When adding an air return to the basement for the sake of habitable space, what considerations should I be making?

Question

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.

Answer 1

Use the rest of the house as a guide. Add up the floor areas (example 1400 square feet) and add up all the return grille openings (example 7 grilles with total 14 sq ft,) In this example there is 1 sq ft return grille area per 100 sq ft floor area. So in this example your basement office return grille would be 1.4 square feet. A 6-inch round duct would be the minimum size.