SSU is a switch and Edison base fuse holder on a handy box cover, manufactured by Bussman.
A switch only would work for the furnace.
The advantage of the SSU is that a T rated, time delay fuse with a setting close to the rating of the motor can be used to aford better protection than the branch circuit over current device.
I used one on my water well, going from a 20 amp circuit breaker to a time delay 8 amp fuse at the pump. It has been in service this way for over 25 years. The fuses cost about $ 3.oo each now but that is still a lot cheaper than a new pump motor.
