Put the box fan in a window facing out.

Putting a box fan in a window blowing outward is the single most effective placement to improve your AC’s efficiency.

A window AC or central system works best when it’s not fighting against hot, stale air. By placing a box fan in a window on the hottest side of the house (or just the side with the least airflow) and having it blow outward, you create negative pressure. This sucks cooler air from the rest of the house towards the AC and pushes the hot air out. The AC doesn’t have to work as hard because it’s pulling in less heat.

Another decent option is pointing the box fan toward the AC’s return vent (if you have central air). That helps circulate the cool air more evenly, but it does less for overall efficiency than exhausting hot air. The window exhaust trick is the move.

Don’t point the fan into the room from a window unless you’re trying to cool a single space without AC — that’s a different game. For AC efficiency, exhaust is your friend.