We recently had a power outage that lasted several days and the house was down to 41 degrees and expected power restore was several days in the future.

I purchased a 3500 watt (5250 starting watts) generator with the hopes to heat my house.

I only wanted to power the furnace and did not want to worry about having it connected to the house at all. I purchased a single outlet and a plug and added them into the furnace line. So when I want to plug in the furnace I first have to unplug it from the houses power and then hopefully plug it into my generator.

The Inducer fan kicks on, the ignition element heats up and the flames start. Just before the blower fan kicks on the flames go out and the cycle starts over.

I am assuming I have 1 or 2 problems.

1) Grounding issue. 2) Dirty power

I would like to explore the grounding issue but am unable to find much on the internet that matches my symptoms (actually I am finding too much, but nothing that seems to fit exactly). I do have a ground lead on my generator, where do I connect it to? My house does have a few poles outside where things come in. I do see a few green wires connected to a pole with a clamp. can I connect my ground to that? is there an easier way to test if this is it the problem first? I've seen things about creating a neutral bonded ground plug and plugging it into a spare outlet on the generator but some reason shorting the neutral and ground didn't seem safe, but I am new to this.

If my issue is dirty power due to a cheap generator, am I out of luck? Should I just sell the generator?

edit: Furnace is about 20 years old. Generator is not an inverter. Furnace does not seem to have a method of delivering error codes. Furnace works fine when I plug it back into the single outlet I added.

  • My furnace is over 20 years old Feb 12, 2019 at 19:21
  • Also, my generator is not an inverter Feb 12, 2019 at 19:29
  • Hello, and welcome to Home Improvement. I presume it all works when you move it back to house power? (Edit the new info into your question; that makes it easier to read.) Feb 12, 2019 at 21:14
  • No furnace error codes? How do you know the blower is about to kick on? Does it try to?
    – isherwood
    Feb 12, 2019 at 21:33
  • 3
    What size is the blower motor? It may be causing the voltage to droop and thus cycle restart. A 120v furnace may be taking all the available power from that leg assuming it's 240v 3500w rated a 1/3hp motor takes 7.2a once up to speed but getting there it will probably be drawing over 20a.
    – Ed Beal
    Feb 12, 2019 at 21:34

6 Answers 6


I too suspect the (lack of) ground-neutral bond. So far as I'm aware there isn't any industry standard for ground bonding in portable generators: it's almost universal that portables in the US have a 3-prong outlet, and in many units the ground prong connects only to the generator chassis, leaving the hot and neutral to float.

My favorite remedy for ground-neutral bonding in portable generators I learned from Mike Sokol (http://noshockzone.org/). Get a regular 3-prong male cord end and put a jumper wire between the neutral and ground terminals. Reassemble the cord end and label it as a ground bond jumper. Plug it to an unused socket on the generator and test the furnace again.

A 20-year old furnace is likely to have a single LED which lights solid when all is well, or gives a series of flashes when there's some error. The control board is likely to be mounted in the blower compartment, and usually there would be a small window through which the LED would be visible -- though it may be dim, and it may be visible only when viewed on exactly the right axis. It could be easier to see if you remove the blower compartment door, keep all hands, long hair, clothing, etc clear of being drawn into the blower, and hold the blower door safety switch so that the control board can power on.

Supposing an indicator LED is found, it'll need to be decoded. Often there's a wiring diagram on a label fixed to the inside of the blower compartment door; the "magic decoder ring" for translating flash count to a meaningful trouble cause might be as simple (inconspicuous) as a footnote on that schematic.

  • Well, I'm not sure about itty-bitty generators, but anything that's "contractor grade" MUST have its N-G bond factory fitted in order to comply with OSHA regs for jobsite use. Feb 13, 2019 at 23:38
  • @ThreePhaseEel We definitely agree that OSHA requires a bond at the generator in some conditions (source: OSHA Fact Sheet: Grounding Requirements for Portable Generators), but I could not confirm whether OSHA requires the bond must be factory-installed. My very brief survey of current portables on the market found that the Honda EB4000 does have a factory-installed bond (per text in its user manual), while the EU3000iS does not (per the wiring diagram in its manual). Maybe my claim that "many" float goes too far -- but some current models do float. Bonding shouldn't be assumed.
    – Greg Hill
    Feb 14, 2019 at 17:36
  • Indeed -- oftentimes generators get horse-traded... Feb 14, 2019 at 23:28
  • One other note on your suggestion with the bonding plug -- I'd use the L5-30 outlet sometimes provided on larger portable generators for that if it is present. That way, it won't get confused for a regular plug.... Aug 17, 2020 at 23:13

Many flame presence detectors use the relationship between Earth and one side of the Mains to sense a small current passing through the flame. If the voltage is not as expected, the flame presence / failure detection may not work.

So again, establishing the proper relationship between Mains and Earth is critical for proper operation.


