I am making preps to rip out my old fiberglass plenum and ducting and replace it. The current plenum is 18 inches deep, 25 feet long and 10 inches in height. All of the 8 inch flex ducting is coming off the top and then has to make a 90 degree turn to go to each room. This causes the ducting to pinch and restrict the air to certain rooms. The rooms with the longest ducting have minimal airflow coming out.
I plan on using 8 inch ducting from the plenum and then install 8x6x6 wye connectors to split the ducting to the rooms.
I need to know how to determine the correct size for a plenum. Need to know how to determine how many registers are needed based on size of room.