am in the process of having 30 furnaces changed out in some rental units and am in a bit of a disagreement with the HVAC guys about the disconnects. While we both agree that one is required, they want to remove the existing SSU's and just install a switch. I would like to leave the SSU's intact, but see the point that an accessible switch serves the purpose. So, I figured I would pose this question to you guys and see what your opinions and/or facts are.

Thanks in advance

Please forgive me, but what do you mean by the acronym "SSU"?

Service/safety/switched shut-off unit?

I wonder if that is another name for disconnect?

SSU/SSY....the terminology escapes me at the moment, but it is the switch used as a disconnect that also incorporates a fuse block.

Are these gas/oil units or electric? If gas/oil, then a standard switch will suffice. The existing disconnects can be reused, but I'd spend the extra couple of bucks and replace it with a new one. If it it an electric/heat pump unit, then that depends upon the manufacturer's name plate ratings as to whether a fused disconnect is required or not.

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.

OK, now that Alan explained that one; I have not seen one of those in 20 years. Didn't know that Bussmann still made them!

Alan: Is it a true Edison base fuse, or a Fusestat?