Some of the oldest houses we work on
An old house was designed to survive a Texas summer without mechanical cooling: high ceilings to let heat rise away from people, tall windows for cross ventilation, deep porches for shade. All of that is intelligent, and none of it anticipated ductwork. There are no wall cavities meant for supply runs and often no attic space that will take an air handler comfortably.
So systems went in wherever they fit, decades after the fact, and the result is usually a compromise. High ceilings also mean stratification: the hottest air in the room sits above head height while the thermostat five feet off the floor reads something entirely different, so the system stops before the room feels right.
And some of the newest
On the other side of town the housing is recent, two story, tight, and built with the air handler in the attic. Those homes have the modern set of issues: long flex duct runs, upstairs rooms that lag behind, and builder specified equipment reaching the point where repairs start stacking up.
On a new house the equipment is usually the story. On a hundred year old house the equipment is rarely the story.
What we look at first in an older home
Before we talk about replacing anything, we want to understand the air path: where return air is coming from, whether the supply runs are actually delivering, and how much conditioned air is being lost into unconditioned space along the way. In a lot of older homes the equipment is adequate and the delivery is not.
Ductless mini split systems are worth knowing about in this context. They put a small indoor head directly in the room that needs it and connect back to a compact outdoor unit through a small line set, with no duct run at all. For a converted attic room, a sun porch, an addition, or a room in an old house that has never been comfortable, that is often the cleanest answer available.