I have a tri-level with a sub basement. The lower level has no HVAC return and the cold air just collects at this level. The HVAC registers for the lower level are on the ceiling. I can add a HVAC return on the partial shared wall (due to tri-level layout) between lower level and sub basement. The new HVAC return would have to cross below several basement joists then two 90s to turn and get up into a joist bay to travel towards the HVAC trunk. Once at HVAC trunk I have two choices: A) topside takeoff but would have cut a hole in the bottom of the HVAC return to cut out for the takeoff on top of the HVAC return and then repair the trunk OR B) pass over the HVAC trunk return and use two 90s into the available side of the HVAC trunk. I know each 90 impacts airflow. While I am adding this new HVAC return should I also add a HVAC return for the basement. Thoughts on what option to use.