Got a friend that has a room with very low airflow.
Assuming things have been balanced....
What are peoples experience with installing duct booster fans? Are they effective? If so what brand have you used? Looking at a 4 5 inch diameter.
It could be a broken elbow or poor design, like a bedroom with no return air vent or a bedroom over a garage with only 1 supply vent.
I've had builders supply these and they did improve airflow. We would attach a junction box to the side of it and wire it into the furnace.
https://www.homedepot.ca/product/vortex-powerfan-vortex-vat-series-5-inch-axial-tube-inline-fan/1001690352?rrec=true