Quick answer
A gas tankless water heater needs 120-volt power for its control board, fan and igniter, so it stops in an outage and sometimes stays off afterward. The usual causes are a tripped GFCI receptacle, a latched error code that needs a power-cycle reset, or an internal fuse blown by the surge when power returned. Unplug it for 30 seconds, plug it back in and check the display.
A tankless water heater not working after a power outage is rarely damaged; start at the receptacle and the display before assuming the worst. Gas keeps flowing during an outage, but a tankless cannot use it without electricity: the board decides when to fire, the fan moves the combustion air and the igniter makes the spark. When the power comes back the unit has to restart cleanly, and three things commonly get in the way.
No display at all: a tripped GFCI, a breaker or a blown internal fuse
A blank display means no power is reaching the board. Work from the wall inward.
- The receptacle. Many tankless units plug into a GFCI receptacle, or into one wired downstream of a GFCI elsewhere in the basement or garage. Power returning can trip it. Press Reset and prove the outlet with a lamp.
- The breaker. Look in the panel for a tripped breaker and push it fully off, then on.
- The unit's own power button. Some controllers come back in standby and only need switching on.
- The internal fuse. Tankless boards are protected by small fuses on the board or in the wiring harness. A surge or a brown-out as the grid comes back can blow one, and the unit stays dark even though the outlet is live. A fuse that blows again straight after replacement means the board or another component took damage, so treat this one as a service call.
Display on but no hot water: clear the latched error code
If the display is lit and showing a code, the unit tried to restart, failed and locked out. Power flickering on and off during restoration can interrupt an ignition sequence and leave a fault latched. The reset is a power cycle.
- Close any open hot taps.
- Switch the unit off at its power button and unplug it, or turn off its breaker.
- Wait 30 seconds.
- Restore power, switch the unit on and confirm the setpoint appears.
- Open one hot tap all the way and listen for the fan, then the burner.
- If the code comes back, write it down and stop resetting.
A code that returns is reporting a real fault. Ignition codes are covered in why a tankless water heater is not igniting, and the full brand list is in our Navien error code guide. If the furnace is also down, see why a furnace will not turn on after a power outage.
Settings an outage can wipe: recirculation timer, clock and setpoint
Hot water that works but arrives slowly or at the wrong temperature after an outage is usually a lost setting, not a fault. Depending on the model, the controller can lose its clock, switch recirculation off or fall back to the 49 °C factory setpoint. A recirculation schedule that runs off the unit's clock or a plug-in timer then runs at the wrong hours, or not at all, and the upstairs taps go back to a long wait. Reset the time, re-enable recirculation and compare the setpoint with what you had before.
On a private well, as on rural lots in Flamborough, Kilbride and Caledon, the well pump was down too. Let the pressure tank recover and run a cold tap to clear the air before judging the heater: a sputtering line can keep the flow below what the unit needs to fire.
Winter outages: freeze protection loses power too
The electric freeze-protection heaters inside a tankless work only while it has power. In a winter outage, a unit in a garage, a crawlspace or on a cold exterior wall can freeze, and cold air dropping down the vent makes it worse. A frozen heat exchanger can split, and manufacturers generally exclude freeze damage from warranty.
If the house will be without power and heat for hours in freezing weather, drain the unit. Your manual has the exact steps, and the outline is the same on most models.
- Close the gas valve to the unit and unplug it.
- Close the cold-water isolation valve feeding the heater.
- Open a hot tap in the house to relieve the pressure.
- With a bucket underneath, open the drain caps on the service valves, then remove the inlet water filter and let the unit empty.
- Leave the drains open until power and heat are back, then reverse the steps and check for leaks before restarting.
If the unit already froze, do not fire it. What to do when a tankless water heater is frozen covers safe thawing.
Is a surge protector or a small UPS worth adding?
Yes for the surge protector, and sometimes for the UPS. A gas tankless draws little electricity compared with any electric heating appliance, and the rating plate gives the wattage. A plug-in surge protector guards the control board, the costliest electrical part, against the spike when Burlington Hydro, Alectra or Hydro One re-energizes the line.
A UPS sized to that wattage carries the unit through short outages and keeps freeze protection alive for a while. Runtime is limited, and a recirculation pump or the freeze heaters shorten it, so decide what you want it to carry before buying one. For a long outage a generator is the realistic backup.
Call for service if
- The display stays dark with a live outlet.
- The same error code returns after one reset.
- The breaker or GFCI trips again when the unit starts.
- There is water under the unit after a freezing outage.
- You smell gas or scorched electronics at the cabinet.
ZK Mechanical repairs Navien, Rinnai and Bosch tankless heaters, with no overtime, weekend or holiday surcharge on the 24/7 line at (647) 801-1252. Have the model number and the error code ready. Board, fuse and igniter work is described on our tankless water heater repair page, the diagnostic is $90–$150 against the repair, and the contact form works if you would sooner write than call.