I have been working on a property recently and we were getting ready for our framing inspection for new ductwork and even though this is the only “change” we were doing that wasn’t cosmetic the inspector came in and told us that a lot of the work that was previously done in the house (almost 12 years ago) was not permitted and he plans on making me responsible for it.

@Peter Morgan there are dampers in the duct work that can be adjusted to “throttle” the incoming air.

We are getting new ductwork/heaters for all 3 units so now I'm wondering...should we take that one step further and do central a/c too?? 

After walking the property with my HVAC contractor he recommended a mini split may be the way to go instead of adding duct work for an HVAC.

-Create another kitchen-Create another laundry room-Create a firewall in attic and main level -Add 2 hvac units with duct work-Add an additional water heaterOn top of all that we had to remodel everything and fix all its issues to make it an attractive rental.

I have never seen a cooling coil installed in ductwork like the original installation, its a pretty sketchy original installation. 

The home was built in 1930, and, like many homes built during this time, has ductwork in the basement that has the old asbestos wrap on some of the 6 inch ducts as well as some of the boots.  

Is there HVAC service - with ductwork installed? 

You will have to give the tenants the EPA required lead based paint paperwork anyway because its better that testing the whole house.I use mini-splits for HVAC in old houses--the ductwork will not fit through the crawl spaces and attics with all the room additions, so its much cheaper to put in the mini-splits and the tenants love having the individual room controls.For what you are proposing to do, you might as well just buy a vacant lot. 

Do not expect to be able to put HVAC duct work in the house if it has a series of add ons--the attics and crawl spaces will have areas that are blocked.Asbestos and lead based paint will be throughout the house.