If the AC unit were a removable window unit, then you could use
5 years, since the window unit could be considered an appliance.
But if the "AC/furnace" is part of the structure and
is what is called "central heat and air", then the unit is part of
the structure itself, and must be depreciated over 27.5 years.
Please see Table
2-1 in IRS Publication
527.
