Furnace Ground

[MasterTheNEC]

GD,

Here is where I am having a problem.

The controls are inside a metal furnace along with the sensor itself and the related wiring.

Where / how is any kind of 'noise' being introduced into this equipment that an EGC would fix?


If I seem to be a bit of a PITA about this it is only because the OP is asking for an answer and IMO we have only provided anecdotal stories about what might be.

OH GAWD...don't get me started on the important values of isolated grounding circuits for noise reduction.:angel:

 

[iwire]

OH GAWD...don't get me started on the important values of isolated grounding circuits for noise reduction.:angel:

:lol::lol:

Computers can't work without it ...

 

[GoldDigger]

GD,

Here is where I am having a problem.

The controls are inside a metal furnace along with the sensor itself and the related wiring.

Where / how is any kind of 'noise' being introduced into this equipment that an EGC would fix?


If I seem to be a bit of a PITA about this it is only because the OP is asking for an answer and IMO we have only provided anecdotal stories about what might be.

I agree with your point, and based on that I have this to say to the OP:

asphalt,
Yes, in terms of the concerns of the NEC the EGC in a circuit is not expected to carry current during normal operation. But that does not mean that it will never, in the real world, be carrying current related to stray or capacitively induced voltages or low level fault currents, so there may be some truth to what they are saying.
Your observation that the idealized EGC does not carry current is not enough, by itself, to win the argument.

 

[mbrooke]

OH GAWD...don't get me started on the important values of isolated grounding circuits for noise reduction.:angel:

Isolated grounds are revolutionary! You have no idea how much they help!!!!! If you have a standing neutral to ground fault; running an isolated ground and making sure the equipment is on rubber feet fixes everything. No need to correct NEC issues and I get to sell something while everyone thinks Im really smart Simply Brilliant!














Im kidding of course

 

[iwire]

I agree with your point, .

That I am a PITA?

 

[GoldDigger]

That I am a PITA?

Of course not. Only that you seem to be a PITA.
Precision of language, you know. (Which makes me seem to be a PITA at times too.)

 

[fmtjfw]

GD,

Here is where I am having a problem.

The controls are inside a metal furnace along with the sensor itself and the related wiring.

Where / how is any kind of 'noise' being introduced into this equipment that an EGC would fix?


If I seem to be a bit of a PITA about this it is only because the OP is asking for an answer and IMO we have only provided anecdotal stories about what might be.

Last attempt:

A furnace, depending on its size (residential ... multistory large school) has a bunch of electrical components in it that define the electrical "noise" environment:

1) Electric ignitor transformer.
2) Electrically operated valves (pilot, main, safety).
3) Electric draft fan(s), electric purge fan(s); warm air blower fan(s), circulation pump(s).
4) Electronic controls (or heaven for fend -- electromagnetic).
5) Electric Flame Detector (case in point).

1) runs at a high voltage until pilot lights.
2) controlled by electronic controls.
3) may be timer controlled from controls, pre-ignition; thermostatic control.
4) sit in the middle
5) tries to detect small currents

Now the goals:

1) Don't blow up the building: Make sure that gas doesn't flow in any appreciable amount before you have an ignition source and for not long thereafter if no flame is detected.
2) Heat the building when the building system calls for heat.
3) Don't burn down the building: Shutdown the system if an over-temperature is detected.

The electronic flame sensor walks a very narrow path electrically. IT MUST SHUTDOWN ON FLAME MISSING. It has better not shutdown when flame is present.

Without proper grounding, the electrical noise from the furnace: motors starting, high-voltage sparks from the ignition transformer and external electrical noise can overwhelm it.

 

[kwired]

@asphalt

I am sure I am going to mirror what others have stated but the requirement for an equipment grounding conductor goes back quite a ways so any system with a branch circuit supplying it not having an EGC would be..well a violation at the least, poor design at best. As others have stated, the EGC is essential to aid in clearing the OCPD and without it increases the user and installers risk. Not to mention that it is a code violation and will probably result in another site visit, added cost to re-do work (no one bids a job to do it twice...and survives) and a nice rejection ticket if the inspector is competent.

