How does a furnace know...?

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mbrooke

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United States
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How does a residential gas furnace know you have reverse polarity? I mean if the electronics ares fed via transformer, how could it know? Same goes for an open ground, how does it know when the ground isn't allowed to carry power?


The mystery of it all:lol::dunce:
 

arcsnsparks98

Senior Member
Location
Jackson, TN USA
What I mean is that a simple circiit can determine polarity based on a grounded reference. Correct polarity would show no voltage to the reference from the grounded source conductor. Reverse polarity would because L1 would be there instead.
 

GoldDigger

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What I mean is that a simple circiit can determine polarity based on a grounded reference. Correct polarity would show no voltage to the reference from the grounded source conductor. Reverse polarity would because L1 would be there instead.
Detecting the condition when you have an EGC or other ground (even capacitive) available is not difficult. I think that the underlying question is why the furnace cares?
It has been suggested that it has something to do with the electronic flame sensor used by the electronic ignition circuits, but there has not been a satisfactory detailed explanation AFAIK.
 

Sierrasparky

Senior Member
Location
USA
Occupation
Electrician ,contractor
If the hot and neutral are reversed. The unit will display error codes:

The furnace is fed by the 120v supply. There should be hot , N,and G. There probably is a circuit that knows if the Neutral is live compared to ground.
 

Dennis Alwon

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It used to be that tv,s needed the correct polarity. I assume it has something to do with the electronics. It has got to be pretty hard to mess up the polarity on a 120v furnace
 

mbrooke

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Location
United States
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Technician
It used to be that tv,s needed the correct polarity. I assume it has something to do with the electronics. It has got to be pretty hard to mess up the polarity on a 120v furnace


Id say it is something with the electronics. The issue has been fixed though, it was in a hidden J box:roll:



Out of curiosity, anyone have a link to that thread that explained why furnaces need an EGC to work?
 

Dennis Alwon

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Id say it is something with the electronics. The issue has been fixed though, it was in a hidden J box:roll:



Out of curiosity, anyone have a link to that thread that explained why furnaces need an EGC to work?
Certainly the older furnaces didn't need an ECG. I am sure that an electric furnace would not need an ECG so I am surprise that a gas furnace would need an ECG.
 

kwired

Electron manager
Location
NE Nebraska
Id say it is something with the electronics. The issue has been fixed though, it was in a hidden J box:roll:



Out of curiosity, anyone have a link to that thread that explained why furnaces need an EGC to work?

I don't have a link but originally it had to do with the operation of the flame sensor. Inadequate or no equipment ground would result in intermittent or total failure of proper flame detection and cause shutdown of burner controls. They may have in recent years put more advanced detection in the controls to detect improper polarity and or loss of/missing EGC to actually put the more advanced controller into a lockout situation rather then waiting for a flame sensor failure to lock it out.

Why reverse polarity between the ungrounded/grounded conductor should matter IDK, but for no EGC, the flame sensing rod does pick up a millivolt signal that is developed between the flame and the grounded burner housing. Why it should matter that the housing be actually grounded doesn't exactly make much sense either.

I have had furnaces before that I used steel conduit for EGC, and the HVAC guy has combustion problems and tells me he needs a grounding conductor pulled in the conduit. The times I have pulled an EGC in to make them happy they still continued to have combustion troubles. But I generally now pull a EGC just so I don't get called back and have to deal with such issues, I think it is just easier for them to scream at the electrician that there is no obvious ground then it is for them to actually find what is the true problem. I remember one such furnace finally started working when they poured water in the P trap in the condensate line - which apparently that open trap was effecting some air pressure sensor - been a while can't remember all the details on that one.
 

infinity

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New Jersey
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This seems rather strange why a piece of AC equipment would care if the hot and neutral were reversed, and why are they even calling this "polarity"?
 

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
Factory Training

Factory Training

I'll say it again.
Many types of HVAC systems, small and large will not START if the wiring is reversed or if they are don't see a ground connected, OR if the shield wire on say larger Centrifugal chillers etc. is not landed on ground on BOTH ends.
The service schools always say this, but they don't say why. So it is drilled into the HVAC guys to be sure the power is as thus.
A lot of HVAC Techs are thrown out there with insufficient training and overloaded to run volume. The younger less experienced ones often have not perfected how they cope with all the stress along with the increasing complexity of many types of equipment and all its quirks and myriad instabilities. Many in this boat don't take enough time to look at simple-basic things when thoubleshooting, say such as a loose neutral junction at the machine etc or loose connections in the control chain, or low voltage on the control circuit......
With that said, MOST matters can be solved in a straight forward way, but the incidence of unusual types of scenarios is and has been growing for some time. In those cases the ROOT of the matter will be veiled under some complexity that is not something obvious or seen every day. There are threads on this board which deal with similar matters in the electrical realm.

Many times its possible to call factory Engineering on a problem and they will not know what the hell is going on with low end residential equipment. Once you get into larger equipment such as medium tonnage chillers etc. the Tech Support Engineers will start talking with expertise and things can be worked out IF the Tech can communicate effectively.

The Rod type sensing systems work by FLAME RECTIFICATION, where an AC signal becomes DC when there is flame on it.

Because everything has been monetized and also due to Engineering trends that have been steadily declining over the last 20-30 years, a great deal of the control boards out there are complete junk. [Just like much of the electrical gear being forced on us]. Many will not stand the slightest bit of dirty power or fluctuation on the line, after which they will lock out with FORCED manual reset.
The old school controls such as the Honeywell RA-890 etc. did not behave this way. The older stuff was a LOT more stable. The Engineers are not being held accountable for making things reliable.
Nowadays equipment is running people rather than vice versa, and everyone knows it.
Its a useless, time wasting state, and we the people need to reverse it.
It starts with calling a spade a spade when things don't work and waste your time which is valuable.
 

GerryB

Senior Member
It used to be that tv,s needed the correct polarity. I assume it has something to do with the electronics. It has got to be pretty hard to mess up the polarity on a 120v furnace

Ahh! the kind of "age old question", who should be working on what here. Yes, it should be pretty hard to mess up the polarity, except say the plumber is swapping out the boiler and doing the electric, which from what I know is ok if everything is existing.
 

kwired

Electron manager
Location
NE Nebraska
Ahh! the kind of "age old question", who should be working on what here. Yes, it should be pretty hard to mess up the polarity, except say the plumber is swapping out the boiler and doing the electric, which from what I know is ok if everything is existing.
That may depend on local rules and exactly what is needed to be done.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
This seems rather strange why a piece of AC equipment would care if the hot and neutral were reversed, and why are they even calling this "polarity"?

Tell me about it. Same goes for gas direct vent water heaters. They need a grounded conductor and a grounding conductor... :blink:
 
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