Most tankless heaters that stay cold after an outage have lost power at a tripped breaker or GFCI outlet, are holding an error code from the interruption, or were switched off at the controller. Check power first, then read the display and follow the manual's reset step. Repeated trips or codes need a technician.
Why does a gas tankless need electricity at all?
Gas does the heating, but electricity runs everything that controls it. A. O. Smith explains that gas tankless heaters need power for the electronic ignition, control board, display and safety systems, and that without electricity the unit can't start up or heat water safely. Rinnai lists its units at 120 volts AC and less than three amps.
That's why the hot water stopped the moment the lights went out, even though the gas meter kept running. It's also why the heater has to boot back up correctly before it will fire again. If you are wondering what other heaters do during an outage, our page on whether a gas water heater works during a power outage covers tanks too.
What should you check first?
Start with power, because a dead outlet looks exactly like a dead heater.
- 1Look at the display or controller. Lit and showing a temperature? Skip to the next section. Blank? Keep going.
- 2Check the outlet the heater plugs into. Many tankless units plug into a nearby receptacle, and outdoor or garage outlets are often GFCI type. An outage, or the surge when power returns, can trip them. A. O. Smith's troubleshooting advice for a blank display is to check your circuit breaker and GFCI outlet, reset if needed, and call an electrician if it keeps tripping.
- 3Check the breaker panel. Find the breaker for the water heater and reset it once if it tripped.
- 4Check the controller's On/Off button. On some models the display goes dark simply because the controller is off. Our blank tankless display guide explains how to tell an off controller from a power fault.
One reset, not five. A breaker or GFCI that trips again right away is telling you something is wrong. Leave it off and get it looked at.
The display is on, so why is there still no hot water?
Read the screen before you touch anything else. If the unit logged a fault during the outage, the code tells you where to look.
Rinnai code 03 is a good example. In Rinnai's N Series manual, code 03 means a power interruption during bath fill, and the note says water will not flow when power returns. The fix it lists is to turn off all hot water taps and press ON/OFF twice. That's it. Simple as that.
Rinnai code 11 (no ignition) and 12 (no flame) point to gas supply checks first, such as confirming the gas is on at the heater and meter. That matters after an outage if your gas was also interrupted, or if someone closed the gas valve while the power was out.
Navien units keep your settings through an outage. Navien's NPE-2 manual says the water heater will retain your settings during a power outage, and that for an error message you can try a reset by pressing the Back button. If a reset doesn't clear it, the code goes to a technician.
Some codes on other brands also clear by cycling the main power to the heater, but only when the manual for that code says so. Our pages on Rinnai tankless error codes and Navien error codes break down the common ones.
What if it fires but the water is only warm or cuts out?
Then the outage probably isn't the cause anymore, and normal tankless troubleshooting applies. Rinnai notes its units need at least 0.4 to 0.6 gallons per minute of hot water flow to ignite. A low-flow faucet barely cracked open may never trigger the burner. Open a shower or tub tap fully as a test.
Other everyday causes, like a clogged inlet filter or scale in the heat exchanger, are on our common tankless water heater problems page. Phoenix hard water makes scale a real factor here, outage or not.
When is it time to call for service?
Call when the checks above don't bring the heater back, or when any of these show up:
- A breaker or GFCI that trips again after one reset.
- An error code that returns after a reset, or a code your manual says needs a qualified technician.
- A burning or electrical smell, scorch marks, or a unit that went dark during a storm with a lot of lightning.
- Any gas smell. Leave the house and call the gas company from outside first.
Leave the cover on. Rinnai's manual warns that some of its diagnostic checks should be performed by a trained and qualified professional, and that consumers should never attempt any action they are not qualified to perform. Internal wiring, fuses and board testing fall in that group.
If an outage is likely to last in cold weather, the makers also want the heater protected from freezing. Rinnai's FAQ says its built-in freeze protection works only while the appliance has both power and gas, and Navien says to drain the heater if power or gas will be off for an extended period. That's rarely an issue in Phoenix, but an outdoor unit on a January night is worth thinking about.
HQ services and replaces gas tankless water heaters across the Valley, and we'll read the code before we quote anything. Start at our water heater service page.