Tankless Water Heater Freeze Protection: Why It Needs Power, and When to Drain It

Most gas tankless water heaters have built-in freeze protection: small electric heaters inside the unit that switch on in the cold. The catch is in Noritz's words: "Freezing cannot be prevented when the power plug is unplugged." The same goes for a power outage. And every maker we checked says freeze damage isn't covered by the warranty. So the plan is simple: keep it powered, protect the pipes outside the cabinet, and drain it if the power is about to go out in a freeze.

How Built-In Freeze Protection Works

• Rinnai lists freeze protection "down to -22°F (-30°C) for indoor models or as cold as -4°F (-20°C) for outdoor models," "when protected from direct wind exposure and connected to both power and gas."
• Noritz rates its internal freeze prevention at −30°F indoors and −4°F outdoors, and says its built-in electric heaters "must be plugged in. Without a power source, they won't work."
• Rheem says multiple ceramic heaters protect select models down to −30°F.
• Takagi says its heating blocks switch on below 36.5°F, and "there has to be electrical power to the unit."
• Navien says freeze protection "requires electricity," to keep the power and gas valves on, and not to close the gas valve, which limits additional freeze protection.

Wind matters too. Rinnai's manual warns that "any wind or circulation of the air on the water heater will reduce its ability to protect itself from freezing."

The Pipes Outside the Unit Aren't Protected

The heaters protect the inside of the cabinet only. Rinnai says freeze protection "will not prevent the external piping and valves from freezing," and that those pipes "must be heat traced and insulated." Takagi says "any water pipes (hot or cold) located outside the unit will not be protected," and Navien adds the condensate drain line on condensing models. Noritz lists insulation, heat tape, and pipe covers. If you add heat cable, follow the CPSC's rules: a UL-listed cable on a GFCI-protected outlet, applied directly to the pipe, never wrapped over itself unless the maker allows it, and no foam or vinyl insulation over it. Our heat tape guide covers the choices.

If the Power Goes Out in a Freeze

Navien: "If the power fails, the freeze protection function cannot operate. If you expect a power outage during a freeze, you should drain the unit immediately." Rheem and Rinnai say the same, and Rinnai "highly recommend[s]" a drain-down kit that drains the unit automatically when it loses power. Navien and Noritz suggest backup power, and Noritz specifies a UPS or generator with "pure-sine wave output." A portable generator runs outdoors only, at least 20 feet from the house; see what to do when the power goes out.

Draining It

Follow your own manual; the valves differ by model. Rinnai's outdoor-model steps give the general shape:

1. Shut off the cold water supply and the gas supply.
2. Turn off the temperature controller and disconnect the power.
3. Wait for the unit to cool. Rinnai warns to "wait until the equipment cools down before draining."
4. Remove the drain caps on both isolation valves and open both valves.
5. Remove the water filter to drain the cold side.

Rinnai adds draining the condensate trap, drain line, and pressure relief valve on condensing models.

Vents and Location

• Cold air down the vent. Navien says indoor units can freeze from back drafting that "draws cold air into the unit via the exhaust pipe," and Takagi says an indoor unit freezes only if cold air enters through the venting from negative pressure or strong winds. Noritz says to face horizontal vent terminations away from prevailing winds.
• Outdoors or in an unheated garage. Navien says not to install a tankless water heater outdoors where freezing is common, or in an unheated garage in cold regions. If yours is outside, its limit is the outdoor rating, about −4°F for Rinnai and Noritz outdoor models, and only with power and gas on.

