As for depreciation, if they are part of the central HVAC system you have to depreciate them over 27.5 years. If they are stand alone units, more like window AC units (i.e. not a part of the structure of the building) then you can depreciate them over a seven year period.
However Landlords may use the De Minimis Safe Harbor to deduct any cost as substantiated by a invoice as long as that cost does not exceed $2,500 ($5,000 if you have applicable financial statements).
This dollar limit is applied to each item on the invoice, not the invoice in the aggregate. As a result, it is critically important to itemize all invoices.
The limit was originally $500 when released in 2013 but the IRS later retroactively applies an increased $2,500 limit.
There is an anti-abuse rule which prevents landlords from breaking down the cost of a component into each of its individual parts on an invoice. For example, you cannot deduct the cost of a new HVAC if the total cost was $4,000 by breaking up that cost into the HVAC’s individual parts on the invoice.
