I do not know how the below is dealt with on *commercial* rental real estate, as I've not researched it. But I would expect it to be treated exactly the same as is required for *residential* rental real estate.
There is a different kind of A/C system referred to as "ductless". With this type of system a single compressor unit is located outside of the structure. Then individual cooling units are located inside one or more rooms of the house. You only have about a 2" hold in the wall or ceiling for the refrigerant lines to pass through to the compressor located outside. Hence, no air ducts.
These types of units are generally more economical because each inside unit has it's own thermostat and only operates when activated by the thermostat. These ductless setups are treated exactly the same as any other central A/C setup. The ductless units are not portable and do "in fact" become a physical part of the structure. So they classified as residential rental real estate and depreciated over 27.5 years. Take note that this does *not* qualify for the Special Depreciation Allowance either.
As for the SEC179 deduction, residential rental real estate and it's associated assets do not qualify for that at all.
