It's easier to run 240v wiring than tearing into walls to run new duct work for forced air furnaces. 

I'm talking new roof, new HVAC and duct work, new electrical panel, new paint, new PVC & PEX piping throughout the house, new doors, some new windows, landscaping and gravel is all fresh, new plastic in the crawl space, new outlets and switches throughout ... new garage door, roof, and paint on the single detached garage which matches the house. 

The same HVAC Tech did a bid on a rehab and told me it would be $10,000 to install a new 3 ton outdoor unit, indoor unit, gas furnace and duct work.

We are switching out electric baseboards for electric wall fan heaters in those units that don't have natural gas and/or do not have ductwork. 

It was about the same price as forced-air heat but saved me space I didn't have for duct work.  

The duct work isn't even connected under the cabin which is another issue.

Proper ductwork (aka no leaks) will increase the life of your compressor as they are rated by how many times they turn on and off.

This may involve adding ductwork, installing a separate HVAC unit, or using portable heating and cooling units.5.

Most all have been converted to forced air and a/c with the duct work in the ceiling .

So duct work can not be put in above or below.