Why won't my furnace complete the startup sequence when powered by a portable generator?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.