Not to mention it is hard pressed to find any cable product today without an EGC present...did not say impossible just hard to find. So summary, smile and say they are right (code-wise) but newer HVAC system probably not technically wise.

If any of that makes sense....

Most of us agree that an EGC is necessary by the NEC and for good reasons. The question here is why does a flame sensor sometimes not operate correctly in the absence of an EGC, and not why does the NEC require one? Or sometimes this is an issue even when you use a raceway for EGC instead of a copper conductor?

0-6 micro-amps. Doesn't take a lot of "noise" to screw that measurement up.

http://www.selas.com/files/docs/B7515-How-to-choose-and-use-flame-sensors.pdf

I am on Bob's side right now, where is this noise coming from? In a typical residential type furnace or other gas burning appliance with such a flame sensor we have a sensing probe and a controller that are usually only a couple feet of conductor apart at the most.

I have an industrial customer that has a burner on a product "dryer" where the flame sensing probe is probably 60 - 75 feet away from the controller - the burner is at the top of this machine and the controller is at the lower level. All that runs to this probe is a single conductor, return path is via the stainless steel shell of the vessel that is the main part of the machine - and any incidental EGC's to motors, and other electrical components as well as steel structure of the building that is essentially bonded to it by structurally being a support. That single conductor is not in a raceway with power conductors but is in PVC conduit running next to other PVC conduits that do contain power conductors. Seems like this would be inviting many problems with flame sensor according to HVAC experts - this thing has been in operation with no flame sensing problems (related to grounding issues anyway) for about as long as I have been in this trade - which is nearing 30 years.


I have had furnace guys want me to run an EGC, whether there is none at all or to add one to a metal raceway that is otherwise acceptable to NEC for EGC purposes. They are having flame sensor or other problem they can't solve when they request it. I can't recall a single one of those instances where I added the EGC where their problem went away afterward either, it is just one item on the checklist and they are hoping it will work.

 

[MasterTheNEC]

Most of us agree that an EGC is necessary by the NEC and for good reasons. The question here is why does a flame sensor sometimes not operate correctly in the absence of an EGC, and not why does the NEC require one? Or sometimes this is an issue even when you use a raceway for EGC instead of a copper conductor?

Well I will go with this response (since I have no idea how a flame sensor works)...I will do like the people who blame AFCI's for everything...it's a problem with the manufacturer of the "flame-thingies"..have them fix it...pronto !

 

[don_resqcapt19]

...
Without proper grounding, the electrical noise from the furnace: motors starting, high-voltage sparks from the ignition transformer and external electrical noise can overwhelm it.

The connection to earth doesn't really do anything to reduce electrical noise.

 

[Galt]

Electrons flow from the flame rod to the pilot burner which is grounded. Burnt gas conducts way better than unburnt gas thus more current flows through the flame the control module interprets a flame present and allows the gas to keep flowing.

 

[Birken Vogt]

Furnace techs are making a generalization here. They need a good EGC/bond for some of the units they work on to be reliable. Other units will not care.

They don't know or care which units are the finicky ones. It has been mentioned here that some units check for EGC on startup and the microprocessor control will lock out and give an error code if the hot/neutral are reversed, or if the EGC is not connected.

These are the same units that require perfect 60 Hz sine wave to function. If you connect them to a generator that has some harmonic distortion but is perfectly adequate to run every other appliance and computer in the house, they will refuse to function. This is evidently because there is so little rectification/filtering in the power supply part of the control, they cannot deal with anything other than a perfect sine wave. In other words they are built cheap with little leeway for anything outside of the ordinary.

 

[Mgraw]

Furnace techs are making a generalization here. They need a good EGC/bond for some of the units they work on to be reliable. Other units will not care.

