Replace both when both are old (within a few years of each other and past about 12), when the new outdoor unit needs a blower or coil the old furnace can’t provide, or when doing both saves enough on shared labor and gives you a matched, fully warranted system; replace only the failed half when the other is young and compatible. In North Carolina there’s a third option that’s often the best one: if the AC failed and you have gas, put in a heat pump instead of an AC and keep the furnace as dual-fuel backup — and if you don’t have gas, a heat pump with an air handler replaces both at once.
Reasons to do both
Age parity: an AC and furnace installed together are usually failing together; replacing one buys a year or two before the other. Compatibility: modern outdoor units, especially variable-speed and new-refrigerant systems, need a matched indoor coil and often a blower the old furnace can’t provide; a mismatched system loses efficiency and can void the warranty. Cost: one crew, one permit, one visit; savings of roughly $1,000–$2,500 versus separate jobs. Warranty: a full matched system carries one clean warranty.
Reasons to do one
The other unit is under about 8–10 years and healthy; the furnace blower and a new matched coil can support the new outdoor unit; budget. Replacing just the AC on a young furnace is routine; replacing just the furnace on a young AC is too.
The North Carolina angle
A failed AC on a gas home is the classic moment to go dual-fuel: a heat pump replaces the AC, the furnace stays as backup, and you get heat pump economics for most of the winter plus rebates on the heat pump. A failed AC or furnace on an all-electric home is the moment to replace both with a heat pump and air handler. Heat pump vs furnace in NC and dual-fuel explained.
Costs
AC only: AC replacement cost. Furnace only: furnace replacement cost. Both, or heat pump: heat pump cost. Ask each contractor to quote the option you asked for and the alternative; the comparison is where the value shows. Every contractor on the North Carolina registry shows the license status we found.