The fact that your furnace is on a 15 amp breaker, does not mean that it consumes 15 amps, indeed the NEC requires that circuits supplying continous loads are not loaded to more than 80% capacity, therefore if instaled to code the furnace should draw 12 amps or less, probably a lot less.

The actual draw may be determined from the manufactuerers, or if cord and plug connected, by measurement.