If the water is only off for a few hours and the tank stays full, you can leave the heater alone. Turn off the breaker or gas control first if the tank will be drained, drawn down or opened up for a repair. Refill the tank completely and purge air before power or gas goes back on.
Why a water heater must never fire empty
The manuals for both electric and gas tanks say the same thing: the tank has to be full of water before heat goes on. Bradford White's electric manual says: "NEVER OPERATE THE WATER HEATER WITHOUT FIRST BEING CERTAIN IT IS FILLED WITH WATER." A.O. Smith's gas manual for its residential condensing models says: "Never use this water heater unless it is completely full of water. To prevent damage to the tank, the tank must be filled with water." The same manual adds: "Water must flow from the hot water faucet before turning ON gas to the water heater."
A.O. Smith gives the reason in terms of the tank: the tank must be filled "to prevent damage to the tank." So a tank that is partly or fully empty and running is the problem, not the outage itself.
What to do, by situation
| Situation | Heater action |
|---|---|
| City or neighborhood outage, you did nothing | Usually nothing. Don't open the drain valve, and don't run a long hot-water draw that pulls the tank down. |
| You closed the main to fix or replace a fixture | Leave it if only a faucet or toilet part is involved and the tank is not touched. |
| A plumber will replace the heater, a valve on the tank, or work on the tank's piping | Power or gas off first. The plumber handles it. |
| You will drain the tank (flushing, anode rod, repair) | Breaker off or gas control off, then close the cold supply, then drain. |
| Leak or burst line drained the house | Power or gas off, then refill before restoring. |
Bradford White spells out the drain order. "TO DRAIN THE WATER HEATER," it says, starts with "Disconnect the power supply to the water heater," then "Close the cold-water supply shut-off valve," then opening the drain valve and "a hot water faucet to allow air to enter the system." Power first, always. For gas, the equivalent is turning the gas control or the manual gas shutoff to off before the tank empties.
When the water comes back
This is where the damage usually happens, because the house wants hot water right now and someone flips the breaker. Do the refill in order instead.
- 1Close the heater's drain valve if you opened it.
- 2Open the cold-water supply valve at the heater.
- 3Open several hot-water faucets to let air escape. Bradford White says to leave them open until "a steady stream of water flows through the faucets," then close them.
- 4Check for leaks at the drain valve, the T&P valve and the hot and cold connections.
- 5Only then restore power or gas. For a gas heater, relight by following the label on the heater, not from memory.
Expect sputtering and cloudy water for a minute or two as trapped air clears. If your faucets keep spitting air, see why air in the pipes makes faucets sputter.
Is my water heater at risk in a Phoenix outage?
A city main break or planned work drops the supply to your street, but a closed-up tank generally stays full, so the heater can usually be left alone. A power outage in a monsoon storm is a separate issue: if the tank is still full, an electric heater simply resumes when power returns, while a gas heater with a pilot may need relighting per the label.
To report a water or sewer emergency such as a main break, the City of Phoenix says to call its 24/7 Emergency Hotline at 602-261-8000.
When to call a plumber
Call if the heater is leaking, if the tank may have been drawn down during a repair and you are not sure it refilled, if the burner or elements won't restart after you follow the refill steps, or if you ever smell gas. HQ Plumbing and Air replaces and repairs tank and tankless water heaters, with 24/7 emergency service. See our water heater services.
For a trip away rather than an outage, see whether to turn the water heater off on vacation. For the shutoff itself, how to shut off a leaking water heater covers the cold-water valve and the breaker or gas control.