Reverse polarity in gas furnace

A few years ago I replaced my heat pump HVAC system with a gas furnace and AC unit. I had the installers bring the furnace feed wire outside the panel box and install a male/female connection between the breaker and the furnace so I could run an extension cord from a small generator and power the furnace in the event of a power outage. I thought I had tested it with the generator but recently, during an outage, the furnace would not run. The thermostat was getting power (from the furnace) but there was a reverse polarity error code on the furnace. I have been using the furnace for years so it clearly was the result of my temporary power supply. However, the generator outlet and all cords from the generator to the outlet near the circuit panel are all three prong cords so I don’t understand how the polarity could be reversed. Is this a misleading error code or am I missing something?

Get a polarity tester. Cheap ones start around ten bucks or even less, perhaps adequate for confirming that everything is wonderful. Since you are diagnosing an actual problem, I would look at the better ones in the $18 to $23 range.

So something like this, or this.

1 Like

I hope you have a meter to check electrical connections? The Generator you have has multiple electrical outlets?
So it looks like you have the positive and negative leads switched somewhere in the circuit. So the right side of the outlet (Black) should have the power on it. Stick your lead of your meter into the right side and the other lead on your meter to a ground to make sure you see power coming out. It should be close to 120vac. Check each electrical outlet on your generator to make sure they all are good. If your generator is good then the lead from your Input to your furnace (from outlet to furnace has been reversed) you will have to go through each section that the wire is ran and connected to make sure the Black wire is on the Brass (right side of the outlet) and the white wire is on the left(shiny silver screw). Some where in the circuit the leads have been reversed.

For info:(in an AC circuit there isn’t a positive or negative but when it goes to DC there is and that is why you are getting a message about polarity).


1 Like

Thanks. Yes I have a multimeter and am familiar with it. The generator has multiple outlets and I did try a different outlet to see if the problem was one outlet. That sounds like a logical plan. It did occur to me after I posted that the generator itself may have the polarity reversed. I bought it at Cabela’s and it was fairly inexpensive. I will update the thread when I have run through the process.

1 Like

That’s where I’d look. If the installers wired both the male and female connections backwards, things would come out right for the furnace. But as soon as you connect your correctly wired female (from the generator and through a good extension cord) to the reverse wired male, you’ve got your problem.

But a question before you go poking around. Is your problem consistently repeatable? If you switch the furnace back and forth between the connection to the breaker and your generator, does the problem happen every time to connect to the generator? If so, I’d check what I suggested. But if it comes and goes, the problem might be the somewhat “dirty” power from the generator.

The power coming from the utility is usually pretty “clean”. That is, it’s at a steady voltage and a consistent rate of switching on the AC current (the 60 Hz rate generally used in North America). A generator can have a lot of spikes up and down in the voltage, and can fluctuate around that 60 Hz standard. Those quick and sometimes large variations can be a problem for some electronic equipment.