Does an furnace electronics require a ground?

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tom baker

tom baker

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One of my students installs furnaces. The newer types with electronic circuit boards require a "ground", however, I suspect what is required is an equipment grounding (bonding) conductor in the supply circuit. Apparently the furnace will lock out without a "ground", as is the case when a new furnace is installed on a 2 wire circuit in an existing house.
So, does an furnace electronics require a ground?,or is it just a misuse of the term ground and the furnace really requires an equipment gronding (bonding) conductor?
If thats the case, the correct solution would be a new branch circuit that contains an EGC to allow an installaion per NEC section 250.104
 
Re: Does an furnace electronics require a ground?

I can't even remotely imagine a "ground" being "required" by any circuitry. I'd guess it's an interlock.
 
Re: Does an furnace electronics require a ground?

Can you even use an EGC to run a circuit? I'm wondering if you can even measure the EGC for an interlock?
 
Re: Does an furnace electronics require a ground?

I'd bet the circuit is checking for a "ground". If its not there it won't run.
The equipment has to have an equipment grounding conductor, thats how it was tested and listed...
If I knew the type of furnace I could check its listing with the on line product directory at UL
 
Re: Does an furnace electronics require a ground?

Tom, by "ground" (and not the EGC), I think you mean a neutral, which would only be required if there is a neutral connection in the wiring compartment.

All metal-encased electrical apperatus requires an EGC. It is possible that the system won't work without it; many ground one side of the 24v x-former.
 
Re: Does an furnace electronics require a ground?

I might be able to offer a little insight on this. I did HVAC work for a while. The "ground" that is required is the grounding path between the main burner and the board (sometimes called the ICM). Most modern furnaces require this path between the burner, through the chassis, and back to the board so that the flame rectification (flame sensing) will work. The systems use a probe in the flame with one wire attached to the probe for one lead, and the ground path to complete the circuit. There are warnings all over the literature for these units about maintaining the ground path, but they really mean chassis bond among the necessary components. I assure you that the furnace will function just fine without an equipment grounding conductor in the branch circuit.
 
Re: Does an furnace electronics require a ground?

All metal-encased electrical apperatus requires an EGC. It is possible that the system won't work without it; many ground one side of the 24v x-former.
Click to expand...
I'm still gonna say that's not allowed.

One side of a power supply or the center of a bipolar power supply are almost always chassis grounded.

But I don't believe it's permissable to use the EGC to bring anything to a piece of equipment other than a connection to the MBJ and the electrode grounding system.

Certainly can't be used as neutral.
 
Re: Does an furnace electronics require a ground?

This brings back a frustrating incident. My nephew installed a new gas hot air furnace in my home, and of course he wanted temporary power to set it up and test it. I had an old 1828 cape cod house with knob and tube wiring. I tapped into the old wiring temporarily and the electronic unit didn't work, changed the electronic board and still no action. We called the company and they told us to make sure we had the grounded conductor connected to the correct terminal. I reversed the wires and lo and behold, IT WORKED.
 
Re: Does an furnace electronics require a ground?

This is interesting. I usually just provide a box for the HVAC guys, of course there's always a ground in it.

Obviously the equipment wouldn't know the difference if the ground and neutral terminals were jumped.

I've never heard of an issue like this.
 
Re: Does an furnace electronics require a ground?

A 24 volt control circuit is allowed to be grounded using the equipment grounding conductor of the supply circuit as the grounding electrode conductor.

Also, the class 2 control transformers in a lot of the furnaces are connected to an automotive blade type fuse on the circuit board hence the other side of the circuit has to be grounded. Otherwise, 2 fuses would be required which would be "more expensive".
 
Re: Does an furnace electronics require a ground?

I don't know what this supply is supposed to be for but it's very typical.
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Not trying to say anything about the HVAC anomaly.

Usually the negative side of the DC supply would be tied to the enclosure. Without the DC one side of the transformer secondary might be tied to the enclosure.

Edit: Along with a seperate EGC connection to the enclosure.
Clearly this supply doesn't give a hoot about an EGC.

[ May 31, 2005, 01:39 AM: Message edited by: physis ]
 
Re: Does an furnace electronics require a ground?

Originally posted by john m. caloggero:
We called the company and they told us to make sure we had the grounded conductor connected to the correct terminal. I reversed the wires and lo and behold, IT WORKED.
Click to expand...
Okay, now that we're talking about a 120v feed and a neutral, I can add some insight: We wired a kitched remodel, one of my helpers swapped the hot and neutral, and the electronic ignition wouldn't work. The fix was the same.
 
Re: Does an furnace electronics require a ground?

By Sam: Clearly this supply doesn't give a hoot about an EGC.
Click to expand...
A supply through a transformer should not effect the operation of equipment but when electronic switching power supply's get involved then it can be a different matter as they are not isolated like a transformer. Most ignition system are electronic and use switching supply's to supply the HV to the spark igniter.
Just a thought.
 
Re: Does an furnace electronics require a ground?

I'm not trying to explain anything, I still don't get it.

The schematic I posted was meant as a reference for what most things look like. Not something to explain igniters. I haven't been able to find anything related to HVAC stuff.

I'm actually looking forward to seeing something that would explain the need for an EGC for the thing to operate.

Using a switching supply doesn't change anything though, your computer uses one and dosen't need an EGC.
 
Re: Does an furnace electronics require a ground?

Yep sam I don't know what I was thinking when I said that about switching power supply's as a igniter is not a switching power supply? :eek:

All I can figure is some how if the Egc is connected to the frame of the furnace and the neutral and hot is switched may be it causes the flame detection circuit to short out and not allow the gas valve to turn on? :confused:
 
Re: Does an furnace electronics require a ground?

Maybe Tom 'll get us some model info and we can track down a schematic.
 
Re: Does an furnace electronics require a ground?

I'm still thinking, as Tom's own posts mentioned, that he's asking about hot and neutral polatity.

Tom?
 
Re: Does an furnace electronics require a ground?

By Tom Baker:

So, does an furnace electronics require a ground?,
Click to expand...
I'm sorry Larry, I just reread everything Tom posted and the quote above is the consistant question.

I didn't see anything about polarity.
 
Re: Does an furnace electronics require a ground?

I don't mean to be argumentative; it's just that it's important to understand a question in order to answer it accurately.
Originally posted by tom baker:
The newer types with electronic circuit boards require a "ground", however, I suspect what is required is an equipment grounding (bonding) conductor in the supply circuit.
So, does a furnace electronics require a ground?,or is it just a misuse of the term ground and the furnace really requires an equipment gronding (bonding) conductor?
Click to expand...
Notice in both sentences, he uses the word "ground" to differentiate from EGC. If I'm wrong, that's what is throwing me.

This is why I mentioned the electronic ignition that wouldn't work unless the proper wire is grounded (neutral). A helper mis-wired the range receptacle.

So, Tom? :confused:
 
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