We moved into our house about a year ago and the second floor in my house never get enough heat in winter or cold AC in the summer. Our first floor can be a 72F deg while our upstair stays at nearly 66F in winter; while in summer the upstairs stays at 88F while the downstairs is 76F. As per the HVAC guy said, We have a Cadillac of HVAC systems and is more than enough to handle our house.
Our attic is well insulated with bat insulation between studs and about 19 inches of blown insulation plus automatic attic fan, so the insulation is good.
We check the vents and we can feel the air flow through the vents without an issue and make sure there is nothing obstructing the air flow from the vents.
We asked the HVAC guy if having a second zone upstarts will be a solution but he thinks that is not necessary and will be expensive to switch to a dual zone system and instead he recommended playing with the return airflow by closing the 2 returns in the first floor, so that the 2 returns in the second floor will force the hot/cold air to travel upstairs but it doesn't seem to be working.
Any one has any advise on what could be the issue and how to fix it?