Zappa

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zappa

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Hey folks,

I'm new to this forum and I have a question.

1) I noticed that my furnace complains when I flip the plug upside down on the outlet. It is a two prong outlet from the 50's with no ground. The plug is a three prong male plug with the ground prong broken off. The furnace LED indicates "polarity reversed". How does it know and Why does it care? If the current is alternating 60 times per second, isn't it true that if it is not correct now wait 1/60th of a second and it will be correct?
Thanks,

Zappa
 
zappa said:
Hey folks,

I'm new to this forum and I have a question.

1) I noticed that my furnace complains when I flip the plug upside down on the outlet. It is a two prong outlet from the 50's with no ground. The plug is a three prong male plug with the ground prong broken off. The furnace LED indicates "polarity reversed". How does it know and Why does it care? If the current is alternating 60 times per second, isn't it true that if it is not correct now wait 1/60th of a second and it will be correct?
Thanks,

Zappa
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Not exactly, assuming this is a 120V household receptacle. One line is ungrounded and its voltage oscillates at 60Hz, while the grounded one (aka Neutral) is a steady state 0V (though there is likely to be small AC voltage due to voltage drop back to the source).

You are correct about the current, but the sensing circuit is likely using voltage to determine reversed polarity.

"It" cares probably because it uses a switched mode power supply, and it switches the ungrounded line to control voltage and/or current. With lines reversed, switching a 0V supply does not produce any output.
 
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Smart $ said:
One line is ungrounded and its voltage oscillates at 60Hz, while the grounded one (aka Neutral) is a steady state 0V (though there is likely to be small AC voltage due to voltage drop back to the source).
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I think it is important to say that the grounded conductor is 0V in reference to ground but is 120V in reference to the ungrounded conductor.

I have no idea why the furnace 'cares' which conductor is 0V to ground. :confused:
 
iwire is correct in that it should not know the difference if there is no ground. It must have a ground reference somewhere to tell the difference.

But, it must be using the housing and connecting duct, pipe, etc, to establish a reference point. Even with the ground wire from the furnace not being connected, the housing and duct work will be coupled to ground in some manner. If not directly connected somewhere, it will at least have a capacitive coupling to ground.
 
ultramegabob said:
could a diode in the circuit cause an issue with polarity of how it is plugged in?
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Only about 1/2 the time.:D

I would not think it would be connected to ground (or the housing in this case) as the ground is not supposed to be used as a current-carrying conductor.
 
zappa said:
. . . It is a two prong outlet from the 50's with no ground. The plug is a three prong male plug with the ground prong broken off. . .
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I am concerned that the furnace is not grounded and neither is the gas pipe. I know the code is not retroactive but, if it were my home, I would replace the circuit, install a grounding receptacle, and ground the furnace. I also wonder why the furnace plugs in instead of being direct wired? I recommend you have an electrician in to install this correctly. :)
 
If the furnace has any sort of modern electronics in it, the board may very much care about polarity. If the manufacturer goes to the trouble and expense of installing the circuitry to indicate reverse polarity, it's there for a good reason.

My next question is, is the furnace listed for connected use?
 
I am fairly certain that cord and plug connected furnaces are the norm in some jurisdictions.

I learned that here on this forum. :smile:
 
480sparky said:
If the furnace has any sort of modern electronics in it, the board may very much care about polarity. If the manufacturer goes to the trouble and expense of installing the circuitry to indicate reverse polarity, it's there for a good reason.
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I don't disagree with your logic but what would be the reason for it? :-?

My PC does not care about polarity why should a furnace?:-?
 
iwire said:
I don't disagree with your logic but what would be the reason for it? :-?

My PC does not care about polarity why should a furnace?:-?
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I don't claim any special knowledge about electronics, especially when it comes to what ol' Dave puts in his products.

If there's an LED to indicate reverse polarity, I'd say it's for the benefit of the service technician and not because someone had some extra LEDs at the factory.

And if the OPs furnace truly doesn't like hot/neutral reversal, then something is going on.
 
480sparky said:
If there's an LED to indicate reverse polarity, I'd say it's for the benefit of the service technician and not because someone had some extra LEDs at the factory.
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And I agree with you ........... I just wonder what the benefit is of designing an AC furnace that is polarity sensitive. :smile:
 
I ran into a commercial microwave about 15 years ago that used the ground as part of the circuit for the electronics. It was a 240 micro. I notified UL of my findings and did get a response but by the time I went thru all the hoops it wasn't worth the trouble. The furnace might use the ground as well in their circuit. However it dosen't make sense being 120 volt.
 
LarryFine said:
I believe the flame detector requires proper polarity.
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But it is run off of DC. This should be behind a transformer, rectifier, and filter and I would not think it would know the difference. The flame detector is probably tied to ground (or the frame since the ground plug was broken).

They may have issues with switching the grounded conductor instead of the ungrounded conductor. There may also be a issue with the ground-to-grounded conductor vs the ground-to-ungrounded conductor insulation rating. Still not clear why.
 
May I direct everyone to this thread..

May I direct everyone to this thread..

Folks, we've been down this road before... check out THIS THREAD.

Despite what logic and common electrical experience tells us, for whatever reasons the manufacturers have, there IS AC-powered electrical equipment out there that IS sensitive to "Polarity".

Please read that thread and hopefully it will help to understand the OP's situation.

Edited to add: Also see my two posts (#'s 42 and 45) on THIS THREAD
 
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My guess is there is a control transformer with a ground that if the incoming power is not polorized--neutral to neutral and hot to hot, the transformer output is effected. Many of the old time intercom systems had this same problem--they had a small two wire(unpolorized) plug connected transformer inside and if you reversed the factory plug, it would give you a wrong polorized warning light and only a HUMMM out of the speakers.
 
It could have something to do with the igniter. Had a stove last summer plugged in to a reverse wired outlet with no ground. Turned unbonded gas pipe into a big 120 vt bus!!
 
iaov said:
It could have something to do with the igniter. Had a stove last summer plugged in to a reverse wired outlet with no ground. Turned unbonded gas pipe into a big 120 vt bus!!
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So you have a gas stove with a neutral bonded to the case?
 
mxslick said:
Folks, we've been down this road before... check out THIS THREAD.

Despite what logic and common electrical experience tells us, for whatever reasons the manufacturers have, there IS AC-powered electrical equipment out there that IS sensitive to "Polarity".
Click to expand...

I did not see one person say it is not possible.:roll:
 
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