Episode: 4282
Title: HPR4282: Backup Power for my Gas Furnace
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr4282/hpr4282.mp3
Transcribed: 2025-10-25 22:26:29

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This is Hacker Public Radio Episode 4282 for Tuesday 31 December 2024.
Today's show is entitled Back Up Power for My Gas Furnace.
It is the 20th show of Trey and is about 12 minutes long.
It carries a clean flag.
The summary is how I modified the power connection to my forced air gas furnace to allow for
backup power use.
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.
Thank you.
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, however, we experienced an ice storm with temperatures dropping into the
single digits Fahrenheit or below, negative 12 degrees Celsius or so.
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 forced air furnaces work, I'm including a link to a helpful
YouTube video by the DIY HVAC guy.
So with the loss of electric power came a loss of heat.
We experienced a full day of being huddled under blankets and drinking warm beverages.
As I could heat up on my gas stove, when I used a match or a lighter to ignite the burner.
Thankfully the next day our power was restored and our home was warmed 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.
Explaining an option for retrofitting the electrical connections to the furnace.
But I never got around to doing it.
The link is in the show notes.
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 battery, an inverter, and a charging system
all bundled together in an easy to carry case.
It provides over 1000 watt hours of power.
It can be charged using portable solar panels, which I purchased at the time, or supported
using external power sources from 12 volts to 48 volts DC.
The retrofit of the electrical feed line into the furnace was easily accomplished by doing
something 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 the 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 120 volts 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 of the furnace so that I could perform the next steps carefully.
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.
You can use the provided link for a more complete description of this naming convention.
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 in 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 hot wire from the house to the brass colored screw on the commercial grade
120-volt 15-amp duplex outlet.
The neutral wire from the house, white wire, 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 in 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 12 gauge, which is 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 a green screw on the
box.
The final installation steps were to connect a quality 120 volt 15 amp 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 used one which displays voltage along with any wiring faults, which also has a button
which can be used to test ground fault circuit interrupts, like the one found in the link
provided.
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 and immediately saw the 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.
Kind of like me.
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 it into the furnace.
However, with more than 40 years of experience building, repairing, modifying, electrical
and electronic devices, I've 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.
Okay, 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 different sources during gaps in operation.
So I connected the solar generator and nothing.
The furnace would not even complete itself test sequence.
Yes, the switch was in the right position.
Did I break something?
I went ahead and plugged the furnace back directly into the power outlet and everything
worked properly.
Well, that's a plus at least.
I didn't break anything.
So I spent the next several hours troubleshooting two different blinking light error codes on the
furnace controller board.
One referenced hot and neutral reverse.
While the other referenced open ground.
This was odd.
Well, I had never actually tested the output of this brand new solar generator.
So I plugged the outlet tester into the solar generator and verified, yes, open ground.
Time for technical support.
This is when I learned something important.
Apparently, it is a common issue with solar generators and as part of their design.
To resolve the issue, I need to simply connect a bonding plug into any of the 120 volt
outlets on the generator.
You can purchase a generator bonding plug, also called a ground neutral shunt and I've
included several links as reference.
I have found them selling anywhere between 10 and 35 US dollars.
But all they are is a 1 to 2 dollar 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.
Keep in mind, 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 prefer for situations like this.
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