I have a room at the end of the house furthest from the HVAC air handler that just does not get enough ac airflow. How can I fix this? I see duct fans at Lowe's and HD, but they seem to all be a fixed speed. I am afraid that will push too much air without having an adjustment. Ducts in attic are not kinked or anything. I think it's just the distance. Any suggestions would be greatly appreciated.

We had this issue with our master bedroom. Farthest from the air handler/manifold, south and west facing roof that gets baked in the afternoon sun. Always the hottest room in the house during summer.

I just had a second supply register and return register installed, and it changed the whole game.

First I would be really sure there is no dampener in that branch of the duct. Sometimes they swing shut and you forget they're there because of being wrapped in insulation.

Second, I've had good experience with this from amazon

Variable speed duct booster

pressure switch

You just install it with a pressure switch to turn it off/on when it notices duct pressure

Thanks for the replies. That adjustable and pressure sensor are exactly what I need. I am going to see the the furnace has an aux output. If not, I'll put that pressure sensor in.

Well, just found the schematic for the furnace and no aux output… Looks like I will need both.

Don't install the switch near the booster because the booster could pull enough negative pressure to keep the switch activated once the ac turns off. I've got mine about 15 ft apart

Thanks for the tip!