When you move into a new home, usually the AC and the furnace were installed at the same time—and usually they’ll need to be replaced at the same time as well. In other cases, either the AC or furnace will run down before the other (air conditioning systems in general don’t last as long as either gas or electric furnaces), and you’ll arrange to have the older unit replaced.
However, there’s a third possibility. What if one of the two comfort systems needs a replacement, and you decide to replace both, even though the other system is still in good working order?
This isn’t an outlandish idea. Homeowners often make this choice on the advice of professional HVAC installers and enjoy benefits from it. It isn’t automatically the right choice, but it’s something to consider.