Electrical Backup For A Gas Furnace

Feature | Closeup of inside a home natural gas furnace showing blue jets of fossil fuel burning | Electrical Backup For A Gas Furnace

Know how to keep your gas furnace and air conditioning problems running even when electric power is down!

RELATED: Having a Backup Power Supply

Gas Furnace | Keep Warm Even When Electricity Is Down

This article was originally posted at totalsurvival.net and is shared with full permission.

The last article described how to keep a side-by-side refrigerator-freezer going after electrical power is lost. This article will describe how I can keep my gas furnace operating and warm my house even when electricity is not available from the local power grid.

What Is a Gas Furnace? A furnace powered by natural gas to provide heat in low room temperatures.

My gas furnace has a variable speed fan motor to push warm air through the vent ducts in our home. As shown in Figure 1, a single power cord (shown on the lower left) connects the furnace fan motor and the thermostat to 120 VAC power.


Fig. 1: Gas furnace with AC power connection.

During a recent cleaning and maintenance check, the service technician took a number of measurements that I recorded for reference and future design planning. The furnace is quite efficient. It draws 2.0 Amps under normal operation and 3.9 Amps when the variable speed motor is operating. The maximum draw is at 5.0 Amps using a clamp-on multimeter such as the one shown in Figure 2. This means that 10.0 Amps is likely the highest surge current expected for the variable speed motor in this furnace.


Fig. 2: Digital clamp-on multimeter for measuring actual AC current draw.

As I mentioned in the last article, my solar power system uses a Sunny Boy 3800TL-US SMA inverter with the secure AC electrical backup socket as shown in Figure 3. The actual socket connection is under the cover in the lower right of the photo.


Fig. 3: Inverter with 120VAC, 12.5A backup power.

This inverter can produce 1500 watts of 120 volts AC and can provide 12.5 amps if grid power goes out and the sun is still shining. I need just 5.0 amps, so the secure power from the inverter works just fine during sunny days. Figure 4 shows the configuration chosen for this application.

Fig. 4: Configuration providing 120VAC backup from the solar array.

Since a solar day here is about 7 hours long, I need another way to produce electricity for the 17 hours my solar panels aren’t producing power. I decided to use the Honda 2000i portable generator shown in Figure 5.


Fig. 5: Gasoline-powered electric generator.

The 2000i has several external power sockets and can produce 13.7 amps of 120VAC using unleaded gas. The 1-gallon gas tank in the 2000i can keep the generator going for about 5 hours—9.6 hours if the economy burn rate is set on the generator. It’s been quite dependable—and quiet. Figure 6 shows why the generator meets the requirements for powering the furnace.

RELATED: The Grid-Tie Inverter: Electrical Backup for Your Refrigerator-Freezer


Fig. 6: Generator driving a natural gas furnace.

Connecting a 6-gallon marine gas tank to the 2000i gave me a 7-gallon power system that can drive this generator for between 35 and over 67 continuous hours—normal setting or “economy” setting. Figures 7 and 8 show how the power cord from the furnace plugs into an extension cord that connects to the generator.


Fig. 7


Fig. 8

Conceptually, Figure 9 shows how the generator and furnace were connected.


Fig. 9: Generator with expanded fuel storage for over 67 hours of 120VAC.

By adding multiple 6-gallon gas tanks, you can run a generator continuously for days, but this single tank design is essentially all I need to provide 24/7 backup and keep my home warm and comfortable while electrical power is out. Multiple gas tanks just mean fewer refills.

During operational tests, the inverter secure power supply and the gas generator worked just fine and provided power to keep the furnace working as desired. Except for the time to disconnect from the SMA inverter and plug the furnace into a power cord from the Honda 2000i generator, I experienced no downtime of significance. I used the economy setting for the generator, which gave me approximately 9.6 hours of run time for each gallon of fuel. During sunny conditions, the generator would be used approximately 17 hours each day, so the 7-gallon fuel supply could easily provide over five days of furnace operation—or indefinitely by refilling the marine gas tank periodically while the sun-driven SMA inverter was providing power to the furnace. Even on sunless days, the generator could drive the furnace for 67 hours continuously before gas refill would be required.

Home heating and air conditioning were major concerns for me, and the SMA secure power and gas generator configuration met the challenge. My wife is happy as she no longer worries about heating and cooling our house and keeping food cold during emergency situations, and I’m happy because the issue of finding a source for emergency power backup for these major appliances has now been resolved.

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Electrical Backup for a Gas Furnace

Maintaining the heat in your home is very important especially in the cold seasons. With the right solar panel setup, the absence of electricity is not a problem to power up your gas furnace. It's a great back up for keeping warm in emergency situations.

Do you have your own electricity back up for your gas furnace at home? Tell us about it in the comments section below!



Editor’s Note: This post was originally published on April 8, 2016, and has been updated for quality and relevancy.

7 Responses to :
Electrical Backup For A Gas Furnace

  1. Elsie says:

    So I have to ask – how do you find fuel during a power outage? It only stores so long at home and in low quantities, especially if you live in a rented duplex. Gas stations can’t pump without power. Gas becomes a major shortage in emergency situations.
    Also, how do you complete these hook-ups to power inverters and generators? A gas generator has to be outside when running and in the last power outage I experienced, generators were stolen right out of the yards (while running) of the owners. How do you secure your generator & fuel tanks?

    1. bob says:

      Check out a product called “PRI-G” for gas and “PRI-D” for diesel. I have kept gas fresh in a can for over 1.5 years. I talked to someone with the company who said they brought 10 year old gas in a can back to a use-able state. I love the stuff. It will keep in its container for many years. Check with places that sell boats or campers. They usually sell it.

  2. Harvey Richardson says:

    Maybe I’m missing something.
    Without power how is the natural gas getting pumped to the furnace?

    1. Otto Leslie says:

      You don’t pump natural gas or liquid propane
      They are pressurised.

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