezing. For a long absence, draining it is an option. After two weeks or more away, flush the hot water line before using it.
What does the vacation setting do?
It drops the tank to a low temperature so the burner rarely runs, without shutting the heater down. American Water Heater's gas manual says: "When leaving your home for extended periods (vacations, etc.) turn the temperature dial to the vacation (VAC) setting. This will maintain the water at low temperatures with minimum energy losses and prevent the tank from freezing during cold weather."
On that model, the VAC mode sets the thermostat "at approximately 55°F." The manual adds that the real water temperature in most installations "will be greater than 55°F due to the surrounding environment and the pilot flame." In a Phoenix garage in July, the tank will sit well above that anyway.
A.O. Smith gas heaters label the same setting differently. Its manual says to turn the dial "to its lowest setting" when you leave for extended periods, and that its LOW position sets the controller "at approximately 91°F." A.O. Smith describes that as a temperature "that prevents the water in the water heater from freezing while still conserving energy."
So check your own knob. It may say VAC, LOW, or show a small vacation mark. The number varies by brand, but the idea is the same: the heater stays lit and ready, and it barely burns gas while you're gone.
### Why not just turn it off?
On a gas tank, turning the control to OFF shuts the pilot off too. When you come home, you have to relight it by following the lighting instructions on the heater's label. A.O. Smith's lighting steps start with "STOP! It is imperative that you read all safety warnings before lighting the pilot," and they include waiting 10 minutes to clear out any gas before you continue. If you'd rather not relight a pilot, the vacation setting avoids that step entirely. If you smell gas when you get home, don't try to light anything. See what to do when a water heater's pilot is out and you smell gas.
In Phoenix, there's rarely a freeze risk to worry about in summer, so the choice between VAC and OFF for a gas tank mostly comes down to convenience. In winter, the vacation setting also keeps a tank in an unheated garage from freezing, which OFF does not.
What about an electric water heater?
An electric tank has no pilot to relight, so shutting it off at the breaker is a reasonable choice for a long trip. You can also turn the thermostats down if you'd rather leave it running. The part that matters is what happens when you turn it back on.
GSW's electric water heater manual is blunt: "Do not turn electrical power on unless you are sure all of the air is out of the tank and the tank is completely full of water. If power is applied before the tank is completely full of water, the upper element will burn out (Dry Fire)." The same manual says to let a hot water faucet run full "for three minutes or longer" to push the air out before the power goes on.
That rule doesn't matter if the tank stayed full while you were gone. It matters a great deal if you drained it or shut the water off and opened faucets. Refill first, purge the air, then flip the breaker. A burned element is a common way to come home to no hot water. Our guide on testing a water heater element covers what to check if it happens anyway.
Should you drain the water heater?
Only for a long absence, or if the tank could freeze while it's shut down. A.O. Smith's manual says the water heater "should be drained if being shut down during freezing temperatures or for an extended period of time." American Water Heater's manual says it "should be drained if being shut down during freezing temperatures."
For a week or two away, draining is more work than it's worth. For a snowbird leaving a house empty for months, it's a fair option, especially with the main water valve off. Here's how the choices line up:
| How long you'll be gone | Gas tank | Electric tank |
|---|---|---|
| A few days | Leave it as is, or turn it down | Leave it as is, or turn it down |
| A week or more | Vacation (VAC) or lowest setting | Lowest setting, or breaker off with the tank full |
| Months, or the house sits empty | Vacation setting, or turn off and drain | Breaker off, and drain if you want |
To drain a gas tank, A.O. Smith's steps start with turning the gas control knob to OFF, turning off the gas supply at the manual shutoff valve, and closing the cold water inlet valve to the heater. Then you open a nearby hot water faucet, connect a hose to the drain valve, run it to an adequate drain, and open the valve. For a tank that will sit drained for a long time, it says the drain valve "should be left open with hose connected allowing water to terminate to an adequate drain."
When you refill, the rule from the electric manual applies to gas tanks too. A.O. Smith's refill steps include making sure the heater is full of water before you follow the lighting instructions to restart it. A draining is also a good time to flush out sediment, which Phoenix's hard water builds up fast. See how often to flush a water heater in Phoenix.
If you're shutting off the main water for the whole trip, the snowbird plumbing checklist for Arizona walks through the rest of the house.
What should you do when you get home?
If the hot water system sat unused for two weeks or more, flush it before you use any appliance on the hot water line. All three manuals warn about the same thing: hydrogen gas.
American Water Heater's manual says: "Hydrogen gas can be produced in a hot water system that has not been used for a long period of time (generally two weeks or more). Hydrogen gas is extremely flammable and can ignite when exposed to a spark or flame." It notes that its vacation mode "can cause a Hydrogen gas build up in the water system," so turning the heater down doesn't remove the risk.
The fix is simple and takes a few minutes:
- 1Open a hot water faucet first. American Water Heater recommends "the hot water faucet be opened for several minutes at the kitchen sink" before using any electrical appliance connected to the hot water system. A.O. Smith says to open the hot water faucet "located farthest away" for several minutes, before using appliances such as a dishwasher or washing machine.
- 2Listen for air. A.O. Smith says that if hydrogen gas is present, "there will probably be an unusual sound similar to air escaping through the pipe as the hot water faucet is opened."
- 3Keep flames away. GSW's manual says not to smoke or have an open flame or other ignition source "near the faucet while it is open."
- 4Turn the heater back up. Return the dial to your normal setting. About 120 degrees at the tap is a common target; see what temperature a water heater should be set to for how to check it.
Give a gas tank some time to reheat, and check around its base for water once it's running. A small leak that started while you were away is easier to catch on day one. If you find water under the tank, our page on a water heater leaking from the bottom explains what it usually means.
These steps come from tank water heater manuals. A tankless unit has its own instructions for long absences and freeze protection, so follow its manual.
If your heater won't relight, trips its breaker, or leaks after a trip, HQ Plumbing & Air handles water heater repair and replacement across the Phoenix area, with 24/7 service and free estimates.
For a building that will sit empty for months, see how to protect plumbing in a vacant commercial building.