Most tankless water heaters stop making hot water during a power outage because their ignition, fan, and controls all run on electricity.
A blackout turns a gas tankless water heater into cold plumbing — the gas is still there, but nothing lights it. Most tankless units pull 120 volts to spark the burner, spin the fan, and light the display, so losing power shuts down the whole system, not just the flame. Rinnai’s manual for its SENSEI RX models is blunt about what that means in cold weather: during a power failure below freezing, the heater has to be drained by hand, or the heat exchanger and water lines can burst. Whether tankless water heater power backup saves your morning shower comes down to two things — how cold it is outside, and whether the unit has a power source sized for its own electrical load.
What A Power Outage Does To A Tankless Heater
A tankless water heater stops producing hot water the second it loses power, because ignition, controls, fans, and safety sensors all need electricity. A gas line running straight to the unit changes nothing — the gas feeds the burner, and nothing lights it without power.
The temperature controller doesn’t help. Rinnai’s manual states plainly that the temperature controller does not control the electrical power, so turning the dial during an outage accomplishes nothing. Most residential units need 120V to operate at all.
Safety systems add another layer. Flame sensors, airflow switches, and high-limit cutoffs all watch the burner while it runs, and without power they can’t confirm a safe burn, so the unit stays shut down rather than guessing.
Winter changes the stakes. Freeze protection on many models runs on that same electricity, and Rinnai’s SENSEI RX installation and operation manual warns that losing it can cause water damage from a burst heat exchanger or water lines. An outdoor or garage install feels an outage fastest; a heated basement buys a little more time. A cold shower is temporary. A split heat exchanger is not.
Tankless Water Heater Backup Power: Which Setups Keep Hot Water Flowing
A battery backup keeps a tankless heater running through an outage only if it can supply the unit’s exact voltage and wattage. Noritz says that number comes off the spec sheet for your specific model, never from a general rule of thumb.
Noritz’s own example makes the math concrete: the EZ111 propane model draws 120 volts AC and 98 watts during normal operation. Match the backup to that load, leave headroom for start-up draw, and decide how many hours of hot water you actually want. The same battery runs far longer under a 98-watt load than under a heavier one.
Not every model is a candidate. Units with standing pilots or low-voltage ignition can sometimes run without grid power. Models that need 120V for ignition, controls, and a powered vent fan can’t. Check your unit’s rating before buying anything.
Freeze protection has its own fix. Rinnai’s Drain Down Kit (part #104000059) drains the heater automatically the moment power is lost, which matters for units installed outdoors or in an unheated garage. If you’d rather compare tested hardware than run the numbers yourself, our breakdown of battery backup picks for tankless water heaters matches each option to the units it can realistically carry. Sizing help comes straight from Noritz’s battery backup guidance.
| Outage Scenario | What The Heater Does | The Step That Helps |
|---|---|---|
| Mild weather, power out | Goes cold and waits | Nothing; it restarts on its own |
| Below freezing, power out | Freeze protection stops working | Drain the unit through the isolation valves |
| Unit or external piping already frozen | Risks a cracked heat exchanger | Keep it off and call a technician |
| Correctly sized battery backup in place | Keeps igniting and heating for a while | Match the backup to the model’s volts and watts |
| Rinnai SENSEI RX with Drain Down Kit | Drains itself the moment power drops | Install kit #104000059 |
| Power back, still no hot water | Breaker, GFCI, or fuse may have tripped | Reset the breaker, then check the GFCI |
| Blank display after resetting | Power supply or internal fault | Unplug the unit, then book service |
What Should You Do When The Power Fails Below Freezing?
Drain the unit before it freezes, and leave a frozen heater off until a technician inspects it. The SENSEI RX manual lays out the sequence, which takes about ten minutes:
- Shut the unit down and disconnect power.
- Close the cold water supply valve and the gas supply valve.
- Set a container under the isolation valves and remove the drain caps on both to let the water out.
Rinnai’s outdoor guidance adds one hard rule: don’t operate the water heater if it or the external piping is already frozen. Firing a frozen unit risks cracking the heat exchanger, and that damage usually shows up as a leak, not an error code.
When power returns, check the circuit breaker and reset it if it tripped, then check and reset the GFCI. If the display stays dark after you unplug the unit and plug it back in, that’s a call to an authorized technician.
FAQs
Can A Gas Tankless Water Heater Run Without Electricity?
Most can’t. Residential tankless heaters use electricity for ignition, the control board, the vent fan, and the safety sensors, so losing power shuts the burner down even though gas is still flowing to the unit. The exception is a model with a standing pilot or a low-voltage ignition system, so check your unit’s spec sheet for its actual requirements.
How Long Will A Battery Backup Keep A Tankless Heater Running?
Run time depends on the battery’s usable capacity divided by your heater’s wattage, so there’s no single answer that fits every home. Noritz’s example unit, the EZ111 propane model, draws 98 watts during normal operation, which a correctly sized backup can carry far longer than a higher-draw model. Work out the hours you want first.
Does A Tankless Water Heater Restart By Itself After An Outage?
Usually, yes. Once power is restored, most units come back on their own and heat normally again. If it stays cold, check the breaker on the heater’s circuit and reset it if it tripped, then check the GFCI. A display that stays blank after those checks points to a power supply problem.
References & Sources
- Noritz. “Battery Back-Up Recommended?” Explains that backup sizing depends on the exact model’s voltage and wattage, with the EZ111 drawing 120 volts AC and 98 watts.
- Rinnai. “SENSEI RX Residential Installation and Operation Manual.” Source for freeze-protection loss, manual drain steps, the Drain Down Kit part number, and post-outage checks.