They don't know or care which units are the finicky ones. It has been mentioned here that some units check for EGC on startup and the microprocessor control will lock out and give an error code if the hot/neutral are reversed, or if the EGC is not connected.

These are the same units that require perfect 60 Hz sine wave to function. If you connect them to a generator that has some harmonic distortion but is perfectly adequate to run every other appliance and computer in the house, they will refuse to function. This is evidently because there is so little rectification/filtering in the power supply part of the control, they cannot deal with anything other than a perfect sine wave. In other words they are built cheap with little leeway for anything outside of the ordinary.

BINGO

These circuit boards are made so cheap the slightest static discharge will blow the board.

 

[StarCat]

10-4

10-4

Furnace techs are making a generalization here. They need a good EGC/bond for some of the units they work on to be reliable. Other units will not care.

They don't know or care which units are the finicky ones. It has been mentioned here that some units check for EGC on startup and the microprocessor control will lock out and give an error code if the hot/neutral are reversed, or if the EGC is not connected.

These are the same units that require perfect 60 Hz sine wave to function. If you connect them to a generator that has some harmonic distortion but is perfectly adequate to run every other appliance and computer in the house, they will refuse to function. This is evidently because there is so little rectification/filtering in the power supply part of the control, they cannot deal with anything other than a perfect sine wave. In other words they are built cheap with little leeway for anything outside of the ordinary.

Vogt, you are spot on here with the " Cheaply " thing.
One thing about Solid state controls. On a 1000 Ton Centravac, they have to work......and as thus the Engineering is better.
However, on smaller split system DX, anything goes and Sorry Electronic Engineering is the rule not the exception in many cases.
I have a mate back in Texas that is dealing with some of Yorks finer product on a facility and its just as you say.
Again. I have watched these matters devolve over the last 30 years. Much of the older generation controls were way better.

All the best.

 

[MasterTheNEC]

The connection to earth doesn't really do anything to reduce electrical noise.

Shhhhhhh....

 

[qcroanoke]

I have had furnace guys want me to run an EGC, whether there is none at all or to add one to a metal raceway that is otherwise acceptable to NEC for EGC purposes. They are having flame sensor or other problem they can't solve when they request it. I can't recall a single one of those instances where I added the EGC where their problem went away afterward either, it is just one item on the checklist and they are hoping it will work.

This is out of the manual for one of the controllers we use:
CONTROLLER GROUNDING
A reliable earth ground must be connected in addition to
any grounding from conduit. Grounding lugs are
supplied for this purpose.

There is no line voltage in the controller. But they require a EGC. Guess what we do.
Also these controllers are not for gas or oil fired units.

 

[asphalt]

Thanks for all the wonderful replies, I've learned a lot. It seems that I was wrong and that an EGC is indeed necessary for the furnace to work correctly. I do want to make one thing clear, however. I am not talking about pulling a circuit without and EGC, I am talking about existing circuits that do not have an EGC like the old cloth romex. I've run into this a few times and opted to bong the gas line and the CWP to provide some sort of ground for the system, bonding on both sides of the gas flex of course. It seems that the only times we run into a furnace without a ground is when the homeowner is financially unable to pay to have a new circuit pulled in, or is unwilling to pay for it. I don't know if my bonding "fix" is appropriate or not, but it seems better than nothing given the situation.

Thanks for all the help.

 

[frank everingham]

harpo

harpo

whether the furnace is new, or a replacement makes no difference. article 250.134 tells you that equipment fastened in place needs an egc. 250.134b tells you that the egc has to be in the cable. .none of the exceptions apply here. the csst gas line is a separate issue.

 

[kwired]

whether the furnace is new, or a replacement makes no difference. article 250.134 tells you that equipment fastened in place needs an egc. 250.134b tells you that the egc has to be in the cable. .none of the exceptions apply here. the csst gas line is a separate issue.

Again most of us don't argue with that. But the OP is not asking if it needs grounded, he is asking why they sometimes do not work properly if they don't have an EGC or even if there is an EGC but isn't low enough impedance.