How to Keep Your Heat On During a Winter Power Outage

Why your heat stops when the power does

It surprises many people: a natural gas or propane furnace goes cold during a power outage even though the gas line is still full. That is because the furnace depends on electricity for nearly everything except the flame itself. The blower motor that pushes warm air through the ducts, the draft inducer that vents exhaust, the igniter, the control board, the gas valve, and the thermostat all need power.

Boilers are similar. The burner may run on gas or oil, but circulator pumps move the hot water, and a control board runs the whole system. Heat pumps are fully electric and among the largest loads in a house. Even pellet stoves, which burn fuel you stack in the garage, need electricity to feed pellets and run fans.

So the question in a winter outage is not just "Do I have fuel?" It is "Can I power the equipment that uses the fuel?" The good news is that gas furnaces, boilers, and pellet stoves draw relatively modest amounts of electricity, and a properly connected portable generator or large power station can run them. Heat pumps are a different story, and this guide explains why.

Step 1: Identify your heating system

Walk to your mechanical room and look. Most homes have one of these:

• Forced-air gas or propane furnace: a metal cabinet connected to ductwork, with a gas line and a vent pipe.
• Oil furnace: similar to a gas furnace, with an oil burner and a supply line from a tank.
• Boiler: heats water or makes steam, with pipes running to radiators, baseboards, or in-floor tubing, and one or more circulator pumps.
• Heat pump: an outdoor unit that looks like an air conditioner, paired with an indoor air handler. It may have electric backup heat strips.
• Pellet stove: a freestanding stove or insert with a hopper, fed by an electric auger.
• Electric baseboard or electric furnace: resistance heating, which is a very large electrical load.

Each has very different power needs. Read the nameplate or data sticker on the equipment for voltage, amperage, and motor ratings. That label is your most reliable source of information.

Step 2: Understand the power each system needs

Gas and oil furnaces

The largest electrical load in a gas furnace is the blower motor. Blower motors vary widely in size and type. Older furnaces often use permanent split capacitor (PSC) motors, while many newer ones use electronically commutated (ECM) motors, which are more efficient. Running loads for a furnace blower commonly fall in the range of a few hundred watts up to around a thousand watts or so, and motors draw more briefly at startup. Add the draft inducer, igniter, and controls, and most gas furnaces can be run by a modest generator.

To estimate, find the blower motor amperage on the nameplate, multiply by 120 volts for running watts, and leave headroom for startup.

Oil furnaces have the same blower plus an oil burner motor and ignition transformer, which add some load.

Boilers

Hot water boilers are usually the lightest load of all. Circulator pumps are small, and the control board and gas valve use little power. A boiler with one or two circulators may run on a small inverter generator or a mid-size power station. Steam boilers may also have a condensate or feed pump.

Heat pumps

Heat pumps are the challenge. The outdoor compressor is a large motor, and the startup surge can be several times its running draw. Most whole-house heat pumps run on 240 volts. On top of that, many systems have electric resistance backup heat strips, which can draw enormous amounts of power, far more than most portable generators can supply.

Running a heat pump on a generator is possible, but it usually requires:

• A large generator with 240V output and enough surge capacity for the compressor.
• Disabling or not calling for the backup heat strips.
• In many cases, a soft-start kit installed on the compressor by an HVAC technician to reduce the startup surge.

We cover this in detail in our comparison of heat pump vs gas furnace on a generator.

Pellet stoves

Pellet stoves run an auger motor, a combustion blower, and a convection fan, plus an igniter during startup. Running loads are typically modest, with a spike during ignition. Many owners run them on small inverter generators or power stations. See our guide to the best generators for pellet stoves.

Space heaters

Plug-in electric space heaters typically draw up to about 1,500 watts on high, which is a significant load for a small generator and quickly drains a power station. They can work as a supplement, but they are an inefficient way to heat a house from a generator compared with running a fuel-burning furnace or boiler.

Step 3: Clean power matters for modern heating equipment

Modern furnaces and boilers use electronic control boards. Some are sensitive to the quality of the electricity they receive. Conventional open-frame generators can produce power with more voltage fluctuation and harmonic distortion than utility power. Inverter generators produce cleaner, more stable power.

Many HVAC technicians and furnace manufacturers recommend clean power for electronic controls. If you are buying a generator specifically to run a furnace or boiler, an inverter generator, or a conventional generator with low rated total harmonic distortion, is the safer choice. Our list of the best generators for furnaces focuses on clean-power options.

The grounding and flame sensing issue

Some furnaces rely on a flame sensor that uses a tiny electrical current through the flame to confirm ignition. This process depends on a proper neutral-to-ground reference. Some portable generators have a floating neutral, meaning the neutral is not bonded to the frame. When such a generator is connected in certain ways, the furnace may fail to confirm the flame and lock out after a few ignition attempts.

