Does a Gas Water Heater Need Electricity: Which Types Work in an Outage

Whether you get a hot shower during a blackout comes down to two details of your gas water heater: how it lights, and how it vents.

Short answer: a standard atmospheric gas tank with a standing pilot needs no electricity. The pilot flame powers its gas valve through a thermocouple or thermopile, so it keeps heating water in a power outage as long as gas and water keep flowing. Power-vent, power-direct-vent, electronic-ignition and condensing tanks, and virtually all gas tankless units, need 120 V and stop when the power goes out.

Which gas water heaters work without power?

Why a standing pilot needs no power

A small pilot flame burns continuously beside the main burner. It heats a thermocouple (or, on some gas controls, a thermopile), which generates a tiny voltage of its own, enough to hold the gas valve's safety magnet open. The thermostat on the valve is mechanical or runs on that same self-generated power. Relighting usually uses a push-button piezo igniter, which needs no outlet either.

If the pilot goes out, the voltage disappears and the valve shuts off the gas. The grid never enters into it.

Why the others stop

• Power vent. A fan on top of the tank pushes exhaust out through a sidewall pipe. A pressure switch has to prove the fan is running before the burner is allowed to fire. No power, no fan, no burner.
• Electronic ignition. A hot-surface or spark igniter and a control board replace the pilot, and both run on household power.
• Condensing tanks use a fan and controls too, and some rely on a condensate pump.
• Tankless. A combustion fan, a flow sensor, a modulating gas valve, an igniter and a control board, all electric. With no stored hot water, an unpowered tankless unit delivers only cold water.

Choosing between these types is covered in gas vs electric water heater and tankless vs tank. This page is only about what happens when the power fails.

How to tell which one you have

Everything here is visible without removing a cover or touching wiring.

1. Look for an electrical connection. A cord plugged into a nearby outlet, or a cable or conduit running into a junction box on the heater, means it needs power. No electrical connection at all means it can't.
2. Look on top of the tank. A motor housing with a fan sitting on the tank is a power vent.
3. Follow the vent pipe. A metal pipe rising from a flared collar (the draft hood) into a chimney or metal flue is atmospheric. Plastic pipe, typically PVC or CPVC, running out through a wall usually means a power-vent or condensing unit.
4. Look for a pilot. A small, steady blue flame visible through the sight window near the bottom, and a gas knob marked PILOT / ON / OFF with a separate igniter button, point to a standing pilot.
5. Read the rating label. If the heater has electrical requirements, the label lists them, such as 120 V and an amp figure.

A wall-hung gas tankless unit needs power unless its manual says otherwise; a few niche models use battery or water-flow ignition.

How to keep hot water during a power outage

Standing-pilot tank: usually nothing to do

It keeps heating as long as gas keeps flowing. Natural gas service usually continues through an electrical outage, and a propane tank doesn't depend on the grid at all. The limit is water supply, covered below.

Power-vent, condensing or electronic-ignition tank

Even with the burner off, the tank is full of already-hot water, and an insulated tank keeps it usable for hours. For a short outage, just go easy on it.

For a longer one, the heater needs a 120 V source: a portable generator or a power station. The load is small (a fan, an igniter and controls), but don't guess it. Multiply the rating label's volts by its amps: 120 V and 3 A would be 120 × 3 = 360 W (example only; use your label). Size the source with headroom, and check the manual for any minimum generator size or clean-power requirement.

Tankless: power every time a tap opens

A tankless unit needs power for every draw, plus two extra considerations:

• Stable power. Tankless control boards are electronics, and some manufacturers specify clean, stable power when running from a generator. An inverter generator or a pure sine wave power station is the safe choice; a conventional open-frame generator may not be.
• Freeze protection. Units outdoors or in unheated spaces often rely on small electric heaters to keep from freezing. With the power out in freezing weather, follow the manual's freeze-protection or draining procedure.

Plugged in or hardwired?

Cord-and-plug connected, common on power-vent tanks: during an outage, unplug it from the wall and plug it into a power station, or into a heavy-duty outdoor extension cord run from a generator placed outside.

Hardwired, common on tankless units: there is no plug to move. Running it from a generator needs a transfer switch or interlock kit installed by an electrician; see interlock kit vs transfer switch. Don't swap the hardwired connection for a cord yourself. Cord connection is allowed only where the manufacturer's instructions permit it.

A power station suits this job: silent, safe indoors, and the load is small. Look for pure sine wave output and UPS-style switchover, and turn off any AC auto-off or eco timer so the outlet doesn't shut down while the heater idles.

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Runtime is the station's watt-hours divided by the heater's average draw, so measure that draw or work it out from the label before counting on a number.

Safety: three rules that don't bend

Carbon monoxide. A generator runs outside, well away from the house: at least 20 feet is the common guidance, with the exhaust pointed away from windows, doors and vents. Never in a garage, even with the door open. Keep battery-powered or battery-backup CO alarms working on every level, since an outage can silence plug-in units without a backup battery.

Never bypass the vent fan. A power-vent heater fired without its fan can spill flue gas, including carbon monoxide, into the house. The pressure switch that stops it is doing its job. Don't jumper it.

No backfeeding. Never feed a generator into a wall outlet with a double-male cord to reach a hardwired heater. That energizes the utility lines and can kill a line worker.

If a standing pilot goes out, relight it using the instructions on the heater's label. If it won't stay lit, or the power is back and the heater still isn't heating, see water heater not heating.

What about the water supply?

On city water, pressure usually holds during a power outage. On a private well, it doesn't. The well pump needs power, and once the pressure tank drains the taps go dry, hot and cold, no matter how hot the water heater is. Keeping water flowing means backing up the pump, which is a much bigger load than any gas water heater; see sizing a generator for a well pump.

Furnaces and boilers are a separate question; see battery backup for a furnace and running a furnace on a generator.

Standards and code reference

The standards behind this guide, for looking up in the edition your jurisdiction has adopted:

• NEC 110.3(B): listed equipment installed and used according to its listing and instructions, including the heater's electrical and venting requirements
• NEC 422.16: flexible-cord connection of appliances, permitted only in specific cases such as appliances intended or identified for cord connection
• NEC 422.33: for a cord-and-plug connected appliance, the plug and receptacle can serve as the disconnecting means
• NEC 702.5: transfer equipment for optional standby systems, which keeps a generator from backfeeding the utility
• NFPA 54 (National Fuel Gas Code) / IFGC: venting of gas appliances, including mechanical-draft (power-vent) systems

Code editions and local amendments vary. Confirm the adopted edition with your AHJ, and treat manufacturer instructions as governing wherever they are more restrictive.

Where to go next

• Backfeeding dangers
• Carbon monoxide safety
• What is an inverter generator
• How to size a portable power station
• Size it properly: generator sizing calculator

More in our heating, cooling and water heating guides.