I am currently planning to create a finished office space in my basement. I understand that habitable spaces require service from both supply and return air. The house, ~2000sqft over 2 habitable floors, is served by two main return air grates, one upstairs in the ceiling of the landing area, one downstairs in the wall in the kitchen.
My assumption going into making plans for air supply and return for the habitable space in the basement is that the current central air system was not specced or balanced for additional air return, however I am not totally sure if this is the case. Can an air return simply be added without worries of unbalancing the system too terribly?
Additionally, how should I go about calculating the size of the return ducting for the habitable space? The current proposed plans have the habitable space at around 140sqft. I have initially opted to have the return grate be a 10" x 10" to 6" duct. However I am unsure if this will constitute enough airflow to be satisfactory to the space.
Thanks in advance.