Electrical Backup for a Gas Furnace
Warm Your Home Even When the Grid Goes Down
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.
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 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 was measured 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 selected on the generator. It’s been quite dependable—and quiet. Figure 6 shows why the generator meets the requirements for powering the furnace.
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 was plugged into an extension cord connected to the generator.
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 down time 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 refrigeration 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 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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