My electric fireplace heater requires a fast blow fuse,,i can only find a slow blow fuse..can this be used? an?

well as to weather or not it will work yes it will work. However it does pose more of a fire hazard. Slow blow fuses do not pop as fast as a fast acting some situations like car amps call for a slow blow but a electric heater if it calls for a fast acting should use a fast acting. If the fuse is poping often it should be looked at by a tech for possable issues with the heating elements or controll board when electronic devices are working correctly it will be very rare for it to pop a fuse.