Furnace Ground

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asphalt

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Ok, I need a little help or explanation because I may have this wrong. I am an electrician for an HVAC company and the installers and techs keep telling me that our furnaces NEED a ground to work and most of the problems they've seen over the years is due to a bad or no ground at the furnace. I've tried to repeatedly explain that under normal circumstances the ground (EGC) does not carry current and any equipment should run just fine without. Am I wrong or do I need to explain it better?
 
solid state might need it

solid state might need it

The only thing I can see is some solid state pbc or systems might need the ground for shield drains.
 
asphalt said:
Ok, I need a little help or explanation because I may have this wrong. I am an electrician for an HVAC company and the installers and techs keep telling me that our furnaces NEED a ground to work and most of the problems they've seen over the years is due to a bad or no ground at the furnace. I've tried to repeatedly explain that under normal circumstances the ground (EGC) does not carry current and any equipment should run just fine without. Am I wrong or do I need to explain it better?
Click to expand...

IMHO... The Equipment should run just fine without it.... However... Not having a EGC, is not code compliant or safe. You might need the EGC for Shield drains in the control circuits if they need it, but I think that Safety would be the number 1 issue.
 
There is a very slight chance that the flame sensor will not work properly if the nozzle and the surrounding metal are not grounded.
Perhaps similar to the need for grounded metal close to some linear fluorescents for reliable starting.

Tapatalk!
 
HVACR Schooling

HVACR Schooling

asphalt said:
Ok, I need a little help or explanation because I may have this wrong. I am an electrician for an HVAC company and the installers and techs keep telling me that our furnaces NEED a ground to work and most of the problems they've seen over the years is due to a bad or no ground at the furnace. I've tried to repeatedly explain that under normal circumstances the ground (EGC) does not carry current and any equipment should run just fine without. Am I wrong or do I need to explain it better?
Click to expand...

When you attend Factory classes for modern furnace controls, they will invariably tell you that the solid state controls will not work correctly without a ground.
Experience says this is often the case. The other thing is, the Engineering has in fact gotten so devolved that many of the modern SS controls will go down and lock out at the slightest hint of dirty power or power flux on the line....and as such require hard re-set of power to function again.
 
iwire said:
Can someone draw the circuit / current flow that requires an EGC from the panelboard be present at the furnace?
Click to expand...

I'll describe the operation:
1) The gas pipe or orifice form one end of the "one wire" detector.
2) A rod is positioned where the flame ought to be, it is connected to the "one wire".
3) As I understand it a "high" voltage AC is applied the "one wire" and the current is monitored.
4) If the flame is present, a DC current flows from the "one wire" to the detector.
5) If the flame is absent, no DC current.
6) If the "one wire" is shorted AC current is present.
7) Only DC current is the flame present condition.
8) This signal is very small.
9) History: Flame rectification hearkens back to very early radio.

If the gas pipe is not well grounded, noise is picked up by the detector and things are flaky.
This has been my experience. I learned this at the knee of a very good HVAC guy and I have fixed these problems typically when the furnaces over flamed and melted the control unit or 3rd-party contractors didn't bother to connect good grounds.
Since it has to do with a wire it is often considered the electrician's fault.

The plan is not to leave the gas valve open when there is no flame -- sometimes known as a air-fuel bomb.
 
iwire said:
How would that be possible?

But regardless none of what you explained requires a EGC from the source panelboard.
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1 -- Gas lines have insulating joints between the outside piping network and the interior piping. Some exterior gas pipe is plastic.

2 -- NEC requires us to connect all grounding together to reduce differing voltages on different grounds. You are not allowed to just ground something to a driven rod without connecting the rod or thing grounded by it to the main grounding point.

3 -- The controls reference the EGC for their signal ground. The other end of the "one wire" circuit (the pipe) needs to be reliably grounded. Pull the EGC and the controls are flaky.

4 -- Why do I feel like a doctoral candidate -- needing to defend every "thesis" I propose -- even the simplest?

Electronic flame sensor can be really touchy about good grounding.
Click to expand...
 
iwire said:
How would that be possible?

But regardless none of what you explained requires a EGC from the source panelboard which seems to be the OPs question.
Click to expand...

Well, any EGC would have to come from the source panelboard (not the service panelboard, just the source of the furnace branch circuit.)
And we know that for reduction of AC noise as well as providing a fault return path a metal EGC is a lot better than a local ground rod.

If the gas piping is not CSST and is not likely to be come energized, there is no requirement to bond it past dielectric unions or other breaks in the ground continuity. That leaves the grounding of the furnace sheet metal even shakier in the absence of an EGC.

What is arguable is the HVAC tech's statement that a ground is needed when what is really needed is a bond. :)
 
fmtjfw said:
1 -- Gas lines have insulating joints between the outside piping network and the interior piping. Some exterior gas pipe is plastic.
Click to expand...

Those issue certainly possible but a truly isolated metal gas line would be unusual.


2 -- NEC requires us to connect all grounding together to reduce differing voltages on different grounds. You are not allowed to just ground something to a driven rod without connecting the rod or thing grounded by it to the main grounding point.
Click to expand...

And that has what to do with the EGC and the flame sensor?

3 -- The controls reference the EGC for their signal ground. The other end of the "one wire" circuit (the pipe) needs to be reliably grounded. Pull the EGC and the controls are flaky.
Click to expand...

So you picture a metal gas line or orifice inside the furnace that has no connection to the metal of the unit and therefore the controls of the same unit?

4 -- Why do I feel like a doctoral candidate -- needing to defend every "thesis" I propose -- even the simplest?
Click to expand...

Why do you feel like a doctoral candidate? :D

I don't know but I am not going to feel the least bit bad about asking for the circuit / current path that requires an EGC to operate this sensor.
 
iwire said:
Oh 'noise' of course the vague catch all answer. :D
Click to expand...

Yes, and sometimes it actually does catch something. :)

I can define the circuit path for current flowing in the grounded metal reflector of a linear fluorescent, but it is not an all metallic path and is not easy to diagram using conventional circuit symbols.
If I knew more about the design and operation of the typical flame sensor I could probably do the same there. But I am content (lazy) enough to accept the "noise" explanation for now.
 
asphalt said:
Ok, I need a little help or explanation because I may have this wrong. I am an electrician for an HVAC company and the installers and techs keep telling me that our furnaces NEED a ground to work and most of the problems they've seen over the years is due to a bad or no ground at the furnace. I've tried to repeatedly explain that under normal circumstances the ground (EGC) does not carry current and any equipment should run just fine without. Am I wrong or do I need to explain it better?
Click to expand...

@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....;)
 
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. :)
 
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