The air handler.

The air handler is the indoor half of a split system. Inside it are four things that matter: the blower that moves the air, the evaporator coil that gets cold, the filter slot, and the drain pan that catches what condenses off the coil.

In a Phoenix house it is usually in one of three places. A hall closet, an attic, or a garage. Where it sits decides a lot about what a repair costs, because a part that takes ten minutes to reach in a closet is a different job in a 150 degree attic in July.

What it is doing

Warm house air is pulled through the return grille, through the filter, across the cold evaporator coil, and pushed back out through the supply vents. Heat and moisture come out of the air at the coil. The heat goes outside through the refrigerant lines. The moisture runs down into the pan and out through the drain.

That last part is why an air handler in an attic is worth paying attention to. There is drywall under it.

How it fails

The blower capacitor goes. The motor hums and will not start, or starts and runs hot. It is a cheap part in an awkward place.

The blower wheel loads up with dust. It still spins, it just moves much less air. This is one of the two things that freezes a coil.

The drain plugs. Algae and dust close the line, the pan fills, and a float switch shuts the system off before the water finds your ceiling.

The coil gets dirty. A coil packed with dust cannot absorb heat properly, and it is a far bigger job to clean than the outdoor one.

In a package unit there is no air handler

Plenty of older Phoenix houses, especially flat-roof ranch homes, run a package unit instead: everything in one cabinet on the roof, with ducts dropping into the house. Same functions, same failures, one box.