we recently had a new unit installed and I think it is a little oversized. It does push out more CFM‘s then the way the ductwork was designed. I have a room that I recently added that I would like to run a branch to. I can run a 6” from the plenum and another 6” from a trunk supplying the living room which has too much airflow right now. The reason why I think it has too much airflow is because the thermostat is in that room in the unit cycles on and off a lot. Plus the air is really pushing out. After making the holes in the ductwork the airflow is still sufficient.
My question is can I take these two 6 inch runs and combined them into an 8x6x6 wye so I can run an 8 inch into the new room. The new room has west facing windows so it is going to need more than a six-inch. By combining them into an 8 inch will I lose pressure since I’m going from a smaller flex to a bigger flex? Thank you