When a generator is connected through a properly installed transfer switch or interlock, the home's neutral-ground bond is usually preserved, which avoids the issue. If you are experiencing lockouts on generator power, have a licensed electrician or HVAC technician evaluate the connection. Do not improvise bonding plugs or modifications without professional advice.

Step 4: Connect safely

A furnace is hardwired, not plugged in. That means you cannot run an extension cord to it unless an electrician has installed a proper connection. The safe options are:

1. Manual transfer switch or interlock kit on your main panel. A licensed electrician installs an inlet box outside and a transfer switch or interlock at the panel. You plug the generator into the inlet and select which circuits to power, including the furnace. This is the most common and flexible solution. See our guide on how to connect a generator to your house.
2. Dedicated furnace transfer switch. A single-circuit switch installed near the furnace, with a plug or inlet for the generator. It is simpler and cheaper for people who only want to back up the furnace.
3. Whole-home standby generator with automatic transfer switch. The most convenient, but a separate category from portable generators.

Step 5: Size your generator

Add up the loads you want to run at the same time.

1. Heating system: from the nameplate, including startup surge.
2. Refrigerator and freezer.
3. Sump pump, if you have one. Winter thaws can make it essential.
4. Well pump, if you have one. Often 240V with a large surge.
5. Lights, internet, phone charging.
6. Anything else essential, such as medical devices.

Then add headroom. Generators should not run at their maximum rating for long periods. Many people target a load that is comfortably below the rated running watts.

For a typical home with a gas furnace, a refrigerator, a sump pump, and lights, a mid-size generator in the several-thousand-watt range with a transfer switch often covers essentials. Homes with well pumps or heat pumps need more.

Step 6: Plan your fuel

Winter outages can last days. Your heating system may run for hours at a time in cold weather, so plan fuel carefully.

• Gasoline: stores poorly without stabilizer. Rotate stored fuel and add stabilizer. Store it only in approved containers, away from the house.
• Propane: stores indefinitely and does not degrade. Dual-fuel generators let you run on propane. Propane performs less well in extreme cold, so larger tanks help.
• Natural gas: some generators can be converted or are designed for natural gas. Unlimited supply as long as the gas utility is running.

Read our guide on how long a generator can run for runtime and refueling guidance.

Carbon monoxide: the number one danger

Portable generators produce carbon monoxide, a colorless, odorless gas that can kill quickly. The U.S. Consumer Product Safety Commission estimates that an average of about 80 people die each year from carbon monoxide poisoning associated with portable generators.

Winter outages are especially dangerous because people are tempted to run generators in garages, basements, or near windows to keep them out of the snow.

• Run generators outdoors only, far from the house, with the exhaust pointed away from doors, windows, and vents. CPSC recommends at least 20 feet from the home.
• Never run a generator in a garage, even with the door open.
• Install battery-operated or battery-backup CO alarms on every level of your home and outside sleeping areas.
• Choose a generator with CO shutoff. Our guide to the best generators with CO shutoff and our explainer on UL 2201 vs PGMA G300 explain what these certifications mean.
• Use a generator tent or cover rated for use while running, so you can protect it from snow without moving it closer to the house.

Keeping the heat in

A generator can run your heating system, but reducing heat loss makes everything easier:

• Close off unused rooms and gather in one area.
• Close curtains at night and open south-facing ones during the day.
• Put draft stoppers at exterior doors.
• Keep the thermostat a few degrees lower than usual to reduce runtime.
• Protect pipes. If the house gets very cold, let faucets drip on exterior walls and open cabinet doors under sinks.

Battery power stations as heating backup

Large LiFePO4 power stations can run boiler circulators, pellet stoves, and even some furnace blowers for limited periods, silently and indoors, with no fumes. They cannot run a heat pump for long or sustain electric resistance heat. They pair well with solar charging and with a generator for recharging.

A winter outage checklist

1. Have your transfer switch or interlock installed before winter.
2. Test your generator monthly, and do a full test with your furnace connected.
3. Store fuel with stabilizer, or keep propane on hand.
4. Check CO alarms and replace batteries.
5. Get a generator cover rated for use while running.
6. Know how to switch your panel over to generator power.
7. Keep extension cords rated for outdoor use and the load you plan to run.
8. Keep flashlights, warm blankets, and a plan for the most vulnerable people in your household.

Before the next storm: a one-weekend prep plan

Saturday morning: find the nameplates on your furnace or boiler, well pump, and sump pump. Photograph them and write the amperage on a card you keep with the generator.

Saturday afternoon: call a licensed electrician about an interlock kit or transfer switch if you do not already have one. Ask whether your furnace circuit is on its own breaker and whether a dedicated furnace transfer switch makes sense for your house.

Sunday: start the generator, run it under load for a while, and check the oil. Buy fuel stabilizer, confirm your fuel containers are in good condition, and put fresh batteries in every CO alarm. That single weekend handles most of what goes wrong in winter outages.