If It Froze

• Don't run it. Rinnai: "Do not operate the water heater if it or the external piping is frozen. Close the gas and water valves and turn off the power. Wait until the water thaws." For electric tankless models, EcoSmart warns: "DO NOT SWITCH THE BREAKER 'ON' IF THERE IS A POSSIBILITY THE WATER IN THE HEATER IS FROZEN."
• Signs. Navien lists no flow from the hot tap, and error codes on condensing models when the condensate line freezes. EcoSmart's A74 code means the water is below 35°F.
• Thawing. Navien suggests opening the nearest hot faucet and warming the unit with a hair dryer; keep electric tools away from any water. Never use an open flame.
• Check for leaks once it thaws, as Rinnai says, and call a qualified service technician if anything leaks or you smell gas. If you smell gas, leave and call from outside; see our gas smell guide.

If You'll Be Away in Winter

The makers say to drain it. Takagi: "Completely drain… Disconnect power." Navien says to drain the unit, disconnect the power and gas supply, and shut off the cold water supply, and Rinnai says a unit that won't be used during possible freezing weather should be drained. If you're only away briefly and keep the house heated, Noritz and Navien say to leave it plugged in so its protection keeps working. Our leaving the house empty in winter guide covers the rest of the house, including keeping the heat no lower than 55°F.

Frequently Asked Questions

Will my tankless water heater freeze if the power goes out?

It can. Its freeze protection needs electricity, and Navien says that if the power fails the function can't operate. If an outage is likely during a freeze, drain the unit, or install a drain-down kit or backup power.

Is freeze damage covered by the warranty?

Generally no. Rinnai, Navien, Noritz, Takagi, Rheem, and EcoSmart all exclude freeze damage, so keeping it powered and draining it before an outage is the only protection.

Do I need to insulate the pipes on a tankless water heater?

Yes, if they're in a cold space. The built-in heaters protect only the inside of the unit. Rinnai says the external pipes and valves must be heat traced and insulated, and Takagi says pipes outside the unit aren't protected.

Should I turn off the gas to protect it from freezing?

Not if it's staying in service. Navien says to keep the power and gas valves on, because closing the gas limits freeze protection. Shut off the gas only when you drain the unit.

The Bottom Line

A tankless water heater's freeze protection works only with power, and gas on some models, and it doesn't cover the pipes outside the cabinet. Keep it powered, insulate and heat-trace exposed pipes by the rules, and drain it before a power outage in a freeze or a long winter absence. If it freezes, don't run it; let it thaw, check for leaks, and call for service.

Sources

• Rinnai — residential FAQ (freeze protection to −22°F indoors and −4°F outdoors with power and gas, away from wind; external pipes must be heat traced and insulated), N-Series condensing manual (PDF) (wind reduces protection; external piping not protected; drain on power failure; drain-down kit; freeze damage not covered), V53De outdoor manual (PDF) (draining steps; don't operate if frozen; check for leaks), and winterizing bulletin (PDF).
• Navien — tankless water heater frozen (needs electricity; keep power and gas on; drain before an outage; backup power; external pipes and condensate line not protected; not covered by warranty; signs and thawing; drain when away) and using tankless water heaters in cold climates (not outdoors or in an unheated garage in cold regions).
• Noritz — winter freeze (−30°F indoor and −4°F outdoor ratings; no protection unplugged; external plumbing not protected; backup power; vent terminations away from wind; freeze damage not covered) and preventing freezing (heaters must be plugged in; draining when away).
• Rheem — winter prep tips (ceramic heaters on select models to −30°F; heat tape on piping; drain before an expected outage) and winter weather tips (freeze damage not covered).
• Takagi — T-H3 installation manual (PDF) (heating blocks below 36.5°F need power; outside pipes not protected; cold air through the vent; drain and disconnect power; freeze damage from power loss not covered).
• EcoSmart — ECOS Element manual (PDF) (A74 code; don't switch the breaker on if frozen; freeze damage not covered).
• U.S. Consumer Product Safety Commission — heat tape guidelines (UL-certified tape; GFCI outlet; on the pipe; no foam or vinyl over it).

Last reviewed October 24, 2026. General information, not professional advice — gas, venting, electrical, and internal repairs on a tankless water heater belong to a licensed plumber or the maker's authorized service technician.

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