Return air.

Air conditioning is a loop. Air leaves through the supply registers, moves through the house, and has to get back to the air handler to be cooled again. That return path is the return.

It gets a fraction of the attention supply gets, and it causes a large share of the complaints.

Why it limits everything

A blower can only push what it can pull. An undersized return means the blower is fighting a restriction on the inlet side, which reduces total airflow no matter how generous the supply ducting is.

Less airflow across the evaporator coil means less heat moved, longer run times, and a colder coil, which is one of the two causes of a coil freezing.

Many older Valley houses have one central return sized for the original equipment, and have since had a larger system fitted without the return being touched.

The closed door problem

This one is worth knowing because it is free to fix.

A bedroom with a supply register and no return needs a path for air to get back out. Usually that is the gap under the door. Close the door, fit thick carpet, and that path mostly disappears.

The room pressurises, airflow into it drops, and it gets warm while the rest of the house gets slightly cold. People reasonably conclude the duct to that room is bad, and the one room guide covers how to tell.

Fixes: undercut the door, fit a transfer grille, or add a return to the room.

What to check yourself

Nothing in front of the return grille. Sofas, bookcases and curtains against a return do more damage than people expect.

Filter clean and correctly sized, since the filter usually lives at the return.

The grille itself not painted shut, which happens.

Doors, as above.

What a measurement shows

Static pressure on the return side separates “the system is tired” from “the return is too small”. That matters because the first is a repair and the second is a duct modification, which is off the list here and goes to a licensed Arizona contractor.