Ground Connection For Furnace And Generator

[DerbyDad03]

Oh boy, another ground question. These discussions are always lively. :-)

I lost power from the wind storm that hit the Great Lakes area on Wednesday. Last night I borrowed a generator from a friend to get the furnace going. I simply unhooked the wires from the breaker to the switch for the furnace and used an extension cord (with the receptacle end cut off) and wire nutted the hot to hot, neutral to neutral, and ground to ground from the generator to the wire that goes from the switch to the furnace. Everything worked fine, other than the fact that the power came back on less than 3 hours later. It figures.

Since then, I've done some reading and found these 2 "facts":

1 - I read a generator related post in an HVAC forum that says that the control board in many HE furnaces require a really good ground in order to operate properly.

2 - The manual for the generator says:

"Grounding

The generator system ground connects the frame to the ground terminals on the power panel.

- The generator (stator winding) is isolated from the frame and from the AC receptacle ground pin.

- Electrical devices that require a grounded receptacle pin connection will not function if the ground pin is not functional. "

The manual also says that the ground terminal on the power panel can be remotely grounded by using a ground rod.

So, this indicates to me that I was running my furnace without a ground, which sort of (sort of) goes against claim #1 that the control boards need a good ground to operate properly. Of course, maybe my furnace *seemed* to have been operating properly with the generator but really wasn't, or maybe my control board doesn't even need a ground.

Comments?

[makolber]

Excellent question...

the usual reason the controllers need a ground is for flame sensor. This u sually applies to GAS fired devices. The electronics senses the flame as it touches metal parts that are grounded. If the electronics does not have a good ground, this may not work reliably.

The solution would be to use a three prong plug with a ground to connect t he furnace to the generator. Connect the white and black wires as you did. Connect the green wire to the ground in the outlet box where you tapped i n which should also connect to the frame of the furnace.

If the flame sensor does not work correctly, then add a jumper wire to conn ect the white and green together.

My generator actually already has the white and green tied together at the outlet box.

If your furnace worked OK as it was, then I wouldn't worry about it. THe fl ame sensors need only a high impedance ground to work correctly. If it ain 't broke, don't fix it.

Mark

[DerbyDad03]

usually applies to GAS fired devices. The electronics senses the flame a s it touches metal parts that are grounded. If the electronics does not ha ve a good ground, this may not work reliably.

the furnace to the generator. Connect the white and black wires as you di d. Connect the green wire to the ground in the outlet box where you tapped in which should also connect to the frame of the furnace.

I didn't say anything about an "outlet box". I said I connected the generat or wires at the *switch* for the furnace.

What am I missing here...

I did use a 3 prong plug. As I said, I connected hot to hot, neutral to neu tral and ground to ground from the generator to the wire that goes from the switch to the fur nace. Even if the ground wire in the switch box goes to the frame of the furnace, if the gene rator ground pin is not really grounded, then the furnace isn't grounded.

I don't see how your suggestion is any different than what I actually did. Am I missing something or did you?

nnect the white and green together.

e outlet box.

flame sensors need only a high impedance ground to work correctly. If it a in't broke, don't fix it.

[gfretwell]

The ground is for surge/transient protection. If you did not remove the grounding connection in the switch box, you are still grounded.

A quasi legal way to do this is to put in "inlet" in a box next to the switch (or use a 4x4 box) then replace the switch with a 3 way. The black (common) goes to the furnace and the other two switch from line power to the inlet. Grounding and neutral are not affected and since the neutral is not bonded in the generator, it is not a separately derived system so this is legal. That meets the intent of the "transfer equipment" rules even if it is not part of the manufacturer's listing and labeling (110.3 (B) violation)

[gfretwell]

A generator is not "grounded" anyway unless you drive a rod or connect it to another grounding electrode. In the case of a portable generator, using cords, you are better not having it grounded since you do not have a fault path to ground. I suppose you could do this

[DerbyDad03]

I did in fact remove the ground screw from the switch and attached the generator only to the ground wire going to the furnace. Therefore I was not grounded.

That leads to the next question, just as learning experience...

Let's say I don't want the add the "inlet" mentioned below (more on that later) and just use the extension cord like I did last night. If I leave the ground from the furnace to the panel connected in that switch box, should I also attach the ground wire from the generator to those ground wires? Wouldn't that ground the generator frame to the panel ground as well as provide a ground for the other receptacles on the generator?

I watched a video in which a "15 year HVAC tech" added a 3 prong appliance cord to the furnace and installed a dedicated receptacle after the furnace switch. The purpose was so that the furnace could just be unplugged from the receptacle and plugged into a standard extension cord from the generator. Based on what you just said, that not only doesn't sound legal, but it also ungrounds the furnace unless the generator is grounded in some other manner, correct?

