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Something I'm curious about . . .


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After our recent ice storm, I connected the furnaces of a couple of friends to gasoline-powered generators. One friend had a fairly new unit, and after I'd made the connections and ran the cable, the furnace fired for a few seconds, then shut down.

I phoned a friend who's an HVAC guru, and he explained that newer furnaces have sophisticated circuit boards that sense when the electrical connections don't contain good grounds. Generators, of course, have no ground rods, and the furnace sensed that the electrical ground was inadequate. To correct this, one has to attach a grounding clamp to a water pipe, and then connect the clamp's wire to a screw on the furnace's cabinet. After I did this, the finicky furnace purred along perfectly.

What I'm curious about, though, is the method by which a furnace senses the ground is bad. Is there some kind of inherent test mechanism like one finds in a SureTest device? Also, how does a generator create a ground? Or does it? I've since reconnected both furnaces to restored electrical services, and wish now I'd stuck a three-light tester or my Inspector II--which is a horrible SureTest knockoff--into one of the generators' outlets to see what kind of result was displayed.

Edit: Mike, I had HVAC on my mind when I wrote this. If it belongs in the Electrical forum, ship me on over.

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Did the generator have a 3 prong outlet? Did you use a 3 prong extention cord to connect the furnace to the generator?

I'm trying to figure out how a furnace power supply would see anything different in power supplied from a generator grounded to the frame of the generator vs the regular electric supply which uses an earth ground.

If just a 2 wire 2 prong extension cord was used, then this would be easy to detect.

Otherwise what is the difference between a frame ground on the generator and an earth ground as the power supply / circuit board would see it?

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I connected a three-prong plug to a strand of 14 gauge wire, and hardwired the other end to the furnace connections. The generator itself contained four three-prong outlets.

The generator had a ground lug, but there was snow and ice on the ground, it was reeaally cold outside, and I didn't want to mess around with driving a rod. I suppose I could have connected the ground lug to something else, but my expertise with generators is limited and I was trying to supply heat to a house that was freezing inside.

As for the difference between a frame ground and an earth ground, and how the furnace knows the difference, that's what I was wondering about.

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. . . I phoned a friend who's an HVAC guru, and he explained that newer furnaces have sophisticated circuit boards that sense when the electrical connections don't contain good grounds. Generators, of course, have no ground rods, and the furnace sensed that the electrical ground was inadequate. To correct this, one has to attach a grounding clamp to a water pipe, and then connect the clamp's wire to a screw on the furnace's cabinet. After I did this, the finicky furnace purred along perfectly.

What I'm curious about, though, is the method by which a furnace senses the ground is bad. Is there some kind of inherent test mechanism like one finds in a SureTest device? Also, how does a generator create a ground? Or does it? I've since reconnected both furnaces to restored electrical services, and wish now I'd stuck a three-light tester or my Inspector II--which is a horrible SureTest knockoff--into one of the generators' outlets to see what kind of result was displayed.

I share your wonder about this one. It's unlikely that the furnace or its circuit board has any idea about whether or not the generator is connected to the earth. The grounding electrode system (earth ground) and the equipment grounding system are two completely different things. The grounding electrode system has nothing whatsoever to do with the normal day-to-day operation of the electrical system.

How did you connect the generator to the furnace? Did you plug the furnace directly into the generator receptacles or did you backfeed the generator into a breaker panel?

- Jim Katen, Oregon

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I'm actually a pretty good electrician, and have completely rewired my 1922 bungalow.

I disconnected the house wiring in the furnace's junction box and connected the 14 gauge wire to the internal wires. At the opposite end of the 14 gauge wire, I attached a three prong plug--the industrial kind in which you slide down the housing and wrap the wires around the screws for each individual prong.

The furnace does, indeed, have some manner in which to sense that the ground is inadequate. The friend I called for help, who owns an HVAC company, said he had gotten several calls from electricians who had had similar results to my own when they tried to connect newer furnaces to generators. Similar to what happened with me, the electricians had to ground the furnaces to water pipes to render the furnaces operable.

I don't understand this stuff as well as you do, Jim and Chad, but apparently newer furnaces have some sort of mechanism by which they can distinguish between a good and not-so-good ground.

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I'm actually a pretty good electrician,...

I had no doubt about that. The only reason I mentioned it is because I hacked a condensing unit I had (sans venting and proper gas plumbing) into service in my shop by poking bare wires into a receptacle. The unit wouldn't fire until I corrected the polarity, but did fire w/o a ground.

What has me wondering is, the gas piping is bonded and the furnace case is bonded so how does it know where the ground is coming from?

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I'm actually a pretty good electrician,...

I had no doubt about that. The only reason I mentioned it is because I hacked a condensing unit I had (sans venting and proper gas plumbing) into service in my shop by poking bare wires into a receptacle. The unit wouldn't fire until I corrected the polarity, but did fire w/o a ground.

What has me wondering is, the gas piping is bonded and the furnace case is bonded so how does it know where the ground is coming from?

I've made my share of mistakes, plenty of them, but the furnace in question was wired correctly.

