Depending upon the type of furnace it is certainly possible.

Gas fired furnaces often run on 120V circuits.  These units normally have a lower electrical draw because they typically only need to power the igniter, a few solenoids, and the fans as the actual heat is provided by burning natural gas or propane.  Most of these units will be “hard wired” into the home’s wiring and will require the use of a transfer or interlock switch.  Some installations may feature a plug whip the is used with a standard duplex outlet.  If you are lucky and have this setup, you may be able to run an extension cord from the furnace directly to the generator.

Heat pumps or other electrically based furnaces will normally require much more power from a portable generator.  Most of these units will need 240V, so a larger generator will be required.  The best starting point for these types of furnaces will be to check with the furnace manufacturer for the required power in watts.  This information can also often be found on the service tags on the equipment directly.  As these units will typically require 240V, it is best to plan for installation of a transfer or interlock switch by a qualified electrician.  The electrician will also be able to help you determine the best size generator for your equipment.

For emergencies, generators like our MM4350 and MM9350 series machines can be used with appropriately sized space heaters to keep you warm, even if you do not have the proper setup for use with your furnace!