This is a complicated tax issue, having many different aspects to it, based on a particular transaction. The following is a general overview.

Section 1245 property. This type of property includes tangible personal property, such as furniture and equipment, that is subject to depreciation, or intangible personal property, such as a patent or license, that is subject to amortization.

Section 1250 property - depreciable real property (like residential rental buildings), including leaseholds if they are subject to depreciation. 

The most common examples of §1250 property are buildings and ..... deck, shingles, vapor barrier, skylights, trusses, girders, and gutters. ... of the cost of construction of the building and depreciated over the life of the building.

Depreciation is not a freebie – it is a loan • Taxable gain at disposition consists of two parts: increase in value of the asset (appreciation & inflation) and reversal of prior depreciation deductions (recapture) • These two parts are taxed differently, resulting in extra taxes. 

Related information: Publication 544 (2016), Sales and Other Dispositions of Assets - IRS.gov

a $12k repair of a central air including all ducts would be considered an expense or can it still be depreciated

if it is a rental property-- (a) to the extent the repair/replacement of the A/C does not increase the value of the property, it is repair;  (b) the portion of the repair/replacement  that improves the value of the property  and  or extends the life of an asset would generally be considered  improvement and therefore must be depreciated.

For example, in  a rented building, the old roof  is leaking and  needs to be repaired .  Roof had  remaining life of 5 years.  The available roofing material  is with 10 year life.   You chose to upgrade to 30 year life  roofing material  and that  cost you an extra  $3000 --- the whole job of "tear down , haul-away and replace cost your a total of 20,000.   In such a scenario, I would use  $17,000   ( $20,000 less upgrade cost $3000 ) as repair and add $3000  additional  depreciation basis  and $3000 to the  cost basis of the property.

Does that make sense ?