Can You Run a Furnace Off of a Generator?

During a winter power outage, the temperature inside your home can drop quickly, and discomfort isn’t the only problem this can cause. Pipes can freeze — and even burst — in an unheated house, leaving you without running water and putting your house at risk of flooding.

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But, with the right generator, you can run a household furnace and avoid problems related to cold-weather blackouts. Here’s what you should know.

Can a Whole-House Generator Run a Furnace?

Whole-house generators are permanently hardwired into your electrical panel and can typically handle 22,000 watts or more, so you can supply electricity to your entire home. They're often referred to as standby generators because they turn on automatically when the power goes out, which means the lights stay on and appliances keep running, with minimal interruption. These high-capacity units usually run on propane or natural gas and may be attached to an available mainline, so you never have to worry about filling a tank.

Because these permanent units can supply uninterrupted power to your home for extended periods, they’re safe for running most residential furnaces. However, to ensure their reliability, they should be maintained regularly by replacing belts, hoses, batteries and fluids, as needed. Unless you have experience with electrical wiring and plumbing that involves flammable liquids, you should leave the installation and maintenance of a hardwired generator to a licensed professional.

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Can a Portable Generator Run a Household Furnace?

If you lose electricity, gas-powered portable generators can fuel essential appliances, such as refrigerators, microwaves and space heaters. It’s also possible to run a furnace off of a portable generator. However, because electric furnaces can take up to 25,000 watts to start and run, you’ll need a higher-capacity generator to power them. Additionally, because most portable generators have integrated outlets that let you plug appliances directly into them, you may need to take additional steps to connect a furnace to a generator.

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What Size Generator Do You Need to Run a Furnace?

If you want to power your furnace during an outage, you’ll need a generator with a capacity that’s equal to or larger than the wattage required to start and run your furnace fan. For example, a furnace with a 1/4 horsepower fan typically takes about 600 watts to run and an additional 1,000 watts to start, whereas a furnace with a 1/2 horsepower fan uses about 875 watts to run and an additional 2,350 to start.

If you also plan to run other appliances, sum up the running-wattage requirements of all the items you want to power, then add on the highest additional starting watts of those appliances. Your generator needs to have a capacity equal to or greater than the total. However, by staggering the use of your appliances, you may be able to use a smaller generator.

How Do You Run a Furnace Off a Generator?

If you have a hardwired generator installed, you won’t need to do anything. Because the unit is connected to your electrical panel, it can power everything in your home. However, if you have a portable generator, you’ll need a transfer switch to run your furnace. This power switch lets you transfer power for multiple circuits from your main source of electricity to your generator.

A licensed electrician can easily install a transfer switch next to your home’s main electrical panel. However, savvy DIYers with electrical experience may also be able to execute this fairly simple installation.

Safety Considerations When Using a Portable Generator to Power a Household Furnace

Safety should always come first when using a portable generator to power a furnace or household appliance. Keep your unit outside on a flat surface, at least 10 feet from your home, and run heavy-duty extension cords through an open door or window to reach the appliances you want to power. A carbon monoxide detector can alert you to a dangerous buildup of exhaust inside your home. Before you attempt to connect a portable generator to your furnace, refer to the manufacturer’s instructions for specific safety information about your model.