Matched systems – those designed to work together – are the most efficient and deliver the best climate control.
If the units haven’t been replaced together in the past, they’ll be different ages. And if, for example, the AC is much newer than the furnace, it might be fine to leave it as long as it would be compatible with the new furnace. When both the furnace and AC are 12-15 years old or older, then we recommend replacement of both units.