Air Conditioning - Isn't Central AC zonable?

I inherited a conventional forced air split central air system that had been added by previous owners. Blower is in attic and they did not use long rectangle plenum with takeoffs, they ran flexiduct from all over back to the unit. My outdoor unit is blowing fuses and I already posted regarding that, so I'm having work done now. Since the system never cooled the first floor while second floor gets very cold we will redo the attic plenum.

I thought, as long as we're installing new plenum run, why not do 2 plenums with motorized dampers? I know I had this in a previous home with forced air AC and heat.

My local installer says cannot do 2 zones cause units are sized for whole house and when dampered, coils would freeze from being way oversized.

Is he right? Or can I have upstairs run without downstairs at night?

Thanks
Randy

For a given size unit, you must move a given amount of air. Any significant reduction in the amount of air flowing thru the indoor coil will cause it to freeze. With zoned systems, they maintain that given airflow. If one zone is calling for cooling while the other is not & there is not sufficient air thru the calling zone, air has to be dumped somewhere. In a nutshell it can be done but it is expensive. You would be better off using a ductless for downstairs if there is no place to run ductwork for a second system.