A properly installed metal conduit can serve as the equipment ground between the panels.

It's near impossible to pull wires in conduit that already have wires in it.

I suspect there is some other problem with the furnace installation. HVAC guys are notorious idiots when it comes to power connections (or even low voltage thermostats).

Post a photo of the inside of the sub-panel if you would.

I agree that the conduit, if it's continuous and properly bonded on both ends, would serve as the ground.

I'm not going to agree with ronnatalie about the HVAC guys...

What I'm curious about is the possibility of an open neutral. A photo of the inside of the sub-panel would help.

An unbonded or floating neutral would present the symptoms you are seeing.

There is tons of info on-line in discussions about using portable electric generators to provide power to gas furnaces during storm outages. (Many portable generators have unbonded neutrals and igniter problems exist when trying to power the furnace.)

This post was edited by saltidawg on Thu, May 8, 14 at 11:36

Thank you for the initial responses.

I don't think its a neutral problem, as I plugged the furnace into another circuit, which is uses BX wiring, with the little bonding strip inside it, and the furnace did the same thing.

Adding a separate ground from a piece of Romex that I recently ran to the main panel solved the problem.

In terms of the conduit, it is not continuous. It is multiple sections, attached by the couplers that use the single set screw. The conduit is a little dusty and dirty on the outside, so I can only imagine that the ground path is just not very good or has a good bit of resistance.

I'm happy to post pictures, and will be at the house tomorrow.

Assuming that the neutral looks good, how do improve the ground path from the subpanel, through the conduit, and to the main panel? Do I need to loosen and sand the conduit and connectors so there is clean bare metal? Can I run a ground wire back to the main panel and attach it to the outside of the conduit? Are there grounding straps that I can bond between each piece of conduit in order to eliminate the need for the screw type connectors to carry the ground path?

The tech guy at the furnace company assumed it was a bad ground after I described the problem, and adding a different ground solved the problem, so I can only image that the ground path through the conduit simply has too much resistance for the delicate diagnostic system in the furnace.

I'll post pics tomorrow of the inside of the subpanel.

Many thanks!

malgyver:

"I don't think its a neutral problem, as I plugged the furnace into another circuit, which is uses BX wiring, with the little bonding strip inside it, and the furnace did the same thing."

This was another circuit off of the 40 amp sub-panel?

This is just a guess at this point without seeing the inside of the sub-panel... But I think the neutral is open and possibly you have a high resistance ground. I think when you ground the furnace back to the main panel, you are actually providing a neutral path for the furnace. The transformer, controls and such may be able to function because they are low current - but as soon as the blower or anything else kicks on, it would throw errors.

What I'm interested to see is if the neutral and ground buses are bonded in the sub-panel.

malgyver

I'm interested in how this turned out. Let us know if you resolved it. Otherwise, add a pic of the inside of the subpanel, Esp show the neutral and grounding buss.

The way I read it, is that an old 40 amp circuit is in pipe, to a box, not sub panel, and two Romex cables are spliced onto it, to derive two circuits from the old 40 amp 3 wire circuit. The description doesn't sound to me like a sub panel at all. If this is the case, that breaker better not still be a 40 amp, or you have a house fire in the making. Also, if it is as I stated, there's no guarantee that the neutral is connected if the old dryer was straight 240 volt. If you are missing the neutral, you have a 240 volt circuit with a microwave and furnace in series. This will cause you all sorts of trouble.

To avoid a fire or killing yourself, you may want to call a professional ASAP.

Joe