[DerbyDad03]

und, which sort

od ground to

oard doesn't

his usually applies to GAS fired devices. The electronics senses the flam e as it touches metal parts that are grounded. If the electronics does not have a good ground, this may not work reliably.

ect the furnace to the generator. Connect the white and black wires as you did. Connect the green wire to the ground in the outlet box where you tap ped in which should also connect to the frame of the furnace.

rator wires at the *switch*

neutral and ground to

furnace. Even if the

enerator ground pin

d. Am I missing something

connect the white and green together.

the outlet box.

He flame sensors need only a high impedance ground to work correctly. If i t ain't broke, don't fix it.

A bucket of coffee grounds would have been even funnier. ;-)

[Al Dente]

While technically not to code, our local electrical inspector would look the other way if the receptacle was mounted in a grounded metal electrical box on the furnace cabinet. The electrical box had to be mounted using 10-32 machine screws, star washers (to ensure a good ground connection) and lock nuts.

[gfretwell]

Short answer is there is never a reason to remove the ground from the furnace, leave it connected to the house wiring. Just switch over the power and if you use the DPST (3 way) solution you are really switching the hot and leaving the neutral connected to both. If you did go with a cord and plug solution, leave the ground connected and connect the generator ground to the box. In reality, the box probably stayed grounded anyway, even if you moved the wire from the switch. It is usually connected to the back of the box first and then that continued on to the furnace. Back when I had a furnace, the box was actually mounted on the furnace and that would be a connection.

[Tekkie®]

[e-mail] posted for all of us...

Yeah that's the idea!

[gfretwell]

There should be a grounding conductor in the cable going to the furnace.

[makolber]

nd, which sort

is usually applies to GAS fired devices. The electronics senses the flame as it touches metal parts that are grounded. If the electronics does not have a good ground, this may not work reliably.

ct the furnace to the generator. Connect the white and black wires as you did. Connect the green wire to the ground in the outlet box where you tapp ed in which should also connect to the frame of the furnace.

ator wires at the *switch*

eutral and ground to

urnace. Even if the

nerator ground pin

. Am I missing something

well if everything worked, my suggestion was to not fix it so yes, it's not any different that what you did.

[DerbyDad03]

That all makes sense but it eliminates the "plug and play" aspect of the solution, making the "15 year HVAC tech's" method nonviable.

Here it is. You don't have to watch the whole thing...the plug and receptacle can be seen at 0:24

Once he unplugs the cord from the receptacle and plugs it into the extension cord from the generator (which is electrically the same thing that I did at the switch) the furnace is no longer grounded, unless the generator was grounded in some other manner.

Assuming the cord and plug solution is legal in the first place, in order to maintain the ground, he would need to run a ground wire from the receptacle box to the furnace chassis or mount the receptacle box *on* the chassis, correct? That way even though the ground wire from the generator isn't doing anything, the furnace would still be grounded.

If the receptacle was mounted on a "stud" like in the video, would an inspector take issue with a ground wire running from the receptacle box (or from the panel) to the furnace chassis as a permanent installation?

That's a good point. I just put a meter on my switch box, which is mounted on the return duct work. Zero ohms to the furnace chassis and zero ohms to the ground pin of a nearby receptacle which is on a different circuit. However, it is grounded only when the switch is attached to the box because the ground wires go directly to the switch, not to the box first.

[clare]

If the generator has the neutral bonded to the ground wire (most portable generators used for construction etc do) you do not need a greound rod - the furnace sees the "virtual ground) and functions properly. If you have a generator designed to be connected to the house wiring as a backup generator they are generally NOT bonded and require a ground rod connection.

[clare]

Just remember MOST portable generators ARE bonded - might be a good idea to cut the bonding wire and put a switch in the line to allow "lifting" the ground.

[clare]

Converting a non-bonded generator to a bonded generator is simple - just take a plug and install a wire between the white (neutral) and ground. Plug it into one outlet when a "bonded" genset is required (generally required by ohsa on construction sites)

[clare]

By code if you do that you need to "lift" the ground on the generator (disconnect the genset ground from neutral)

[clare]

Better yet, just smear a handful of mud on the generator frame and you have a "portable wireless ground"

[hubops]

Leave the ground wires attached - as found -

- but add < connect > the generator cord's ground wire. Don't over complicate this. John T.

[gfretwell]

For what we are talking about, bonding the generator really has little effect on anything if the generator does not have a ground electrode system.