Items that use a 230 volt motor like a well pump draw several times their rated wattage upon startup, which may tax some generators that do not have adequate SURGE watt capacity. By electric furnace, do you refer to a fuel fired type (gas or oil)? The motors in those are not beyond the capacity of a 3500 watt generator...
Want to pass along a tip, for those who have a gas grill, keep an extra tank or two of LP fuel around, if your power goes off for an extended period you can heat a LOT of types of food as well as get reasonably hot water by using the grill to heat things...but keep some old, scuffed up pots and pans to use...the flames will ruin the exteriors of good ones in no time....