My house is a 2 story in Brampton On. The upstairs baathroom vent fan is controlled by 2 three-way switches.?

One switch in the bathroom, the other downstairs by the thermostat for the furnace. Is this for convenience? - I have heard that the main floor switch is for the fire dept. to turn on the fan in case of a fire. I would like to replace switch in bathroom with a countdown timer but would have to eliminate second switch.I don't want to do this if it is a safety feature.- no 3-way timers available.