Will a UPS Keep a Gas Tankless Water Heater Working in a Power Outage?

Yes, a UPS will run a gas tankless water heater, and it does not need to be a big one. Rinnai's FAQ states the problem plainly: "If you experience a power outage, your Rinnai Tankless Water Heater will not operate." The gas keeps flowing, but the control board, combustion fan, electronic ignition and flame sensing all run on 120 V. Those loads are small. The hard parts are elsewhere: how many showers a battery buys, a freeze protection system that can draw more power than the burner, and heaters that have no plug to put into a UPS.

What the manuals say a tankless heater draws

Within each Rinnai series, outdoor versions draw less than indoor ones (the RE199i is rated 96 W, the RE199e 74 W). Navien publishes one maximum rather than a figure per mode, so its row is a ceiling, not a typical value.

The amp ratings are not running draw either. Rinnai's FAQ says its heaters need "120 volts AC, 60 Hz and less than three amps", while every Rinnai manual above lists "Max Current 4 Amps". Four amps at 120 V is 480 W, about five times the largest firing figure in the table. Size and estimate from the watt figures, or measure: an indoor unit with a cord works with a plug-in meter, as described in how to measure power draw.

What size UPS a tankless heater needs

Rinnai answers this with its own product. The RE/VE manual lists a battery backup accessory, part R-UPS350A, described as a "Tankless battery backup system, 500VA/350W uninterrupted power supply (UPS)". The FAQ calls the Hugo X-1 "a battery backup system" for tankless water heaters, "also known as an Uninterrupted Power Supply (UPS)", designed "to run your Rinnai tankless unit without power for multiple days".

A 350 W rating clears every figure in the table with room to spare, including Rheem's 200 W freeze protection and Navien's 200 W maximum. The exception is a Navien with an external recirculation pump on its pump connector, where the manual's ceiling rises to 350 W. That would run a 350 W UPS at full load, so step up to a unit rated 500 W or more, or keep the pump off battery. Read the watt rating, not the VA figure (VA vs watts explains why). Beyond that, watts are easy to satisfy. What you are really buying is battery energy, because that sets how many showers you get.

Waveform

The Rinnai RE and N Series manuals list a flame rod among the parts to inspect. Flame rods prove flame by rectification, the same method furnace boards use, and the gas furnace guide explains why a stepped (simulated sine) output can make a healthy flame read as no flame. None of the manuals cited here states a waveform requirement, so this is our recommendation rather than a manufacturer rule: buy a pure sine wave UPS, and test a full shower on battery before you rely on it.

Worked example: showers per battery

Take a Rinnai RE199i at its rated 96 W and two common UPS sizes. The method follows why UPS runtime is not linear: 85% inverter efficiency and a Peukert exponent of 1.15 for the AGM batteries.

Small UPS, one 12 V 9 Ah battery (108 Wh nominal). To deliver 96 W, the battery supplies about 113 W, or 9.4 A at 12 V.

1500 VA class UPS, two 12 V 9 Ah batteries (216 Wh nominal). The same 113 W at 24 V is 4.7 A, a gentler rate where the batteries give up more of their charge:

Doubling the battery more than doubled the burner time, because the lighter draw per battery loses less to the Peukert effect. Plug your own heater's figure into the runtime calculator.

Now the winter case. If the N Series freeze protection ran continuously at 152 W, the small UPS battery would supply about 179 W, or 14.9 A, and last roughly 21 minutes. The manuals do not say how often freeze protection cycles, but in cold weather a small UPS buys time to shower or drain the heater, not a night of protection.

Checking Rinnai's "5-7 days"

The R-UPS350A entry promises "Hot water usage of 1 hour per day with sufficient power for 5-7 days." At the RE199i's 96 W, one hour of firing a day for five to seven days is 480 to 670 Wh, plus about 1 W of standby for the other 23 hours each day, roughly 600 to 830 Wh at the outlet before any inverter loss. That is six to eight times the nominal energy in a single 12 V 9 Ah battery. The manual does not state the R-UPS350A's battery capacity or what heater load the claim assumes, and the claim is stated for hot water use, not freeze protection. A generic 500 VA desktop UPS will not reproduce the "days" figure. To decide how many hours you actually need, see how much runtime do you need.

Plug-in or hardwired: check this first

Whether you can simply plug the heater into a UPS depends on where it is installed:

For a corded indoor unit, put the UPS within reach of the cord and plug in directly. Rinnai's manuals say "DO NOT use an extension cord or adapter plug with this appliance," and Navien says extension cords are "not recommended". A hardwired or outdoor unit is a different job: the options are an electrician-installed receptacle where code allows it, an inlet with a transfer switch, or a hardwired UPS. Hardwired vs plug-in UPS covers those choices. Do not leave a desktop UPS outdoors next to an outdoor heater.

Recirculation pumps change the math

Rinnai offers a recirculation pump accessory (GTK03U) for its Circ-Logic system, and Navien's pump connector is rated for up to 150 W, which is why its maximum rises from 200 W to 350 W. On battery, the pump and the reheating it causes both come out of the same few hundred watt-hours. Our recommendation: turn recirculation off for the duration of an outage, or leave the pump off the UPS,.

Freezing weather: what the manufacturers actually tell you to do

Rinnai says it protects to -22 °F (-30 °C) for indoor models and -4 °F (-20 °C) for outdoor models, "when protected from direct wind exposure and as long as the appliance has both power and gas supply." A UPS supplies only the first half of that.

When power fails, the manuals do not tell you to run the heater on batteries. Rinnai: "In the event of a power failure and/or gas interruption at temperatures below freezing, the water heater should manually be drained of all water to prevent freezing damage." It recommends an optional drain-down kit that drains the heater "upon loss of power". Navien: in cold areas where power failures are common, "you must completely drain the water heater to prevent damage if the power will be off for any extended period of time," adding that "a battery back-up or generator may be used to supply hot water during periods of power outages." Both exclude freeze damage from the warranty.

Two more limits from the manuals: freeze protection does not protect external piping and valves (Rinnai recommends insulation, and its FAQ says the pipes must be heat-traced), and on the N Series the condensate trap must be drained too.

Where this answer stops applying

• Electric tankless heaters. Everything above is about gas units. An electric tankless heater heats the water with electricity directly, at loads far beyond any UPS in this class.
• Homes on a well. The heater can fire on a UPS, but with no well pump there is no water pressure to shower with. See UPS vs generator for whole-house options.
• Long outages. For more than a day, a large battery system or a generator on a transfer switch fits better than a UPS, and UPS vs portable power station covers the battery route.
• Models not listed here. Read the electrical data table in your own manual. The figures vary by series, by indoor or outdoor version, and by whether a pump is attached.