Like I said, area electricians who were connecting furnaces to generators were calling my HVAC friend 'cause furnaces weren't operating like they should. Older units work fine, apparently, but newer ones won't accept the generator's ground. As for the why and how, that's what I didn't understand, either. I'm assuming the generator has something going on with the ground and neutral that approximates a bootleg ground.

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Hi John,

Did you try connecting a jumper cable between the gas pipe and the ground lug on the generator? The gas pipe should be bonded back to the panel and ground so that should enable the machine to sense a better ground, no?

If anyone is interested, I can paste the entire ground section from a U.S. Army portable generator manual here.

ONE TEAM - ONE FIGHT!!!

Mike

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When trying to figure out something like this, it's best to go back to the beginning, look at the facts and discard the assumptions.

As I understand it:

  • You disconnected the furnace entirely from the household electrical wiring.

You connected the furnace to the generator wiring. This included a hot wire a neutral (grounded) wire and a grounding wire.

You made no other connections to the generator.

The furnace started, ignited, fired for a short while, and then shut down.

You then connected the furnace cabinet to a water pipe.

After that, the furnace worked fine.

Is this the correct series of events?

Did I leave anything out?

- Jim Katen, Oregon

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When trying to figure out something like this, it's best to go back to the beginning, look at the facts and discard the assumptions.

As I understand it:

You then connected the furnace cabinet to a water pipe.

- Jim Katen, Oregon

Oh! I missed that. What are you thinking; they never bothered to bond the gas pipe and the equipment ground at the furnace is kaputt?

ONE TEAM - ONE FIGHT!!!

Mike

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Exactly correct, Jim. Except for one important item.

There were power outages all over town. I live in an old neighborhood and have two 100' oak trees in my front yard that I was praying wouldn't topple over like countless other trees around me already had. I was also anticipating losing power myself--I didn't, unlike during the 2003 ice storm when I was juiceless for two weeks--and was trying to help out a friend.

Maybe I haven't expressed this very well, but when the furnace wouldn't operate they way it was supposed to, I had no idea what the problem was. The notion of a ground wire didn't even enter my mind. What did enter my mind, was to call a very good friend who has masters HVAC and Electrical licenses. I knew that if the glitch was a common one, he'd be aware of it. And if not, he would have stayed on the phone with me and offered suggestions until we got the bloody thing working. And . . . although I wouldn't have let him do it, if I'd fallen short, he no doubt would have driven to where I was to help out because he's wonderful.

Anyhow, my friend immediately asked how old the furnace was--2 years--and when I told him, he explained what I had to do vis a vis the ground wire. He went on to say that he'd fielded several calls the past couple of days from electricians who'd encountered similar problems when they connected newer furnaces to generators and wanted to know why the furnaces wouldn't operate.

So the thing is, I wasn't exactly curious about how to connect a furnace to a generator, I was merely curious about the method by which the furnace senses the ground is inadequate.

I hope this makes sense.

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  • 2 years later...

OK, First let me say hi. Second Please don't roast me for dredging up this 2 year old post. I have been reading thru some of these older post for some entertainment as well as knowledge. I have the answer you are looking for here. It is actually very simple.

The furnaces that won't work properly have flame sensors that resemble a short piece of stiff wire that is in the spark lit pilot or burner flame. This is the wire that sometimes needs to be cleaned when the furnace does not light properly. Most people mistakenly think these work like conventional thermocouples...They do not. They are a ground path for a milivolt current that passes thru the flame from the controller. The controller is looking to see if there is a ground path and therefore will keep the gas valve open. The carbon in the flame acts as the conductor. If there is no ground present the controller thinks that there is no flame and shuts down the burner. Since a generator is not connected to a ground (usually) it can not sense the ground. I can't tell you how many homeowner installed furnaces I have to fix because of no ground circut. I also have to fix many in the late part of fall after the handyman decided to rewire his basement over the summer. The local pool supply company also has me on their speed dial to fix pool heaters for the same issue.

Hope this helps, Dan

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OK, First let me say hi. Second Please don't roast me for dredging up this 2 year old post. I have been reading thru some of these older post for some entertainment as well as knowledge. I have the answer you are looking for here. It is actually very simple.

The furnaces that won't work properly have flame sensors that resemble a short piece of stiff wire that is in the spark lit pilot or burner flame. This is the wire that sometimes needs to be cleaned when the furnace does not light properly. Most people mistakenly think these work like conventional thermocouples...They do not. They are a ground path for a milivolt current that passes thru the flame from the controller. The controller is looking to see if there is a ground path and therefore will keep the gas valve open. The carbon in the flame acts as the conductor. If there is no ground present the controller thinks that there is no flame and shuts down the burner. Since a generator is not connected to a ground (usually) it can not sense the ground. I can't tell you how many homeowner installed furnaces I have to fix because of no ground circut. I also have to fix many in the late part of fall after the handyman decided to rewire his basement over the summer. The local pool supply company also has me on their speed dial to fix pool heaters for the same issue.

Hope this helps, Dan

That makes perfect sense if there is no equipment grounding conductor (EGC - that's the bare wire that you run with the other circuit wires.)

However, it makes no sense if there's no grounding electrode conductor (GEC - that's the wire that runs to the earth.)

Whether or not a furnace circuit is properly grounded has nothing to do with the electrical system's connection to the earth.

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