HPR4282: Backup Power for my Gas Furnace

Hacker Public Radio

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Hello, again. This is Trey.

Before I begin, please note: In the show notes, I include links to several informative videos, and to specific products which might be used for a project like this. I do not endorse, nor support any of the products or influencers linked. I do not profit from anything on or associated with any of the links. They are provided merely for additional information or for reference.

I live in the eastern part of the state of Tennessee in the southeast United States. Our summers are fairly hot and humid, but our winters are generally mild. Last winter, we experienced an ice storm, with temperatures dropping into the single digits (Fahrenheit) or below -12 degrees Celsius. This was not a problem until tree limbs heavy with ice began to break off and fall, on occasion taking power lines with them. As a direct result, we lost power at my home. The icy road conditions made it difficult for service vehicles to reach impacted areas, and also made it impossible for us to leave our home.

My house is heated by a natural gas forced air furnace. However, the fan which blows hot air throughout the house, and the electronics which control everything from telling the furnace to turn on to igniting the gas, all require electricity. For anyone unfamiliar with how a forced air furnace works, I am including a link to a helpful YouTube video by The DIY HVAC Guy Link

So, with the loss of electrical power, came a loss of heat. We experienced a full day of being huddled under blankets and drinking warm beverages. These I could heat up on my gas stove, when I used a match or lighter to ignite the burner. Thankfully, the next day our power was restored, and our home warmed up again.

But that got me thinking, and searching for solutions. I needed to provide an alternative electrical power option for my gas furnace so that it would continue to work when the electricity was interrupted. I found a really interesting video, also by The DIY HVAC Guy ( Link ), explaining an option for retrofitting the electrical connections to the furnace, but I never got around to doing it.

That changed, a couple months ago, when I found a real deal on a brand new portable solar generator. It is effectively a large lithium iron phosphate (LiFePO4) battery, an inverter, and a charging system, all bundled together in an easy to carry case. It provides over 1,000 watt hours of power. It can be charged using portable solar panels (which I purchased at the same time), or supported using external power sources from 12v to 48v DC.

The retrofit of the of the electrical feed line into my furnace was easily accomplished by doing something very similar to what was done in the video linked above. I turned off the circuit breaker for the furnace in the house's main electrical panel, and then located the switch box mounted on the side of my furnace. I removed the switch plate cover and tested the wires inside using a non-contact voltage tester, which would light up and scream at me if there was still any power to any of the wires. No screaming. No power. Good.

Next, I removed the switch from the box, disconnected the black power source "hot" wire from the switch. This would carry 120v AC directly to the switch and allow a user to turn the furnace on and off. The wires going into the furnace included a black "hot" wire, connected to the other side of the switch, and a white "neutral" wire which was connected using a wire nut to the white "Neutral" wire from the house wiring. I disconnected all of these wires, and also the ground wire from the house wiring.

I removed the front service panel from the furnace so that I could perform the next steps safely. I removed the "single gang" electrical box (which had housed the on/off switch) from the side of the furnace, where it had been attached using a single sheet metal screw. The term "single gang" describes a box which is only wide enough to accommodate a single standard sized switch or a single duplex outlet. A "double gang" box is wide enough for two such devices, and a "triple gang" is wide enough for three. Use the provided link for a more complete description of this naming.

I chose to replace the single gang box, which only held a switch, with a double gang box to accommodate both a switch and a duplex outlet. I could have used the existing box and used a single gang outlet/switch combination (Like the one at the provided link ), but I wanted to use a heavier duty switch and outlet.

I mounted a new double gang box to the side of the furnace using multiple sheet metal screws. Then I passed the Romex house wiring through a knockout hole in the side of the box and a protective wire clamp. I wired the black "Hot" wire from the house to the brass colored screw on a commercial grade 120v 15A duplex outlet. The neutral wire from the house went to the silver colored screw on the outlet, and the bare ground wire from the house connected to the green ground screw on the outlet. This was now connected just like any other outlet in the house, and controlled by the breaker in the house's main panel. I wrapped this duplex outlet with electrical tape, as I like to do any time I am working in a metal box or a box containing more than one device. Then I secured the duplex outlet to the box with included screws.

I knocked out another one of the side holes in the box, and installed another wire clamp, through which I passed some heavy duty 12ga (About 2mm conductor diameter) 3 conductor flexible cable. The black wire from this went to one screw connector on the switch, and the black wire from the furnace connected to the other screw connector on the switch. I connected the white wires (from the flexible cable and from the furnace) together using a wire nut, and connected the ground wire from the flexible cable to the green screw on the box.

The final installation steps were to connect a quality, 120v 15A plug to the other end of the flexible cable.

Now for the first test. I turned the breaker back on, and used an outlet tester to verify the newly installed outlet on the side of the furnace was working and wired correctly. I use one which displays voltage along with any wiring faults, and which also has a button which can be used to test ground fault circuit interrupts (GFCI) like the one found at the provided link .

Everything checked out properly, so I plugged the furnace into the outlet, using the flexible cord. Nothing happened, so I flipped the switch to the "On Position". I immediately saw blinking lights on the furnace controller board. Next, I adjusted the thermostat for heat, and within a minute, the furnace had ignited the burners and was blowing hot air.

Good. All my connections were verified, so I turned off the furnace, unplugged it, and installed the faceplate on the box. Yes, I know I could have done this before I plugged in the furnace. However, with over 40 years of experience building, repairing, and modifying electrical and electronic devices. I have learned to never put back together more than you must for a specific test. Otherwise, you will just need to go back and take it apart again when your test fails.

What was next? More testing, of course. I needed to figure out how much power the furnace used when generating heat. Because of the way the furnace was now wired, it was simple to connect an inline power meter between the outlet and the furnace plug. I plugged one in and turned everything back on again. The measured peak power use was 375 watts. The solar generator can easily handle this for a couple hours and I can charge it from various sources during gaps in operation.

So, I connected the solar generator and... Nothing. The furnace would not even complete its self-test sequence. Great... Did I break something? I plugged the furnace back in directly to the power outlet, and everything worked properly. Well, that's a plus, at least.

So, I spent the next several hours troubleshooting two different blinking light error codes on the furnace controller board. One referenced "Hot & Neutral Reversed" while the other referenced "Open Ground". That was odd. Well, I had never actually tested the output of this brand new solar generator, so I plugged my outlet tester into the solar generator, and it verified "Open Ground". Time for a technical support call.

THIS is when I learned something important. Apparently, this is a common issue with solar generators, and is part of their design. To resolve the issue, I need to simply connect a bonding plug into any 120v outlet on the generator.

You can purchase a Generator Bonding Plug, also called a Ground-Neutral Shunt, and I have included several links as reference. ( Bonding Plug or Ground Neutral Shunt ). I found them selling for anywhere between $10 - $35 USD. But all they are is a $1 - $2 USD plug with a wire connecting the ground terminal to the neutral terminal. So, naturally, I built one using a spare plug I had. I connected it to the solar generator, and the outlet tester was very happy.

With fingers crossed I connected the furnace back to the solar generator, with the shunt installed. This is very difficult to do, by the way, when your fingers are crossed. Everything started up properly, and I ran the furnace for 2 hours using just the generator.

Hopefully, I will not need it, but If I do, I have options.

I would love to hear your episode about what you do to prepare for situations like this.

Provide feedback on this episode.