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Looking to power my gas furnace blower with an unbonded inverter generator. Furnace has a hard-wired on/off switch (like a light switch) at the furnace. I don't want to power the circuit at the main breaker with a transfer switch for reasons.

I've seen what appears to be a code-compliant fix on youtube where the "light switch" is converted into a switch-and-plug outlet, and the load wiring is converted into a plug

                     ----------
so, from:  line ---- | switch | ---- furnace
                     ----------
              -----------------
to: line ---- | switch / plug | --- physical plug ---- furnace 
              -----------------

so when the power goes out, you must physically unplug the furnace, and then you can directly plug the furnace into the generator's extension cord to power it. I think this provides the correct isolation so you're not back-feeding electricity from the generator.

this is where my question is:

apparently modern furnaces check for ground/neutral bonding and will not start if they are unbonded. i've seen various solutions to bond the generator:

  • re-wiring the internal wiring of the generator (something i don't want to mess with)
  • buying a plug that bonds the two within the plug (won't work for as it will occupy the only other outlet on the generator that i need for another application)

my understanding is that you also can't double-bond ground and neutral, so you can't bind them at the switch/plug because - in normal operation - the circuit is already bonded at the main breaker.

the only solution i can think of is putting another switch next to the furnace switch/plug:

                    -------------------------------
mains line ---------| switch            / plug    |
                    -------------------------------  --- physical plug --- furnace
generator line ---- | input plug (male) / plug    |
                    | (inside, N and G connected) |               
                    -------------------------------

so when the power goes out, you physically unplug the furnace from the main circuit, plug it into the generator circuit, then connect the generator, and inside that switch (and only that switch) the ground and neutral are connected.

is that right?

is there another solution?

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  • Is mounting a transfer switch at the furnace acceptable? Commented Mar 2 at 21:33
  • The switch is often included in the furnace circuit so the furnace can be turned off "remotely" without going to the furnace or the circuit panel, probably in case of fire or gas leak. The generator plug/socket arrangement should be before the switch. FWIW I put a plug/socket setup on my furnace, too, before the switch setup, works just fine. Commented Mar 2 at 22:17
  • @ThreePhaseEel - I suppose so, but the switches are spendy and I imagine saying anything to an electrician related to "generators" is going to bump up the cost as well Commented Mar 2 at 22:23
  • 1
    I have an inverter that doesn't connect its neutral to its ground. It could be balanced output, where connecting the neutral to ground might burn out the inverter. The model number of your generator might be necessary. Commented Mar 2 at 22:28
  • You could connect the air handler through a divider cord (two female plugs to one male). Connect the bonding plug to the unused female plug. Commented Mar 3 at 1:25

1 Answer 1

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Temporary power arrangements for a furnace seem to be fraught with trouble. Cord-and-plug connection seems reasonable, however, there may be some electrical code issues with that scheme. One possible issue is that code requires a listed appliance to be installed as directed by the manufacturer instructions -- and if those instructions describe only a hard-wired installation then cord-and-plug seems not to be allowed.

Another possible frustration is that furnaces are often located in basements and receptacles in basements must be GFCI. So a hard-wired furnace does not require GFCI but a cord-and-plug connected one may.

One way to navigate through the maze is to use a proper transfer switch. That could be a small subpanel with an interlock, or it could be a purpose-built thing. A web search for "furnace transfer switch" will turn up a few good options. One of them fits neatly on a 4-square junction box.

That said, I'm not going to be the one to stand in the way of a well-done cord-and-plug arrangement using a good SO-type cordage and a quality 15/20 amp receptacle.

As far as the neutral bond in the generator: just build Mike Sokol's generator bonding plug and get yourself a multi-tap adapter (outlet strip) so that you can connect it and your several cords to the generator.

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