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chop existing duct work into two separate systems?

I am wondering if I can/should split the existing ductwork into two separate systems to create dual zones?

I am planning to add a small two-story an addition to my home. TV room, bathroom, and closet downstairs, bedroom and walk-in closets upstairs. Existing house is about 1,600 Sq ft, two story in HOT south Florida. Second floor is I'm guessing five degrees hotter than first.

Central AC is single zone with air handler located roughly in the center of the house downstairs. The ductwork appears to be as follows: An extended supply plenum rises straight up through the second floor all the way up through the ceiling line of second story. A set of flex ducts branch out from the plenum in the first story ceiling/Second story floor and connect to ceiling registers to cool the first floor. A second set of flex ducts eight feet higher up spider out of the plenum (riser?) from near the top of the plenum in the second story ceiling to registers in the second story ceiling. Return air is a single intake near the air handler on the first story.

I am considering capping the plenum after the first story spider ducts and installing a second unit on the roof to tie in the existing second story spider ducts plus a few new ones. And tie in a few new flex ducts into the existing system to feed the downstairs addition. Aside from the logic and comfort of creating downstairs/upstairs zones, I want to do it this way because the existing supply plenum (or riser duct?) to the second floor stands exactly where I need to have a hallway to access the second story addition. So a using a baffle to make the existing single unit serve downstairs/upstairs zones does not solve my problem of the second story riser duct being where it can't be. (I know that simply having a second unit/system to serve the new addition would be tremendously easier to design, and cheaper! Also not a fan of using mini splits because, in part, I need to send air to new bathroom. etc.)

Is it unheard of to chop an existing duct system into two to create two separate systems?

Thanks in advance.

Re: chop existing duct work into two separate systems?

With a large addition, the existing system usually needs an upgrade anyway so the question becomes moot as you're going to have to reconsider the whole thing. Rarely is there enough capacity to avoid over-working the existing system leaving you needing an upgrade PDQ anyway.

I would do dual-zone as balancing temps between floors is a bugger otherwise leaving nobody happy. And I'd call in a HVAC contractor to advise you on the best route to take here and the options you might have. We can give you general advice but being there to see things like floorplans, window layouts, and places where the sun hits hard is best done on-site.


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