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Wiring a Furnace for Emergency Generator

Posted by bshanley (My Page) on
Wed, Mar 9, 11 at 10:31

My main concern is keeping my furnace running during a power outage. I have a new generator, but have not set anything up. I prefer not to open the main panel and install a transfer switch, so how bad is this idea? The furnace is wired directly from the panel on its own line and a 20 amp circuit breaker. What if I were to install a junction box and power outlet along the line, and just plug the furnace into the box? Then in a power failure, I just unplug the furnace from the house wiring and run it off the generator, so no backfeeding? Thanks for any advice/caution.


Follow-Up Postings:

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RE: Wiring a Furnace for Emergency Generator

What furnace do you have and how " clean " is your generator ?


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RE: Wiring a Furnace for Emergency Generator

The furnace is a Trane XE 90, about 12 years old. Aren't all portable generators "dirty?" A transfer switch wouldn't clean it up, would it? Thanks for your interest in my post and for your reply.


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RE: Wiring a Furnace for Emergency Generator

There are small devices made just for that application, see the link. Some furnaces are more tolerant than others regarding voltage, frequency etc.


A properly engineered, high quality portable set can produce "cleaner" power than what you get from most power companies. Conversely, a thrown together set, made with the cheapest parts available will often produce something barely resembling a sine wave.


Here is a link that might be useful: 20amp 1 circuit transfer device


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RE: Wiring a Furnace for Emergency Generator

"Then in a power failure, I just unplug the furnace from the house wiring and run it off the generator, so no backfeeding? Thanks for any advice/caution."

I know it seems tempting, but don't do it. There's no way you can reconcile this approach with code. For one thing, you can't legally stick a plug on the the end of the permanent circuit wiring which is most likely either metallic sheathed (AC or MC) or NM-B. For another, the furnace installation specs almost certainly call for hard-wiring on a dedicated circuit.

Wayne's solution is by far the best way to go, IMO. Aside from the fact that you'll probably need to replace the circuit wire with longer length(s) (since the switch and inlet will mostly likely be located on the outside of the house), wiring the x-fer switch is not much more difficult than installing a junction box and receptacle.

As far as the "dirty power" concern, there's another factor to consider, in addition to generator quality. As often as not, so-called "dirty-power" problems happen when a generator is pushed to (or even a bit beyond) its rated running capacity. This often occurs briefly when motors start up. Because both the voltage and frequency (hertz) are tied to motor RPM in most generators (generators with inverters excepted), when the generator engine speed bogs down, power quality suffers.

Anyway, to avoid dirty power problems, don't skimp too much on generator capacity.

One final note: Before finalizing plans for reworking the circuit to include a transfer switch, make sure to factor in how the thermostat(s) get power. Every once in a while, you hear of someone installing a transfer switch without realizing that the transformer for the low-voltage thermostat gets it power upstream of the transfer switch. It's not all that likely you'd encounter this problem, but it is possible and, thus, should be part of your "checklist".


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RE: Wiring a Furnace for Emergency Generator

I knew that smarter heads would prevail. Thank you. Will do.


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