I'd consult with the furnace manufacturer regarding whether the furnace electronics will function properly on generator power that might not be pure sine wave. If that's the issue you're just wasting your time. It's also possible that you could damage the electronics running on generator power. There are ways to clean up the generator power, but none that will be convenient for your testing purposes.

I'd look at your generator's instructions carefully with respect to the neutral to ground bond rather than using a plug with neutral and ground bonded. For safety and proper function of the equipment grounding system this needs to be done correctly. I imagine if the generator is used standalone as you are using, rather than with a transfer switch, the neutral to ground bond needs to be in place.

It's possible that your furnace's control wiring in some place uses ground as a return path, and without the neutral to ground bond that path is not available. (If this is the case it's likely a wiring issue that should be corrected, even though the furnace runs.) But if this is the case, and the neutral to ground bond is missing, it's possible this could correct your issue.

Connecting the generator's ground lug to your existing ground rod may or may not correct the issue but again I'd defer to the generator manufacturer's instructions. The furnace is probably always connected to ground by it's plumbing so bonding the generator's ground lug may keep everything at the same reference.

I keep referring back to the manufacturer's instructions for safety, I'd not get creative, you could wind up with damaged electronics or a serious safety issue.

  • I was on a project where someone plugged a high end surge suppressor into a standard (non-inverter) generator. The surge suppressor did a great job until it started smoldering from all of the power it was dumping to clean up the waves. Someone noticed and unplugged it before it caught fire, though. But yeah, dirty power + sensitive electronics can be really, really bad.
    – Joe
    Feb 13, 2019 at 18:43

Had the same problem with my generator and gas furnace. When powered by generator (through a transfer switch) the fan kept cycling when no heat was called for. I was quite puzzled. Did some research and discovered the bonded neutral in the generator was causing stray current flows. (Neutral and ground are already bonded in the house panel) Unbonded the generator neutral and the furnace worked fine. I fixed up a dummy 3 prong plug with the neutral and ground connected to make the generator bonded neutral again when not connected to the transfer switch.

  • Welcome to Home Improvement. I'm glad you got your situation working, however, your answer doesn't seem to add anything (except some narrative) to the already accepted answer from 18 months ago. Care to edit your post to add any additional details that may be helpful?
    – FreeMan
    Aug 17, 2020 at 17:10

==> Very helpful thread, thx: It was the grounding in my case. <==

  1. Straight winds took power out of our neighborhood some 10 years ago for a full 4 days. Bought a modified wave inverter (2500W/5000W peak) at a truck stop ($250) to run our fridge etc.
  2. This winter (Feb 2021), power off again thanks to the SPP membership of our state with 13 others. Tried to run the gas furnace but it did not work. I blamed the inverter (modified wave = dirty power) and did not think of a grounding issue.
  3. Bought a "Xantrex-817-2000-Freedom-X-2000-p" PURE SINUS wave inverter from DonRoweCOM to protect the electronics in the 2015 Trane gas furnace. It did not work either. Very frustrated, I searched the WWW.
  4. I just today connected the ground to the chassis of the furnace (metal) which in turn connects to the ground that to the 15A fuse, then to the gas furnace. Now it works.
    You may not need a 2000W inverter just for the gas heater alone (~700 Watt ave.). But do get a quality pure sinus wave inverter for clean electricity. I run it from a 12 Volt battery that sits in a 2015 VOLT. The BIG battery charges the 12 Volt battery constantly from the BIG battery pack (battery for driving) when Volt is in PARK position. The same should work with a Nissan Leaf, I assume. No need for a battery bank ($$$) when you have 17 kWh parked in the garage. On the Volt, I can draw up to 1400 Watts before the single care battery's voltage drops to <11 Volt. A big battery bank would do better. Thanks again to everyone who contributed to this thread.

Was looking at another forum with similar issues. From what I can make out of it, when Furnace is wired to breaker panel (normal configuration), the board is looking for the neutral to be bonded with the ground in the main panel.

If a generators three wires (hot, neutral, grnd) are connected to the furnace, the board does not 'see' the neutral bonded at the panel along with ground.

One suggestion was to make up your standard 3 prong plug from furnace to a receptacle going to the panel for normal operations. When running generator, run a neutral jumper wire from the primary receptacles neutral and share with gen neutral. Then when the plug is put in generator receptacle the neutral wire is seeing the gen AND the panel neutral. No harm, no foul, right?

If not done like that, the furnace's board doesn't 'see' that ground and neutral are actually bonded, .... as it would if coming from the main panel.

I shall contact my HVAC company for clarification. During the Texas Winter Blackout, I came very close to just wiring 12/2 from gen to furnace and just use wire nuts on panel/feed side, but something at the last minute made me not do it as I was more concerned about damage to the furnace board (not even thinking about it not running). Would have taken a lot longer for HVAC guys to bring a new board than just waiting it out till power restored. At least had double see-thru gas log fireplace and used 8 inch 12v computer fans to push some of the heat out. Was NO fun, but survived 4 days with no power. Did use Generator for inside fridge, coffee maker, WOK and blender for some margarita's :-]

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.